# Presentation: 948210

## A Historical Perspective on the Role of NWP Models in the Prediction of Severe Local StormsSteven J. Weiss and Joseph T. Schaefersteven.j.weiss@noaa.gov

*Where Americas Climate and Weather Services Begin*

## Severe Weather Forecasting

- Severe weather is defined as severe local storms (thunderstorms) producing:
-  *Tornadoes*
* **Large hail (> 3/4 inch diameter)*
* **Convective wind gusts > 50 kt or wind damage*
* **In the U.S. heavy rain/flash floods not included*
- Focus will be on _**prediction**_ of severe storms (forecasts) rather than _**detection**_ of existing severe storms (warnings)
- The presentation will be from the perspective of national center forecasting (SPC/NSSFC)

**Severe Weather Forecasting**

## The Beginning of Modern Severe Weather Forecasting

- NWP and tornado forecasting activities both began in the middle of the 20th century
*NWP focused on synoptic scale prediction*
*Tornado prediction focused on mesoscale and storm scale phenomena*
*Despite having similar origins in time, it would take several decades before the paths of NWP and severe weather forecasting would become closely aligned** *

## The Beginning of Modern Severe Weather Forecasting

*On March 25, 1948 Ernest Fawbush and Robert Miller (USAF) issued the first modern tornado forecast*

## The Beginning of Modern Severe Weather Forecasting

*20 Mar 1948*
*25 Mar 1948*

_**Gen.  Borum**_: *“Are you going to issue a tornado forecast?”*
_**Maj.  Fawbush**_: *“Sir, it sure looks like the last one, doesn’t it Bob?”*
_**Capt.  Miller**_: *“Yes, E.J.,it is very similar to last week.”*
_**Fawbush & Miller**_: *“But no one has ever issued an operational tornado forecast”*
_**Gen.  Borum**_: *“You’re about to set a precedent”*

## US Weather Bureau Tornado Forecasts

- Based on their success March 25, 1948, Fawbush and Miller were assigned to issue USAF tornado forecasts
- By early 1952 complaints from public and media, and Congressional pressure forced Weather Bureau to issue tornado forecasts
- A temporary group of research forecasters and supervising analysts at the Washington DC Analysis Center formed initial Severe Weather Unit (SWU)
- The first public tornado forecast was issued March 17 1952 (it did not verify)

## The First Successful USWB Tornado Forecasts and Tornado Reports

- Issued March 21-22, 1952
*28 tornadoes killed 204 people*
*(From Corfidi 1999)*

## US Weather Bureau Tornado Forecasts

- The first tornado forecasts were similar to current tornado and severe thunderstorm watches in time/space
*Short-term forecasts (0-8 hours) covering parts of states*
*Based entirely on observational data (hourly surface data, 4 times daily raobs and PIBAL data) and detailed manual analyses*
- Dynamic and thermodynamic considerations guided the forecast process
*Moisture, instability, and lift sufficient for parcels to reach LFC*
*Upper level jet dynamics, interaction of low and upper jets*
*Application of basic concepts: climatology, pattern recognition, parameter evaluation *
- Forecasters focused on understanding and predicting the _**synoptic scale / mesoscale environment**_ and its relationship to thunderstorms

## SELS Kansas City Operations in 1950s

## Important Severe Weather Papers (1956)

## Mesoscale Analysis of Surface Data

*Research (Fujita et al. 1956) Transferred to Operations (Magor 1959)*

## Introduction of Digital Computers – 1960s

- IBM 1620 computer installed at SELS in 1963
- System upgraded to CDC 3100 computer in 1965
*Used to automate plotting of surface and upper air charts (analysis still done by hand)*
*Allowed computation of fields such as vorticity and divergence from upper air data*
*Automated sounding plots and calculation of stability indices *
- Local computers used to increase forecaster efficiency, and provide new diagnostic information

## Examples of Early Dynamic Computations at SELS

*      **500 mb Vorticity                           300-200 mb Mean Divergence*

## Examples of Computer Plotted Surface Charts – 18z 3 April 1974

*MSL Pressure / Wind                             Temperature / Dew Point*

## Examples of Computer Plotted Surface Charts  2 hr Changes – 18z 3 April 1974

*    **Altimeter Changes                       Temperature / Dew Point Changes*

## Actual Tornado Watch Formulation  21z 3 April 1974 Superoutbreak

*    *

## Early Use of NWP at SELS

- By the late 1960s NMC was running an operational 6-layer Primitive Equation model
*Basic large scale fields (e.g.,500 mb height/vorticity, MSL pressure/thickness) available via facsimile*
*Used by SELS forecasters to provide guidance on synoptic scale features - upper troughs, surface lows, fronts*
*First time that NWP model data was used as part of Convective Outlook preparation process*
*No direct impact on formulation of Tornado and Severe Thunderstorm Watches*
- Very coarse model resolution and limited output fields (North American domain at 12 hr intervals)

