Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 359 PM EST Thu Jan 11 2024 Valid 00Z Fri Jan 12 2024 - 00Z Mon Jan 15 2024 ...Central Plains to the Great Lakes into the interior Northeast... Days 1-3... Sharp trough will exit the Southwest this evening and take on a negative tilt early Friday as a double-barreled jet becomes more buckled and S-shaped across the Great Lakes. This will drive strong upper level divergence and rapid surface cyclogenesis with a deepening area of low pressure lifting from the Ozarks to the Midwest then over the L.P. of MI tomorrow night. Snow will rapidly increase in coverage and intensity tonight into Friday north of the low over the central Plains and then spread eastward from IA through MI. As the low tracks away on Saturday, intense lake-effect snow will develop, covering all five Great Lakes by Saturday night. Lead northern stream shortwave will help spread light snow over NE tonight as the surface low organizes near the MOKSAROK, to be combined into the broad precipitation shield tomorrow morning. Initial WAA over colder sfc air to the east of the low will likely support a wintry mix in the rain to snow transition zone from northern MO into northern IL before the system becomes dominated by just a rain-snow delineation. North in the snow zone, significant snowfall is expected on the northwest side of the low as a deformation band forms and slowly pivots across the area, coincident with an intense TROWAL as the WCB wraps into the low. High-end potential is >18" per PWPF from peak development and subsequent lake enhancement in southeast WI and northeast portions of the L.P., driven by strong UVV into a sufficiently deep DGZ. The powerful cyclone reaches peak intensity over the L.P. Friday night with MSLP in the low 970s making for very strong winds and blizzard conditions north and then west of the low. WPC probabilities for at least 8 inches of snow for this system are at least moderate (40-70%) from the Corn Belt northeastward across nearly all of WI and into all but southeastern Lower Michigan. Within this area, WPC probabilities for at least 12 inches are greater than 50% over southern WI into the northern portions of the L.P., with additional higher probabilities in the U.P. snow belts favored on N-NW flow. There, strong northwest to eventually west flow on the back side of the low envelopes all of the Great Lakes with LES beginning off Lake Superior Saturday and the rest of the Great Lakes by late Saturday night and continuing in earnest through at least Sunday night. Day 2.5-3 snow probs for >6" are >50% for typical NW and W snow belts, with WSW flow over lakes Erie/Ontario favoring areas just south of BUF and into/just north of the Tug Hill. On the eastern side of the approaching front, cold air mass in place will slowly be eroded as broad southerly flow scours out sub-freezing temperatures. The uneven erosion in the column will likely lead to a period of snow to freezing rain to rain in most areas outside northern Maine as a triple point low moves across the area. Before the changeover, several inches of snow will likely accumulate over terrain in interior portions of the Northeast with day 2-3 WPC snow probabilities for >4" highest (generally >70%) over the Adirondacks, Green & White Mtns, and interior Maine (North Woods). ...The West... Days 1-3... An active wintry pattern will continue throughout much of the mountain ranges along the West Coast and throughout the Intermountain West thanks to a pair of strong shortwave troughs; an elongated disturbance over British Columbia diving south into the Northwest this evening, and a more compact Pacific storm system approaching the Pacific Northwest coast Friday night into Saturday. The shortwave trough over British Columbia will have the the most frigid and bitterly cold air-mass of the winter season to date at its disposal as a strong area of Arctic high pressure slowly advances south through northern Washington, the northern Rockies, and Northern High Plains this afternoon and into the interior Northwest on Friday. Northwesterly flow off the coast of British Columbia will strengthen as the longwave upper low over Alberta and an expansive ridge cresting as far north as the Arctic Circle funnel Pacific moisture throughout the Northwest, the Columbia River Gorge, and northern Rockies this evening. With a strong dome of Arctic high pressure (possibly approaching record high pressure observations for the time of year in southwest Canada and northern Montana), a rich plume of 850-700mb moisture running into the boundary, and the added help of strong topographic ascent along ranges oriented orthogonally to the mean flow, heavy snow will unfold in the mountain ranges of the Northwest. Latest WPC PWPF shows moderate-to-high chances (>50-80%) for >18" of snowfall in the Oregon Cascades, with similar chances for >12" in the Blue Mountains and Boise Mountains. This stream of moisture will become advected as far inland as the Wasatch throughout the day on Friday and lingering into Friday night. WPC PWPF shows high chances (>70%) for >12" of snow in the Wasatch on Friday. In addition to the heavy