NWSRFS Snow Modeling

Cold Regions WorkshopNovember 2004

Andrea HolzNCRFC

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

PhotoCourtesyof St.PaulCorps ofEngineers

ExcavatingConnellyDitch,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

ObservedSnow Depth

ObservedSWE

ObservedAESC

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 OutputPrecipitation 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 5.0March 28 - 29

Energy Exchange 0.85

Snow Water Equivalent Change ModObserved values are listed in this table. Notice 3.9 inches on the 29th.

3.9” Observed Value

Water Equivalent Change ModChanging 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 ModelNo Modifications

Too Much Peak

Missed peak

FS

Same Example moving forward in time

SNOW-17 Model Output

Watton SWE NOHRSC AESCWatton 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 ExchangeLow Energy Exchange

About 15mm of melt modeled in any 24 hours

Melt Factor Correction ModMarch 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 ExchangeIncreased 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