Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 408 PM EST Thu Jan 06 2022 Valid 00Z Fri Jan 07 2022 - 00Z Mon Jan 10 2022 ...Pacific Northwest to the Central Rockies... Days 1-2... Moderate to strong AR noted by IVT eclipsing 500 kg/m/s will continue into the Pacific Northwest through D1 before weakening quickly by D2. This IVT will be driven by both a Pacific Jet streak sinking southward across the area transporting moisture to the east, and mid-level confluence downstream of a shortwave which will dig onto the WA/OR coast Friday morning. PWs above 0.75" are likely to persist across the coast of WA/OR until the trough axis swings eastward the latter half of D1, which will also cause a slow wane to precipitation coverage and intensity going into early D2 /Friday night./ WAA within this AR will drive snow levels to as high as 6000 ft west of the Cascades, and 3000-4000 ft to the east of the Crest. Snow levels will drop late D1 as the trough swings eastward, but this will be accompanied by weakening forcing and reduced moisture, so the heaviest snow is likely during the period of highest snow levels. WPC probabilities for 12 inches are moderate to high on D1 in the WA/OR Cascades, the Northern Rockies and Sawtooth, with moderate probabilities for 8 inches encompassing the Olympics and the Tetons and Wind Rivers of WY. Local maxima of 3-4 ft is likely in the highest terrain. By D2, moisture and forcing wane rapidly, but an additional moderate snowfall is likely, with WPC probabilities for 6 inches reaching 80% in the WA Cascades, and 40% in the Northern Rockies. Additionally, east of the Cascades, cold surface high pressure will remain locked in place, leading to a setup favorable for light freezing rain across the Columbia Basin and into some of the sheltered elevated valleys of ID. WPC probabilities D1 indicate a low potential for 0.1" of ice accretion. ...Great Lakes... Days 1... Mid-level trough axis swinging across the Great Lakes and Ohio Valley tonight will produce a 24-hr period of cyclonic flow and CAA across the Lakes. This will produce a favorable setup for impressive LES in the typical NW flow snow band areas. Instability over the Lakes is progged to reach as high as 750 J/kg, driving low-level inversion heights towards 9000 ft. As 850mb temps crash to -15C to -20C, steep lapse rates will envelop the lowering DGZ, into which intense forcing is likely, and periods of heavy LES are expected downwind of all the Lakes. WPC probabilities are high for 6 inches in parts of the U.P. and east of Lake Erie, but are most impressive where a pivoting single band will likely drop across Lake Michigan and into parts of the SW L.P. Snowfall rates may reach 2"/hr at times, and locally more than 12 inches is likely SE of Lake Michigan. Shortwave ridging will follow the mid-level trough closely, bringing an end to the favorable CAA and ascent needed for heavy LES early on D2. ...Ohio Valley/Central Appalachians through the Northeast... Days 1-3... An impressive shortwave moving through the TN VLY this evening will tilt negatively and shift to off the NJ coast Friday morning. As this occurs, a 250mb jet streak lifting along the Northeast coast will amplify to leave the most robust RRQ diffluence east of the DelMarVa tonight. This setup suggests the primary surface low moving into the OH VLY/Central Appalachians will weaken this evening as a coastal low strengthens and lifts northeast towards the New England coast. This secondary low will likely become the primary as it lifts just outside the Benchmark (40N/70W) Friday morning and spreads a swath of heavy snowfall for the I-95 corridor. The primary low is already weakening over the TN VLY noted by rising PVU, but continued favorable moisture with residual synoptic ascent aided by increasing upslope into the WV terrain will produce an area of heavy snowfall through early Friday morning, and WPC probabilities for 6 inches are high across eastern WV on D1. As this primary low weakens and the secondary low develops and intensifies, there is likely to be a relative minimum of snowfall across the Mid-Atlantic states, but there is still likely to be a quick moving band of snowfall with rates that may reach 1"/hr noted by near 0 theta-E lapse rates. This moisture will be transient, but with cold temperatures, WPC probabilities are high for at least 2" along the I-95 corridor from Washington, D.C. to New York City. As the secondary low strengthens, both synoptic and mesoscale ascent will intensify, as WAA spreads moisture across eastern New England. There remains some uncertainty into how far inland the heavy snow will spread, but it is likely that 1"/hr snowfall rates will spread from eastern CT northeast through Boston, MA and along the coast of ME. Exactly where this low tracks will determine how much snowfall occurs, but a modest TROWAL angling onshore from eastern MA northward should lead to a relative maxima in snowfall across this region. WPC probabilities support a moderate risk for 6 inches along I-95 and eastward the latter half of D1. Isolated totals approaching 10 inches are possible as noted by the 90th percentile from the WSE and NBM. After this low pulls away D2, cold high pressure will settle across the Mid-Atlantic and Northeast D2 into D3. As this high begins to shift offshore late Saturday, return flow and some WAA associated with a warm front draped from a low moving across Canada will spread precipitation back into the region. This precipitation will move into a region that initially is cold and dry at the surface, but is well above 0C aloft through the aforementioned WAA. Wet-bulb temperatures are forecast to remain below freezing so precipitation is likely to fall as freezing rain across parts of the Mid-Atlantic and into the Northeast. As of now, the total accretion does not look too significant, but WPC probabilities do feature a low risk for 0.1" just NW of the I-95 corridor. Weiss ~~~ Key Messages for the Tennessee and Ohio Valleys to the Northeast Winter Storm ~~~ - Moderate to heavy snow will continue across the eastern Ohio Valley and Central Appalachians this evening, spreading into New England on Friday. - Snow will likely be heavy, reaching 1"/hr at times. This will produce a swath of snow reaching 6+ inches across the terrain of WV, with an additional band of more than 6 inches likely from eastern Long Island through coastal New England, including Boston, MA. - This snowfall, combined with cold temperatures and gusty winds, will create dangerous travel due to significantly reduced visibility and slippery roads.