Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 411 AM EST Sat Mar 04 2023 Valid 12Z Sat Mar 04 2023 - 12Z Tue Mar 07 2023 ...Northeast... Day 1... The low that tracked through the Midwest yesterday is filling with energy now translating to a coastal low over Long Island that will shift east today. A strong easterly component north of the low center will allow dynamical cooling in banding associated with low level fgen of moist Atlantic air resulting in continued heavy snow across north-central New England where there are high probabilities of an additional 6" after 12Z, particularly for the southern Maine coast to the White Mtns and Merrimack Valley of NH. The low rides the 110kt jet east, bringing an end to most wintry precip over New England this afternoon. ...Pacific Northwest through Central California and the Intermountain West... Days 1-3... Building upper ridging into Alaska will help stall upper level low pressure off the WA coast today through the middle of next week with multiple waves of mid-level vorticity rounding this gyre and pushing into northern CA with the first shortwave trough axis crossing the far northern CA coast this morning. At the same time, a pronounced and intensifying Pacific jet streak, progged to reach 150+kt by tonight, will overlap with the mid-level confluence to surge moisture across northern/central CA and east across the Great Basin and much of the Intermountain West. The overlap of height falls, PVA, and left jet exit diffluence along the upper jet will provide widespread deep layer synoptic lift to support rounds of moderate to heavy snowfall through Sunday night before rates decrease with the very occluded system. Snow levels will rise up to 3000ft over CA ahead of this first wave, but then lower levels shift south from the Pacific Northwest, remaining below 2000ft through the event. Impactful snow is likely at many of the mountain passes with at least moderate accumulations in the coastal ranges and light snow likely even in the lowlands of CA with cold air and generally light onshore flow over the Pacific Northwest leading to generally diminishing snow rates there. With the heavy snow focus now from the OR border south through the Sierra Nevada, Day 1 snow probabilities are high for over a foot in the Klamath and northern Sierra Nevada with moderate probabilities over the Wasatch and highest Cascades. For Day 2, the probabilities for over a foot are limited to the Sierra Nevada where the are high. 2 to 4 feet are forecast over much of the Sierra Nevada through Sunday night. The very stalled pattern and focus of the Pacific jet farther east allows the heavier snow over the Sierra Nevada to become much more intermittent, but the onshore flow does last through midweek which will likely hinder efforts to help Sierra Nevada communities buried in recent snows. Day 3 snow probabilities for over 8 inches are moderate over the northern Sierra Nevada. ...Northern Plains through Upper Midwest/Northern Great Lakes... Days 2-3... A lobe of vorticity from the shortwave trough currently approaching the far northern CA coast will reach northern WY Sunday as it is amplified by the intensifying eastward-translating 150+kt Pacific jet streak over the Intermountain West. The northern Plains will be in the left exit region of this jet which will increase lift north of a lee-side low tracking east over the SD/Neb border on Sunday. Moist advection ahead of this low will lift up the MS Valley and into the Northern Plains as PWs surge to around 0.5in (+1 sigma) just north of a warm front. At the same time, a cold front will dig out of Canada in advance of a 1040mb surface high pressure centered over the northern Alberta, with increasing fgen beneath the upper jet supporting a stripe of moderate to heavy snowfall developing from west to east. Recent guidance remains in good agreement on a heavy snow axis near and north of the SD/ND border Sunday/Sunday night that will carry eastward into northern WI and the U.P. of Michigan Sunday night/Monday. The 00Z consensus is a bit south from the previous which puts the heavy snow focus more over northern WI than the North Shore of MN (though there still will be lake enhancement in easterly flow off Superior). Day 2 snow probabilities are moderately high for over 6" across all of central and south-central ND through the border with SD with moderate Day 2.5 probabilities focused over northern WI up into the North Shore and Day 3 probabilities moderate across the U.P. and the Tip of the L.P. Mitt. Jackson ...Key Messages for Midwest through Northeast Winter Storm... --Heavy Snow will continue to shift east from northern Upstate NY over north-central New England this morning with heavy snow tapering off over New England this afternoon. An additional 6-10" of snow is forecast for much of NH and southern Maine.