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 No modifications FS  Observations Simulation 

SNOW-17 Output Precipitation on March 29 is 0.28 inches of snow 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

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  Energy Exchange 0.85

Snow Water Equivalent Change Mod Observed values are listed in this table. Notice 3.9 inches on the 29 th. 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 SWENOHRSC 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 = No Modifications

About 15 mm of melt modeled in any 24 hours Low Energy Exchange About 15mm of melt modeled in any 24 hours

Melt Factor Correction Mod March 26 – 30, 2004.MFC Z – Z 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 Energy Exchange Values Increase... Stimulating increase in melt Energy Exchange Values Increase.36 – 1.54

45mm of melt  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