NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Wind Energy Forecasting: 

A Collaboration of the National 

Center for Atmospheric Research 

(NCAR) and Xcel Energy

Keith Parks 

Xcel Energy

Denver, Colorado

Yih-Huei Wan 

National Renewable Energy Laboratory

Golden, Colorado

Gerry Wiener and Yubao Liu 

University Corporation for Atmospheric Research (UCAR)

Boulder, Colorado

Subcontract Report

NREL/SR-5500-52233

October 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Wind Energy Forecasting: 

A Collaboration of the National 

Center for Atmospheric Research 

(NCAR) and Xcel Energy

Keith Parks 

Xcel Energy

Denver, Colorado

Yih-Huei Wan 

National Renewable Energy Laboratory

Golden, Colorado

Gerry Wiener and Yubao Liu 

University Corporation for Atmospheric Research (UCAR)

Boulder, Colorado

NREL Technical Monitor: Erik Ela

Prepared under Subcontract No. AFW-0-99427-01

Subcontract Report

NREL/SR-5500-52233

October 2011
This publication received minimal editorial review at NREL.

NOTICE

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express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

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iii

Acknowledgments 

The following institutions and individuals played key roles in development and integration of the wind energy 

forecasting system. Xcel Energy recognizes their extraordinary efforts to build an innovative system. 

National Center for Atmospheric Research

Barbara Brown

William Y.Y. Cheng 

Arnaud Dumont 

John Exby 

Tressa Fowler

Kent Goodrich 

Sue Ellen Haupt 

Tom Hopson

David Johnson 

Brice Lambi

Seth Linden 

Yubao Liu

Yuewei Liu

Bill Mahoney

Luca Delle Monache

William Myers

Julia Pearson

Matt Pocernich

Becky Ruttenberg 

Gregory Roux 

Doug Small 

Jenny Sun 

Biruk Tessema

Tom Warner

Gerry Weiner

Xcel Energy

Drake Bartlett

Russ Bigley

Michael Boughner 

Ryan Cline 

Efrain Davila 

John DeRosier 

Nick Detmer

Kasen Huwa

Sam Jenkins 

Carolyn Lee 

Terri Miller 

Shane Motley 

Keith Parks

Dain Patterson

Eric Pierce

Annie Rhoads 

Cassandra Smith

Jason Sweeney

Robert Wagner 

John Welch

National Renewable Energy Laboratory 

Erik Ela

Yih-Huei Wan
iv

Table of Contents

ACKNOWLEDGMENTS .................................................................................................................................................................. III

TABLE OF CONTENTSFIGURES.................................................................................................................................................. IV

FIGURES ............................................................................................................................................................................................... V

TABLES................................................................................................................................................................................................. V

INTRODUCTION .................................................................................................................................................................................. 1

WIND FORECASTING SYSTEM OVERVIEW ............................................................................................................................... 2

DATA ACQUISITION.......................................................................................................................................................................... 3

Data Requested .................................................................................................................................................. 4

Data Collection System ..................................................................................................................................... 6

Data Quality ....................................................................................................................................................... 7

WIND FORECASTING........................................................................................................................................................................ 8

Real-Time Four Dimensional Data Assimilation (RTFDDA) .......................................................................... 8

Other Public Forecasting Models ...................................................................................................................... 9

Dynamic Integrated ForeCast (DICast) ........................................................................................................... 10

POWER CONVERSION ....................................................................................................................................................................10

Level 1 and 2 Wind Plants ............................................................................................................................... 10

Level 3 Wind Plants ........................................................................................................................................ 11

NREL: Four Empirical Methods ..................................................................................................................... 11

NCAR: Empirical by Turbine by Plant ........................................................................................................... 13

OUTPUT...............................................................................................................................................................................................15

Meteorological Outputs ................................................................................................................................... 15

Operator’s GUI ................................................................................................................................................ 19

RESULTS .............................................................................................................................................................................................20

ONGOING WORK..............................................................................................................................................................................22

High-resolution Mesoscale Ensemble Prediction Model (EPM) .................................................................... 23

Analog-Based Kalman Filter Bias Correction Algorithm (AnKF) ................................................................. 24

Wind Ramp Prediction .................................................................................................................................... 24

CONCLUSIONS ..................................................................................................................................................................................25

REFERENCES.....................................................................................................................................................................................26

CONFERENCE PROCEEDINGS .....................................................................................................................................................27

ADDITIONAL REFERENCES ..........................................................................................................................................................29
v 

Figures 

Figure 1 – Xcel Energy Wind Forecasting System. .............................................................................................. 3

Figure 2 – Turbine Generation and Nacelle Wind Speed for Fifty Turbines. ....................................................... 6

Figure 3 – Schematic of Turbine-Level Data Collection. ..................................................................................... 7

Figure 4 – Data Quality by Counterparty. ............................................................................................................. 8

Figure 5 – RTFDDA-WRF Model Domains. ........................................................................................................ 9

Figure 6 - Wind Plant Loss Curves. .................................................................................................................... 12

Figure 7 – Same Turbine Type at Four Different Wind Plants. .......................................................................... 14

Figure 8 – Surface Air Temperature, Wind Vector; 10-km Domain; 25-hr Forecast. ........................................ 16

Figure 9 – Total Preciptable Water; 10-km Domain; 25-hr Forecast. ................................................................. 17

Figure 10 – 700mb Height Temperature and Wind Vectors; 10-km and 3-km Domain; 25-hr Forecast. .......... 18

Figure 11 – Wind Speeds at 80m; 10-km and 3-km Domain. ............................................................................. 18

Figure 12 – Operator’s GUI. ................................................................................................................................ 20

Figure 13 – PSCo monthly day-ahead forecast error. ......................................................................................... 21

Figure 14 – NSP monthly day-ahead forecast error. ........................................................................................... 21

Figure 15 – SPS monthly day-ahead forecast error. ............................................................................................ 22

Figure 16 – E-RTFDDA Model Domains. .......................................................................................................... 23

Tables

Table 1 – Summary of Data Granularity. .............................................................................................................. 4

Table 2 – Power Conversion Algorithms. ........................................................................................................... 10

Table 3 – MAE of the Four Models. ................................................................................................................... 12
1 

Introduction 

At the end of 2010, Xcel Energy managed the output of 3372 megawatts of installed wind energy. The wind 

plants span three operating companies1

, serving customers in eight states2, and three market structures3. The 

great majority of the wind energy is contracted through power purchase agreements (PPAs). The remainder is 

utility owned, Qualifying Facilities (QF), distributed resources (i.e., “behind the meter”), or merchant entities 

within Xcel Energy’s Balancing Authority footprints. Regardless of the contractual or ownership 

arrangements, the output of the wind energy is balanced by Xcel Energy’s generation resources that include 

fossil, nuclear, and hydro based facilities that are owned or contracted via PPAs. These facilities are committed 

and dispatched or bid into day-ahead and real-time markets by Xcel Energy’s Commercial Operations 

department. 

Wind energy complicates the short and long-term planning goals of least-cost, reliable operations. Due to the 

uncertainty of wind energy production, inherent suboptimal commitment and dispatch associated with 

imperfect wind forecasts drives up costs. For example, a gas combined cycle unit may be turned on, or 

committed, in anticipation of low winds. The reality is winds stayed high, forcing this unit and others to run, or 

be dispatched, to sub-optimal loading positions. In addition, commitment decisions are frequently irreversible 

due to minimum up and down time constraints. That is, a dispatcher lives with inefficient decisions made in 

prior periods. In general, uncertainty contributes to conservative operations – committing more units and 

keeping them on longer than may have been necessary for purposes of maintaining reliability. The downside is 

costs are higher. In organized electricity markets, units that are committed for reliability reasons are paid their 

offer price even when prevailing market prices are lower. Often, these uplift charges are allocated to market 

participants that caused the inefficient dispatch in the first place. Thus, wind energy facilities are burdened 

with their share of costs proportional to their forecast errors.

For Xcel Energy, wind energy uncertainty costs manifest depending on specific market structures. In the 

Public Service of Colorado (PSCo), inefficient commitment and dispatch caused by wind uncertainty increases 

fuel costs. Wind resources participating in the Midwest Independent System Operator (MISO) footprint make 

substantial payments in the real-time markets to true-up their day-ahead positions and are additionally 

burdened with deviation charges called a Revenue Sufficiency Guarantee (RSG) to cover out of market costs 

associated with operations. Southwest Public Service (SPS) wind plants cause both commitment inefficiencies 

and are charged Southwest Power Pool (SPP) imbalance payments due to wind uncertainty and variability. 

