Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 326 AM EST Wed Dec 07 2022 Valid 12Z Wed Dec 07 2022 - 12Z Sat Dec 10 2022 ...Great Basin through the Central Rockies... Day 1... Mid-level trough over the Great Basin this morning will move into the central Rockies tonight and into early Thursday. 130kt southwesterly upper jet will combine with a moisture plume out of the Eastern Pacific streaming northeastward across AZ/NM (though focused south of the heavy snow area) and a surface front draped across the region to continue the ongoing snow for another day. Orographic upslope flow, especially on southwest to south-facing slopes of southern UT and southwestern CO, will aid in additional snow accumulations of more than six inches with some CAMs indicating >1"/hr possible over the Flat Tops, Sawatch, and San Juans. 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 1-3... A two-part upper low and trough over the Gulf of Alaska and northeast Pacific, respectively, will move southeastward into British Columbia and the Pacific Northwest by Thursday along with a trailing vort max to its southwest. This will bring a cold front into Washington/Oregon on Thursday with a narrow band of modest moisture (IVT ~ 250-400 kg/m-s into the Olympics). Snow levels 2000-3000ft just 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. That upper trough will move quickly eastward and spread modest snows to the Blue Mountains and into the central Idaho ranges. This upper trough will be replaced by another one out of the central North Pacific but from a more southerly source region (south of the Gulf of Alaska) by Friday. This will bring yet another cold front to the WA/OR coast and, given its more southerly source, more moisture to the region (continuing beyond this forecast period). Through 12Z Sat, the GEFS shows the probability of IVT values >250 kg/m-s >50% over much of coastal norCal as the ~100kt upper jet noses into the region. Increasing upper divergence will promote modest to heavier snowfall for the northern Sierra into the Shasta/Trinity ranges and also into the Oregon Cascades. WPC probabilities for more than 6 inches of snow is above 50% over the Olympics on Day 1, along the Cascades and into the northern CA ranges Day 2 where they maximize by Day 3 (with a renewed increase over the Olympics and WA Cascades). Three-day totals will likely exceed a foot along the Cascades southward to the northern Sierra. ...Central Plains to Upper Midwest.. Days 2-3... 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. Exiting Rockies trough will ride along the LFQ of a 130kt jet streak and potentially close off again over eastern NE into IA late Thu into Fri as a surface low tracks eastward across MO. There remains a good deal of uncertainty in the exact evolution and strength of the upper feature as well as the QPF and thermal profiles (i.e., p-type and SLR) but the signal remains for a modest strip of snow along the IA/MN border where WPC probabilities for at least 4 inches are around 10-20%. Likelihood of some appreciable freezing rain has increased per some of the 00Z guidance, especially in a zone from south central NE northeastward, perhaps into IA. WPC probabilities for at least 0.10" icing in this area are around 10-40%. This setup is highly susceptible for a tight gradient in snowfall amounts, as rates will determine where snow versus rain will occur, as well as accumulate. Fracasso/Snell