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Performance of Various Operational and Experimental Numerical Forecasts for the March 2003 Colorado SnowstormEd Szoke* Brent Shaw* and Paul SchultzNOAA Forecast Systems LabBoulder, Coloradoedward.j.szoke@noaa.govDave Barjenbruch Boulder WFO*In collaboration with the Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University, Fort Collins, Colorado
Impressive snow in the foothills...87.5” at 9000 feet in Coal Creek Canyon southwest of Boulder
The snow was quite heavy with lots of broken limbs and roof collapses
Ski area snow reports Loveland Ski Area...at Continental Divide on I70
There are MANY interesting aspects of this massive storm • Longer range (8-15 days) model forecasts indicated potential 240 h forecast from the GFS run initialized at 0000 UTC on 8 March and valid 0000 UTC on 18 March
Longer range (8-15 days) model forecasts indicating potential • Drifting to poorer forecasts in the mid-range GFS 132 h forecast initialized 0000 UTC on 13 March and valid 1200 UTC on 18 March. This forecast produces a major storm in the Midwest but very little snow for the Front Range of Colorado.
Use of GFS and Canadian ensemble forecasts as well as multi-model “ensembles” to make a better forecast. GFS ensemble shown below for same 132h forecast.
Then quite good NWP forecasts in the < 2-3 day range. This Eta 84h forecast issued 1200 UTC Fri/14 Mar and valid 0000 UTC Tue/18 Mar is quite good. 500 mb height, vorticity and wind Surface pressure, thickness, and precip
Outline • Then quite good NWP forecasts in the < 2-3 day range • Including impressive 5+ ft. model forecasts for the foothills • Excellent 1-1.5 day NWS snowfall forecasts for historic event • Included forecast of 3-8 ft. of snow for the foothills • Smaller scale details…including the “Lyons snow hole”
Outline...continued • A significant forecasting aspect of the storm was rain versus snow at the lower elevations where most of the population is located • Although a Winter Storm Warning was issued for the Front Range cities as early as Sunday (valid from Monday night into Wednesday)...there were conflicting signals as late as Monday evening about when (if?) the rain would change to snow! • The operational models (Eta, RUC) were indicating that temperatures would be too warm for snow perhaps until late Tuesday • However, it was noted that actual surface temperatures were colder then forecast or even analyzed by the models (at 00z/18 Mar) • The focus of the rest of this talk will be on this issue and examining the 0000 UTC/18 March analyses and forecasts
But first...a quick overview of the storm. 500 mb analysis at 0000 UTC/18 Mar
Focus on the 0000 UTC/18 March time period • Critical forecast issue was when the rain would change to snow. • First a look at some observations
1800 UTC/17 March IR image and surface observations Most of the precipitation is north of CO with a surface low taking shape over se CO.
1800 UTC/17 March radar and surface observations Mild conditions along the Front Range with some light rain. Quite warm over eastern CO. Main precip area is in WY with cooling conditions there.
0000 UTC/18 March radar and surface observations Mild conditions along the Front Range with some light rain. Quite warm over eastern CO. Main precip area is in WY with cooling conditions there.
0600 UTC/18 March radar and surface observations Still raining along most of the Front Range but barrier jet that had been forming to the north producing strong northerly flow down the Front Range and advecting colder air southward from se WY.
1200 UTC/18 March radar and surface observations Precipitation has changed to snow with several inches of accumulation in many areas along the Front Range.
1800 UTC/17 March radar and surface observations Mild conditions along the Front Range with some light rain. Quite warm over eastern CO. Main precip area is in WY with cooling conditions there.
Composite radar with obs at 21z 17 Mar Convection becoming widespread on the Plains
Eta cross-section of RH (image), wind, theta, T, 18h forecast valid 06z 18 Mar Still has the nly winds along the Front Range. Interesting reverse flow w of the Divide…does this effectively raise the mtn height and coupled with the very moist and deep ely flow help yield the S++ west of the Divide?
Closeup reveals the wind is captured well but temps too warm
Eta analysis of surface T and wind with radar for 00z 18 Mar Even the analysis misses the colder temps near the Front Range.
00z/18 Mar Eta 6h forecast of surface T, wind and pcpn with radar for 06z 18 Mar The warmer temps near the Front Range in the analysis carry over through 6h.
00z/18 Mar LAPS analysis of surface T and wind with radar for 00z 18 Mar The LAPS analysis did do a good job, so in theory a fine scale model initialized with this analysis should do better.