Wind energy forecasting helps mitigate these costs. Wind integration studies for the PSCo and Northern States 

Power (NSP) operating companies have projected increasing costs as more wind is installed on the system due 

to forecast error [1][2]. It follows that reducing forecast error would reduce these costs. This is echoed by large 

scale studies in neighboring regions and states that have recommended adoption of state-of-the-art wind 

forecasting tools in day-ahead and real-time planning and operations [3][4]. Further, Xcel Energy concluded 

reduction of the normalized mean absolute error by one percent would have reduced costs in 2008 by over $1 

million annually in PSCo alone [5]. The value of reducing forecast error prompted Xcel Energy to make 

substantial investments in wind energy forecasting research and development. 

Wind energy variability, especially rapid changes in wind energy production, hereto called ramps events, are 

both a cost and reliability concern. A rapid ramp-up event requires conventional generators to turn down with 

comparable speed and vice versa for ramp-down events. Unfortunately, conventional generators have a finite 

 

1 Northern States Power (NSP), Public Service of Colorado (PSCo), and Southwestern Public Service (SPS) 2 Colorado, Michigan, Minnesota, New Mexico, North Dakota, South Dakota, Texas, and Wisconsin

3 Market participant in Southwest Power Pool (SPP) and Midwest Independent Transmission System Operator (MISO) and as a 

Balancing Authority engaged in bilateral transactions in the Western Electric Coordinating Council (WECC).
2 

capacity to absorb wind ramp events at any one time. To better understand ramp event behavior, ramp events 

in PSCo and SPS were studied by the National Renewable Energy Laboratory (NREL). It was found that ramp 

events have seasonal and time-of-day patterns and can have a skewed distribution between up and down events 

[6]. 

Frequently, there is poor to no notification of significant ramp events due to imprecise modeling of the 

underlying meteorological conditions. Large scale events, such as cold fronts, may be predicted by weather 

models, but the timing is frequently incorrect by tens of minutes to hours. Smaller scale events, such as strong 

outflows associated with convective activity, are frequently not forecasted at all. The uncertainty of timing and 

magnitude of wind ramp events requires operators to stage the balance portfolio conservatively despite wind 

ramps infrequent occurrence.

Xcel Energy contracted with the National Center for Atmospheric Research (NCAR) to develop a corporatewide wind forecasting system. The program began in December 2008 with a period of performance of 18 

months, ending in June 2010. A complete and stable system delivered in October 2009. The new forecasting 

system was adopted by Xcel Energy’s Commercial Operations department for production purposes over the 

ensuing months. The system continued to be improved and refined through the period of performance. Since, it 

has continued to be the primary wind forecasting tool used at Xcel Energy. The focus of this report is the wind 

forecasting system developed during this contract period with results of performance through the end of 2010. 

The report is intentionally high-level, with technical details disseminated at various conferences and academic 

papers (see references). 

Wind Forecasting System Overview

The Xcel Energy wind forecasting system consists of four parts (see Figure 1): Data Acquisition (blue), Wind 

Forecasting (yellow), Power Conversion (orange), and Output (red). Each component initializes the 

downstream process. A brief overview follows; standard meteorological data augmented by turbine-level 

nacelle wind speeds are fed to the weather forecasting cluster - a multi-processor computer system. The cluster 

utilizes the Real-Time Four Dimensional Data Assimilation (RTFDDA) process combined with a proprietary 

configuration of the Weather Research and Forecasting (WRF) model adapted for wind forecasting 

applications. The forecasted wind speed from the cluster, forecasted wind speed from publicly available 

weather models (together called the foundation forecasts), and nacelle wind speed gathered from the Xcel 

Energy wind plants are input into the Dynamic Integrated ForeCast (DICast). DICast generates a consensus 

wind speed forecast based on historic performance of the various forecasted data streams. The consensus 

forecast is sent to the power conversion module which converts wind speed to power using real-time power 

and wind speed data to initialize the calculation. Lastly, data are communicated to users via a CSV file and 

ultimately a forecast display for operators.
3 

Figure 1 – Xcel Energy Wind Forecasting System. 

Wx Forecasting Cluster

(high resolution forecast)

Xcel Wind Farm Data 

(status, power generation, etc.)

DICast asks:

Given what I know 

now...what is my 

“best forecast”

Operations Wind 

Forecasting Display

Input MET Data & Observations

(NOAA model output and a variety of observations)

Xcel Users and Operators

Data & Observations from External Sources

Internal Xcel Data

Other Wx Models

RUC

GFS

NAM

etc Px CONVERSION

CSV File 

Output

Automated Processes

Wx Forecasting Cluster

(high resolution forecast)

Xcel Wind Farm Data 

(status, power generation, etc.)

DICast asks:

Given what I know 

now...what is my 

“best forecast”

Operations Wind 

Forecasting Display

Input MET Data & Observations

(NOAA model output and a variety of observations)

Xcel Users and Operators

Data & Observations from External Sources

Internal Xcel Data

Other Wx Models

RUC

GFS

NAM

etc Px CONVERSION

CSV File 

Output

Automated Processes

The system produces two forecasts for each forecast node: a 72-hour forecast with hourly resolution updated 

every 15 minutes and a three-hour forecast with 15-minute resolution updated every 15 minutes. A forecast 

node is a group of turbines with a common point of interconnection – this is the most granular level of 

interest for operational purposes. There are 45 forecast nodes in the forecast system ranging from 2MW to 

575MW. To contrast, the term wind plant is defined as a group of turbines under a common purchase power 

agreement, qualifying facility designation, or owned resource. While wind plants and point of interconnect 

(and thus forecast node) are frequently the same groups, sometimes wind plants cross multiple forecast 

nodes and multiple plants are behind one forecast node. Forecast nodes are then aggregated by Xcel 

Energy’s three operating companies to give the system view most frequently used by operators.

This report discusses each of the four components (Data Acquisition, Wind Forecasting, Power Conversion, 

and Outputs) separately. A results section discusses year-over-year forecast improvements. For more 

information regarding the entire system, see [7].

Data Acquisition

A unique feature of the Xcel Energy wind forecasting system is the unprecedented acquisition of real-time 

turbine level data for use in a wind forecasting system. At the end of 2010, Xcel Energy was collecting power 

and meteorological data from 1929 turbines (74% of total), accounting for 2692 MW of generation (80% of 

total) on a sub-minute basis. In addition, static information, such as latitude, longitude, and hub height, was 
4 

gathered from the counterparty or through the Federal Aviation Administration’s Obstruction 

Evaluation/Airport Airspace Analysis (OE/AAA) database.4

 

The level of collected wind plant data determined the technique used to forecast power output of those 

turbines. Wind plants fell into three types of data detail. First, there are plants that had no turbine and nodallevel generation or meteorological data; we label these Level 1 wind plants. These include plants that were not 

included in the data gathering effort and are net metered − thereby conflating their output with load − or 

market participants within the Balancing Authority whereby nodal generation was unavailable due to the 

separation of transmission and marketing functions within Xcel Energy.5 Second are wind plants with only 

total power output at the interconnection point (Level 2). These tend to be plants that were not included in the 

data gathering effort but are metered. These are plants that are technically incapable of providing real-time data 

and/or are small in installed power. For example, turbines built prior to 2003 tended to be technically incapable 

of providing data via the architected data collection system and plants less than 10MW were not pursued 

unless bundled together with other larger plants. Lastly, there are plants that provide turbine-level and nodallevel detail (Level 3). The last category makes up 80% of the installed generation (Table 1). Xcel Energy 

continues to pursue integration of real-time data in 2011. 

Table 1 – Summary of Data Granularity. 

Type Turbine Detail Nodal Power MW % Total

Level 1 No No 194 6%

Level 2 No Yes 486 14%

Level 3 Yes Yes 2692 80%

Total 3372

Data Requested

The data gathering effort was undertaken starting in October 2008 to gather real-time turbine-level data and 

meteorological data at the wind plant. Since the majority of Xcel Energy’s wind energy is provided by 

counterparties through Power Purchase Agreements (PPAs), data needed to be separately negotiated and was 

acquired via each of the contractual counterparties. 

