Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 AM EST Sun Dec 25 2022 Valid 12Z Sun Dec 25 2022 - 12Z Wed Dec 28 2022 ...Great Lakes through the Interior Northeast... Days 1-2... ...Lake effect blizzard conditions ease today... Low pressure filling as it lifts north from James Bay and the high weakening as it shifts down the Plains will continue to weaken the pressure gradient over the Great Lakes today with Blizzard strength winds generally tapering off by this afternoon. The broad cyclonic flow will maintain lake effect snow through tonight with additional light snow on Monday from the northern fringe of a clipper system. WNW flow over the upper lakes will favor multi-band snows over the U.P. and western Lower Michigan while SW to W flow off Lakes Erie/Ontario will maintain single banding before stalling and weakening by Monday with the heaviest snow off Lake Ontario where flow will be strongest. 1-2"/hr rates are likely tonight near Buffalo and into Watertown as that band shifts southward out of the Thousand Islands region. Day 1 WPC probabilities for at least 6 additional inches are low (20% or less) in the easter U.P., but high north from Tug Hill and south from Buffalo. Snow rates decrease Monday with the only Day 2 probabilities for 6 or more inches near Watertown, NY. ...Pacific Northwest and Northern Rockies... Days 1-3... Arctic air remains in place over much of the Interior PacNW along and east of the Cascades during a lull in precip between waves. The next wave approaches today with warm, onshore flow overrunning the trapped cold air across the Columbia Basin, so some interior areas will see another round of ice later today. Day 1 WPC probabilities for at least 0.10" of additional icing after 00Z tonight are generally 20-30% over east-central Washington, through it is noted that METARs in the Columbia Gorge are still in the upper 20s and icing is possible there today. A larger, system moves over the PacNW through Tuesday with Day 2 ice probabilities for 0.1" or more again around 30% in the Columbia Basin to the Gorge. Snow levels will rise from 5000 to 7000ft through Monday (as precip rates peak) before decreasing to around 4000ft Tuesday (as precip rates also decrease) to around 3000ft Tuesday night as the fairly cold core low approaches Monday and crosses WA Tuesday night. Day 2 snow probabilities are moderately high for 6 or more inches over the WA Cascades. However, the widespread moisture surging inland with lowering snow levels makes for expansive areas for heavy mountain snow for Day 3 with the Cascades and northern Rockies through northern UT all having high probabilities for 6 or more inches. California... Days 2-3... The strong, but progressive atmospheric river will be shunted south down the entire length of CA Monday through Tuesday night as the low pressure system shifts across the PacNW. Snow levels will be quite high, 8000-10,000ft in the core of the moisture plume, limiting snow to the High Sierra where there is a high Day 3 probability for a foot or more (two feet in the most favored areas). ...Northern Plains to the Mid-Mississippi Valley... Days 1-2... An Alberta Clipper will move through the Northern Great Plains today and through the Mid-Mississippi Valley by late tonight before being sheared apart by a following trough in the 130kt+ NWly jet late Monday, greatly weakening the wave as moves over the Ohio Valley. A modest push of milder air atop the cold surface layer in advance of the wave will support a chance of light icing on the southern/western side of the precip shield with >50% probabilities for measurable icing (after 12Z today) stretching from western ND MT through western MO/eastern KS. A band of locally heavy snow is expected from North Dakota southeastward into the Midwest on the order of a couple or few inches. Day 1 WPC probabilities of at least 4 inches of snow have increased to 40-70% over east-central ND and northeast SD into southwest MN with probabilities for 2 or more inches stretching southeast through IA to its northern IL border. Jackson Key Message for Blizzard --Lake Effect Blizzard A powerful winter storm will continue to produce lake-effect, wind-driven snow across the Great Lakes today with winds generally decreased through the day. The heaviest accumulations are expected in the snow belts east of lakes Erie and Ontario.