Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 335 PM EST Mon Feb 20 2023 Valid 00Z Tue Feb 21 2023 - 00Z Fri Feb 24 2023 ...Long-duration, widespread, major winter storm expands from the Northwest Monday night through much of the West and northern tier states through next week... ...Northern Great Lakes through New England... Days 1-2... Deep upper low over northern Hudson Bay will carry a shortwave across the western Great Lakes this evening/overnight and into the Northeast on Tuesday on the LFQ of a 150+kt jet streak off the Mid-Atlantic. Low pressure over northern WI this evening will move eastward and weaken through Canada as the mid-level vorticity streaks out to the southeast, favoring a coastal low near the 40/70 benchmark that quickly pulls away from New England as the trailing vort slips through central New England late Tuesday. Snow will spread across the Great Lakes, Canada, then the Northeast D1 into early D2 with some lake enhancement as the low passes by and the winds turn to northeasterly then northerly before waning in advance of the central CONUS system. Combined with some upslope into the northern shore of the U.P., totals may exceed several inches as the system moves through. WPC probabilities of at least 4 inches of snow are greater than 50% over parts of the U.P. and into the Tug Hill Plateau with low probabilities (generally less than 40%) for the Catskills and Green and White Mountains. ...Western United States through the Northern Plains, Upper Midwest, into the Northeast... Days 1-3... A complex evolution of the mid-level pattern this week will yield widespread heavy snow across nearly all of the West down to low elevations, with an impressive snowstorm developing over the Northern Plains on Tuesday in the first of a two-part event. Lowering snow levels, strong height falls, and well above normal moisture will combine to produce a major winter storm with far-ranging and long-lasting impacts for much of the region. Building central North Pacific ridging will help dig a downstream trough over Vancouver Island to dig strongly down the I-5 corridor D1 as the upper low off Baja opens into a wave over northern Mexico. Pacific front into the NW will sink southward and eastward as an arctic boundary over western Canada drops southward into MT and points south/east, driving in much colder air to the northern tier and up and over the Divide through much of the West (though the coldest air will be east of the Divide). 500mb heights will approach -4 sigma (and lowest in the CFSR climo period for mid/late-February) over the Great Basin, helping to lower snow levels down to all valley floors in the interior and well down into the foothills over much of CA into D2. Ex-upper low over the southern Rockies will eject northeastward, tapping the Gulf and bringing round 1 of precipitation to the Midwest/Corn Belt into the Great Lakes with strong WAA on nearly 50kts 850mb flow but with a 1032 high to the north over Canada. Exiting ~200kt jet (southern Canada) and incoming 150kt jet (central Plains) will provide broad lift with efficient divergence aloft as the first part of the system unfolds. By D3, strengthening upper jet across the Southwest into the Upper Midwest will promote cyclogenesis out of southeastern CO that lifts northeastward into the Great Lakes by late Thursday. Plentiful moisture via the Gulf and eastern Pacific will fuel the impressive dynamics with the system (2nd part, especially), along with strong WAA and FGEN with a strong +3 sigma upper high over Florida to the southeast. Across the west, continued ascent and much lower snow levels will favor snow even into the valleys and lowlands of the West, with moderate to heavy accumulations in even some of the lower terrain, including the coastal terrain around San Francisco, CA through many coastal ranges. High probabilities for more than 6 inches exist each day through the forecast period, spreading southward with time from the Cascades/Northern Rockies/NW WY ranges D1, OR Cascades and much of the Intermountain West D2, and Sierra/San Bernadinos D3. Widespread 3-day snowfall of several feet is likely above 2000 ft elevation, with impactful snow spreading into the passes as far south as southern CA. As impressive as the snowfall will be in the West, the heaviest and most impactful accumulations are focused on the northern High Plains and into the Upper Midwest D2 with the second part of the event the most prolific D3 from the Upper Midwest eastward into the northern Great Lakes. The first part favors eastern MT through southern MN Tuesday onward along the boundary via strong omega through isentropic upglide, leading to strengthening mid-level 700-600mb fgen just beneath the DGZ, supporting efficient snow growth, while the low level WAA helps to produce a deepening isothermal layer for efficient aggregate maintenance. The signal for excessive snow rates above 1"/hr within this band is modest due to modest conditional instability above the DGZ, but a translating band of heavy snow is likely into D2 and WPC probabilities for more than 8 inches D1-2 are near 50% in some areas, especially across the High Plains of WY, the Black Hills of SD, and into the Coteau des Prairies, Buffalo Ridge, and southern MN. However, by D3 /Wednesday/ the event really begins to ramp up as impressive warm/moist advection surging into a TROWAL aloft will enhance both elevated instability and moisture, resulting in expansive heavy snow from the central Plains through the Upper Midwest. SLRs may be above climo values especially in efficient bands with >1-2"/hr rates lifting across MN into WI. Two-day WPC probabilities for more than 12 inches of snow are high (>70%) over southern MN (including the Twin Cities) and into central/northern WI. Locally up to 2 feet of storm total snow is possible where the bands persist with the longest temporal duration, suggesting a top 5 event is possible. Farther downstream, increasingly impressive meridional moisture advection within the amplifying theta-e ridge on WAA out of the Gulf of Mexico will yield an expanding precipitation shield into the Great Lakes and Northeast D3. A potent warm nose with 850mb temps > 0C will lift well northward, and as a piece of the elongated Canadian high pressure arcs eastward towards northern Quebec and Ontario, an overrunning/wedge scenario appears to be unfolding resulting in a mixed precipitation event of snow, sleet, and freezing rain. Uncertainty into the exact thermal evolution create lowered confidence this far out into the forecast period, but it is becoming increasingly likely that impactful snow and ice (and a band of sleet) will spread across the L.P. of MI, northern PA and Upstate NY, and into New England on Thursday. Models continue to shift the low-level thermal boundary in response to the strength of the upstream parent low, but a modest middle-ground solution still yields appreciable icing and sleet accumulation south of the heavier snow. The snow axis will likely continue eastward into northern NY and VT/NH and eventually Maine as the system stretches out by D3 through the eastern Great Lakes at the nose of a strong/wide/elongated 185kt jet. For snow, WPC probabilities of at least 6 inches are greater than 50% for much of the North Country in NY eastward across VT/NH and into western ME. Ice accumulation may be significant, and WPC probabilities of at least 0.25" of freezing rain range are highest across southern Lower Michigan but also along the WI/IL line. In between, an axis of heavy sleet exceeding 1" is also possible though ensemble p-type probabilities show all p-types possible at this lead time in many area; details will be refined with future guidance closer to the event. Fracasso/Weiss ...Key Messages for Major Coast-to-Coast Winter Storm... --A prolonged major winter storm will spread a large swath of heavy snow from the West Coast to the Northeast this week. --Confidence is high that this winter storm will be extremely disruptive to travel, infrastructure, livestock, and recreation in affected areas. Power outages and tree damage are possible. --The combination of heavy snow rates and strong wind gusts may result in blizzard conditions in parts of the West and Midwest. In some places winds gusts will exceed 50 mph. --Farther east, heavy snow and a wintry mix of snow, sleet, and freezing rain will impact the Great Lakes and Northeast late Wednesday and Thursday. Treacherous travel conditions are likely, with scattered power outages possible. --Very cold temperatures are expected from the West Coast into the Northern Plains behind this system, with record lows and dangerous wind chills possible. The potential also exists for a flash freeze in portions of the northern Rockies on Tuesday.