Turbine Data

The requested data points changed over time as we refined our vocabulary. Ultimately, we settled on the 

following ten real-time data points:

1. Turbine Generation 

2. Nacelle Wind Speed

3. Turbine Availability Status (or equivalent)

4. Generator Status (on/off-line)

5. Nacelle Position (relative to true north)

6. Wind Direction (relative to true north)

7. Yaw Error

8. Rotor RPM

9. Blade Pitch

10. Voltage 

 

4 Obstruction Evaluation/Airport Airspace Analysis (OE/AAA), https://oeaaa.faa.gov/oeaaa/external/portal.jsp 5 FERC Order 880 separates the energy marketing and transmission functions in the utility sector. Since the wind forecasting tool is 

primarily used by Xcel Energy’s energy marketing department, real-time output of other generators (like competing wind farms) 

within the Balancing Authority (BA) are restricted.
5 

And the following static data points: 

1. Latitude (NAD83)

2. Longitude (NAD83) 

3. Hub Height (m) 

4. Turbine Manufacturer and Model

5. Manufacturer’s Power Curve 

The first seven real-time data points were considered important for wind forecasting purposes. The remaining 

real-time data points were requested with the intention of possibly extending research at some future time. 

While many points were requested, a subset of requested points was typically collected. Additionally, the 

requested dataset improved over time. For example, nacelle position is not a quality indicator of wind direction 

and had to be further refined to include relative to true north in the request. Wind direction and yaw error were 

added to the request after working with two wind plants. Even so, nacelle position relative to true north and 

wind direction are not typically part of the turbine system – i.e., they were never collected.6 At a minimum, 

turbine generation, nacelle wind speed, and turbine availability status or equivalent were collected. In the case 

of turbine availability status, Xcel Energy translated numerous equivalent codes into a binary variable 

indicating the availability of the turbine. 

Fortunately, the first three data points were the most useful for forecasting purposes. While wind direction was 

considered a critical meteorological data feed, and would be useful for future forecasting efforts, the teams 

quickly recalibrated data expectations to focus on the first three, most reliable data feeds. An example of the 

turbine generation and wind speed data for fifty turbines coincident with a slow-moving front is shown in 

Figure 2. 

 

6 It was later learned that nacelle position can be calibrated to true north − either by direct manipulation of the turbine system or 

through a calculated offset. It is a lesson learned that turbines are able to provide true north through coordinated activity or 

calculation, but is not part of the standard outputs available in the wind farm SCADA systems. 
6 

Figure 2 – Turbine Generation and Nacelle Wind Speed for Fifty Turbines. 

 -55 -50 -45 -40 minutes -20 -15 -10 -5 

Plot-0 2/18/2010 2:45:04 PM

-200

-100

0

100

200

300

400

500

600

700

800

 -55 -50 -45 -40 minutes -20 -15 -10 -5 

Plot-0 2/18/2010 2:45:04 PM

-1

1

3

5

7

10

Turbine 

Generation

Wind Speed

 -55 -50 -45 -40 minutes -20 -15 -10 -5 

Plot-0 2/18/2010 2:45:04 PM

-200

-100

0

100

200

300

400

500

600

700

800

 -55 -50 -45 -40 minutes -20 -15 -10 -5 

Plot-0 2/18/2010 2:45:04 PM

-1

1

3

5

7

10

Turbine 

Generation

Wind Speed

The turbine data are used throughout the wind forecasting system to initialize the weather forecasting cluster, 

calibrate wind speed forecasts, initialize power conversion, and establish performance metrics. 

Meteorological Tower Data

When available, meteorological tower data was gathered at the wind plant and communicated via OSISoft’s PI 

system. Data requested included: 

1. Wind Speed**

2. Wind Direction**

3. Temperature

4. Pressure

5. Air Density

**at all sensor heights 

Meteorological tower data was infrequently available at wind plants. In many instances where data was 

available, the data quality was poor or incomplete due to broken or poorly maintained sensors. 

Data Collection System

Xcel Energy gathers turbine-level data and meteorological data from Xcel Energy owned and contracted wind 

plants on a real-time basis. The data are collected and stored in numerous PI Systems

7

. A centralized PI 

System is placed outside the Xcel Energy corporate firewall (called the DMZ), thereby enabling easy 

 

7 The PI System is an energy industry-standard product produced by OSISoft (www.osisoft.com). The PI System stores and manages 

data intensive time-series data in an efficient, easy-to-use format.
7 

communication with outside counterparties. Data are retrieved from wind plants via one of two pathways 

which are described in detail in the next sections: 

1. Direct from the wind plant - a direct feed from an Xcel Energy owned server installed at the wind plant. 

2. PI-to-PI Connection - A PI-to-PI connection to a counterparty’s PI System that already collects the 

required turbine-level and meteorological data.

Direct from the Wind Plant

Data are securely transmitted from the wind plant to Xcel Energy over the Internet (Figure 3). An Xcel PI 

Interface Node – a server connected to the wind plant supervisory control and data acquisition (SCADA) 

system – spools and translates turbine-level data into PI System format. The data are pushed across a Virtual 

Private Network (VPN) tunnel secured by two CISCO ASA 5505 devices with ready access to the Internet. 

Data are stored on the Xcel Energy PI DMZ System located outside the corporate firewall. Data are then 

spooled to the Corporate PI System server for long-term storage. New data on the PI DMZ System eventually 

overwrites the oldest data. The expected life of data is estimated to achieve steady state at approximately three 

months. 

Figure 3 – Schematic of Turbine-Level Data Collection. 

PI-to-PI Connection

The second, more common method is a direct connection to another existing PI System. If the counterparty 

already has implemented a PI System solution, then a VPN tunnel is established between the counterparty’s PI 

System and the PI DMZ System. Data are streamed across this PI-to-PI connection in real-time.

Data Quality

Timely, high quality data are a key part of the Xcel Energy wind forecasting system. To ensure quality, 

separate data quality calculations are performed on each set of turbine data. Range checks are performed on 

generation and wind speed data. For example, wind speeds are assumed to be positive and generation must be 
8 

less than 120% of rated capacity. If the range check fails, the turbine data are flagged as bad. If the range check 

passes, then a time lag check is performed. If the last turbine-level data are greater than 5-minutes old, the data 

are flagged as bad. If the turbine passes all tests, the data quality passes as good. In addition to providing a 

real-time measure of data quality, performance can be measured as the percent of time the turbine had good 

data quality. Data are summarized by wind plant, counterparty, and operating company. Counterparties that 

drop in performance can be notified and mitigating action taken. 

Data quality can be plotted historically by counterparty (see Figure 4), though counterparty names are 

excluded to protect their identity. Real-time data quality tended to perform around 90% overall with occasional 

degradation near 80%. Periodic data outages occurred, some lasting weeks. Since it is unrealistic to expect all 

data to flow all the time, a guide of 90% compliance was adopted at Xcel Energy – only counterparties that 

performed below 90% were informed of their performance. In rare occasions, technical problems associated 

with older wind plant technology caused a counterparty to be much below the target. 

Figure 4 – Data Quality by Counterparty. 

Turbine Data Collection Performance

20%

30%

40%

50%

60%

70%

80%

90%

100%

Jan-10 Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10

Month

Percentage of Valid/Timely Data

OVERALL DATA QUALITY

Wind Forecasting

The Wind Forecasting component is the most complex function in the Xcel Energy wind forecasting system. 

This component consists of the forecasting cluster (RTFDDA-WRF), other public forecasting models, and the 

DICast systems.

Real-Time Four Dimensional Data Assimilation (RTFDDA) 

The Real-time Four Dimensional Data Assimilation (RTFDDA) version of the Weather and Forecasting 

Mesoscale model (WRF) is a mesoscale numerical weather prediction model designed for high-resolution 

applications, featuring rapid forecast updates and continuous real-time assimilation of observed data. A 

customized version for Xcel Energy was optimized for wind energy applications. The model domain covers 

the entire western United States (D1 resolution = 30-km), with two nested domains (D2 resolution = 10-km; 

D3 resolution = 3-km) (see Figure 5). The model includes 41 vertical levels for all domains. D1 and D2 

domains operate for the entire 72 hour forecast cycle with hourly outputs. The D3 domain is used for the first 

24 hours of the forecast cycle, with 15 minute outputs. The model has a cold start once a week on Saturdays, 

with restarts every three hours (i.e., warm starts). The warm RTFDDA-WRF ingests new observations, 

including real-time nacelle wind speeds and meteorological tower information collected by Xcel Energy. 
9 

During forecast operation, model outputs − most importantly hub height wind speeds − are written in situ and 

passed to DICast for post-processing. Additionally, numerous meteorological displays are generated for use by 

staff meteorologists. The meteorological displays are discussed later in this paper. 