## Example of PE Model Lifted Index Forecast for 3 April 1974

*00 hr*
*24 hr*

*12 hr*
*36 hr*

## The LFM Model Plays a Larger Role in Severe Weather Forecasting

- Limited-area Fine Mesh (LFM) model introduced in 1971
*First regional model at NMC*
*Similar to PE with half the grid spacing (190.5 km) and run over smaller domain*
*More output fields and increased resolution provided improved guidance for Convective Outlook preparation*
*First time SELS forecasters produced forecast composite charts based on model data*
*provided more structured basis for Convective Outlook*
*Again, relatively coarse resolution and inability to resolve convective systems provided little short-term (watch) guidance*

## Examples of LFM Facsimile Chart Output

*500 mb Height / Vorticity                            MSL Pressure / Thickness*

## Examples of LFM Facsimile Chart Output

*700 mb Height / Mean RH                    700 mb Vert. Vel / 12 hr Precip.*

## NWP / Technology Advances in the 1980s

- Technology
_**Automation of Field Operations and Services (AFOS - 1980)**_
- Designed to enhance display of NWP model data via electronic monitors
- Increased communications bandwidth allowed dissemination of more model fields (compared to FAX)
- Used at SELS primarily for Convective Outlook preparation
- Frequency and forecast intervals of NWP output similar to FAX (12 hour intervals), although smaller geographic domain could be specified

## NWP / Technology Advances in the 1980s

- Technology
_**Centralized Storm Information System (CSIS - 1982)**_
- McIDAS-based interactive workstations specifically designed for severe weather forecasting at SELS
- Unique advantage was integrated display of observational data sets
*Satellite, radar, surface and upper air*
- Objective analysis of hourly derived fields (lifted index, mstr convg.)
- Facilitated better application of traditional forecasting procedures developed in 1950s-1970s
- Revolutionized short-term severe weather forecasting through access to real-time data
*Resulted in more accurate timing and placement of severe weather watches*
- Had limited utility for NWP output display => less impact on forecasts beyond ~6 hours (such as Convective Outlooks)

## CSIS Workstation – 1980s

*IR Satellite and Mstr Flux Convg.                   Radar and Surface Data*

## NWP / Technology Advances in the 1980s

- NWP
_**Nested Grid Model (NGM - 1985)**_
- Triple nested model with 80 km grid spacing inner domain and 16 vertical levels and forecasts to 48 hrs
- Increased resolution resulted in overall improved forecast skill compared to LFM
- Allowed extension of SELS Convective Outlook product into the Day 2 time period (1986)
- Instability (lifted index) forecasts introduced concept of “Best” lifted index to account for elevated instability (good news), but...
*NGM LI’s were often less accurate than LFM, particularly when extreme instability considered*
*Caused by systematic errors in temperature / dew point profiles*
- SELS forecasters still focused on synoptic scale environment

## Links Between Observable Scales and Storm Scale Remained Largely Unknown

*Thanks to  Sidney Harris*

- Synoptic scale

- Storm scale

## NWP / Technology Advances in the 1980s

_**Research Cloud-Scale Models**_
- Provided physical insights about links between observable scales and storm scale phenomena
- Helped explain dynamics of supercell thunderstorms
- Emphasized role of CAPE and vertical shear on storm evolution

*Relationship of CAPE / Shear ratio (BRN) and modeled storm type  **(from Weisman and Klemp 1986)*

## NWP / Technology Advances in the 1990s

**NWP and Computer Workstations**
_**Eta and RUC Models**_
- Rapid increases in model resolution, physics complexity, model frequency, amount of output, and skill levels
- For the first time, models became useful sources of information for short-term convective forecasting (severe storm watches)
*Output complemented real-time observational data*
- Model forecasts of key convective parameters such as CAPE, vertical shear, and helicity
*Forecasters could more directly identify synoptic / mesoscale environments favorable for supercell development*
_**Advanced Operational Workstations**_
- National-center AWIPS (NAWIPS)
- Designed to display gridded model data plus observational data

## Eta Model 24 hr Forecast of CAPE / Shear / Helicity 

- CAPE     0-3 km Helicity contours    0-6 km Shear vectors

## NWP / Science &Technology Advances in the 1990s

- Improvements in scientific understanding of severe storms through field programs such as VORTEX, coupled with better NWP guidance, resulted in improvements to SPC product suite
_**Eta Model**_
*Issuance of Probabilistic Severe Storm Outlooks to complement traditional Categorical (Slight, Moderate, High Risk) Outlooks*
*Extension of Convective Outlooks to Day 3 Time Period*
_**RUC Model**_
*Real-time 3D hourly analyses by combining surface data and RUC data*
*Objective analyses of hundreds of convective parameters*
*Extensively used at SPC for short-term severe weather forecasting and shared with WFOs via SPC web page*

## Example of RUC - “SfcOA” Output 

## Recent Applications of NWP to Severe Storm Forecasting

- Co-location of SPC with NSSL in 1997 enhanced collaboration between research and operations
_**Annual Spring Programs**_
- Identify issues of mutual interest for both severe weather forecasters and researchers, leading to improved forecasting
- Provide for efficient testing and subsequent delivery of program results to SPC operations

