Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 354 PM EST Mon Feb 27 2023 Valid 00Z Tue Feb 28 2023 - 00Z Fri Mar 03 2023 ...Northern Great Lakes through the Northeast... Days 1-2... A deamplifying shortwave trough over the Midwest will track across the northern Mid-Atlantic tonight with translation of energy to a coastal low tracking south of Long Island while the surface low over the Midwest fills. A building surface high over the Gulf of St. Lawrence will continue to provide sufficiently cold enough temperatures to support snow tonight from central PA/northern NJ and across much of NY (including NYC/Long Island) into New England. The 12Z HREF continues to have mean one hour snow rates of 1" across these areas into the overnight. Atlantic moisture funneling around the north side of the surface low can lead to a more prolonged period of banded snowfall from the Tri-State area to southern New England early Tuesday morning. Given the timing of when the periods of snow occur, this will likely lead to reduced visibility and slick travel conditions on roadways for the Tuesday AM commute from NYC through Boston. Day 1 snow probabilities for 4 or more inches are moderate from northeast PA/northern NJ, just north of NYC and north, with moderate probabilities for 8 or more inches for the Mohawk Valley into the southern Adirondacks, the Catskills, the Berkshires, and the White Mtns of NH into Maine. Warm air advection ahead of the Midwest low brings a risk for pockets of freezing rain tonight over northern PA where sufficient cool air in sheltered valleys freezes the rain. There are 10 to 20% Day 1 probabilities for a tenth inch of ice over North-central PA. ...West Coast through Four Corners States onto the southern High Plains... Days 1-3... A trough extending down the West coast through northern CA will shift east across the Great Basin tonight with a reinforcing trough currently over the Gulf of Alaska that will dig south from Vancouver Island Tuesday, closing into a low over central CA Wednesday before tracking through the Southwest Wednesday night. After this, a ridge builds over the West Coast, providing a break from this active pattern for which there are Key Message as shown below. Heavy snow continues to push down the Sierra Nevada this evening from the trough along the coast with 3"+ snow rates from topographic enhancement and coupled jets structures. The reinforced trough/low will continue amplifying over CA Tuesday night/Wed before pushing into the Southwest on Wednesday night where it will create a strong ~150kt jet streak at 250mb over the AZ and NM. As height falls and temperatures continue to cool in wake of a cold frontal passage, sufficient 700mb moisture flux will be funneled over the Rockies, setting the stage for round of heavy snow throughout the Four Corners region's mountains ranges on Wednesday. Day 2.5 WPC snow probabilities are high for >8" of snow along the Mogollon Rim, the ranges of southern UT, and through the San Juans. The Mogollon Rim of northern AZ features the best chances for >12" totals on Wednesday with 80%+ probabilities. Surface cyclogenesis ahead of the approaching upper low begins over the southern High Plains Wednesday night with upslope convergence north of the low occurring over the eastern CO/NM border with bands extending northeast into western KS where there are low Day 3 probabilities for >4". Key Messages continue to be generated for this event. The latest key messages are below. ...Northern Plains to the Upper Midwest... Days 1-2... The upper trough currently over the West Coast will eject east, crossing WY late Tuesday with lee-side cyclogenesis that crosses SD Tuesday night. A southerly/southeasterly flow of low-level moisture will be directed north to the Dakotas and western MN. There is a weak Canadian high pressure area to the north helping to lock in sufficiently cold air while 850mb frontogenesis aids in developing east-to-west oriented snow bands. The deformation axis is likely to setup on the northern flank of the 850mb front with heavy snowfall rates up to 1"/hr possible and since the motion will be well oriented with bands, sufficient duration of snow for localized double digit snowfall is forecast. Day 1.5 snow probabilities are 50 to 60% for 6 or more inches roughly from the SD/ND border to the I-94 corridor of central ND. The front begins to undergo frontolysis over MN which will lead to diminishing snowfall rates. Still, cold enough conditions for high SLRs and moisture aloft could lead to some areas across central MN and northwest WI to see plowable snow through Tuesday night where there are 30% probabilities for >4". Jackson ...Key Messages for Winter Storms pushing through the Southwest... --A series of winter storms will continue to impact the West Coast through Wednesday with the final storm in the series pushing over the Southwest U.S. Wednesday into Thursday. --Mountains from southern Oregon to central California will heavy snow rates of 2 to 3 inches per hour at times and additional snowfall of 3 or more feet for the Sierra Nevada and southern California ranges. Combined with high winds, blizzard conditions and dangerous to impossible travel are expected there. --Heavy snow will push across terrain of the Four Corners and southern Rockies and as far south as northern Arizona where hazardous travel conditions are expected Tuesday night into Thursday. --If you plan to travel through any mountain passes in the Southwest states, be prepared for rapidly changing conditions and have winter driving supplies.