Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 AM EST Tue Jan 10 2023 Valid 12Z Tue Jan 10 2023 - 12Z Fri Jan 13 2023 ...Western U.S... Days 1-3... Another in this seemingly unending string of atmospheric rivers (AR) will move onshore today into CA downstream of closed low pivoting into OR tonight, with yet another progged to lift onshore again D2 into D3, but this second AR should be removed north of the previous, sparing CA the latter half of this forecast period. For D1, as the closed low moves into OR Tuesday evening, the associated IVT of 250-500 kg/m/s will be wrung out by WAA and height falls, with moisture spilling out as far east as the Central Rockies thanks to impressive upper diffluence as the zonal Pacific jet stream persists. This WAA will modestly raise snow levels to 4000-5000 ft, but will be offset by the antecedent cold front pushing eastward into the Intermountain West, so snow levels will only modestly fluctuate D1. Still, the impressive deep layer ascent atop the anomalous IVT and aided by upslope where low/mid-level flow orthogonally interacts with the terrain, will result in heavy snowfall. The highest accumulations are likely in the Sierra, Shasta/Trinity/Siskiyou region, Uintas/Wasatch of UT, and into the Wind Rivers and CO Rockies. WPC probabilities are high for 8 inches in all of these ranges, with more than 3 feet possible near Mt. Shasta and the southern Sierra. Shortwave ridging builds in briefly behind this first trough bringing a short respite to the heavy snow, before the next shortwave and associated AR moves onshore. This is progged to be much farther north than the previous ARs, focusing the heaviest precipitation northward into the Pacific Northwest, although heavy snow is again likely in the Shasta region of Northern CA as well. The northward displacement of this shortwave and AR will allow warm air to flood eastward into the Pacific coast, driving snow levels to above 7000 ft for most of WA/OR/CA, keeping heavy snow above most of the pass levels. However, impressive moisture and deep layer ascent will still result in heavy snow, with WPC probabilities indicating a high chance for more than 6 inches in the CO Rockies, residual from the leading impulse on D1, and locally 1-2 feet near Mt. Shasta. D3 probabilities for more than 6 inches reach 80% in the WA Cascades, although primarily above the passes, with a 10-30% chance for 6+ inches in the Blue Mountains. ...Northeast and Great Lakes... Day 3... Phasing northern and southern stream energy will result in an amplified longwave trough centered east of the Mississippi River Thursday night. Divergence downstream of this trough axis combined with height falls and increasing upper diffluence within the LFQ of a downstream and poleward arcing jet streak will drive a strengthening surface low from the Missouri Valley into Ontario, Canada by Friday morning. To the east of this deepening low, WAA from the Gulf of Mexico will spread northward to expand precipitation from the Mid-Atlantic to the northeast, while to the west some modest fgen and CAA in the wake of the low will produce moderate snowfall across the Great Lakes. For the Great Lakes area, increasing N to NW flow behind the low will drive an increasingly favorable environment for lake effect snow (LES), initially to the south of Lake Superior and east of Lake Michigan, before expanding to be downstream of Lakes Ontario and Erie late. While inversion depths and 850mb-sfc delta-Ts are progged to be modest, there should still be ample support for at least moderate LES snowfall Thursday and Thursday night thanks to increased moisture pulled from the Lakes. Additionally, a stripe of enhanced mid-level fgen within a weak deformation west of the upper trough should cause a more rapid changeover from rain to snow through dynamic cooling to result in moderate snowfall accumulations. WPC probabilities on D3 are 30-50% for 4+" in the U.P. and northern L.P. of MI due to LES, and 10-20% from northern IL through southern WI and into lower MI where the better ascent due to fgen occurs. Farther to the east, impressive WAA surging meridionally along the Atlantic coast will increase and enhance precipitation into New England and Upstate New York. This WAA will eventually drive a warm nose >0C northward while a Canadian high pressure retreats to the east. This setup suggests that while precipitation may start as snow across central and northern New England, it will likely change to rain everywhere except the White Mountains of NH and northern portions of ME, especially at higher elevations. There is still considerable uncertainty into how far north that warm nose will penetrate, but current WPC probabilities indicate a high risk for more than 4 inches across northern NH and much of northern ME, with local maxima approaching 8" possible in the higher terrain. The probability of significant icing greater than 0.25 inches is less than 10 percent. Weiss ...Key Messages for Early January Atmospheric Rivers... --Strong Atmospheric River Continues into Tuesday Heavy rain, heavy mountain snow, and damaging winds up to 60 MPH will continue over California and western Nevada today as the focus of the atmospheric river moves slowly southeastward. Flash flooding and debris flows are possible over burn scar areas. Scattered power outages are also likely. --Widespread Considerable Flood Impacts The cumulative effect of successive heavy rain events will lead to rapid water rises and landslides over northern and central California and portions of far western Nevada. Isolated major river flooding is expected through early this week, with record high river levels already observed, and may continue into Tuesday. --Dangerous Mountain Travel Through Tuesday 1-4 feet (localized amounts greater than 6 feet possible above 7000 feet) of additional snowfall through Tuesday, with blowing snow at times, will lead to dangerous travel above 4000 feet in the mountains and passes of northern and central California. --Increasing Risk of Infrastructure Impacts and Avalanches Mountain communities will be at increasing risk of infrastructure impacts and avalanches as snow load continues to increase. Two week snow totals have reached or exceeded 100 inches in many of the higher elevation areas of the Sierra Nevada. --Additional Precipitation Tuesday Another round of rain and mountain snow will move through the region on Tuesday. Some flash flooding is possible along with heavy snowfall in the mountains.