Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 310 AM EST Mon Jan 09 2023 Valid 12Z Mon Jan 09 2023 - 12Z Thu Jan 12 2023 ...Western U.S... Days 1-3... The parade of atmospheric river (AR) events continues, almost without a break, this period as two additional troughs and associated ARs shift into the West. The first will continue to center across CA D1 with IVT exceeding 750 kg/m/s, and the associated WAA pushing snow levels up to 8000 ft. The core of this AR will shift southward today into Tuesday as the associated vort swings eastward, helping to spread more anomalous PWs into the Great Basin and Central Rockies. While snow levels will initially be quite high, they will fall to around 4000 ft as the associated surface low and cold front drop southeast before dissipating. This impressive AR /moisture/ and accompanying large scale ascent will result in heavy precipitation falling as snow at rates of 3-5"/hr in the Sierra according to the WPC snow band tool, with rates even above 1"/hr shifting along an axis of enhanced fgen into the Great Basin and Northern Rockies. WPC probabilities D1 are high for more than 8 inches in the northern CA ranges and Sierra, as well as into the Uintas, Sawtooth, and multiple ranges of NW WY. The heaviest accumulations will occur across the Sierra however, where heavy wet snow will likely accumulate to more than 5 feet resulting in extreme impacts, primarily above 7000 ft due to snow load. Another wave and associated AR will follow quickly behind the first into CA, and although the intensity of this AR will be more modest than the first, the associated anomalous moisture and deep layer ascent will produce additional heavy snow, generally above 4000 ft, for the NorCal terrain and Sierra once again, with moisture spilling as far east as the Wasatch and San Juans resulting in heavy snow across these areas as well. WPC probabilities D2 are moderate to high for an additional 8+ inches of snow across these areas, with the Sierra again receiving locally 2-3 ft in some areas. Yet a third AR will approach the Pacific coast D3, but the core of this IVT is progged to be much farther north than those occurring D1-2. This will drive snow levels to 5000-7000 ft along the entirety of the coast west of the Cascades/Sierra, with heavy snow lifting northward from the Shasta/Trinity/Siskiyou region of northern CA into the WA Olympics and Cascades. Heavy snow exceeding 6 inches is likely across this area on D3, with, flow angling more to the south supporting more than 1 foot near Mt. Shasta. ...Great Lakes... Day 3... A northern stream shortwave moving along the international border will amplify Wednesday as it moves towards the Great Lakes. Some interaction with a southern stream impulse may help drive surface low development across the Dakotas and into the western Great Lakes, with PVA and modest height falls helping to provide ascent. To the east of this low, WAA from the Gulf of Mexico will increase to spread higher PWs northward, resulting in expanding precipitation across the region. While most of this snow is likely to be light, SE flow across Lake Superior will locally enhance moisture, which will then upslope into the Arrowhead of MN to lead to locally higher snowfall totals. WPC probabilities for 4+ inches are as high as 20% in the far NE tip of MN. 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 Arrives Sunday Night: A powerful atmospheric river arrives Sunday Night, peaking in intensity Monday into Tuesday over California and western Nevada with additional heavy rain, heavy mountain snow, and damaging winds up to 60 MPH expected. Flash flooding and debris flows are possible over burn scar areas. Scattered power outages are also likely. --Threat of Heavy Rain, Flooding Continues: The cumulative effect of successive heavy rain events will lead to additional considerable flood impacts, including rapid water rises and mudslides, over northern and central California, and portions of far western Nevada. Isolated major river flooding is expected through much of this week, with record high river levels possible. --Dangerous Mountain Travel To Continue 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. --Emergency Preparedness: Never drive across flooded roads †turn around, donâ€t drown †and be sure to have both an emergency preparedness kit and evacuation plan in place. More heavy precipitation returns late week.