Figure 5 – RTFDDA-WRF Model Domains. 

The RTFDDA-WRF runs on 53 servers (49 x Dell PowerEdge 1950; 4 x Dell PowerEdge 2950) installed on 

two racks with 3 switches (2 x Dell Power Connect 5448 switch; 1 x Myrinet M3 E64 switch). It is referred to 

as the deterministic cluster, or XCEL-C1. For more information on the RTFDDA-WRF subsystem, please see 

[8].

Other Public Forecasting Models

Besides the RTFDDA-WRF, the Xcel Energy wind forecasting system utilizes other public forecasting models 

including: 

1. North American Mesoscale model (NAM)

2. Global Forecast System (GFS)

3. Rapid Update Cycle (RUC)

4. MAV-MOS

5. MET-MOS

6. LAMP-MOS

These models are maintained for the public good by the National Center for Environmental Prediction 

(NCEP). Only the wind speeds from grids coincident with turbine locations are extracted. The ingest process 

must manage the various refresh times and forecasted time horizons of the various models in preparation for 

the DICast component. 

D1

D3

D2
10 

Dynamic Integrated ForeCast (DICast) 

The Dynamic Integrated ForeCast (DICast) system ingests RTFDDA-WRF model output, output from other 

public weather forecasting models (together called foundation forecasts), and nacelle wind speed 

observations to generate a consensus forecast. DICast performs a two-step optimization to generate the 

consensus forecast. In the first stage, DICast performs regression statistics to remove model bias from each 

of the foundation forecasts, better known as dynamic model output statistics (DMOS). The DMOS 

calculation is performed weekly with a training period over the last ninety days. In the second stage, DICast 

assigns weights to various adjusted foundation forecasts based on their recent performance. This calculation 

is performed daily using the last day’s dataset as the training set. The weights are calculated for every 

forecasted time step, for every issue time, at every nacelle in the Xcel Energy service territories. While 

weights are equal at the initial condition, DICast changes weights daily to favor the better forecasts. Note 

that the maximum weight change is restricted to maintain stability. For turbines that have no real-time 

nacelle wind speed data, the weights never change from the initial conditions (i.e. the average of the 

foundation forecasts). The consensus forecast is the inner product of the DICast weights with the available 

adjusted foundation forecasts, divided by the sum of the weights of the available forecasts. The consensus 

forecast is a major input into the power conversion module.

DICast provides robustness by always producing a consensus forecast. If the real-time data ceases, DICast 

stops optimization and propagates the latest weights until such time that real-time data are reestablished. If a 

foundation forecast is missing, DICast will continue to optimize with the available forecasts. For more 

information on the DICast subsystem, please see [9] 

Power Conversion

The power conversion algorithm depends on availability and granularity of real-time data. Three data detail 

categories were determined throughout the course of the project (see Table 1). In total, three power conversion 

algorithms were explored; two by NCAR − one for Level 1 and 2 plants and one for Level 3 plants − and four 

by NREL for Level 3 wind plants. The system ultimately employed NCAR’s power conversion technique for 

Level 3 turbines.

Table 2 – Power Conversion Algorithms. 

Count Institution Turbine Detail Power Conversion

1 NCAR Level 1 and 2 Manufacturer's Power Curve

2 NCAR Level 3 Direct Estimate by Turbine by Plant

3 NREL Level 3 Equivalent Power Curve

4 NREL Level 3 Directional Equivalent Power Curve

5 NREL Level 3 Direct Estimate by Turbine Type

6 NREL Level 3 Neural Network

Level 1 and 2 Wind Plants 

The manufacturer’s power curve is used for power conversion for Level 1and 2 plants. This is a simple and 

rudimentary approach. There are many reasons more sophisticated models were not created. Level 1 and 2 

wind plants make up a minority (less than 20%) of the total portfolio. Wind forecasting vendors have already 

developed sophisticated algorithms with limited real-time data. Further, Level 1 plants – especially thirdparties within the Balancing Authority (BA) – may provide real-time data to Xcel Energy in the future either 

per a recent FERC Notice of Proposed Rulemaking (NOPR)8

 or through other commercial agreements. Level 2 

plants tend to be older plants with incompatible SCADA systems for the data collection process that, as they 

 

8 November 18, 2010. FERC NOPR Docket No RM10-11-000 Integration of Variable energy Resources. 
11 

upgrade their systems, will become data-rich (Level 3). Future plants will be Level 3 plants because provisions 

in the PPA now require specific data to be provided. Thus, little effort was placed on improving techniques for 

Level 1 and 2 wind plants.

Level 3 Wind Plants 

The availability of real-time turbine-level data affords opportunities for novel power conversion 

methodologies. Both NCAR and NREL pursued power conversion methodologies. NREL developed four 

empirical power conversion methodologies for an entire wind plant, controlling for wind direction, 

temperature, pressure and wind speed from two sources: meteorological towers and nacelle wind speeds. 

NCAR developed a turbine specific methodology, empirically associating nacelle wind speed to turbine 

generation. The two methods are discussed below, followed by a discussion of their merits and pitfalls. 

NREL: Four Empirical Methods

While output of a single wind turbine can be characterized by the manufacturer’s power curve, an equivalent 

wind plant power-curve becomes highly desirable and useful in predicting an aggregated output for a given 

wind speed forecast. However, unlike the single-turbine power curve, it is difficult to capture all the nuances of 

a wind plant consisting of tens or even hundreds of turbines with a single curve. A model capable of fully 

characterizing the complex input/output relationship of a wind plant to account for the effects of different wind 

directions, local terrain, and asymmetric turbine layout in a wind plant may consist of a set of curves or some 

other mathematical models. The wind speed information from on-site meteorological tower and metered plant 

output are used to construct an equivalent power curve for the entire plant. As expected, the resulting curve 

takes a similar shape of the single turbine power curve. Four empirical power curves were developed: 

equivalent power curve, directional equivalent power curve, direct estimate based on turbine type, and neural 

network. 

The data used for this effort include 10-minute time series of average individual turbine generation, nacelle 

wind speed, total metered wind plant output, wind speed and direction from on-site meteorological (MET) 

towers, temperature and barometric pressure. Quantity and availability of these data vary significantly 

throughout the studying period. Consequently a large amount of effort is devoted to clean the data to arrive at 

consistent data streams.

Equivalent Power Curve

For a wind plant with hundreds of turbines, wind speed forecast for every turbine will be difficult to obtain. A 

more likely scenario is to have only one wind speed forecast for the entire plant site. In this project, the wind 

speed information from on-site MET tower and metered plant output are used to construct an equivalent power 

curve for the entire plant. As expected, the resulting curve takes a similar shape of the single-turbine power 

curve. 

Directional Equivalent Power Curves

The asymmetric layout of turbines in wind plants indicates they are designed to minimize the wake effect of 

the site’s prevailing wind direction. The data show the plant outputs vary with the directions of winds, and it 

suggests that a set of power curves, each associated with a specific wind sector may provide a better power 

conversion result. The wind resource data from the on-site MET tower were separated into eight wind sectors. 

The directional wind data along with their corresponding metered plant output are used to construct eight 

power curves for the wind plant. These eight directional power curves also resemble the single-turbine power 

curve. However, the differences between the equivalent power curves of the prevailing wind direction and the 

least frequent wind direction are significant.
12 

Direct Estimate Based on Turbine Types

The specific wind plant for this project has two types of turbines – one with a hub-height of 80 m and the other 

69 m – and thus two different turbine power curves. Another approach is to use wind information at the 

corresponding hub heights and the two turbine power curves to estimate the output of two groups of turbines 

directly. Outputs of one turbine from each group are estimated based on the hub-height wind speed forecast. 

