Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 402 AM EST Thu Dec 22 2022 Valid 12Z Thu Dec 22 2022 - 12Z Sun Dec 25 2022 ...Mid-Atlantic into Northeast... Day 1... A mid-level shortwave approaching the southern Appalachians early this morning is developing a coastal low over the Carolinas this morning, with a surge of moisture lifting up over the Mid-Atlantic today and the Northeast tonight. Divergence increases aloft over the Eastern Seaboard today east of the developing cold core low over the north-central CONUS which will promote lift across the Mid-Atlantic into the Northeast. SSE flow will bring in milder temperatures and above normal moisture to the region on 40-55kt 850mb flow which will lift up and over the colder surface boundary layer with cold air damming in place from a high centered over New England. This will yield an area of freezing rain and some sleet over the central Appalachians including Northwest Virginia, eastern West Virginia, through western MD and into south central PA where Day 1 probabilities for a tenth inch of ice are moderately high (low 20 to 30% probs for 0.25" ice over the Potomac Highlands. The high pressure drifting into Atlantic Canada will attempt to hold in colder air at the surface but this will eventually be eroded as low pressure from the south moves northward along I-95 in VA and I-81 in PA in advance of the arctic front to the west. Snow is expected mainly a little farther west than the ice, near the crest of the Appalachians (near the Allegheny Front through the Laurels of Pennsylvania) where there are around 20% Day 1 snow probabilities for over 4 inches. Southerly flow over the Northeast and New England tonight will allow moderate to locally heavy snow before changing to rain for higher mountains such as the Adirondacks, Greens, and Whites with low probs for less than a tenth ice of ice over the interior northern Mid-Atlantic and New England. ...Midwest through the Interior Northeast... Days 1-3... ...Powerful blizzard with extremely cold temperatures, dangerously frigid wind chills, and a flash freeze likely... A powerful and likely historic winter storm is set to unfold across the Midwest and the Great Lakes today with the powerful wind field, and lake effect snow, continuing through at least Christmas Day. The combination of intense wind gusts and dangerously cold temperatures with this cold core low will exacerbate impacts. The mid-level low is over the Dakotas which is well behind the surface arctic cold front which has reached the Texas Panhandle and is nearly through Iowa at the time of writing. Anafrontal snow will continue in a large swath well behind the cold front with an inverted trough stretching up the Midwest will become the focus for surface low development late this afternoon. This anafrontal snow will be the main wintry weather for the southern Plains and Midwest today with coverage and intensity increasing, particularly over the Midwest, as the surface low develops. Day 1 snow probabilities for 2 inches are moderately high from southeast Kansas, northeast Oklahoma through Illinois with probs for 4 or more inches generally limited to Indiana then through Michigan and far eastern Wisconsin where the Great Lakes will enhance developing wrap around bands that pivot tonight. Day 1 snow probabilities for over 8 inches are generally limited to the U.P. though there is a low chance near the Indiana/Michigan border southeast of Lake Michigan. Farther south, the fast moving Arctic front will prompt periods of rain to occur within the warm sector, but once the front passes through, temperatures will plummet to subfreezing levels late Thursday into Friday. This allows for precipitation to change over to a combination of graupel/rain, then eventually all snow as the atmospheric column quickly cools below freezing. While cold air chasing moisture is rarely a setup for snow accumulation, the rapidity at which the column cools enables the DGZ to remain saturated even as temps fall to below freezing near the surface. This combined with dynamic cooling through impressive ascent could result in light accumulations anywhere along the front, but with a period of difficult driving/travel likely from a flash freeze. The occluded low drifting north of the Great Lakes Friday/Saturday produces a large shield of snow that is enhanced from the rather ice-free Great Lakes. Day 1.5 snow probs for 6 or more inches are high in lake effect snow belts from the cyclonic flow over the lakes, but moderate probabilities cross the L.P. of Michigan, northern Ohio, and much of Upstate New York. By Saturday night, when the jet streak rounding the occluded low lifts north of the Lakes, the focus for heavy snow is just the lake effect snow belts. Day 3 snow probabilities for 6 or more inches are over the north coast of the U.P. far northwest L.P., with single bands off Lakes Erie and Ontario likely producing feet of snow near Buffalo and Watertown, New York. ...Pacific Northwest... Days 1-3... A shortwave