Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 PM EST Mon Dec 19 2022 Valid 00Z Tue Dec 20 2022 - 00Z Fri Dec 23 2022 ...Arctic cold front pushes down the Plains with Blizzard conditions for portions of the Plains and Midwest... ...Pacific Northwest, Northern Rockies, and Northern High Plains... Days 1-2... An upper trough currently over the Gulf of Alaska will wrap around a deep upper low currently centered over the Canadian Rockies through Tuesday before shifting southeast down the northern Rockies and northern High Plains Tuesday night and Wednesday. A plume of Pacific moisture ahead of the Gulf of Alaska wave will shift across WA and the northern ID/MT Rockies tonight before shifting south to northern OR and southern ID/MT Rockies. Snow levels near sea level and moderate to locally heavy precip rates along a nearly stationary arctic cold front look to cause notable accumulations across Washington tonight including the Seattle metro area along with enhanced snowfall in mountains. Moderately high Day 1 snow probabilities for over 4 inches are across the Puget Sound area with high probabilities for a foot or more for the Olympics and WA Cascades with moderate probabilities for over 8 inches in the northern ID/MT Rockies including the Bitterroots. The favorable upper forcing and strong low-to-mid level frontogenesis will support heavy mountain snow spreading east from the Pacific Northwest into the northern and north-central Rockies Tuesday through Wednesday. Day 2 snow probabilities for more than 8 inches are high again for the Bitterroots as well as the Absarokas, Tetons, and Wind River Range in WY with 48hr totals in both these mountainous areas having high probabilities for over 18 inches. In addition, as the upper trough begins its push southeast down the northern Rockies Tuesday night the advancing arctic cold front looks to be accompanied by lines of heavy snow and likely snow squalls that spread over ID/WY Tuesday night/Wednesday and possibly northern Colorado late Wednesday. Please see Key messages below. By Tuesday afternoon, favorable upper divergence associated with a left-exit region of a strong NWly 250mb jet streak coming in from the North Pacific will help spread overrunning precipitation into the northern High Plains of Montana. Surface temperatures will be extremely cold and well below zero during this timeframe, which may actually temper SLRs from exceeding 20 to 1. Day 1.5 snow probabilities for 6 inches or more are high for much of west-central Montana mountains and valleys with the Big Belt Mountains forming much of the eastern boundary. ...Northern and Central Plains through the Midwest/Great Lakes... Days 2-3... The aforementioned upper trough currently over the Gulf of Alaska that pushes down the northern Rockies/northern High Plains Tuesday night with an arctic cold front pushing south into the central Plains Wednesday into early Thursday will have impressive fgen, enhanced upper level jet support, and very cold air. This will allow for development of widespread moderate snowfall amounts Wednesday over the eastern Dakotas that then spreads south through Nebraska and east through much of Minnesota and northwest Iowa. Day 2 snow probabilities for more than four inches are centered near the Buffalo Ridge of southwest MN. Lee-side cyclogenesis ahead of the arctic cold front dips south to the TX Panhandle late Wednesday before shooting east to the MO/AR border by early Thursday then rapidly intensifying as it turns northeast up the Midwest through Thursday. This will allow for further expansion of the snow areas along with development of heavy snow bands. This is seen in the Day 3 snow probabilities which for 4 or more inches are moderate or higher from west-central KS to central MO and northeast to the western Great Lakes. Embedded are moderate probabilities for 8 or more inches from southeast Neb, northwest MO through central IA to southeast MN. The presence of an existing wave moving along the Gulf Coast today through Tuesday will limit the amount of Gulf moisture available to the mid to late week system. Gulf moisture begins to stream ahead of the system starting on Wednesday which will allow further development of heavy snow bands for Thursday. Enhanced lift within a deep DGZ for much of the central Plains and into the Upper Midwest will support very efficient SLRs to make up for the lack of initial moisture, along with surface temperatures around zero behind the arctic cold front. High SLRs above 20 to 1 may be short-lived for much of the central High Plains, as strong low to mid-level winds above 50 kts fracture the falling dendrites. However, the gusty winds (and extremely low wind chills) will add to the potential for dangerous travel as visibility decreases and blowing snow ensues. This system will continue to progress eastward and become a large and powerful storm set to impact much of the Midwest and Great Lakes with potentially heavy snow through late this week and into the Holiday Weekend. Of particular note is the slow motion which will allow for long duration of heavy snow bands, particularly over the central Great Lakes. ...Southern/Central Appalachians... Day 3... In response to the deepening trough over the central U.S. on Wednesday, moisture return along the East Coast will enter the southern Mid-Atlantic and ride up along the southern and central Appalachians Wednesday night into Thursday morning. A strong surface high depicted by all guidance over New England during this time frame will support cold air damming into Thursday morning and the at least localized freezing rain over mainly the higher terrain west from the Blue Ridge Mountains. Day 3 WPC probabilities for at least a tenth of an inch of ice accretion are moderate across western NC, southwest Virginia, eastern WV, into west-central PA. The threat of freezing rain is expected to persist into Thursday night until the arctic cold front blows through and changes to accumulating snow. Key Messages for Blizzard --Significant Blizzard Developing this Week An area of low pressure will develop Wednesday night and then strengthen while lifting northeast into the Great Lakes by Friday morning. This will result in blizzard conditions across portions of the Central/Northern Plains, Upper Midwest, and Great Lakes. --Extremely Dangerous Travel Conditions Heavy snowfall rates of 1-2 inches per hour at times, along with wind gusts of over 50 mph will result in near zero visibility and considerable blowing and drifting of snow. This will lead to dangerous, to at times impossible, land and air travel leading up to the holiday weekend. --Potential for Impacts to Infrastructure The combination of heavy snow and strong wind gusts could lead to significant infrastructure impacts as well, including scattered tree damage and power outages. --Dangerous Cold along with the Blizzard Dangerous wind chills will accompany the blizzard, creating a significant hazard for anyone that becomes stranded. Prepare now for extreme cold and ensure outdoor animals and livestock have sufficient shelter. --Flood Threat for the Interior Northeast Farther east, heavy rain falling onto a melting snowpack will cause flooding concerns across the Northeast. Key Messages for Cold Front and Snow Squalls --Dangerous Cold for Much of Country A combination of frigid temperatures and gusty winds behind an Arctic cold front will bring dangerously cold conditions across most of the country this week, and to the Eastern U.S. by weekâ€s end. --Rapid Temperature Drops with Front Arctic air will arrive quickly behind the cold front, with temperatures likely to drop 25 to 35 degrees in just a few hours as the front passes a given location. --Hazardous Snow Squalls in the West Snow squalls may lead to hazardous travel in the Western U.S. from Tuesday Night into Wednesday. Snow rates over 1 inch per hour and gusty winds may lead to sudden whiteout conditions. --Flash Freeze Possible Farther East From the Mid-South to the East Coast, rain before the Arctic front could freeze in the rapidly falling temperatures and lead to icy roads and hazardous travel. Jackson