Single turbine output is multiplied by the predicted numbers of on-line turbines for each group to calculate the 

group output. 

If wind speed can be predicted for every turbine location within a wind plant, the power conversion process 

becomes a simple exercise of summing up all turbine outputs (obtained through manufacturer’s turbine power 

curve) minus plant losses. Figure 6 below shows the wind plant loss curves from the available data. 

Figure 6 - Wind Plant Loss Curves. 

The plant output is the sum of two group outputs minus plant losses with the losses being estimated based on 

Figure 6. 

Neural Network Model

Finally, the neural network technique is used to simulate the complex relationship between wind speeds, 

directions, temperature, and pressure and the wind plant output. In this report, the neural network is a 

straightforward feed-forward network with back-propagation using all available wind resource data and 

weather information from the on-site MET towers.

The results of these four approaches are compared by calculating the mean absolute error (MAE) between the 

predicted and actual wind plant outputs. Table 3 below lists MAE of all four approaches. 

Table 3 – MAE of the Four Models. 

Equivalent 

Power Curve

Directional 

Equivalent 

Power 

Curve

Neural 

Network

Direct 

Estimate

MAE (MW) 26.4 23.1 18.5 11.9

MAE (% of plant

capacity)

8.8% 7.7% 6.1% 4.0%
13 

There are many issues with characterizing wind power plant operations with an equivalent power curve or a set 

of such curves, especially for large wind plants with many turbines spread over a wide area at different 

elevations. The problems arise because the output of a plant is influenced by many variables. Wind speed is the 

most critical variable in determining the plant output, but no single wind speed can adequately represent the 

wind conditions for the entire wind plant. Many wind speed values are required to characterize the plant

operation. Depending on how the wind speed values are obtained, using them to characterize the plant 

operation can result in large uncertainty. 

There are other variables besides wind speed that affect the plant performance and output levels. The results of 

directionally equivalent power curves demonstrated that separating the wind resource into major sectors 

marginally improves the representation. More data are required to reduce the noise in the resulting equivalent 

power curves. Finer wind sectors may incrementally improve the accuracy further, but also incurs the problem 

of determining the wind directions that are representative of the wind conditions for the entire wind plant. 

The neural network technique appears to be well suited for the task of representing the complex relationship 

between input variables (wind speeds, direction, etc.) and plant output level. The reason that the neural 

network model did not perform significantly better than the equivalent power curves could be due to the input 

data quality. The locations of the two MET towers may not be optimal to characterize the wind conditions for 

the entire wind plant. A more rigorous quality check may be required on all input data, although it is not clear 

how much improvement can be gained with additional cleaning of the input data. Using a more complex 

dynamic neural network model may offer greater performance improvement, especially during high wind 

periods. Individual wind turbines exhibit hysteresis behavior around cut-off wind speeds, and therefore 

individual turbine output is determined not only by current conditions of wind speed and direction, but depends 

on previous turbine states. A dynamic neural-network model would be able to simulate this behavior. 

The direct estimate approach produces the best results compared to other approaches tested. This approach is 

straightforward and the concept behind it is simple. However, its success depends on the quality of turbinelevel data. 

A detailed description of NREL’s estimation effort can be found at [10].

NCAR: Empirical by Turbine by Plant

Due to the availability of turbine-level data and the encouraging results of the NREL investigation, a turbinelevel power curve methodology was implemented. An initial study was completed that demonstrated each 

turbine type within a plant behaved similarly given the paired wind speed and generation data. However, the 

behavior of the same turbine type over different plants was disparate (see Figure 7). The deviation from the 

manufacturer’s power curve is also peculiar to each wind plant. 
14 

Figure 7 – Same Turbine Type at Four Different Wind Plants. 

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A representative turbine by turbine type by plant was selected for data mining. Wind speed and generation data 

was averaged over 15-minute intervals. The most predictive equation for current generation was derived from 

the previous generation and wind speed and the current wind speed. When forecasting, the forecasted wind 

speed is substituted for the current wind speed in a recursive equation (1a). The previous 15-minute observed 

power and wind speed initializes the equation (1b). 

However, due to technical and operational vagaries, observed power and wind speed were not always 

available. In the absence of a valid observed power data point, an estimate is made using the observed wind 

speed and the manufacturer’s power curve (1c). In lieu of any real-time data, the forecast wind speed is 

transformed to power through the manufacturer’s power curve (1d). 

Equation 1 – Empirical by Turbine

( ) 1 100 ,, t+ = WSWSPfP t+ (a)

0 = obs 0 = WSWSPP obs (b) 

( ) 0 = obs 0 = WSWSWSMP obs (c)

( ) 0 = 1 0 = WSWSWSMP 1 (d) 

The forecasted power at the turbine-level is summed by connection node. The connection nodes are summed 

by operating company. This technique was internally referred to as the sum-of-turbines approach. 

The sum-of-turbines approach has inherent flaws. As described in the above section, the losses between the 

turbine and the point of interconnect can be significant. These losses can be as large as 4-5% for plants with 

long radial transmission lines. Typically, wind plants are close to the point of interconnection making losses 

nominal, or the loss equation was estimated and applied outside the modeling environment. Second, the 

approach tended to assume full availability. Though, this can be mitigated by carefully choosing a training 
15 

dataset with periods of maintenance, forced outages, and curtailments. If the training set included such periods, 

the data mining processes tended to derate the forecast-based current availability or curtailed output. This has a 

down side as derates tend to be propagated based on historic precedent and not current operational realities. 

There are inherent benefits as well. The turbine-level datasets, when available, tended to be clean and usable 

with little data quality work. The method is simple. When connection nodes were expanded with new wind 

turbines, the turbine-level model could be added for the additional turbines, rather than retraining the entire 

connection node output. 

Output 

The Xcel Energy wind forecast system produces many weather related outputs including RTFDDA-WRF 

outputs, CSV files containing forecasted energy, and an Operator’s graphical users interface (GUI) based on 

the CSV files. 

Meteorological Outputs

The RTFDDA-WRF produces many weather related snapshots over the forecasted domains (D1 - 30-km; D2 - 

10-km; D3 - 3-km) and forecast times. Xcel Energy meteorologists use these snapshots to better understand 

meteorological conditions to forecast wind and load, and characterize possible risks to day-ahead and real-time 

operations. The images are updated every 3-hrs and are archived for three days. 

Figure 8 displays surface air temperature and wind vectors. It has a resolution of 10-km and is forecast hour 25 

after the current hour. This one image contains a wealth of information about the pressure, temperatures, and 

winds at the surface. The images at different forecast steps can be invoked by rolling a mouse over a series of 

hyperlinks. This creates a “moving image” that gives meteorologists a quick overview of the forecast in both 

the geographic and time domains. 
16 

Figure 8 – Surface Air Temperature, Wind Vector; 10-km Domain; 25-hr Forecast. 

Conditions of interest go beyond temperature and wind speeds. Figure 9 is an image of precipitable water for 

the 25-hour forecast. Precipitable water indicates the amount of moisture in a given environment. It indicates 

clouds and precipitation – or the likelihood thereof. It can help predict convective activity, identify fronts, and 

cyclones.
17 

Figure 9 – Total Preciptable Water; 10-km Domain; 25-hr Forecast. 

Figure 10 contains images of the 700 mb height, temperature, and winds. The first image has a coarser 

resolution (10-km grids). The second image is from the same 25-hour forecast, but the resolution is finer (3-

km grids). The nested domain of the 3-km grid is smaller in the higher resolution map. 
18 

Figure 10 – 700mb Height Temperature and Wind Vectors; 10-km and 3-km Domain; 25-hr Forecast. 

Figure 11 includes four images that indicate the wind speeds at 80 meters. The wind barbs indicate direction 

and speed while the color scheme indicates a finer resolution of wind speed. The first image is the full nested 

grid with 10-km grid spacing while the next three are the individual maps for each service territory at the 3-km 

grid spacing. Eighty meters was chosen because this is the dominant hub height for wind turbines in Xcel 

Energy’s territory. These are superior to surface wind speeds, allowing meteorologists to view a slice of model 

output at the height of most significance over each of the service territories.

