Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 331 PM EST Wed Dec 07 2022 Valid 00Z Thu Dec 08 2022 - 00Z Sun Dec 11 2022 ...Great Basin through the Central Rockies... Day 1... Mid-level trough over the Great Basin this afternoon with a strong subtropical 130kt southwesterly upper jet drawing moisture from the Eastern Pacific will allow for snow to continue mainly over the CO Rockies tonight. Orographic upslope flow, especially on southwest to south-facing slopes of western CO, will aid in additional snow accumulations of more than six inches with 12Z HREF consensus for >1"/hr rates over the Flat Tops, Sawatch, and San Juans. Mountain snow will quickly diminish Thursday as the aforementioned wave ejects onto the central Plains. ...Central Plains to Upper Midwest.. Days 1-2... A compact and strong upper-level shortwave progresses east-northeast from northern CO across Neb Thursday, IA Thursday night, and IL/IN Friday. Warm air advection ahead of the wave draws Gulf moisture ahead and around the wave and developing 700mb low creating changeover from plain rain to a wintry mix then all snow with latitude. A swath of light freezing rain accumulation is forecast from the central Neb/KS border ENE through eastern Neb and western IA to southwest MN Thursday into Thursday night. Day 1 ice probabilities for over a tenth inch are 20-40% in this swath, highest in central Neb where diurnal effects look to aid accretion Thursday morning. It is noted that the cold front currently moving through Neb is quite potent with teens noted this afternoon in north-central Neb. A developing comma head around the 700mb low track looks to bring banded snow along the extend of the Neb/SD border into central SD and east through a more mature low (along with nocturnal effects) make the probabilities for 4 or more inches greatest along the IA/MN border (20 to 30%) in the Day 1.5/2 snow probabilities. Pivoting bands should allow locally higher amounts too. Mostly minor sleet is expected in pockets generally between the freezing rain and snow areas. ...Pacific Northwest through central California through the northern Intermountain West to the Northern Rockies... Days 1-3... A negatively tilted trough currently extending from the Gulf of Alaska is to well off Vancouver Island will dive to western WA tonight with a reinforcing shortwave trough crossing the OR/northern CA coast Thursday. This wave shifts east over the northern Rockies through Friday. A decent fetch of Pacific moisture accompanies this progressive wave with frontal precip with snow levels around 3000ft in the Cascades arriving late tonight to the WA/OR coast before it shifts south to northern CA and across the northern Intermountain West. Day 1/2 snow probs for 6 or more inches are moderately high for the length of the Cascades, northern Sierra Nevada, Klamath Mtns, and mainly the ID northern Rockies. However, a more amplified/deeper trough shifts southeast from the Aleutians Thursday night, arriving at the Pacific Northwest coast Friday night with reinforcing waves then closing the wave into a low that tracks across northern CA Sunday. This will bring another cold front to the WA/OR coast and, given its more southerly source, more moisture to the region as it becomes a full-latitude trough/low with much needed moderate to locally heavy precip shifting through all of CA Friday night through Sunday. Snow levels rise ahead of the wave Friday night to generally around 6000ft along the CA coast, but drop to around 3000ft under the upper trough through Saturday. Day 3 snow probs are high for 8 or more inches for the Klamath, CA Cascades, and northern/central Sierra Nevada. Heavy snow continues to shift south in terrain through Sunday. 3 day snows of over two feet are expected for the Klamath/Shasta/Trinity/CA Cascades/northern Sierra Nevada tonight through Saturday. Jackson