Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 411 AM EST Thu Jan 06 2022 Valid 12Z Thu Jan 06 2022 - 12Z Sun Jan 09 2022 ...Pacific Northwest to the Central Rockies... Days 1/2... Two systems are combining to promote a surge in moisture into western Washington early this morning with precipitable water values of 1.25 inches on the coast which is +3 sigma. This will moisture plume will shift inland across Idaho and into Wyoming/Utah today to produce heavy snow above the snow level will be around 5000ft. A trough axis shifts inland over WA/OR Friday with a wave of additional heavy snow then through Friday night across the Cascades over to the Sawtooths/Absarokas with snow levels decreasing behind the trough as precip rates slowly decrease Friday night. This trough weakens some by Saturday when it affects the CO Rockies. Ridging overspreads the western US Saturday/Sunday with a welcome break from the La Nina onslaught for the NW through particularly cold air settles in east of the Divide and over the Northern Plains. ...Great Lakes... Days 1/2... Northerly to northwesterly winds will allow a favorable long duration lake effect regime through tonight across the Great Lakes. The much colder air and maximum lift well within the DGZ supports SLRs 20:1 or even a bit higher in the more intense snow bands. As a result, total accumulations in excess of 8 inches are expected off the Lakes, particularly for the favored snow belt areas where isolated totals of a foot or more are possible. Snows are expected to diminish on Friday from west to east as a shortwave ridge moves east across the region. ...Tennessee and Ohio Valleys through Mid-Atlantic/Northeast... Days 1-2... An amplifying shortwave trough currently pushing ESE from the central High Plains will swing to the lower OH valley by this evening before taking on a negative tilt and shifting east to the Mid-Atlantic coast tonight before lifting northeast just off the Northeastern Seaboard Friday. Coastal low pressure develops over Cape Hatteras this evening, rapidly developing as it shifts north to Nova Scotia through Friday afternoon. 00Z Guidance is in fairly good agreement on this progressive system with a just offshore track that would allow brief heavy snow potential along the I-95 corridor northeast from Northern Virginia through eastern Maine. Computer server issues on site here at College Park did not allow for the full ensemble of data to provide snow probabilities, so a few important notes will be made here. So far, the system is performing strongly over KS with bands of moderate snow. Guidance continues to indicate a period of strong forcing supported by low-to-mid level frontogenesis overlapped by a coupled upper jet to support banded heavy snow (1+ inch/hr rates as supported by the 00Z HREF) developing and translating east across central/southern Kentucky and northern Tennessee starting later this morning and across the southern into the central Appalachians late this afternoon through the evening. South of the heavier snow, a stripe of up to around a tenth inch of freezing rain this morning centered over northern MS/AL with some over eastern TN this afternoon. By midnight, as energy translates from the northern stream trough to the coastal low develops moderate to locally heavy snow breaks out over the north-central VA Piedmont and shifts east across the greater DC metro before turning northeast up the I-95 corridor. The 00Z HREF mean 1-hr snowfall indicates a few hours of snow over 0.5"/hr with embedded areas of 1.0"/hr for one or two hours from north-central VA to southeastern New England tonight, continuing along eastern coastal New England Friday morning and through the afternoon for Down East Maine. Jackson ~~~ Key Messages for the Tennessee and Ohio Valleys to the Northeast Winter Storm ~~~ - A winter storm is likely to impact areas from the Mid-South to the Northeast through Friday. - Several inches of heavy snow and a zone of mixed precipitation, including freezing rain, is likely to cause travel disruptions and hazardous to potentially dangerous road conditions from the Mid-South to the southern and central Appalachians Thursday and Thursday night. - Accumulating snow, heavy in places, is expected as the system moves through the Mid-Atlantic and Northeast Thursday night into Friday, continuing through Friday for eastern Maine.