Figure 11 – Wind Speeds at 80m; 10-km and 3-km Domain. 
19 

Operator’s GUI

The Operator’s GUI displays historic and forecasted energy time-series data with numerous drill down, look 

back, and configurable views. The observed output is displayed in a 15-minute average power time series to 

the left of the real-time line. Forecasted wind energy (both the 3hour/15minute and 72hour/hour forecasts) is

shown to the right of the real-time line with a band indicating recent performance. The performance band is the 

75% cumulative absolute error distribution over the last seven days above and below the expected forecast. 

The graph supports power (left) and percent capacity (right) on the y-axis. The time scale (x-axis) can be 

adjusted to accommodate different time zones and historic and forecasted time frames. Forecasts are rolled up 

to a system level view by the operating company. A user can drill down to every point of interconnection to 

view the observed and forecasted time-series data. Lastly, the user can look back at prior forecasts up to 72 

hours in the past overlaid with observed data. 
20 

Figure 12 – Operator’s GUI. 

Results

The NCAR forecasting system was integrated into day-ahead and real-time operations from November 2009 to 

April 2010. PSCo and NSP were integrated simultaneously first, with SPS’ integration second. As such, a 

comparison of 2009 versus 2010 is largely a comparison of wind energy forecasts without and with the NCAR 

forecast system. Installed wind energy capacity did grow slightly over the comparison period, complicating a 

head-to-head comparison. To mitigate, installations at the very end of the study period, such as the installation 

of 200 MW Nobles Wind Plant in NSP in December 2010, are excluded from the error metrics. Remaining 

growth is only 233MW across all three systems (PSCo, 174 MW; SPS, 34.5 MW; NSP, 24.5 MW). Thus, error 

metrics are normalized by installed capacity hereto called the Mean Absolute Percent Error (MAPE). We focus 

on the day-ahead forecast. 

The day-ahead forecast is used in the day-ahead commitment process. This process has different forecast 

horizons depending on the market structure. For example, NSP bids into MISO’s day-ahead market. MISO’s 

day-ahead market operates everyday – Monday’s bids and awards for Tuesday, and so on. In PSCo and SPS, 

the day-ahead commitment process is through the next business day – Monday for Tuesday, and so on, until 

Friday, which is a 3-day forecast through Monday. Forecasts extend longer for holidays. The implications of 

the day-ahead commitment process are many. Generators are informed of the intention to run the next day, 

natural gas nominations committed, and incremental and decremental prices are set up for day-ahead trading. 

Prior to the NCAR forecasting system, Xcel Energy performed its own day-ahead forecast. Forecasted wind 

speeds were downloaded from the North American Mesoscale Model (NAM) as made available by 

Expected Wind Energy

Confidence Band 

Observed Output

Adjustable 

Timescale

Aggregated vs

Wind Plant View

View Previous Forecasts

Expected Wind Energy

Performance Band 

Observed Output

Adjustable 

Timescale

Aggregated Wind 

Plant View 

View Previous Forecasts
21 

Pennsylvania State University’s Bufkit Data Distribution System (BDDS)9. Since all NAM gridded output is 

unavailable through the BDDS, data soundings nearby major wind centers were used as proxies for a wind 

plant’s wind speed forecast. The forecasted wind speed between 45m and 80m above ground level were used 

to approximate hub height wind speed. The manufacturer’s power curve was used to transfer hub height wind 

speed to power output. The power output was scaled by the number of turbines at the wind plant and further 

adjusted for wind plant turbine availability. 

The day-ahead forecast improved annually across all three systems. In Figure 13, PSCo saw reductions in error 

in every month, with a total reduction of from 18% (2009) to 14.3% (2010) – an absolute reduction of 3.7% - 

or a 20% decline in forecast error. Figure 14 demonstrates similar gains in NSP. Improvements were made 

every month except June and September. Annual forecast error was reduced by 3.5% from 15.65% (2009) to 

12.2% (2010). SPS also demonstrated lower forecast errors (Figure 15). All months had lower forecast errors 

except February and March. Annual forecast error modestly reduced from 16.4% (2009) to 14.0% (2010) – an 

absolute reduction of 2.4% − or a 14.7% reduction. Note that SPS was the last system to fully integrate the 

NCAR modeling system. Also, improvements in the RTFDDA-WRF parameterization schemes implemented 

in June 2010 are believed to have contributed to further lowering the forecast errors. Overall, months with 

lower production (July through September) tend to have lower forecast errors. January and February 2010 in 

NSP and PSCo had unusually low capacity factors which contributed to the lower forecast errors. Lastly, icing 

events are not forecasted in the current system and contributed to large forecast errors. 

Figure 13 – PSCo monthly day-ahead forecast error. 

0.0%

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NovemberDecember

MAPE

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9 More information regarding Pennsylvania State University’s Bufkit Data Distrbution System can be found at 

http://www.meteo.psu.edu/bufkit/CONUS_NAM_12.html) 
22 

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Figure 15 – SPS monthly day-ahead forecast error. 

0.00%

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2010

Ongoing Work

Since October of 2010, there have been a number of new developments in the forecast system. In particular, an 

NCAR mesoscale ensemble prediction model and the Canadian Global Environmental Multiscale (GEM) 

model from the Canadian Meteorological Centre (CMC) have been incorporated into the Xcel Energy 

forecasting system and are providing improved guidance. The GEM model inclusion involved accessing GEM 

model output and then subsequent incorporation into the DICast system. The mesoscale ensemble prediction 

model involved new model development and is discussed in more detail below. 

Figure 14 – NSP monthly day-ahead forecast error. 
23 

High-resolution Mesoscale Ensemble Prediction Model (EPM)

It is known that atmospheric processes are chaotic in nature. This implies that even small errors in the model 

initial conditions combined with the imperfections inherent in the NWP model formulations, such as truncation 

errors and approximations in model dynamics and physics, can lead to a wind forecast with large errors for 

certain weather regimes. Thus, probabilistic wind prediction approaches are necessary for guiding wind power 

applications. Ensemble prediction is at present a practical approach for producing such probabilistic 

predictions. An innovative mesoscale Ensemble Real-Time Four Dimensional Data Assimilation (E-RTFDDA) 

and forecasting system that was developed at NCAR (Liu et al., 2008c, 2009, 2010, Pace et al., 2010) was used 

as the basis for incorporating this ensemble prediction capability into the Xcel forecasting system. 

In particular, a 30-member E-RTFDDA system was implemented for wind power prediction. This system 

produces 6-hour analyses and 48-hour forecasts using 4 forecast cycles a day. Because the ensemble model 

requires significantly more computing power than a single model, the XCEL ensemble system contains only 

two domains, consisting of a coarse domain covering the same area of the deterministic forecast system’s

Domain 1 (cf. Figure 5) and a fine mesh domain that is the same as the deterministic forecast system’s Domain 

3, but at 10-km grid intervals (see Figure 16). A preliminary suite of probabilistic wind products was produced 

and provided to users by means of web pages. The suite includes ensemble mean, spread, spaghetti maps,

meteograms, wind roses, likelihood-of-ramp event magnitudes and timing, and exceedance probabilities for 

given wind thresholds. The real-time E-RTFDDA wind predictions for 10 major wind plants located in 

different geographical regions across Colorado, Minnesota, New Mexico and northern Texas are now being 

generated and provided to the DICast post-processing system to improve wind power forecast accuracy and for 

estimation of forecast uncertainty. 

Figure 16 – E-RTFDDA Model Domains. 

D2

D1
24 

Analog-Based Kalman Filter Bias Correction Algorithm (AnKF)

To deal with the bias of the mean and the spread of E-RTFDDA forecasts, an analog-based Kalman filter bias 

correction algorithm (AnKF) (e.g., Homleid, 1995, Delle Monache et al., 2006, 2008, 2010) was implemented 

for bias correction of E-RTFDDA wind predictions at the wind plants. For post-processing the E-RTFDDA 

forecasts at wind plants, the hub-height wind predictions of each ensemble member are first processed with the 

AnKF scheme for bias correction. Then the NCAR quantile regression (QR) calibration technique (Hopson et 

al. 2010) is employed to calibrate the hub-height wind prediction of E-RTFDDA at the wind plants using the 

AnKF output. The QR algorithm has been formulated using a step-wise forward selection framework. Model 

selection for each quantile relies on both the QR cost function and the binomial distribution, leading to 

ensemble forecasts with both good reliability and sharpness. In addition, a second pass is performed to recalibrate over separate intervals of self-diagnosed forecast instability, leading to a calibrated ensemble forecast 

with an informative skill-spread relationship. 

