Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 315 PM EST Tue Jan 09 2024 Valid 00Z Wed Jan 10 2024 - 00Z Sat Jan 13 2024 ...Midwest through the northern Great Lakes... Days 1-2... The deep/strong low pressure over the Midwest this afternoon will continue to move northeast through the Great Lakes tonight and Wednesday morning. Downstream of this low, intense moisture advection will continue to surge out of the Gulf of Mexico on 290-295K moist isentropic ascent, driving PWs up towards +2 sigma according to NAEFS. This moisture laden air is then progged to wrap cyclonically within the WCB into a modest TROWAL which will pivot back into the Great Lakes and Upper Midwest, resulting in some enhanced elevated instability to increase ascent and drive more impressive snowfall rates that could exceed 1-2"/hr from far northern IL, southern WI through western to northern L.P. of Michigan later tonight. Generally along/northwest of the 850-700 mb low track, a swath of heavy snow remains expected and for the D1 period (00Z Wed-00Z Thu) where the latest WPC snow probabilities for 6 inches are moderate (40-60 percent). The snow and snow rates will be combined with gusty winds (30-40 mph), making for dangerous travel conditions due to low visibility and snow covered roads. As the low pulls into Canada, the best synoptic ascent and deepest moisture will shift out of the region, but increasing N/NW flow may result in some area of heavy lake effect snow (LES), especially across the eastern U.P. and northwest L.P. of MI. Inversion depths in regional soundings are modest and winds are quite strong which will reduce parcel residence time, but multi bands with heavy snow rates are likely, and WPC probabilities for more than 6 inches are above 70% in the Huron Mountains, and 30-50% across much of the eastern U.P. and near the Tip of the Mitt. ...Northeast... Days 1-2... The strong/potent storm system currently over the Ohio Valley to the southern Appalachians will continue to bring widespread precipitation through the Eastern U.S. over the next 12-24 hours. The very anomalous upper low (between 3-4 standard deviations below normal) will take on a negative tilt as it swings through the Northeast later today through tomorrow morning. Very impressive moist advection/transport ahead of the system along with very favorable upper level diffluence will bring a large shield of precipitation to the region. The main winter precipitation areas will be confined to the interior Northeast, particularly VT/NH and Maine, tonight where heavy snowfall is likely/expected to total several inches. Snowfall rates up to 2"/hr will be possible (based on the WPC snowband forecast tool prototype). Accumulations through Wednesday are expected to total 8-12"+ across the White/Green Mountains through much of Maine (away from the coast) where WPC probabilities are very high. Localized amounts in the 12-18" range are possible, for the higher peaks of the White Mountains in NH. The snow will be very wet/heavy and when combined with the very strong winds expected (gusts over 50 mph), this will create a dangerous situation where tree damage and downed trees/power lines resulting power outages will be possible. Finally, as the system departs Wednesday, very strong northwest flow winds will create a favorable upslope flow event and enhanced lake effect snow for the favored locations. The still relatively warm waters of the Great Lakes combined with the strengthening cold air advection will likely lead to narrow intense snow bands, especially downwind of Lake Ontario where totals could exceed 6-12". Meanwhile, in the upslope region of the central Appalachians, totals 4-6"+ will be possible based on the latest WPC snow probabilities, especially in the favored areas of eastern WV. ...The West... Days 1-3... An extremely active pattern continues across the Pacific Northwest and into the Northern Rockies/Great Basin through late week as a series of shortwaves drop onshore from the Pacific. The first of these systems is ongoing and will drive a surface low/associated cold front through the Pacific Northwest and as far east as the Northern Rockies tonight. Although the synoptic forcing responsible for driving this feature eastward will weaken with time late tonight into Wednesday, the modest IVT (less than +1 sigma according to NAEFS) will still push elevated mid-level RH above 90% across much of the Northwest, which will manifest as snow above snow levels that will continue to fall through the day 1 period, reaching as low as 500 ft east of the Cascades, 1500 ft in the Cascades. Following immediately in the wake of this first shortwave, a more intense shortwave and accompanying vorticity maxima will drop along the PacNW coast and come onshore near the OR/CA border before amplifying into a more potent longwave trough near the Four Corners Thursday night. This feature will again be accompanied by impressively confluent mid-level flow to surge moisture inland, and although PW anomalies are modest, intense ascent in the vicinity of a translating low and strong surface cold front, and within the LFQ of a more potent jet streak will drive strong ascent to wring out available moisture. Snow levels may briefly rise on the warm/moist advection, but will generally continue to fall, and will likely reach the surface across much of the interior Northwest, and fall to around just 1000 ft near the coast of WA/OR, higher into the Sierra. The heaviest snow with this second wave will likely shift a bit southeast from the greatest accumulations on D1. By D3 the pattern begins to shift a little as the primary trough amplifies over Alberta and then digs southward, driving a surface cold front into a more NW to SE orientation as it drops into WA/ID/MT by the end of the forecast period. South of this trough, mid-level flow becomes increasingly pinched with W/NW flow impinging again into the Cascades. Although the highest IVT/PWs will remain just offshore by the end of the period, increasing moisture being acted upon by intensifying fgen along the cold front and amplifying upper diffluence ahead of an approaching jet streak should result in widespread snowfall once again spreading across the Northwest, with snow levels low enough to support light accumulations even into the lowlands around Seattle, WA by the end of the forecast period. WPC snow probabilities for at least 6 inches of snow are high (greater than 80-90%) for all 