Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 254 PM EST Fri Mar 11 2022 Valid 00Z Sat Mar 12 2022 - 00Z Tue Mar 15 2022 ...Mid-Mississippi Valley, Ohio Valley, Mid-Atlantic, and Northeast... Days 1-2... A rapidly deepening surface low will eject from near the Gulf Coast, through the Mid-Atlantic, and up across New England tonight through Sunday morning. This low will strengthen in response to impressive height falls ahead of a longwave trough which will amplify in response to shortwave phasing of the southern and northern streams. As this trough sharpens across the OH VLY and shifts eastward, extremely impressive jet streak coupling will occur as the LFQ of a subtropical jet streak with near record intensity according to SPC sounding climatology couples with the RRQ of departing northern stream jet energy moving into Canada. The overlap of the jet streaks with the height falls above an intensifying baroclinic zone along a surface cold front will drive explosive cyclogenesis, and this low is likely to bomb as it moves into Maine/Canada. Robust WAA will drive increasing moisture from the southern Appalachians into New England, which will help expand the precipitation shield and yield to a large swath of heavy snow from northern MS through northern New England. Within this swath of heavy snow, a pronounced and intensifying band of fgen is likely in response to strong CAA on gusty NW winds, and within this band there are signals for CSI or potentially upright convection /thunder snow/ as far south as TN. Where this band translates from SW to NE, snowfall rates will likely reach 2"/hr as shown by HREF and NBM probabilities, with the greatest snowfall rates likely from TN through eastern KY, with a second maxima possible in VT/NH/ME. Additionally, this strengthening fgen combined with a slightly further east track which appeared in the guidance at 12Z today suggests the rain/snow line will collapse more rapidly eastward, and this has led to an increase in snowfall for I-95 and points east. Here, the precipitation will initially be rain but will likely change over to a brief period of sleet and then quickly heavy snow with rates of 1"/hr or greater, which should overwhelm the antecedent hostile conditions to rapidly accumulate. While amounts are likely to be much less for the urban corridor, impacts could still be substantial. The entire system will pull away quickly Sunday morning with lingering moderate to heavy snow across Maine Sunday before exiting to the northeast thereafter. WPC probabilities for heavy snow on D1 are above 50% for 6+ inches in NE TN, eastern KY, and the higher terrain of WV, where locally 8-10" is possible. A secondary max of snow is likely from northeast PA into Upstate NY and much of northern New England where WPC probabilities are above 60% for 6+ inches and locally 12 inches is likely in the higher terrain of VT/NH and ME. The PWPF does not account for melting at snowfall onset, especially for the I-95 corridor, so may be a bit overdone, but the impressive rates should still lead to several inches of snowfall on Saturday, and WPC probabilities for 2+ inches are above 70% for Washington, D.C. to Philadelphia, and above 50% for NYC, but lower for Boston. By D2 the heavy snow will be confined to far northern Maine. Behind the system, a pronounced cold pool aloft within the impressive trough will increase instability across the Great Lakes and Ohio Valley. The high-res guidance is in good agreement that afternoon snow showers and snow squalls will develop and advect southeast through the evening, potentially reaching the Central Appalachians later Saturday evening. Although snowfall accumulation will be light, intense snowfall rates of more than 1"/hr and gusty winds will likely produce widespread periods of severely limited visibility and difficult travel. ...Pacific Northwest... Days 2-3... A potent closed low will drop out of the Gulf of Alaska and then move onshore the Pacific Northwest coast Saturday night while slowly filling. This feature will open into a positively tilted trough and advect through the Northern Rockies and then flatten into the Northern Plains by Sunday night. This feature will combine with weakly coupled jet streaks as a the RRQ of a departing jet streak shifts eastward and a Pacific jet streak moves into CA. The overlap of these height falls with the upper coupled jets will provide ascent into a modestly moistening column to produce moderate to heavy snow in the terrain of the Northwest. D2 WPC probabilities are high for 6+ inches in the Olympics and WA/OR Cascades, with lesser probabilities into the Sawtooth and Northern Rockies. By D3 the primary trough shifts eastward, but residual jet-level ascent will continue to produce moderate snow in the WA Cascades. Light to moderate snowfall accumulations are likely at the Cascade Passes across WA and OR on Sunday. Weiss ~~~ Key Messages for the Tennessee Valley through Northeast Winter Storm ~~~ - A significant winter storm is likely for portions of the Tennessee and Ohio Valleys, northeastward into the Mid-Atlantic and New England tonight through early Sunday. - Snow rates of 1-2"/hr combined with strong and gusty winds up to 50 mph will produce blowing and drifting snow even after the snow ends. Severely reduced visibility and white-out conditions will make travel extremely dangerous at times. - Snow accumulations of 6-12+ inches are likely over much of the interior Northeast. Up to several inches are possible closer to the coast which will fall quickly Saturday morning. - Strong winds and the heavy wet snow could lead to scattered power outages and tree damage. Untreated surfaces will freeze over as temperatures drop sharply. - Record cold is likely for much of the Southeast Sunday morning as temperatures fall below freezing along the Gulf Coast and through northern Florida.