Wind Ramp Prediction

A difficult issue for wind power forecasting is consistently and accurately predicting wind power ramps (Ela 

and Kemper 2009). Although the core wind/power forecasting system described above has skill foreseeing 

power ramps generated from large-scale weather events (e.g., cold fronts), there is a need to fine-tune this 

capability to accurately predict the time, magnitude, and duration of intermediate and smaller-scale events 

including thunderstorm outflows. To that end, a short-term (0-6 hour) ramp forecasting subsystem is in the 

process of being incorporated into the overall wind/power prediction system. This ramp detection subsystem 

involves two additional components to the existing system: the four dimensional Variational Doppler Radar 

Analysis System (VDRAS) and an observational-based system that analyzes publically available 

meteorological data in the vicinity of the wind plant. At the same time, ongoing research and development 

efforts are being pursued to improve the identification of ramp events within WRF-RTFDDA and the 

Ensemble RTFDDA systems.

Wind Ramp Nowcasting Using VDRAS

VDRAS assimilates radar reflectivity and radial velocity data into a numerical cloud-scale model and produces 

high-resolution boundary layer wind fields (Sun and Crook 1997). Case studies are being conducted to 

evaluate and verify VDRAS performance for wind ramp ‘now-casting’. A preliminary study of two cases over 

northern Colorado has shown that the frequently updated (18-minute) VDRAS wind analysis reveals wind 

ramps that were approaching the wind plants, suggesting that VDRAS could be a useful tool for generating a 

0-2 hour warning of ramp events. Verification of the VDRAS analysis against turbine hub height wind 

measurements showed close agreement for the two cases studied. More cases will be evaluated and verified. 

Zero to 2-hour now-casting algorithms will be developed and tested in the near future. 

Observation-based Ramp Forecast Techniques

A short-term ramp forecast expert system was developed that uses publicly available observational data in 

eight concentric rings centered on the wind plant with 50-km spacing. The current configuration is built to 

predict weather patterns advancing from the northwest, the predominant direction for synoptic patterns in this 

region. This rule-based expert system searches for wind ramp signatures in upstream observations and uses 

these observations to infer the time and magnitude of the wind ramp that is expected to affect the wind plant. 

For each site and each historical hour, a ramp metric is computed using the current hour and the previous 

hour’s observations. The observed wind at 10 meters is extrapolated to hub height (80 meters) and changes in 

wind speed and direction are evaluated. The percentage of sites that indicate a ramp (defined here as a change 

of at least 25% of capacity) are tabulated. These percentages are averaged across rings for each lead time. A 

ramp indicator is computed that depends on a threshold of that average percentage. The expert system is 

applied for up to 6-hours lead time and results are displayed to advise system operators of imminent ramping 

events.
25 

WRF-RTFDDA Ramp Studies

The high-resolution WRF-RTFDDA modeling system attempts to forecast wind ramps associated with 

different weather processes via a physical approach by incorporating high resolution terrain and land surface 

forcing, and regional and wind plant observations with 4-dimensional data assimilation into the full-physics 

WRF model. Studies are being conducted by employing a feature-based verification approach to assess the 

Xcel 3.3-km WRF RTFDDA wind ramp forecasts at four selected wind plants during the summer and winter 

seasons, and the initial result for the Cedar Creek wind plant in the northern Colorado with complex terrain 

indicates that the WRF-RTFDDA model captured 50–70% of the major ramps for 0 to12-hour forecasts. The 

model’s ability to forecast these ramps degraded by 10–15% from 0 to 3-hour to 9 to 12-hour forecast ranges. 

Further studies to better identify and display the WRF RTFDDA ramp forecasts to Xcel Energy operators are 

being performed. 

Part of the challenge for forecasting ramps is connected with the limited predictability of many mesoscale 

weather processes, such as convection and mountain waves over complex terrain. Ensemble forecasting 

systems, such as the Xcel Energy operational 10-km Ensemble RTFDDA system, provides a viable ability to 

address predictability issues by predicting the probabilities of the weather processes and associated wind 

ramps. Ongoing research is being performed to extract, derive and verify the probabilities of ramps in terms of 

their occurrences, timing, duration and magnitudes from the 30-member E-RTFDDA forecasts incorporating 

ensemble calibration approaches. The goal is to derive and present intuitive probabilistic forecast products 

signaling ramps for Xcel Energy operators. Finally, for future development guidance, NCAR is pursuing 

research to investigate the trade-off between finer-resolution deterministic forecasting and coarser-grid 

ensemble prediction, and then to optimize wind plant data assimilation for both approaches. 

Conclusions 

The Xcel Energy wind forecasting system has significantly reduced forecast error in all three systems. The 

day-ahead forecast errors were reduced by 22%, 20%, and 15% from 2009 to 2010 for NSP, PSCo, and SPS, 

respectively. The meteorological displays provide additional information to staff meteorologists specific to 

wind energy production with geographic focus in the wind producing regions. The turbine-level power 

conversion approach is reasonable, and possibly superior to other power conversion methods. None of the 

material gains would have been possible without substantial investment in real-time turbine-level power and 

wind speed data acquisition. Working with NCAR and NREL has proved a productive and worthwhile 

collaboration for Xcel Energy resulting in significant benefits for customers. Ongoing work has continued in 

this area with promise for further reductions in forecast error across all forecast time horizons.
26 

References

[1] EnerNex Corporation, Final Report – 2006 Minnesota Wind Integration Study Volume I, November 30, 

2006. 

[2] EnerNex Corporation, Wind Integration Study for Public Service of Colorado Addendum Detailed Analysis 

of 20% Wind Penetration, December 1, 2008. 

[3] GE Energy, Western Wind and Solar Integration Study, May 2010. 

[4] California ISO, Integration of Renewable Resources, August 31, 2010. 

[5] Keith Parks, 2008 Public Service of Colorado Wind Uncertainty Costs: The Value of Better Wind 

Forecasting, June 2, 2009. 

[6] Erik Ela, J. Kemper, Wind Plant Ramping Behavior, NREL/TP-550-46938, December 2009. 

[7] Susan E. Haupt, A Wind Power Forecasting System to Optimize Power integration, COST ES1002 

Weather Intelligence for Renewable Energies State-of-the-Art Workshop, Nice, France, March 22-23, 2011. 

[8] Yubao Liu, T. Warner, Y. Liu, C. Vincent, W. Wu, B. Mahoney, S. Swerdlin, K. Parks, J. Boehnert, 

Simultaneous Nested Modeling from the Synoptic Scale to the LES Scale for Wind Energy Applications, J. 

Wind Eng. Ind. Aerodyn., doi:10.1016/j.jweia.2011.01.013. 2011. 

[9] William Myers, G. Wiener, S. Linden, S.E. Haupt, A Consensus Forecasting Approach for Improved 

Turbine Hub Height Wind Speed Predictions, American wind Energy Association (AWEA), 2011. 

[10] Yih-Huei Wan, E. Ela, K. Orwig, Development of an Equivalent Wind Plant Power Curve, NREL Report 

No. CP-550-48146, 2010. 
27 

Conference Proceedings

The National Center for Atmospheric Research reported on their Wind Power Prediction research and 

development at the 91st Annual Meeting of the American Meteorology Society in January 2011. The topics 

ranged from numerical weather prediction through post-processing techniques to predict and calibrate wind 

turbine hub height winds and the resulting power output. The bulk of these papers were presented as part of the 

Second Conference on Weather, Climate and the New Energy Economy, but the team was also represented at 

several of the other conferences. These references go into more detail on the components of the forecasting 

system mentioned in this report. A list of these papers and links are provided below. 

Second Conference on Weather, Climate and the New Energy Economy

"An overview of NCAR's advanced wind forecasting system for integrating wind resources into the new energy 

economy"

David B. Johnson, B. Mahoney, Y. Liu, G. Wiener, W. Myers, and K. Parks 

http://ams.confex.com/ams/91Annual/webprogram/Paper186427.html

"Wind energy forecasting with the NCAR RTFDDA and ensemble RTFDDA systems"

Yubao Liu, W. Y. Y. Cheng, G. Roux, Y. Liu, L. Delle Monache, M. Pocernich, B. Kosovic, T. M. Hopson, A. 