trough reaches the Pacific Northwest coast tonight, running into a ridge currently set up just offshore. Moisture will funnel inland within a modest atmospheric river progged by CW3E to exceed 250 kg/m/s. While the column is initially cold, by the time the precip reaches the coast later this afternoon warm air nosing in from the Pacific at 850mb will slowly melt snowflakes aloft to cause a change in p-type. The airmass ahead of this wave is quite cold and dry noted by dew points in the teens, which will support rapid wet bulb effects as precip falls this evening. While snow is likely in most areas to start, the warm nose aloft will cause a rapid transition to IP and then ZR. Light to moderate precip rates which generally feature efficient accretion will be enhanced by robust easterly winds, especially along and through the Columbia Gorge and into the Willamette Valley and surrounding lowlands. This could result in extremely efficient ice accretion with the Day 1.5 risk for 0.25" now 50-80% for the Coastal Ranges of Oregon and southwest Washington and points east through the Columbia Gorge and the lowlands of SW Washington. Continued onshore flow Friday maintains light precip/accreting ice that should generally erode from the west/shift east, though gap winds through the Columbia Gorge are known to cause mixed precip type concerns. Day 2 probabilities for 0.25" ice are focused near the Columbia Gorge with low probs extending north from Portland to Seattle. Day 2 snow probabilities are limited to the Washington Cascades and Bitterroots of northern Idaho with snow levels for the Cascades near 6000ft. The next shortwave trough approaches Friday night with renewed onshore flow and increased moisture that continues through Saturday. Day 3 ice probabilities for 0.25" are 20-40% again near the Columbia Gorge and east into the Columbia Basin (QPF will be the main limitation in the drier areas east of the Cascades). This will result in extremely dangerous travel for the Friday before the holiday weekend. Farther to the north, heavy snow will likely remain the primary p-type across the WA Cascades where Day 3 WPC probabilities for more than 6 inches are above 50% for higher snow levels (around 7000ft) than Day 2. Jackson Key Messages for Blizzard --Widespread Very Gusty, and Potentially Damaging, Winds A powerful winter storm system will bring widespread wind gusts over 40 MPH to a large portion of the Central and Eastern U.S. over the next few days. In some spots, the wind gusts could approach or exceed 60 MPH. This could lead to damage and power outages. --Blizzard Conditions Where Wind Combines with Snow The winter storm will produce new snow primarily in the Midwest, Great Lakes, and interior Northeast. Even small amounts of snow could combine with the gusty winds to produce blizzard conditions. Additionally, existing snow cover in the northern Plains could combine with the winds to produce a ground blizzard, even in the absence of new snowfall. --Extremely Dangerous Travel with Whiteout Conditions Where blizzard, or near-blizzard, conditions occur, expect periodic whiteouts with near zero visibility and considerable blowing and drifting of snow. Traveling in these conditions will be extremely dangerous, to at times impossible. You risk becoming stranded and more vulnerable to the bitterly cold weather. --Life-threatening Cold Temperatures Dangerous wind chills will create a significant hazard for travelers that become stranded. If you must travel, prepare for extreme cold. In some areas, being outdoors could lead to frostbite in minutes. Ensure outdoor animals and livestock have sufficient shelter. --Coastal and Inland Flooding Threat for the Northeast Heavy rain falling onto a melting snowpack may cause flooding impacts in urban areas or along rivers and streams, and minor to isolated major coastal flooding will be possible due to onshore winds. Key Messages for Cold Front and associated Snow Squalls/Flash Freeze --Dangerous Cold for Much of Country Frigid temperatures and gusty winds behind a powerful Arctic cold front will bring dangerously cold conditions across most of the country this week, and to the Eastern U.S. by the weekend. --Increased Impact if Power Outages Occur The impact of the cold will be increased if any power outages occur due to the winter storm. --Rapid Temperature Drops with Front Temperatures are likely to rapidly drop 20 to 35 degrees in just a few hours as the front passes a given location. --Considerable Travel Impacts Possible Snow squalls, or a brief burst of moderate to heavy snow (1 to 2 hours), are likely to occur immediately behind the Arctic cold front from the Intermountain West, to the Central Plains, Ohio Valley, Mid Atlantic, and Northeast. -Snow will be accompanied by gusts to 40 MPH, potentially creating sudden whiteouts. -Snow will coincide with rapid temperature drops, which will likely lead to flash freezing and icy spots developing on roadways. Any flooding in the Northeast may also re-freeze.