Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 PM EST Sat Jan 13 2024 Valid 00Z Sun Jan 14 2024 - 00Z Wed Jan 17 2024 ...Great Lakes and Interior Northeast... Days 1-3... Multiple vort maxes and/or upper lows over the Upper Midwest/Great Lakes/Hudson Bay will consolidate into one main, though elongated, center on Sunday over James Bay, retrograding through northern Ontario Mon-Tues. This will lead to a prolonged lake effect snow event downwind of all five Great Lakes (already underway over Michigan) that will only slowly diminish into next week. Cold air through the column (850mb temps <-18C) over the still nearly unfrozen lakes will lead to more than steep enough lapse rates for snow, but the cold air will generally favor lower than maximum SLRs (past the DGZ on the cold end) also from dendritic fracturing in the gusty winds. D1 snow will be enhanced by an advancing stream of vorticity across the Corn Belt into the Midwest and Northeast, which may also help produce snow squalls over parts of PA/NY into New England and even the northern Mid-Atlantic on Sunday (snow squall parameter per some of the models >1 or 2). WPC probabilities for at least 6 inches of snow on just D1 are high (>70%) over the eastern U.P. of Michigan, northwestern Lower MI, and off of Lakes Erie/Ontario in WSW flow (favoring near and just south of BUF) where double-digit totals are quite likely. By D2, flow will become northwesterly to westerly over much of the western lakes, supporting multi-bands into much of Lower Michigan but still some single bands into the U.P. where WPC probabilities for at least another 4 inches are around 50%. Intense single bands off Erie/Ontario will waver a bit and lift northward but weaken, still allowing for several more inches of snow. WPC probabilities are highest in the Tug Hill. By D3, snow will continue but generally with light amounts over the region with probabilities for at least 4 inches less than 50%, focused near the Keweenaw Peninsula. ...Northwest to the central Rockies... Days 1-2... A compact shortwave moving southeast off of the top of the northern Pacific/Gulf of Alaska ridge will support additional organized precipitation as it dives southeast into Oregon and Northern California later today. This system is forecast to quickly deamplify as it moves onshore, but continue to support moderate to locally heavy precipitation as it interacts with the remnants of an atmospheric river and moves southeastward along the strong low-to-mid level baroclinic zone associated with the arctic airmass that is already in place. Impacts from this system include additional freezing rain across portions of northwestern Oregon and southwestern Washington, with additional ice accumulations likely. An additional 0.10 inch of ice is possible, especially for portions of northwestern Oregon. Additional heavy snow accumulations are possible for portions of the Oregon Cascades and northern Sierra Nevada, through the Intermountain West into the central Rockies. The heaviest accumulations beginning late this afternoon through late Sunday are expected to fall along the northern to central Utah and the western Colorado ranges. WPC PWPF shows widespread high probabilities for 8 inches or more, with more localized high probabilities for a foot or more across these areas. Even as the initial shortwave moves downstream, northern stream energy dropping into the base of the longwave trough will support additional periods of snow with heavy snow possible across portions of the central Rockies. The PWPF shows high probabilities for additional accumulations of a foot or more late Sunday into Monday, with the highest probabilities centered over the northwestern Colorado mountains. As the amplified ridge centered over the eastern Pacific shifts east, dry weather is expected across much of the West beginning Sunday that will spread east into the Rockies by Tuesday. ...Mid-South/TN Valley/southern Appalachians... Days 1-3... Shortwave exiting the Four Corners tomorrow night on the southwest periphery of a broad trough over the eastern 2/3rds of the CONUS. Multi-stream 130-kt jet from the Ozarks eastward will promote broad upper divergence over the region into the Mid-South on Monday as the main and minor vort maxes push through the area. Arctic front will have pushed into the Gulf by late Sunday/early Monday, bringing cold (near and sub-freezing) temperatures to eastern TX through the Lower MS Valley. However, 850mb flow by then will allow Gulf moisture to surge northward over the Arctic-sourced airmass in place, favoring a broad area of a wintry mix of sleet and freezing rain on Monday. Farther north