3 days across much of the terrain areas in the West from the Olympics, Cascades, into the Sierra and eastward across the Great Basin and even into the Wasatch and central Rockies. For the Day 1 period (00Z Wed-00Z Thu), the greatest accumulations will be found across the Cascades and into the Sierra where localized 2-3 feet totals will be possible for the highest peaks. By Day 2 and especially Day 3, the next few storm systems will hone in on the Pacific Northwest, bringing the bulk of the accumulating snow to the Olympics and Cascades before spreading eastward across the northern Great Basin and portions of the Rockies. The approach of an Arctic front seeping cold air southwestward into even the lower elevations will bring the threat of accumulating snow to the valleys and lowlands, including the Seattle metro area where the signal for at least a few inches of snow is increasing (WPC snow probabilities for 4 inches is at 20-30 percent. Finally, the strong cold front moving across Oregon tonight into Wednesday will continue to shift rapidly southeast across the Great Basin and into the Central Rockies by Thursday morning. This could spread convective snow showers and snow squalls across the area, with the CIPS snow squall parameter showing values above +1 spreading as far south as the Four Corners states. Evaluation of the fields which drive the snow squall parameter suggest coverage of snow squalls may be modest as there exists only pockets of overlapping sufficient 0-2km RH/925mb fgen/SBCAPE>100 J/kg to support intense squalls, but scattered to widespread convective snow showers are possible, which could cause significant travel impacts even beyond the areas of heavier synoptic snow. ...Central Plains to Great Lakes... Day 3... The potent/strong storm system dropping through the Pacific Northwest mid-week will reach the Four Corners region by Thursday/Thursday night then quickly lift through the Central U.S. Friday and Friday night. The latest guidance has come into better agreement with the large scale pattern with the main shortwave energy but there remains timing/speed differences as well as thermal/temperature profiles as colder air races southward on the backside of the deepening low pressure system that is currently forecast to take an Oklahoma to Ohio Valley to western New York state track. Favorable moisture transport ahead of the system and moisture wrapping around the northwest side is expected to drop a swath of heavy accumulating snow. Meanwhile, an increasingly tight pressure gradient developing between the deepening low pressure system and building Arctic high (forecast 1040+ mb) over southern Canada will lead to very strong winds that will create very dangerous/near blizzard conditions with this system. The latest WPC snow probabilities show moderate values (30-50%) for at least 6 inches from near Kansas City through northern MO/southern IA toward northern Illinois including the Chicago area and already shows at least 30 percent chance for at least 8 inches across northern Illinois. Based on the latest WSSI-P cycle, moderate winter impact probabilities are above 30-40 percent for a larger area due to the combined effects of the strong wind, blowing snow components. A narrow area of mixed precipitation (ice/sleet) will be possible with at least moderate probabilities (30-40%) of 0.01"+ ice accumulation across portions of the Plains/Midwest. Stay tuned as the next few forecast cycles will capture the full storm system as it will likely impact a large area from the Plains through the Great Lakes and eventually portions of the northern/interior Northeast. Weiss/Taylor Key Messages for Jan. 8-13 Major Winter Storm -Continuing Blizzard Impacts in the Northwest Periods of heavy snow will continue into early Wednesday in the Northwest, bringing snow totals to several feet in the higher elevations of the Cascades and Olympics. Gusts to at least 60 MPH will create blizzard conditions, and snow levels will fall to between 1000-2000 ft by Wednesday, leading to considerable travel impacts for many mountain passes. -Western Snow Squalls and Heavy Snow Rates Heavy snow exceeding 12 inches is likely (70-90%) for many of the other higher elevation mountain ranges of the West, including the Northern Rockies, Sierra Nevada, and Wasatch. In the Great Basin, snow squalls are likely through Wednesday. Intense bursts of snow of 1-2 inches per hour and wind gusts over 50 MPH will lead to rapid changes in visibility, occasional whiteouts, and pose significant danger to motorists. -Confidence Growing in Central U.S. Impacts The storm will emerge into the Plains by Thursday and strengthen. A swath of heavy snow is likely somewhere in the Plains and Midwest by Friday and Saturday. Blowing snow will also be possible in strong winds. At this time, impacts appear most likely in Missouri, southeast Iowa, Illinois, northern and central Indiana, eastern Wisconsin, and Lower Michigan, but continue to check back for updates as additional forecast changes are typical as a storm comes into view. Key Messages for Jan. 8-10 Major Winter Storm -Dangerous Travel in Heavy Snow Heavy snow bands with rates of 1-2â€/hr will shift from the Upper Midwest this afternoon into the Great Lakes tonight. These snow rates will be accompanied by gusty winds reaching 30-40 mph, making for dangerous travel due to low visibility and snow covered roads. An additional 4-8 inches of storm-total snowfall is possible from the western shores of Lake Michigan to the Upper Peninsula of Michigan. -Impactful Snow in the Northeast Heavy/wet snow will move across the high terrain of the interior Northeast tonight into Wednesday. This snow will cling to trees and power lines, which when combined with gusty winds potentially exceeding 55 mph, could result in power outages and scattered tree damage. -Significant River, Flash, and Coastal Flooding Additional significant river and flash flooding is likely along the East Coast through tonight. Powerful onshore winds will lead to widespread moderate to locally major coastal flooding on Mid-Atlantic and New England tidal rivers and bays through Wednesday. -Damaging Winds To Cause Power Outages Widespread wind gusts in excess of 55 MPH are likely across the central Appalachians, much of the Eastern Seaboard, and New England tonight and into Wednesday. Prepare for numerous power outages. Severe thunderstorms are also possible across the Southeast this evening.