snow, strong wind gusts may lead to blizzard conditions across some portions of the interior Northwest, most notably in southern Idaho. A vorticity maximum shearing off a long wave trough over the northeast Pacific will develop an area of low pressure that tracks towards the Oregon coast Saturday morning. This storm system will tap into a conveyor belt of moisture that has origins into the subtropics. The combination of anomalous moisture and a strong 850mb jet will lead to IVT values approaching the 99th climatological percentile being directed at southern Oregon and northern California. This is a recipe for heavy snowfall in the Oregon Cascades and northern ranges of California. Confidence is highest in heavy snowfall accumulations along the warm front from southern Oregon and northern Nevada to the Wasatch as they also are, coincidentally, at the nose of the strong IVT plume emanating out from the northeast Pacific. The WSSI shows an elongated swath of Moderate Impacts from central Oregon and most of northern California's tallest mountain ranges on east through southern Idaho and into the Wasatch with some embedded Major Impacts. WPC PWPF sports high chances (>70%) for >12" of snowfall on Day 3 for these ranges as well. Similar probabilities exist in the Sierra Nevada, southeast Oregon, far southwest Idaho, and northern Nevada where there is higher confidence in totals topping 8". As the IVT continues gradually drifts south and directs copious amounts of Pacific moisture inland, heavy snow will advance as far inland as the Colorado Rockies where there are high probabilities (>70%) for snowfall amounts >8" through Sunday afternoon. All told, through Sunday afternoon the Oregon Cascades, mountains ranges of southern Idaho and northeast Nevada, and the Wasatch all have high chances (>80%) for snowfall totals >18" through Saturday afternoon with some ridges topping 4 feet in the tallest peaks. In addition, WPC PWPF shows moderate chances (40-60%) for >0.25" of ice in west-central Oregon where a prolonged period of freezing rain is likely to occur starting late Friday night and lasting into Saturday afternoon. WSSI shows Moderate Impacts are possible there, suggesting treacherous travel conditions are possible within the Minor and Moderate areas of west-central Oregon. Mullinax/Fracasso ***Key Messages for Jan 10-14 Major Winter Storm over the Midwest*** -Major Midwest Snowstorm Likely A strong winter storm will emerge into the Plains today and rapidly intensify as it tracks northeast into the Great Lakes on Friday. Heavy snow is expected from the mid-Missouri Valley to the Great Lakes, with over a foot likely across portions of Wisconsin and Michigan. -Blizzard Conditions Possible with Strong Winds Strong winds will spread across the Midwest and Great Lakes on Friday and Saturday. Blizzard conditions are possible, particularly in exposed areas. Winds will increase on Friday night, and the drastically reduced visibility will make travel dangerous to impossible. Wind gusts of 40 to 50 MPH may also lead to some power outages. -Lake Effect Snow Persists This Weekend While the larger snow area will begin to diminish on Saturday, the arrival of colder air will generate heavy snow downwind of the Great Lakes this weekend. Winds will remain strong, posing a risk of significant blowing snow. -Flooding and Severe Storms in the South and East Severe thunderstorms will be possible across portions of the southern Plains and lower Mississippi Valley tonight and into the Southeast on Friday. Additional heavy rain in the Mid-Atlantic and Northeast will lead to renewed rises on rivers and streams and potential flooding. Coastal flooding is also likely in the Mid-Atlantic and Northeast. ***Key Messages for mid-January Arctic Blast*** -Bitter Cold Surges South and Affects Most of U.S. The first significant Arctic outbreak of the winter arrives in the northern Rockies and northern Plains tonight and Friday, before advancing farther south and east through much of the Plains and Midwest this weekend. Numerous daily cold records are likely in the south-central U.S. on Sunday, Monday, and Tuesday. -Dangerously Cold Wind Chills Expected At the peak of the Arctic outbreak early next week, minimum wind chills should fall below zero into Texas and the interior Southeast. Wind chills this weekend across the northern Plains and northern Rockies should be below negative 40 degrees. This will pose an increased risk of frostbite on exposed skin and hypothermia. Have a cold survival kit if you must travel. -Heavy Snow in the West Friday and Saturday Arctic air will gradually lower snow levels in time for the arrival of another storm system in the West on Friday. This will lead to snow and considerable impacts in valleys in Oregon, Idaho, Nevada, and Utah, including the Portland, Boise, and Salt Lake City metro areas. Freezing rain is likely Saturday in western Oregon. -Southern, Northeast Snow Early Next Week Snow is likely on the periphery of the advancing Arctic air mass from the interior South into the Mid-Atlantic Sunday into Monday and possibly over the Northeast into midweek. Stay tuned for further details.