Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Sat Mar 15 2025 Valid 12Z Sat Mar 15 2025 - 12Z Tue Mar 18 2025 ...Northern Plains... Day 1... The rapidly strengthening low moving across the Northern Plains which has been advertised the past few days will be well underway at the start of the period. At 12Z Saturday, the surface low is progged to be near the Twin Cities, MN, and will be lifting northeast, reaching Ontario, Canada by 00Z Sunday. N and W of this low center, a strong and pivoting deformation axis will be helping to drive intense ascent, causing a changeover from rain to heavy snow across western MN, resulting in blizzard, and near-blizzard, conditions through the aftn. Moisture advection downstream of this strengthening cyclone will remain impressive through D1, with isentropic ascent surging the accompanying theta-e ridge into a robust TROWAL which will pivot cyclonically back into the Dakotas and Minnesota today. The overlap of this enhanced moisture and the collocated deformation axis will increase both moisture and ascent to cause a narrow stripe of heavy precipitation, changing rapidly from rain to snow. Dynamic cross- sections across this region continue to suggest an axis of CSI/CI, supported by the elevated instability within the TROWAL, to create snowfall rates that are forecast both by HREF probabilities and the WPC prototype snowband tool to reach 1-2"/hr. Despite hostile antecedent conditions due to warm temperatures and rain, these snowfall rates will rapidly accumulate, and when combined with wind gusts of 50 mph, will likely result in blizzard and near-blizzard conditions through the aftn. This is despite accumulations that will in most areas be modest except within the stripe beneath the pivoting deformation axis, where WPC probabilities are high (>70%) for 4+ inches and locally as much as 8 inches is possible. ...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. The next impulse within this active pattern will push an impressive vorticity lobe eastward from OR through the Great Basin to start Saturday morning, with the resultant trough development driving height falls once again into the Four Corners on D1. PW anomalies that are near normal east of this feature as the maximum IVT downstream of this trough axis is modes and focused generally south into Mexico, but available moisture is still sufficient for a swath of moderate to heavy precipitation, with snow-levels generally 3000-5000 ft allowing for moderate snowfall accumulations above these levels. WPC probabilities D1 are moderate to high (50-90%) for 4+ inches across the CO Rockies, Sangre de Cristos, White Mountains of AZ, and the Sacramentos. As this lead 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 squeezing into WA/OR. Impulses within this flow will help push two distinct surface lows onshore, the first this 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. Resultant IVT with this setup becomes moderate to strong once again, potentially reaching 500-750 kg/m/s as moisture funnels onshore 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 later D2 into D3. Still impressive snow, but at generally above pass-levels, is forecast 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. During the latter half of D3 /Monday evening/ the parent trough digging across CA and into the Great Basin responsible for the snow axis shifting south into the Sierra late D2 will pivot eastward towards the Four Corners/Central Rockies. This will bring a slow end to precipitation across much of the West (at least briefly) but may help to produce another significant central U.S. storm by the middle of next week. ...Central Rockies and High Plains... Day 3... A shortwave emerging from the Pacific will amplify as it moves across CA late Monday and then tracks progressively into the Great Basin, reaching the Four Corners by the end of the forecast period. This shortwave will be accompanied by an intensifying sub-tropical jet streak which will begin to arc poleward downstream of this amplifying trough. This will have the dual-pronged effect of transporting moisture into the Central Rockies while also placing favorable LFQ diffluence atop the region of greatest mid- level height falls. The resultant deep layer ascent will likely interact with a cold front dropping southeast into the area, and the setup appears favorable for developing cyclogenesis in the central High Plains by Tuesday morning. Where the strongest ascent overlaps the greatest moisture, especially in areas of expanding frontogenesis, a swath of heavy snow is likely. WPC probabilities at this time are modest, just 30-50% for 2+ inches outside of terrain features across WY, but are higher in the Big Horns and Black Hills. This system may become the next powerful cyclone across the Plains through the middle of next week. 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