Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Sun Mar 16 2025 Valid 00Z Mon Mar 17 2025 - 00Z Thu Mar 20 2025 ...The West... Days 1-2... The period begins with the Pacific Coast sandwiched between an anomalous ridge to the east across the Intermountain West, and an amplifying trough well to the west over the Pacific Ocean. Between these two features, pinched mid-level flow will continue surge moisture eastward into the Pacific Northwest during the early part of D1, with IVT falling below 600 kg/m/s through this evening, funneling along a cold front and focused into northern CA and the northern Great Basin tonight. As the upstream trough over the Pacific deepens and pivots eastward, it will shed periodic lobes of vorticity into the Northwest, with an accompanying surface low likely moving towards the OR/CA border Monday morning. This surface low will briefly intensify ascent (which will already be impressive through height falls, PVA, upslope flow, and LFQ jet- level diffluence), while also pushing the strongest moisture channel southward into the central Sierra Nevada. While this suggests the highest PW anomalies will pivot south as well towards the Sierra, sufficient upper level flow will maintain elevated moisture well inland through the beginning of D2, providing the impetus for continued moderate to heavy snowfall across the terrain as far east as the NW WY ranges. For D1, the heaviest snowfall is likely within the plume of highest IVT along the cold front from the OR Cascades eastward into the Salmon River/Sawtooth ranges, and south into central CA and the Sierra. Here, WPC probabilities are high (>70%) for more than 8 inches of snow, with locally 1-2 additional feet possible in the Cascades and near the Shasta/Trinity region, although snow-levels indicate the heaviest accumulations will remain above 5000 ft before falling in tandem with weakening snow intensity. During D2, the heaviest snow becomes primarily focused in the Utah ranges and western WY as the main axis of moisture slides inland with the sharp upper trough, where probabilities for more than 8" of snow are also high, mainly above 8000 ft. ...Central Rockies/Plains through the Upper Midwest... Days 2-3... Increasing potential for a strong winter storm to stretch from the central High Plains to the Upper Midwest between Tuesday and Wednesday, with blizzard conditions becoming increasingly likely. See the latest Key Messages linked at the bottom of the discussion for more information. This system will initially develop downstream of a large trough which will cross out of CA and the Great Basin, reaching the Four Corners states Tuesday morning. Downstream height falls and PVA will help drive large scale ascent into the region, which will be additionally enhanced by a subtropical jet streak strengthening through the primary trough axis. As this jet streak pivots poleward, the LFQ will overlap with the mid-level height falls to help drive surface pressure falls, and lee cyclogenesis on Tuesday. However, this jet streak will outrun the primary forcing in response to vorticity shedding rapidly to the east, with secondary troughing and a more pronounced, negatively tilted, trough developing over the Central High Plains later Tuesday. This will result in secondary jet streak development, with even more impressive diffluence overlapping the more structured height falls/mid-level divergence. It is this secondary development around 00Z Wednesday that will potentially become a powerful cyclone with heavy snow across the central Plains and into the Upper Midwest through the end of the forecast period. 00Z NAEFS SAT depicts the low pressure over Kansas Tuesday night below the 0.5th climatological percentile and eventually PWs reaching above the 90th percentile across eastern Iowa and Wisconsin on Wednesday where the stronger surge of southerly flow exists, further highlighting the strength of this system for the time of year. Although the first low is weaker, it will interact with a cold front dropping out of the interior Northwest, leading to some enhanced ascent through fgen and upslope flow across Wyoming on D2. Moisture streaming into the wave from the E/SE, as well as aloft from the S/SW will manifest as precipitation across WY, but with the heaviest snowfall likely occurring in the terrain of the Big Horns, NW WY ranges, and along the axis of greatest fgen over N-central WY D2. However, total snowfall is expected to be modest this period except above 4000 ft in the NW WY ranges and Big Horns, where WPC probabilities for 6+ inches are above 70%. Then during D3, snowfall becomes much more expansive and heavy as the secondary, stronger, low develops and spreads snow from the Central Rockies into the Central Plains and eventually the Upper Midwest. There is still some placement and intensity uncertainty, but it has gradually decreased today with subtle slowing of the shortwave in the GFS/GEFS and faster ejection in the ECMWF. WPC probabilities have increased somewhat and still support a high risk (>70%) of 6+ inches from the Wind Rivers through the Laramie Range and into much of the CO Rockies where heavy snow rates are likely both through fgen/upslope flow. Farther east, increasing deformation over the Central Plains and Upper Midwest by the end of the period will result in increasing coverage of heavy snowfall and rates of 1-2"/hr in the deformation axis northwest of the strong surface low. Current WPC probabilities for at least 4" are low (10-30%) from eastern Colorado through northern Kansas and central Nebraska, before increasing to around 30-60% from eastern Nebraska through north- central Iowa. Then, a more mature cyclone and combination of WAA/deformation leads to higher probabilities for 6"+ of snow from southeast Minnesota through central/northern Wisconsin and the MI U.P., where the latest WPC chances are 40-60%. One factor that may impede the higher end potential of snow with this system is that temperatures will be very warm during the days and even hours prior to snowfall development. Highs on Tuesday are forecast to reach the 70s and 80s across the central Plains and Midwest. Additionally, as this secondary low intensifies, the pressure gradient is progged to become quite intense, regardless of the model preference, and WSSI-P is already featuring a widespread 40-70% probability of moderate blowing snow impacts, suggesting the potential for blizzard conditions in some areas. Additionally, decent overlap in ECMWF EFI snow and wind gust values in the central Plains are usually a good indicator that impacts associated with blowing snow are possible. Snell/Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png