Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Thu Mar 13 2025 Valid 12Z Thu Mar 13 2025 - 12Z Sun Mar 16 2025 ...The West... Days 1-3... ...Extremely active weather pattern will spread heavy snow across most of the West today through Friday, with a second wave approaching for the weekend. This lead system will likely become a significant winter storm in the Northern Plains Saturday. Key Messages are in effect, and linked below... The period begins with a short wavelength but extremely amplified trough aligned on the immediate Pacific coast Thursday morning. This trough will continue to amplify as it moves across the Great Basin Thursday evening and then into the Rockies on Friday. During this evolution, a potent shortwave and accompanying vorticity maxima will swing through the base of this trough, This feature will outrun the northern leg of this trough, resulting in a closed mid-level low pivoting into the Southern Rockies Friday morning and then into the Southern Plains. This will be an extremely impressive feature, as NAEFS ensemble tables indicate 500-700mb heights fall below the minimum within the CFSR database by Friday morning across the Central and Southern Rockies, with a surrounding large area of height anomalies below the 1st percentile across most of the Western U.S. This mid-level height evolution is a clear signal for a powerful and widespread system to bring heavy precipitation across an expansive area. While height falls will drive ascent across a vast region, this will combine with upper diffluence within the LFQ of a subtropical jet streak which will wrap around the base of this trough, producing enhanced deep layer lift, especially into CA and the Four Corners. Additionally, periods of upslope flow on W/SW 700 mb winds will enhance ascent across much of the terrain, first across the Sierra and Cascades, and then spreading east across the Great Basin and into the Rockies Friday. Additionally, W/SW mid- level flow will isentropically ascend the region, leading to additionally enhanced lift across a large portion of the region. In the presence of this pronounced ascent, moisture will steadily increase as an atmospheric river (AR) pivots onshore downstream of the primary trough axis. IVT progs from both the GEFS and ECENS indicate a high probabilities (>90%) for a plume of 250 kg/m/s IVT, although probabilities for 500 kg/m/s IVT are tepid (< 30%). Still, this pronounced moisture plume will surge PWs to above the 90th climatological percentile from CA through the Southern Rockies, with more normal PWs farther north. The overlap of the aforementioned ascent into this moistening column will result in widespread precipitation, with snow falling in the terrain. While confidence is high in widespread precipitation, there is still uncertainty into the snow levels. The steep lapse rates beneath the core of the deepening upper trough suggest heavy precipitation rates will drag down snow levels, indicating lower NBM percentiles are likely more realistic. Using the NBM 25th percentile as proxy, snow levels could fall to 2000-3000 ft during periods of heavy snow across much of the West, with heights as low as 1000-1500 ft across the Cascades. This indicates that travel across many passes will be greatly impacted, especially where snowfall rates reach 1-2"/hr, potentially as high as 3"/hr across the Sierra, Transverse Ranges of CA, and parts of the Great Basin terrain as reflected by the WPC prototype snowband tool. Not only will rates be impressive, they will combine with strong winds to produce substantial impacts, and the WSSI-P is forecasting a high probability (>90%) for at least moderate impacts for much of the terrain, especially from CA through the Four Corners. In these areas, WPC probabilities forecast a high chance (>90%) for at least 8 inches D1 in the Sierra, Transverse/Peninsular Ranges, the Wasatch, Mogollon Rim, San Juans, Uintas, and Wind River Mountains. By D2 lingering snowfall may accumulate above 8 inches once again, but primarily across the CO Rockies. Locally 1-3 feet of event total snowfall is probable in these ranges. As this first impulse departs into the Central Plains D2, /00Z-12Z Saturday/, another wave immediately on its heels will approach the Pacific Coast and spread renewed height falls onshore as far east as the Central Rockies. This second wave appears to be less intense as reflected by weaker height and PW anomalies, and is progged to be shifted north of the initial impulse. However, renewed periods of heavy snowfall are likely from the Sierra and northern CA terrain north and east into the Cascades and then spread east D3 into the Rockies, generally from CO northward as a warm front pivots through the area. Snow levels with this second wave are expected to be higher, rising to 4000-5000 ft in CA and 2500-3500 ft elsewhere, but still low enough to be impactful to many passes as Pacific air is slow to flood eastward. With this second wave, WPC probabilities D2 re-focus in the Sierra, the Shasta/Trinity region of northern CA, and the OR Cascades where they are above 70% for 8+ inches. On D3 these probabilities shift northeast, with 70%+ probabilities extending across the WA/OR Cascades, the Olympics, and into the Salmon River range. ...Central and Northern Plains... Days 2-3... The southern stream closed low emerging from the Four Corners and into the Southern Plains Friday morning will become a formidable winter storm across the Plains Friday into Saturday. As this closed low emerges into the Plains early Friday, it will continue to deepen, featuring 500-700mb height anomalies by 00Z Saturday that are below the minimum percentile within the CFSR climatology between 850-500mb across a large portion of the Plains. This is an exceptionally deep system, and the combination of the intense height falls with the LFQ of a subtropical jet streak rotating through the trough will result in rapid lee-cyclogenesis beginning as early as Friday morning. This surface low will deepen rapidly, approaching March record pressure levels across the Plains as it tracks from eastern CO Friday morning to eastern SD Friday evening, and then the U.P. of MI Saturday aftn. While there is some uncertainty in the exact placement and track of this low, the overall spread is relatively small for a D3 system, leading to high confidence in the evolution. With this low deepening rapidly, significant moisture will be drawn northward from the Gulf on impressive isentropic ascent, and the resultant theta-e ridge is progged to lift into a robust TROWAL which will pivot across WI and into MN/ND/SD Saturday morning. As this occurs, the combination of increasing ascent, aided by a pivoting deformation axis W/NW of the low center, and cooling of the column through both dynamic affects and ageostrophic flow/cold air drainage from the NNW, will result in a p-type transition from rain to mixed to snow. There remains uncertainty into how long this transition will take to occur, but the intense omega into the column Saturday morning suggests the guidance may be under-doing the speed at which this occurs, indicating a faster transition from rain to snow, with less freezing rain/sleet in between. This is more typical of the conceptual model for this type of system as well, so while mixed precip is likely at times, the more impactful wintry precipitation type should be snow, which will likely come down heavy at times. At the same time, very strong winds, which are progged by model soundings to exceed 50 kts in the 800-900mb layer, while help to fracture dendrites leading to low SLRs which will be blow around considerably. So, while total snowfall may be somewhat modest except beneath the pivot point of this deformation axis, impacts will be substantial with near blizzard conditions possible from far northeast Nebraska through northern Minnesota. In this area, 48-hr WPC probabilities from 12Z Fri to 12Z Sunday reach as high as 50-70% for 4+ inches, and isolated totals above 6 inches are possible (10-30%). Additionally, some light freezing rain has a 30-50% chance of accumulating 0.1" or more in a small portion of NW MN. 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