Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 347 PM EST Sun Mar 2 2025 Valid 00Z Mon Mar 03 2025 - 00Z Thu Mar 06 2025 ...California & Great Basin to the Central Rockies... Days 1-3... A progressive but highly amplified upper level pattern will set up across the country through the middle of the workweek. This highly energetic pattern will support widespread wintry weather across the Rockies. Starting tonight, a deep upper level trough will be over California and the Great Basin, with instability induced by the cold air aloft supporting widespread snow across many of the local mountain ranges from the west coast to the Uinta Mountains of Utah. The snow will spread east to the Front Range of Colorado by Monday as the upper level trough pushes east and lee cyclogenesis begins in eastern Colorado. Meanwhile, an upper level front will follow behind the trough across the Pacific Northwest through Tuesday morning. The front will provide the forcing for snow to continue from Washington State east through Montana, while a surface polar high over the Canadian Prairies provides cold air from the north, keeping snow levels low (under 4,000 ft. in some areas) from the Sierras north and east through Montana. Integrated Vertical Transport (IVT) values through Tuesday will generally peak between 200 and 300 kg/ms. These are modest values, implying that at first, lack of available moisture will limit accumulations, especially where orographic uplift is not present to wring out additional snowfall. WPC Probabilities remain high (70-90%) for at least 4 inches of snow through Monday afternoon across the ridges of central and northeastern Nevada. As the upper level low pushes east through Tuesday and the developing Plains cyclone takes over as the primary forcing mechanism for heavy snow, the probabilities of heavy snow will also shift east into Idaho, Wyoming and Montana, as well as into the Front and Park Ranges of Colorado. A shortwave embedded within the upper level trough will locally increase upper level divergence in the favorable left exit region of a 150 kt jet that extends into west Texas. The low level jet will be strengthening from east Texas into Arkansas, drawing Gulf moisture northward. However, in these early stages of the low's development, moisture into the northern half of the storm will still be limited. Thus, the heaviest forecast snow totals will be limited to the Front and Park Ranges and adjacent mountains where orographic uplift will provide a larger fraction of the overall lift causing the heavy snow. There is a moderate (50-70%) chance of at least 6 inches of snow through Tuesday afternoon at 8,000 ft. and higher in the latest WSSI-P. Further north and west across Wyoming and Montana, where the upper level front will be the dominant forcing mechanism and further away from the Gulf, snowfall totals will be lighter. There is a low (10-30%) chance of 4 inches or more of snow in these areas in the WPC probabilities. A second deep trough in the upper level flow will move into the West Coast on Wednesday, with yet another round of heavy snow possible from the Sierras into northern Nevada through the day on Wednesday. A surface low may develop to the lee of the Sierras across southern Nevada as a result of the forcing and leeside flow over the Sierras. The timing of how fast the cold air can fill in behind (north and west) of the low, as well as the extent of the precipitation shield will be critical as to potential snowfall accumulations. There remains considerable uncertainty about those factors, which will become more clear with future updates in the guidance. ...Central Plains to Upper Midwest... Days 2-3... ...Powerful winter storm to cause widespread heavy snow and blowing snow with localized blizzard conditions across the central Plains from Monday night into Tuesday and then into the upper Midwest Tuesday night through Wednesday... The upper level trough that ejects from the Rockies into the Plains Monday night will have an embedded low that moves across Colorado and into Kansas. This will be the focus for where the surface low will form and rapidly intensify during the day on Tuesday. The trough will become negatively tilted Tuesday, resulting in a steadier intensification of the surface low as it tracks northeast from Kansas to northern Michigan by Wednesday evening. North and west of this track, from northeastern Colorado to western Iowa, southern Minnesota, Wisconsin, and northern Michigan, a period of heavy snow is likely as the comma-head of the storm moves over the region. Meanwhile, the surface low will deepen well into the 980s hPa range. With a polar high providing cold air and adding to the baroclinic instability with the remarkably strong LLJ pumping Gulf moisture northward ahead of the storm, very strong winds will accompany the heavy snow, causing widespread blowing snow and possible blizzard conditions. The latest WPC Winter Storm Severity Index (WSSI) shows a large area of Moderate impacts (disruptions to daily life, with extra caution needed while driving) through Wednesday morning from the northeast corner of New Mexico north across almost all of the eastern Colorado Plains, the western two-thirds of Nebraska, and south-central South Dakota. This is being driven almost entirely by the blowing snow component of the WSSI. Due to both the intensity of the storm and its fast movement, there will be a rapid transition of precipitation-type from rain to snow on the west/cold side of the storm. Also due to the strong winds and resultant blowing snow, there is considerable uncertainty as to snowfall amounts from the Plains to the U.P. of Michigan, as blowing and drifting will cause large variability in accumulations from place to place. Thus, for most areas, expect the impacts from blowing snow to outweigh the impacts from snow amounts alone. Further, the high winds will likely break apart any larger snowflakes, resulting in lower SLRs and lower accumulations, but increasing the number of partial flakes in the air and contribute to lower visibilities. There is likely to be an area of lower accumulations in and around eastern Nebraska as the low refocuses and develops. This will occur when the upper level trough supporting the low first turns negatively tilted Tuesday afternoon, and the southern jet streak rounding the base of the trough extends far enough northward ahead of the trough to maximize the upper level divergence by Tuesday evening between that jet streak and a second west-east oriented jet streak extending from the Arrowhead of Minnesota east across the Canadian Provinces of Ontario and Quebec. Wegman ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png