# Presentation: 472393

## NWSRFS Snow Modeling

- Cold Regions Workshop
- November 2004
- Andrea Holz
- NCRFC

## North Dakota, 1966

- Historic NWS Photo

## SNOW-17 Model Overview

- Input Variables
- Air Temperature
- 10 days of maximum/minimum point (CCF) forecasts
- May use gridded temperature forecasts soon    (maybe next spring)
- Precipitation
- 24 Hours of QPF
- 72 Hour QPF Contingency Forecasts
- Output
- Rain + Melt
- Simulated Water Equivalent
- Simulated Areal Extent of Snow Cover
- Simulated Snow Depth

## Other SNOW-17 Model calculations

- Snow Pack Information
- Accumulation of snow cover
- Surface Energy Exchange
- Heat storage in snow pack
- Rain on snow event
- Snow pack ripening
- Liquid water retention
- Ground Heat Exchange

## Photo

- Courtesy
- of St.Paul
- Corps of
- Engineers

- Excavating
- Connelly
- Ditch,
- Minnesota,
- 1997

## SNOW-17

- Calculated on 6 Hour Time Intervals
- Air temperature determines form of precipitation input
- Can specify precipitation type during real time river forecasting
- Can adjust melt rate based upon observed information
- Dewpoints
- Windspeeds
- Observations from the field
- Can change snow water equivalent values based on observed data

## South Dakota, 1966

- Historic NWS Photo

## Example of SNOW-17 Output with observed values listed for comparison

- Observed
- Snow Depth

- Observed
- SWE

- Observed
- AESC

## Can create Snow Depth or SWE “Areal Observations”

- More than one observer in the basin
- 2 or more observers outside of the basin, yet nearby
- If one observation is missing for the day, no value is calculated – considered “missing”

- Historic NWS Photo

## Tolerances used in Updating

- Automatically update SWE or AESC by setting tolerances
- SWE:  Updating occurs if                    | sim – ob | > | tolerance x ob |
- AESC:  Updating occurs if                   | sim – ob | > tolerance

## NCRFC Tolerances

- High tolerances specified in the model prevent forecasters from making run time SWE or AESC modifications
- To prevent automatic updates and to allow forecasters to make real time modifications, NCRFC does not display observed WE
- Having ability to view observed WE would be operationally beneficial

## Historic NWS Photo

## River Forecasting using the Snow Model

- Sturgeon River at Alston
- Upper Peninsula of Michigan
- Snow Melt & Rain on Snow Event
- March, 2004
- For purposes of illustrating mods, observed AESC & WE are displayed

## Sturgeon River at Alston

- 6 hour RAIM around 0.2 inches, March 28 - 29

- No modifications

- FS

-  Observations

- Simulation 

## SNOW-17 Output

- Precipitation on March 29 is 0.28 inches of snow + 0.06 inches of rain

*Keep in mind Energy Exchange value for future example*

-  Energy Exchange 0.5

## Rain Snow Modification

- Field observations of rain instead of snow
- Rain/snow typing might be incorrect if there is an unknown temperature bias
- Timing might be off for an event with rain turning to snow in the evening
- Change precipitation typing in 6 hour increments
- Difficult to determine which 6 hour time period to change since output is in 24 hour time steps

## Change precipitation to Rain March 28 12Z – 29 12Z

## Rain/snow Modification 24 hours

- 6 hour RAIM over 0.5 inches, March 28 - 29

## UADJ Mod – During Rain on Snow Events

- Rain on Snow must exceed 2.5mm per 6 hours for model to simulate an increase in melt (tenth of an inch)
- Modification can increase or decrease energy exchange
- .UADJ March 28 12Z  – 29 12Z                                     ALSM4  5.0

## Rain/snow + UADJ mods

- RAIM near 0.8 inches

## UADJ 5.0March 28 - 29

-  Energy Exchange 0.85

## Snow Water Equivalent Change Mod

- Observed values are listed in this table.  Notice 3.9 inches on the 29th.

- 3.9” Observed Value 

## Water Equivalent Change Mod

- Changing WE during melt automatically changes AESC . . .

- Best approach:  have WE values set before melt begins.

- Near Little South Pembina River, North Dakota, 1997

- Photo courtesy of St.Paul Corps of Engineers

## Updating Snow Model

- No Modifications

- Too Much Peak 

-  Missed peak

- FS

- Same Example moving forward in time

## SNOW-17 Model Output

- Watton SWE

- NOHRSC AESC

- Watton SD

*Melt already started . . .*

## Start run further back in carryover

- Melting begins . . .

## SNOW-17 Output

- Notice difference between Sim WE & Obs WE

- Energy Exchange = .17 - .71

- No Modifications

## About 15 mm of melt modeled in any 24 hours

- Low Energy Exchange

- Low Energy Exchange

- About 15mm of melt modeled in any 24 hours

## Melt Factor Correction ModMarch 26 – 30, 2004

- .MFC 03260412Z – 03300412Z                                                           ALSM4  4.0
- During time of active melt
- Meteorological conditions causing abnormal snowmelt
- High dewpoints
- High windspeeds
- Difficult to forecast MFC into future
- Watch meteorological forecasts closely
- Possible temperature bias
- Rain on snow

- Can also use MFC to slow down melt!

## MFC Mod Applied

## MFC 4.0  March 26 - 30

- Energy Exchange Values Increase . . .

- Stimulating increase in melt

- Energy Exchange Values Increase
- .36 – 1.54

- 45mm of melt 

- Increased Energy Exchange

- Increased Energy Exchange

- ~45mm of melt 

## How does Late April Peak Perform Now?

-  Caught peak

- Within few tenths 

## Current SNOW-17 Model

- Current output in 24 hour time steps
- Java display in 6 hour time steps?
- Possible to display observed WE without automatically updating yet still have ability to make WE mods?
- More Water Equivalent Data!
- Other observations from the field
- Water in ditches
- Runoff under snowpack

## Photo Courtesy of NDSU