Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 355 PM EST Tue Dec 06 2022 Valid 00Z Wed Dec 07 2022 - 00Z Sat Dec 10 2022 ...Great Basin through the Central Rockies... Days 1-2... Weakly-closed low (bordering open wave) near the central CA coast this morning will finally move inland tonight through the Great Basin and into the central Rockies by early Thursday. Southwesterly upper jet will strengthen with time (>130kts by later tonight into early Wed) and a moisture plume out of the Eastern Pacific will stream northeastward across AZ/NM (though focused south of the heavy snow area). Surface front draped through the region will act as a focus for enhanced lift aided by orographic upslope flow, especially on southwest to south-facing slopes of southern UT and southwestern CO. CAMs indicate heavier snow rates >1"/hr possible over the Flat Tops, Sawatch, and San Juans, where 2-day total snowfall will likely exceed 1 foot above 7000 ft. Snow should finally begin to diminish in coverage and intensity on Thursday as the aforementioned upper low ejects into the central Plains. ...Pacific Northwest... Days 2-3... A sharp trough and closed low over northeast Pacific and Gulf of Alaska, respectively, on Wednesday will move southeastward into British Columbia and the Pacific Northwest by Thursday. This will bring a cold front into Washington/Oregon with a narrow band of modest moisture (IVT ~ 250-400 kg/m-s). Snow levels 2000-3000ft ahead of the front west of the Cascades (but below 1000ft along and to the east) will lower by 500-1000ft into Thursday as colder air moves in behind the front but with lower QPF. WPC probabilities for more than 8 inches of snow Wed-Thu are generally between 40-70% between the WA Cascades/Olympics and as far south as the Shasta/Trinity ranges in northern CA. More than 10 inches of snow is likely in the higher terrain with impactful snow possible at pass levels in the Cascades. Additionally, more widespread and lower-elevation snowfall accumulations are likely just beyond this forecast period. ...Central Plains to Upper Midwest.. Day 3... As a potent upper-level shortwave progresses east-northeast through the central Plains on Thursday and into parts of the upper Midwest by Friday, a swath of light freezing rain and potentially moderate snowfall is possible from Nebraska to southern Wisconsin. At the upper levels, a modest and extensive 250mb (110-130 kt) jet stream extending from northern Baja California to southern New England will produce an environment with plentiful Pacific moisture over the middle of the country. Shifting to the mid-levels, the shortwave riding along the northern edge of this jet stream is likely to sharpen and close off by Thursday night along with an associated 850 mb low. A warm nose at 850 mb near and to the east of this low along with cold and dry northeasterly flow at the surface will support a period of light freezing rain at onset of precipitation throughout eastern Nebraska and Iowa on Thursday. WPC probabilities for at least 0.10" of freezing rain are low (10-20%) across this region. As the mid and upper-level lows cross over this region, better upper divergence and Fgen are likely to lead to increasing snowfall rates. Assuming this, dynamic cooling is likely be high enough to substantially cool the column enough to support a localized region of moderate-to-heavy snowfall down to the surface. Better omega also appears to coincide within the DGZ, which could support higher SLRs than guidance depicts. Either way, a localized corridor of around 4 inches of snowfall could be realized throughout the aforementioned region and more specifically along the Iowa-Minnesota border into southern Wisconsin. WPC probabilities for at least 4 inches of snow are between 10-30%. This setup is highly susceptible for a tight gradient in snowfall amounts, as rates determine where snow versus rain will occur, as well as accumulate. Snell