Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Fri Mar 14 2025 Valid 00Z Sat Mar 15 2025 - 00Z Tue Mar 18 2025 ...Northern Plains... Day 1... Rapidly strengthening low pressure analyzed over the central Plains this afternoon will approach or exceed March sea-level pressure records across the Northern Plains as it tracks northeast through Saturday aftn. Not only will SLP records be approached, but 850-500mb height anomalies are progged to fall below the all-time minimums within the CFSR climatology according to NAEFS, further indicative of the intensity of this system. The result of this evolution will be a major winter storm impacting the Northern Plains late Friday through Saturday with a combination of heavy snow and gusty winds. As the surface low tracks northeast from the Central High Plains, it will be driven by an overlap of impressive height falls downstream of the primary closed low and at least modest upper diffluence in the LFQ of a poleward arcing jet streak east of the mid-level center. Together, these will drive intense deep layer ascent aiding in the storm strengthening. At the same time, confluent low-level flow east of the low will draw impressive moisture northward on 300-305K isentropic ascent. This moisture will surge am impressive theta-e ridge northward, which will then rotate cyclonically and lift into a robust TROWAL, pivoting over eastern NE, SD, ND, and western/central MN Saturday. At the same time, an impressive deformation axis will overlap with this TROWAL, supporting an intense axis of maximized ascent to drive heavy precipitation rates. Initially, the theta-e advection will surge warm air into the region such that the column will support primarily rain. However, as the low continues to deepen and move northeast, and mesoscale ascent intensifies, the combination of dynamic cooling and ageostrophic flow on NW winds into the low center will rapidly cool the column. The guidance continues to suggest a period of mixed precip, including sleet and freezing rain, will occur, but this again appears to be a conceptual setup where the transition period is quite rapid. This indicates that while some light icing is possible, the predominant wintry p-type will be snow, which will almost certainly become heavy at times (70% probabilities for 1"/hr according to the HREF). This is supported as well by NAM cross- sections indicating a region of theta-es lapse rates <0C/km in the presence of symmetric instability, suggesting the potential for CSI or even CI (thundersnow). Although antecedent conditions are quite warm and some rain will begin this event, these rates should quickly overwhelm this hostility leading to rapid snow accumulation. Finally, impacts during the period of heavy snow are likely to be impressive due to strong winds that may gust above 50 mph during the period of heavy snow. While the period of heavy snow rates is likely to be somewhat temporally modest except beneath the pivot point of the deformation axis, the overlap of extreme rates of 1-2"/hr and these strong winds will create near blizzard conditions and extremely dangerous travel. The changeover to snow will commence across eastern SD after 06Z Saturday before expanding north-northeast across western and north-central Minnesota between 12Z-18Z. Recent trends have shifted this band of heavy snowfall amounts (>4") eastward by about 75 miles or so as guidance is consolidating on a more compact area of low pressure across the Upper Midwest. WPC probabilities for at least 4" are high (>70%) across southwest to north-central Minnesota, with 40-70% chances extending into far eastern South Dakota and far southeast North Dakota. Locally, a narrow swath of at least 8" of snow is possible (20-40%) beneath the pivoting deformation, especially should any thundersnow occur. Additionally, some light icing across north- central MN has a 10-30% chance of reaching 0.1+" of ice accretion. ...The West... Days 1-3... The active pattern continues across the West with two separate systems producing widespread wintry precipitation across much of the area through early next week. After the significant winter storm departs into the central U.S., any break in precipitation across the West will be short lived as the active pattern sheds another amplifying shortwave across CA/OR tonight. This next impulse will be accompanied by an impressive lobe of vorticity, with the resultant trough development driving height falls once again from CA through the Four Corners on D1. Available moisture with this next impulse is progged to be much less than with the first shortwave, reflected by NAEFS PW anomalies that are near normal east of AZ as the maximum IVT downstream of this trough axis is modest (GEFS and ECENS probabilities for 500 kg/m/s less than 10%) and focused generally south into Mexico. Of course, this is pretty typical with ARs that are oriented more W/NW than W/SW, but enough moisture and continued lowered snow levels of generally 3000-5000 ft will allow for at least modest snowfall accumulations. Interestingly, as this shortwave digs across the Four Corners, the upstream flow becomes more favorable for a surge of moisture and heavy precipitation farther north into the Pacific and Interior Northwest. A trough over British Columbia/Alberta interacts with the trough moving across the Four Corners to squeeze the flow over the Pacific, resulting in fast and zonal flow surging into WA/OR. Impulses within this flow will help push two distinct surface lows onshore, the first Saturday morning and the second early Sunday morning, helping to enhance ascent. Regardless of those surface lows, deep layer lift becomes impressive and moisture increases as a strengthening Pacific jet streak pivots onshore and begins to buckle, lifting meridionally on Sunday downstream of an even more potent trough approaching the coast. Both 12Z ECMWF and GFS depict 500-700 kg/m/s IVT reaching the coast D2, funneling moisture along a cold front which will elongate into the region. Although the accompanying WAA with this atmospheric river (AR) will drive snow levels to as high as 6000 ft south of the cold front, considerable moisture and a sharp gradient in snow levels will likely result in impactful pass- level snow from the Cascades through the Northern Rockies and eventually Sierra Nevada once again on D3, with still impressive snow but generally above pass- levels farther south from the Great Basin into the Central Rockies, as much of the West is again covered in elevation- based snowfall. For the entire forecast period, high WPC probabilities (>70%) for 8+ inches exist from the Sierra Nevada and OR Cascades northward into the Olympics and WA Cascades, and as far east as the Salmon River/Sawtooth area, north into the Northern Rockies, and even as far as the Tetons. Event total snowfall is likely to be extreme in the higher terrain of the OR Cascades where 4-6 feet of snow is possible, with widespread 1-3 feet in the other higher elevations regions of the Northwest and Sierra Nevada. 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