Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 335 AM EST Wed Jan 05 2022 Valid 12Z Wed Jan 05 2022 - 12Z Sat Jan 08 2022 ...Pacific Northwest to the Central Rockies... Days 1-3... Strong upper high northwest of Alaska will favor troughing to its south out of the Gulf of Alaska into western British Columbia and the Pacific Northwest as a series of jet streaks pass through. By the end of the week, troughing from the Gulf of Alaska as well as the northern Pacific in the mid-latitudes will increase broader troughing into much of the Northwest with two attendant surface systems moving into the region. Working in tandem, these systems will promote a surge in moisture into western Washington by late Wed/early Thu with precipitable water values over 1 inch on the coast which is +2 to +3 sigma. This will shift inland across Idaho on Thursday and into Wyoming/Utah as well where snowfall will be significant in higher elevations. Temperatures will moderate as milder air draws northeastward, but initially cold surface temperatures in lowland/valley locations east of the Cascades will support some freezing rain (also perhaps west of the Cascades before the column warms completely). Heavy snow is forecast for the Cascades with significant accumulating snow across the passes Thursday. Snow levels will rise late Thu into Fri over western WA and OR over 5000ft before trending down as the upper trough moves in. WPC probabilities of at least 12 inches of snow are moderate to high each day over the Washington Cascades and mountainous Idaho/western Montana. ...Northern Plains and Great Lakes... Days 1-2... Closed low forming over the Great Lakes and low pressure centered over Wisconsin will track east/northeast through today/tonight. Strong frontogenetical forcing and wrap around moisture on the backside of the deepening low will produce several inches of light, fluffy snow early this morning. Combined with very strong winds, blizzard conditions are expected to persist through portions of the day 1 period. As the low lifts north/northeast of the Great Lakes, northerly to northwesterly winds will increase setting up a favorable long duration potentially significant lake effect regime. The much colder air and maximum lift well within the DGZ supports SLRs 20:1 or even a bit higher in the more intense snow bands. As a result, total accumulations in excess of 8 inches are expected off the Lakes, particularly for the favored snow belt areas where isolated totals between 12-18 inches will be possible. ...Central High Plains... Day 2... Sheared vorticity moving through the developing longwave trough within a zone of increasing upper level diffluence and overlapping low to mid level frontogenesis will bring a period of locally heavy snowfall to portions of southeastern Wyoming into the Nebraska Panhandle today/tonight. The latest WPC probabilities are high for at least 4 inches across southeast WY with even a narrow high probability for 8 inches. Probabilities for heavy snow drop off considerably moving into central Nebraska, Kansas and Missouri where light accumulations are expected. ...Tennessee and Ohio Valleys into Mid-Atlantic/Northeast... Day 2-3... Shortwave energy crossing the central U.S. in the quasi-zonal flow begins to sharpen and dig as it approaches the TN/OH Valley Thursday as an area of low pressure develops off the southeastward moving frontal boundary. This low is expected to track through the southern Appalachians before exiting off the Mid-Atlantic coast early Friday morning. Then on Friday, as the associated shortwave begins to assume a negative tilt and lift northeast, the surface low is expected to rapidly intensify and track northeast, potentially affecting the Northeast/New England region. This forecast cycle showed an eastern trend, keeping the low track far enough offshore such that the QPF/snow forecast trended downward, especially inland/interior areas. With the signal for the system to be 1) more offshore and 2) very fast moving - probabilities of heavy snow have decreased this cycle, however there continues to be some ensemble support for a stronger/closer system so over the next several days the details will ironed out. The latest WPC probabilities show the greatest chances for at least 4 inches across eastern KY through WY, especially for the favored terrain/upslope areas while those probabilities drop off to the slight/moderate range for the Mid-Atlantic. Given the continued uncertainty for southern New England and coastal Maine, the highest probabilities remain right along the coast. Taylor/Fracasso/Pereira ~~~ Key Messages for the Tennessee and Ohio Valleys to the Northeast Winter Storm ~~~ -A winter storm is increasingly likely to impact portions of the Ohio and Tennessee Valleys into the southern/central Appalachians Thursday into Thursday night. -Several inches of snow and a zone of mixed precipitation including freezing rain is likely to cause travel disruptions and hazardous travel conditions. -As the system moves through the Mid-Atlantic and Northeast, accumulating snow is likely Thursday night into Friday where a few inches of snow will be possible. However, there remains a lot of uncertainty in the track and potential amounts, especially for the Northeast and New England.