Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 219 PM EDT Thu Mar 28 2024 Valid 00Z Fri Mar 29 2024 - 00Z Mon Apr 01 2024 ...The West... Days 1-3... An increasingly active period of winter weather is likely across the West into the weekend as an impressive closed low digs along the CA coast, forcing a broad trough to encompass the West with waves of energy and moisture shifting onshore. The period begins with a weakening closed low west of the WA coast shedding vorticity lobes to the east and forcing increasing height falls into the Pacific Northwest as the trough amplifies. At the same time, a potent shortwave rotating around this closed low will dive southeast and interact with the lead closed low, causing it to shear and weaken, while the secondary impulse becomes the primary closed low off the CA coast by Saturday morning. The evolution from there becomes quite complex as this closed low continues to dive along the CA coast into Sunday, while shearing potent shortwaves downstream into the impressively divergent flow over the Great Basin, leading to a positively tilted by amplified longwave trough over the West by the end of the forecast period. The resultant subtropical jet streak pivoting downstream of this trough axis will additionally enhance ascent, and by the weekend there is likely to be pronounced but broad ascent across much of the region. At the same time, moisture will steadily increase into the area as onshore flow becomes increasingly backed leading to deeper moist advection on SW /warm/ flow, with IVT increasing to above 250 kg/ms leading to PW anomalies reaching above +2 sigma, highest across southern CA and the Great Basin late D2 into D3. The expansive synoptic ascent noted above will lead to waves of low pressure moving eastward, including a strong low which may move into southern CA D3. The accompanying fronts will drive varying snow levels as well, although NBM 75th percentiles hover around 5000 ft, and the result of this ascent into the higher moisture content within varying snow levels will result in widespread heavy snow across the West. For D1, WPC probabilities above 70% for 6+ inches extend from the Olympics into the WA/OR Cascades, east to the NW WY ranges and the Uintas, and then down into northern CA and the northern Sierra. By D2 the forcing and moisture shift to become more focused across CA and into the Central Rockies, with WPC probabilities for more than 6 inches reaching 70% or more focused in the Sierra and Shasta/Trinity/Siskiyou region. Heavy snow will continue across the Sierra on D3, but will become more expansive as well, with WPC probabilities above 50% for 6+ inches reaching the Transverse and Peninsular ranges, as well as surging across the terrain of NV and lifting into the Wasatch and Uintas as well. Although most of the snow will be terrain focused, the setup may support some enhanced banding and heavier snow rates along the 700mb WAA/fgen in NV/UT which could produce light snow into much lower elevations as well. Even if that occurs, the heaviest accumulations should remain in the Sierra and other high terrain of CA where 3-day snowfall of 2-3 feet is likely. ...New England... Day 2... Southern stream impulse lifting along the Atlantic coast will phase with northern stream energy near the Mid-Atlantic states, leaving a negatively tilted and eventually closed-off mid-level low moving into the Canadian Maritimes by Saturday morning. This mid- level evolution combined with modestly coupled upper jet streaks and along a surface baroclinic gradient will drive rapidly intensifying cyclogenesis, and an impressive surface low is likely to be positioned well east of Maine by Friday night. The guidance has trended a bit east with this feature, and while this keeps the highest moisture farther offshore, it also allows for colder air to filter more rapidly into northern New England. Additionally, a potent deformation axis strengthening on the west side of this low will help enhance ascent, and combined with the somewhat colder column to produce a swath of moderate to heavy snow from far northern NH into downeast ME. The heaviest accumulations should remain in the higher terrain as the column is still marginal otherwise until dynamic cooling can overlap with the slowly filtering CAA, but WPC probabilities are as high as 50% for far northern NH and northern ME, especially at higher elevations. ...Northern Plains and Upper Midwest... Day 2.... A lobe of vorticity shedding from the closed low west of the Pacific Northwest will surge eastward across the Northern Rockies and then begin to amplify over the Northern High Plains Friday morning. This feature will maintain amplitude as it advects progressively to the east, reaching the Great Lakes Saturday morning, and will interact with modest upper diffluence along the LFQ of a jet streak to the south, to produce a weak wave of low pressure along the surface front. While this wave will be weak and progressive, the accompanying 850-700mb WAA will surge northward and lift isentropically with mixing ratios of 3-4 g/kg. This will expand an area of precipitation from the Dakotas into MN/WI/MI. The column into which this ascent is occurring appears more than cold enough for snow, but the DGZ id elevated and only partially saturated, so expect SLRs will be somewhat compromised as the best moisture and lift is warmer than -12C. This should still result in an axis of accumulating snow, but WPC probabilities for more than 4 inches of snow reach as high as 20-30% across northern ND and into western MN. Weiss