Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 404 AM EST Sun Feb 26 2023 Valid 12Z Sun Feb 26 2023 - 12Z Wed Mar 01 2023 ...Northeast... Day 1... A potent upper level disturbance tracking across New England on Sunday will be responsible for a surge in low-mid level moisture and WAA aloft. This trough is also the reason that a wave of low pressure will look to form along the Northeast Coast by Sunday evening. This system will be fairly progressive, limiting snowfall totals to generally being shy of warning-criteria throughout the region. WPC PWPF's highest probabilities are confined to generally being in the 2-4" range, highlighted with >2" probabilities topping out around 50-70% in the Adirondacks and across the White Mountains. Downeast Maine has the highest odds for >4" of snowfall, topping out between 70-80%. This storm system is not forecast to leave a wide swath of heavy snow, but heavy snow showers would result in reduced visibility and potentially dangerous travel conditions. The experimental PWSSI does contain some 20-40% probabilities for Minor impacts in far northern NY and the higher terrain of northern VT and NH, with up to 60% probabilities for Minor impacts over Downeast ME. Should the coastal low place a stripe of heavier snow bands over Downeast ME, it is within the realm of possibility to some localized totals to surpass 6". ...Upper Midwest/Great Lakes to the Northeast... Days 2-3... A vigorous upper level trough, the same trough responsible for the recent major winter storm in California, will race east across the Four Corners region Sunday and enter the Great Plains Sunday night. By 06Z Monday, the mean 300-700mb trough takes on a negative tilt fostering an impressive surface low tracking into the Midwest. By 12Z Monday, NAEFS shows both MSLP and 850mb heights falling below the observed CFSR for the time of year. This storm system will be accompanied by a strong and southerly 60-70 knot 850mb jet while it encounters the backside of the a cold Canadian high pressure system over Quebec. With boundary layer temps more marginal and an inevitable warm nose >0C aloft, ice will be the main concern over eastern MN, much of northern WI, and into central MI. These areas continue to have deep snow pack, which will aid in keeping temperatures sub-freezing longer. WPC PWPF shows a large area with 80% probabilities of >0.1" of ice accumulation in central WI, with 20-30% probabilities for >0.25" in north-central WI. Farther north, the atmospheric column will stay sub-freezing over the MN Arrowhead, the Michigan U.P., and far northern MI to allow for snow to be the primary precipitation type. WPC PWPF depicts 50-60% probabilities for >6" of snowfall over the U.P. of Michigan and along the coast of the MN Arrowhead. Farther east, strong 290k isentropic glide over the Mid-Atlantic and corresponding strong 850-700mb frontogenesis will result in heavy snowfall over the interior portions of the Northeast. Sub-freezing temperatures will be in place to start with a cold Canadian high located over Quebec. Thermal profiles over north-central PA will be more marginal, causing precipitation to fall in the form of snow initially, then changeover to sleet and freezing rain. From the Poconos on north into Upstate NY, precipitation will stay snow longer with snowfall rates of 1"/hr possible Monday night. While the 850mb low will open up as it tracks into Quebec Tuesday morning, there will remain a steady feed of 850mb moisture ahead of the 500mb shortwave trough tracking north of the Great Lakes. This will keep snow in the forecast across northern New England on Tuesday, although snowfall rates will gradually lighten throughout the day due to the weakening WAA aloft. WPC PWPF does show a large area of probabilities of 50-60% in the Catskills, Adirondacks, Berkshires, and north into the Green and White Mountains for >6" of snowfall Monday night into Tuesday. The Adirondacks and Whites, in particular, feature 40-50% probabilities for >8" of snow. The latest experimental PWSSI shows 40-50% probabilities for Moderate impacts in portions of western MA and northern CT. This is primarily driven by snowfall rates as a developing wave of low pressure off the southern shores of Long Island delivers some modest Atlantic moisture into the region. The key to snowfall potential along southern New England and as far south as the Tri-State area will be where and when this low develops. Latest guidance has come in a little colder over northern NJ, the Lower Hudson Valley, and in southern New England. This is largely due to the surface low forming initially closer to Atlantic City than farther north near the mouth of the Hudson River. There will likely be a sharp cut-of in totals along the coast as more maritime influence cuts down on snow totals, but where snow can remain the primary precip type, WPC PWPF does show 50-60% probabilities from as far west as the Poconos to central CT and central MA. Snowfall rates of 0.50-1"/hr overnight and into the early morning hours Tuesday may result in treacherous travel conditions for the Tuesday morning commute in the areas. ...Pacific Northwest, northern/central California, and Intermountain West... Days 1-3... Over the next several days, a series of upper level disturbances are set to produce rounds of heavy snow along many of the West Coast's mountain ranges, as well as the Intermountain West. The heaviest snowfall during this period will be found along the Sierra Nevada with the WPC 72-hour PWPF showing >80% probabilities for >30" of snowfall in the northern and central Sierra Nevada. More specifically, snowfall amounts could range between 4-6 feet in the higher terrain of the Sierra Nevada. Snow will also be measured in feet along the Coastal Range of northern CA and southwest OR, as well as the Cascade Range and the Olympics. The fetch of Pacific moisture and brisk 35-40 kt winds aloft will allow for upslope flow into ranges such as the Wasatch, Tetons, Sawtooth, and even as far east as the Colorado Rockies. All of these ranges, at varying times between Sunday and Tuesday, feature at least 40% odds for >8" of snowfall. The Wasatch and Tetons are the ranges with the best odds of seeing well over a foot of snow. In addition to the snow, wind gusts exceeding 40 mph will be common in these ranges, providing a recipe (between both snowfall rates and winds) for whiteout conditions and dangerous travel as low as 2,000 feet in elevation or some locations. The WSSI shows a large areal extent of Extreme Impacts over the next few days in the Sierra Nevada, Trinity/Shasta, and into the Coastal Range of northern CA and southwest OR. There are also Major impacts depicted along the Cascades, the Olympics, and the Wasatch through Tuesday. Lower elevations of the Pacific Northwest are also susceptible to disruptive snowfall amounts. Areas located along I-5 from the Seattle metro area to the Portland metro areas could witness a few inches of snow into early next week. The cumulative Days 1-3 WSSI does show Moderate impacts in the Portland metro area with Minor impacts along I-5 south of Olympia. Between light snow chances Monday and Tuesday and temperatures dropping near or below freezing each morning, roads could be slick in some areas and cause hazardous travel conditions. WPC is doing Key Messages for the ongoing parade of winter storms in along the West Coast and are listed below. Mullinax ...Key Messages for West Coast Winter Storms... --A series of storm systems moving southeast across the Pacific Northwest through midweek will produce heavy snow in the Cascades, Coastal Ranges, and Sierra Nevada. --Northern and Central California mountains, including the Sierra Nevada, will see heavy snow rates in excess of 2 inches per hour at times, leading to dangerous to even impossible travel. --If you plan to travel by road at elevations above 2,000 feet through West Coast states, be prepared for rapidly changing conditions and have winter driving supplies.