Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 PM EST Sun Feb 26 2023 Valid 00Z Mon Feb 27 2023 - 00Z Thu Mar 02 2023 ...Upper Midwest/Great Lakes to the Northeast... Days 1-2... A deep closed upper low will continue to move from the Four Corners Region into the southern Rockies this evening, before beginning to lift more to the north ahead of a northern stream trough in the Northwest. As the wave begins to pivot northeast, a rapidly intensifying surface cyclone is forecast develop and track east from the central High Plains into the lower Missouri and mid Mississippi Valleys through the overnight into early Monday. The system will then continue its track to the northeast, reaching the Great Lakes by late in the day. Strong southerly winds, feeding moisture into an area of strong lift supported by low-to-mid level frontogenesis and strong upper forcing, will support precipitation advancing north into the upper Mississippi Valley and Great Lakes early Monday. The warm air aloft surging north over lingering shallow cold at the surface will set the stage for a freezing rain event, especially for portions of Minnesota, Wisconsin, Michigan. With little to lock the cold air in place, expect the surface freezing line to advance steadily to the north through early Monday before the system begins to wrap some colder air in from the north, supporting a transition to snow from northeastern Minnesota to the northern Michigan by the afternoon and continuing into the evening for portions of the region. The latest probabilities reflect a slight northward adjustment, but still indicate that ice accumulations of 0.10 inch or more are likely from east-central Minnesota southeastward into central Wisconsin, with higher probabilities east Lake Michigan over portions of central Lower Michigan as well. By late Monday, warm advection precipitation will begin to spread northeast from the upper Ohio Valley into western and central Pennsylvania and New York by the evening hours. Cold high pressure centered over eastern Canada and extending southeast into the northeastern U.S. will support a wintry mix for parts to western to central Pennsylvania and New York, with mostly snow farther north and east Monday evening. The system is forecast to begin to shear to the east as it interacts with the confluent flow over the northeastern U.S. As it does, energy will begin to transfer to a low developing near the northern Mid Atlantic coast Monday night. This will help to lock the cold air in place, while drawing moisture into a region of enhanced lift supported in part by low-to-mid level frontogenesis on the north to northwest side of the low. This may support mesoscale banding, raising the threat for some locally heavier totals especially for interior portions of southern New York and New England Tuesday morning. This secondary low is expected to quickly slide off to the east on Tuesday. However, precipitation will continue across parts of the Northeast as an upstream shortwave moves from the Great Lakes across the Northeast. This will support additional snows spreading northeast across eastern Upstate New York and New England through late Tuesday into Wednesday. Onshore flow and perhaps a weak wave developing over the Gulf of Maine may help to bolster additional totals across southern Maine Tuesday night. Widespread storm total accumulations of 4 inches are likely from the Poconos and northern New Jersey, through much of eastern Upstate New York, and into the interior portions of southern to central New England. Accumulations of 6 inches or more most likely across Catskills, southern portions of the Adirondacks, and the White Mountains. ...Pacific Northwest and California to the central and southern Rockies... Days 1-3... Even as the system currently impacting the West lifts out to the northeast, a series of shortwaves are forecast to follow, maintaining a broad trough and unsettled weather over the next couple of days. A shortwave trough and its associated frontal band will continue to sweep across the Pacific Northwest and Northern California this evening. While this system is expected to weaken and continue to move quickly to the east, a more-defined system diving southeast from the Gulf of Alaska is forecast to approach the Oregon coast Monday morning. This will bring another round of heavy mountain snows into the southern Oregon and northwestern California coastal ranges Monday morning before extending farther southeast into the northern Sierra by the afternoon. HREF guidance indicates periods of heavy snow reaching above 2 inches/hr across this area. In addition to heavy snow for the mountains, cold air in place will support accumulating snows for the lowlands of eastern Oregon and Washington. Additional energy dropping into the base of the broader scale trough will support additional heavy mountain snows, with accumulating snows in the lowlands across from western Washington to northern California on Tuesday. Energy dropping into the trough Tuesday night is expected to begin to amplify the trough, shifting the axis of deeper onshore flow farther south and bring the threat for heavy snow into the southern Sierra and the Southern California mountains Tuesday night into early Wednesday. By early Wednesday, models show the upper trough axis beginning to shift east, bringing organized precipitation and the threat for heavy snow across the Great Basin and northern Arizona and into the Utah and western Colorado ranges. By late Wednesday, these storms are likely to have produced several feet of additional snow along the Sierra. In addition to the heavy snow, windy conditions will also contribute to extremely dangerous to nearly impossible mountain travel. See below for WPC's Key Messages regarding these storms. ...Northern Plains to the Upper Midwest... Day 3... A well-defined shortwave emanating from the western U.S. trough is forecast to eject east-northeast across the region on Wednesday. While details remain far from certain, this may produce a narrow stripe of significant snows over the area, with strong upper forcing and low-to-mid level frontogenesis supporting the threat for mesoscale banding. The latest WPC guidance centers the greater threat from from the eastern South and North Dakota borders into central Minnesota. Pereira ...Key Messages for West Coast Winter Storms... --A series of storm systems will continue moving across the Pacific Northwest and into California through Wednesday with cold conditions making for lower than normal snow levels. --Mountains from southern Oregon to central California will see the greatest impacts with heavy snow rates in excess of 2 inches per hour at times and additional snowfall on the Sierra Nevada of 4 to 7 feet. Combined with high winds, blizzard conditions and dangerous to impossible travel are expected. --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.