## Spring Programs 2003 and 2004

- <date/time>

**Spring Programs ****2003 and 2004**

- Focused on two current approaches to NWP development, with emphasis on operational application of both modeling concepts
- Examined High Resolution WRF Model predictions of short-term convective initiation/evolution/mode *(Watch Scale Guidance)*
- Explored ability of SREF systems to aid severe weather forecasting *(Outlook Scale Guidance) *
*Help define range of forecast possibilities*
*Help quantify forecast uncertainty*

## Spring Program 2003

- <date/time>

**Spring Program 2003**

- SREF Applications at SPC
- Assessed impact of using traditional deterministic models plus new SREF systems in severe weather forecasting
- Small but positive improvement in experimental Day 2 outlooks when using SREF information
*Forecast linkage between SREF and high resolution  deterministic NWP must be improved*
*Quantity of information can be overwhelming*
- Resulted in development of numerous (~300) special SREF products for SPC forecasting

## SREF Combined Probability

- <date/time>

**SREF Combined Probability**

**Combined probabilities very useful **
**Quick way to determine juxtaposition of key parameters**
**Not a true probability**
**Not independent**
**Different members contribute**

**Prob CAPE >= 1000 j/kg  X  Prob Shear >= 30 kt  X  Prob Conv Pcpn >= .01”**
**                                                                                           ****F036: Valid 21 UTC 28 May 2003**

**Spring Program 2003**

**Notes:**
Interesting concept...useful of further “statistical” investigation.  I.e., just what do we have here?  It’s not a true probability...parameters are not independent.  But, perhaps the lack of internal (to the model) consistency is a good way to get around model biases (if the ensemble size is large enough).

## SREF Combined Probability

- <date/time>

**Severe Reports**
**Red=Tor****; ****Blue=Wind****; ****Green=Hail**

**Prob CAPE >= 1000 j/kg  X  Prob Shear >= 30 kt  X  Prob Conv Pcpn >= .01”**
**                                                                                           ****F036: Valid 21 UTC 28 May 2003**

**Combined probabilities very useful **
**Quick way to determine juxtaposition of key parameters**
**Not a true probability**
**Not independent**
**Different members contribute**

**Notes:**
Interesting concept...useful of further “statistical” investigation.  I.e., just what do we have here?  It’s not a true probability...parameters are not independent.  But, perhaps the lack of internal (to the model) consistency is a good way to get around model biases (if the ensemble size is large enough).

## Spring Program 2004

- <date/time>

**Spring Program 2004**

- Primary focus on Weather Research and Forecast (WRF) model as it relates to severe weather forecasting
- Collaboration expanded to include NCAR and OU/CAPS
- Mesoscale (dx~10 km) and near stormscale (dx~4km) versions of the WRF examined (EMC, NCAR, OU/CAPS)
*Determine impacts of grid resolution, parameterized convection versus explicit microphysics, and data assimiliation*
- Experimental severe storm forecasts addressed initiation, intensity, evolution, _and mode_ *(discrete cells, lines, etc.) *
- Key goal - determine usefulness of very high resolution WRF output to severe storm forecasters
*Provide guidance to model developers on future development efforts*

## 26 hour WRF 4 km  Forecasts Valid 02z 29 May 2004

*WRF-NMM*
*1-Hr Radar*

*WRF-NCAR*
*Severe Rpts*

## 23 hour WRF 4 km  Forecasts Valid 23z 1 June 2004

*WRF-NMM*
*1-Hr Radar*

*WRF-NCAR*
*Severe Rpts*

## Spring Program 2004

- <date/time>

**Spring Program 2004**

- Preliminary Impressions
- On some days 00z “cold start” WRF models with explicit precipitation physics were remarkably accurate predicting details of next day convective initiation, evolution, and mode
- The NCAR Mass Core, and to a lesser extent the WRF-NMM, exhibited a  tendency to produce bowing structures more often than observed
- The low level moisture and instability fields differed strongly between different versions of the WRF
*Suggests a strong sensitivity to PBL or surface parameterizations*
- All WRF’s often had difficulty predicting evolution of significant nocturnal convection during 1st 12 hours of the 00z forecast* *
- Operational SPC forecasters are continuing to view the WRF-NMM and NCAR WRF after conclusion of the Spring Program

## SPC Annual Watch Verification 1970-2002

## SPC Verification for Strong and Violent (F2-F5) Tornadoes 1970-2002

**Tornado Forecast Improvement Since 1970**

## Concluding Comments

- Over the last 50 years, improvements in severe weather forecasting have resulted from:
*Better scientific understanding of convection*
*More accurate NWP models*
*Advanced technology / workstation achievements*
- However, our knowledge of the mesoscale and storm scale remains substantially less than for the synoptic scale
*Mesoscale environment is not well sampled*
*4-D distribution of water vapor is not resolved adequately*
*Deep convective processes are extremely complex and poorly understood*
- Better observational data / data assimilation (IC uncertainties), model physics, and computational capabilities will all play increasing roles to further improvements in severe weather forecasting

## Thank You

**Thank You**