in the deeper cold air, light to locally moderate snow will expand eastward across eastern OK and through AR into TN Monday afternoon, and eventually into the southern Appalachians. Frontogenetic forcing will likely enhance some snowfall with relatively high SLRs (~12-18:1 vs 10:1 Baxter climo), allowing for several inches of snow in a west-east band from central AR ENE across TN. Additional enhancement is likely in the Ozarks thanks to terrain. Models continue to shift a bit with respect to the QPF amounts/placement, but remain in overall good agreement. Amount of warm air aloft will also play a role in how far north the sleet area progresses. WPC probabilities of at least 4 inches of snow are moderate (40-70%) over the Ozarks and from central AR across the Mississippi River into Middle TN, as well as into the southern Appalachians (eastern TN, southeastern KY and southwestern VA). Farther south, cold surface temperatures beneath a warm nose aloft supports an area of freezing rain/sleet from the TX Hill Country eastward across northern LA/southern AR and into MS. This is also the area where the models struggle the most with respect to ptype, and uncertainty is maximized here. It will be a battle between the arctic air beneath the advancing warmer air aloft, and the latest guidance indicated a nudge northward in the snow/mix line, with more room for refinement. For now, WPC probabilities of at least 0.10" ice are moderate (40-70%) from around I-35/Waco eastward into central MS (I-55). Within this area, there are low-end (10-40%) probs of at least 0.25" ice over eastern TX into western LA (between I-45 and I-49 and along/south of I-20). The chance of any (>0.01") icing (>10% chance) extends from central TX eastward into northern GA, and all the way to near the I-10 corridor/Gulf Coast. The entire system will stream northeastward to the East Coast into Tuesday, with light snow for much of the Northeast/Mid-Atlantic, but low (<10%) chances for at least 4 inches of snow. However, this may bring the first inch of snow to some of the I-95 corridor (DC-PHL-NYC) where the snow drought continues. Fracasso/Pereira ***Key Messages for Western U.S. to Mid-South Winter Storm*** --Snow from the West Coast to the Rockies, Ice in Oregon-- Heavy snow, blowing snow, and valley icing will all contribute to poor to impossible travel from Oregon, Idaho, Nevada, and Utah, including the Portland, Boise, and Salt Lake City metro areas. Significant freezing rain is expected today over western Oregon with tree and powerline damage possible. --High Avalanche Danger in Portions of the Sierra Nevada and Rockies-- Heavy snowfall and strong winds have created dangerous avalanche conditions across portions of the Sierra Nevada and the northern to central Rockies. Travel in, near, or below avalanche terrain is not recommended. --Wintry Precipitation into Mid-South by Late Sunday-- Areas of snow, sleet, and freezing rain are likely to develop and spread across portions of the southern Plains, through the ArkLaTex, and into the Tennessee Valley Sunday into Monday. Several inches of snow are possible. Accumulating ice is expected from portions of central Texas through the lower Mississippi Valley. With arctic air firmly in place, impacts from wintry weather may last for several days, resulting in a prolonged period of hazardous travel. ***Key Messages for Mid-January Arctic Blast*** --Dangerously Cold Wind Chills Expected-- Sub-zero wind chills will affect much of the U.S. and reach into portions of the South. Wind chills this weekend will fall below minus 30 degrees from the northern Rockies to the central Plains and mid Mississippi Valley. Values will drop as low as minus 70 degrees from Montana across the western Dakotas. These wind chills will pose a risk of frostbite on exposed skin and hypothermia. Have a cold survival kit if you must travel. --Lake Effect Snow Persists This Weekend-- The arrival of colder air will generate heavy snow downwind of the Great Lakes this weekend into midweek. Whiteout conditions in the lake effect snow bands are expected. An additional 12-24 inches of snow will occur through Monday across portions of western and northern New York State. --Snow Squalls Possible on Sunday-- Snow showers or squalls may develop across portions of the Northeast and the northern Mid-Atlantic. Gusty winds and rapidly reduced visibility will result in dangerous travel if squalls occur. --Hazardous Cold Weather to Persist/Redevelop-- Parts of the Midwest could again experience near record, dangerously low temperatures and wind chills, with subfreezing temperatures possible in the Deep South by late next week.