Bourgeois, G. Wiener, T. Warner, B. Mahoney, and D. B. Johnson 

http://ams.confex.com/ams/91Annual/webprogram/Paper186591.html

"Kalman filter, analog and wavelet postprocessing in the NCAR-Xcel operational wind-energy forecasting 

system" 

Luca Delle Monache, A. Fournier, T. M. Hopson, Y. Liu, B. Mahoney, G. Roux, and T. Warner 

http://ams.confex.com/ams/91Annual/webprogram/Paper186510.html

"Statistical Analysis of intra-farm microscale wind characteristics at selected Xcel wind farms"

Yuewei Liu, NCAR, Boulder, CO; and Y. Liu, W. Cheng, G. Wiener, B. Lambi, and B. Mahoney 

http://ams.confex.com/ams/91Annual/webprogram/Paper186522.html

"Verification and analysis of hub-height wind forecasts from the NCAR-Xcel WRF-RTFDDA"

Gregory Roux, Y. Liu, M. J. Pocernich, W. Y. Y. Cheng, L. Delle Monache, A. Fournier, S. Linden, and W. 

Myers 

http://ams.confex.com/ams/91Annual/webprogram/Paper186547.html

"A comparison of turbine-based and farm-based methods for converting wind to power"

Julia M. Pearson, G. Wiener, B. Lambi, and W. Myers 

http://ams.confex.com/ams/91Annual/webprogram/Paper179783.html

"An evaluation of different data mining methods for forecasting wind farm power"

Gerry Wiener, J. M. Pearson, B. Lambi, and W. Myers 

http://ams.confex.com/ams/91Annual/webprogram/Paper180009.html

"Improving the 0-3 hour wind forecast through wind farm data assimilation in the NCAR/ATEC WRF 

RTFDDA"

W. Y. Y. Cheng, Y. Liu, Y. Liu, B. Mahoney, M. Politovich, T. T. Warner, K. Parks, and J. Himelic 

http://ams.confex.com/ams/91Annual/webprogram/Paper182487.html
28 

"A rapid-updated wind analysis system based on mesoscale model, radar, and surface data for ramp-event 

wind energy forecasting"

Juanzhen Sun, Y. Zhang, G. Wiener, N. Oien, and W. Mahoney 

http://ams.confex.com/ams/91Annual/webprogram/Paper182972.html

"An Investigation into the Spatiotemporal Scale of Two Wind Ramp Events in Northeastern Colorado" 

Theresa A. Aguilar, Y. Liu, Y. Liu, and B. Mahoney 

http://ams.confex.com/ams/91Annual/webprogram/Paper185976.html

Ninth Conference on Artificial Intelligence and its Applications to the Environmental 

Sciences 

"A turbine hub height wind speed consensus forecasting system"

William Myers, and S. Linden 

http://ams.confex.com/ams/91Annual/webprogram/Paper187355.html

Joint Session for the 24th Conference on Weather Analysis and Forecasting and the 20th 

Conference on Numerical Weather Prediction 

"Kalman filter and analog schemes to postprocess numerical weather predictions"

Luca Delle Monache, T. Nipen, Y. Liu, G. Roux, R. B. Stull, T. T. Warner, and P. Childs 

http://ams.confex.com/ams/91Annual/webprogram/Paper185473.html

"NCAR ensemble RTFDDA: real-time operational forecasting applications and new data assimilation 

developments"

Yubao Liu, T. Warner, S. Swerdlin, T. Betancourt, J. Knievel, B. Mahoney, J. Pace, D. Rostkier-Edelstein, N. 

A. Jacobs, P. Childs, and K. Parks 

http://ams.confex.com/ams/91Annual/webprogram/Paper182108.html

"Sensitivity of WRF-RTFDDA model physics in weather forecasting applications: From synoptic scale to 

meso-gamma scale"

William Y. Y. Cheng, Y. Liu, Y. Zhang, Y. Liu, D. Rostkier-Edelstein, A. Pietrkovski, B. Mahoney, T. T. 

Warner, and S. Drobot 

http://ams.confex.com/ams/91Annual/webprogram/Paper182629.html

15th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, 

Oceans and Land Surface 

"The NCAR 4DREKF ensemble data assimilation and forecasting system"

Yubao Liu, L. Pan, Y. Wu, A. Bourgeois, T. Warner, S. Swerdlin, S. F. Halvorson, and J. Pace 

http://ams.confex.com/ams/91Annual/webprogram/Paper185113.html
29 

Additional References

Haupt, S.E., 2011: A Wind Power Forecasting System to Optimize Power Integration, COST ES1002 Weather 

Intelligence for Renewable Energies State-of-the-Art Workshop, 22-34 March, Nice, France, Keynote 

Presentation.

Liu, Y., T. Warner, Y. Liu, C. Vincent, W. Wu, B. Mahoney, S. Swerdlin, K. Parks, J. Boehnert, 2011: 

Simultaneous nested modeling from the synoptic scale to the LES scale for wind energy applications. J. Wind 

Eng. Ind. Aerodyn., doi:10.1016/j.jweia. 2011.01.013. 

Liu Y., T. Warner, W.Y.Y. Cheng, G. Roux, L. Delle Monache, Y. Liu, W. Mahoney, K. Parks, Y.-H. Wan, T. 

Hopson, B. Kosovic, 2011: Analysis and prediction of winds at large inland wind farms: NWP modeling tools 

and challenges. Wind Energy, (in revision). 

Delle Monache, L., T. Nipen, Y. Liu, G. Roux, and R. Stull, 2011: Kalman filter and analog schemes to postprocess numerical weather predictions. Mon. Wea. Rev. (in press).

Liu, Y., T. Warner, B. Mahoney, W. Cheng, Y.W. Liu, G. Roux, L. D. Monache, W. Wu, B. Kosovic, G. 

Wiener, B. Myers, D. Johnson, S. Swerdlin, C. Vincent, M. Pocernich and M. Politovich, K. Parks, Y.-H. 

Wan, 2010: Analysis and Prediction of Wind Power: the State of the Art Modeling Tools. China Wind Power2010 Expo and Conference Proceedings, 11pp. 

Liu, Y., T.T., Warner, W. Mahoney, K. Parks and Y.-H. Wan, 2010: Analysis and prediction of winds for 

inland wind farms: NWP modeling tools and challenges. AWEA WindPower-2010 Expo and Conference 

Proceedings. American Wind Energy Association. 10 pp. 

Liu, Y., W. Y.Y. Cheng, Y.W. Liu, G. Roux, G. Wiener, B. Kosovic, T. Warner, W. Mahoney, J. Himelic and 

S. Early. Improving short-term wind energy prediction with wind farm data using the NCAR WRF-RTFDDA 

models. 10th EMS Annual Meeting and 8th ECAC, Zurich, Switzerland 13 – 17 September 2010. 

Liu, Y., T. Warner, W. Wu, G. Roux, W. Cheng, Y. Liu, F. Chen, L. Delle Monache, W. Mahoney and S. 

Swerdlin, 2009: A versatile WRF and MM5-based weather analysis and forecasting system for supporting 

wind energy prediction. 23rd WAF / 19th NWP Conf., AMS, Omaha, NE. June 1- 5, 2009. 

Liu., Y., T. Warner, B. Mahoney, K. Parks, R. Bigley, Y. Wan, D. Corbus, and E. Ela, 2009: Analysis and 

modeling study of inter-farm and intra-farm wind variations with the NCAR high-resolution multi-scale WRFRTFDDA system. EGU-2009 Assembly: Wind Power Meteorology. Vienna, Austria. 19-24 April, 2009. 

 

Liu Y., T. Warner, S. Swerdlin and J. Pace, 2009: The NCAR/ATEC Operational Mesoscale Ensemble Data 

Assimilation and Prediction System – “Ensemble-RTFDDA”. Sept. 23 – 24, 2009. National Workshop on 

Mesoscale Probabilistic Prediction, DTC. Boulder, CO. 

Myers, William, Gerry Wiener, Seth Linden, and Sue Ellen Haupt; A Consensus Forecasting Approach for 

Improved Turbine Hub Height Wind Speed Predictions; American Wind Energy Association (AWEA) 2011.