Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 AM EST Tue Feb 09 2021 Valid 12Z Tue Feb 09 2021 - 12Z Fri Feb 12 2021 ...Upper Ohio Valley through Northeast and Great Lakes... Day 1... A fast moving shortwave trough embedded within broad cyclonic flow will race from the Midwest this morning and off the New England coast by this evening. Warm air advection ahead of the shortwave combined with mid-level divergence and a band of mid-level frontogenesis stretching west to east from the the Cincy metro area up the rest of the Ohio River and across central New England will continue to produce locally heavy snow rates. Lift will continue reach the DGZ and allow snow rates to locally exceed 1"/hr along this swath as indicated by the 00Z HREF probabilities. Given the movement of these existing bands along their axis of orientation, local maxima above 4" additional snow are possible across southern OH into southwest PA rest of tonight, then disruptions should occur in the terrain of west-central PA. However, farther northeast, southerly orographic lift in terrain of northeast PA such as the Poconos and the Catskills of southern NY allow local enhancements where Day 1 snow probabilities for 4 or more inches area moderate. Coastal low development will begin along the central Mid-Atlantic coast this morning and shift northeast with most development offshore this evening. Repeating bands of snow across Mass as the warm air advection band turns into a TROWAL northwest of the developing coastal low this evening brings moderately high probabilities for 4 or more inches through this evening before the system shifts out to sea. Behind the system, continued CAA will produce some LES in the favored NW snow belts. However, due to the exceedingly cold air, most crystal develop is above the DGZ, so efficient crystal growth is unlikely. Still, there exists a moderate chance for 4 or more inches in the Keweenaw Peninsula of the U.P. of Michigan as well as off the southeast corner of Lake Ontario. ...Freezing Rain in the Southern Plains eastward to the Central/Southern Appalachians... Days 1-3... ...Significant freezing rain event likely Wednesday through Thursday in the Mid-South and Kentucky... Broad cyclonic flow across the CONUS will persist into this weekend with slow expansion of the particularly cold airmass over the north-central states. Meanwhile a trough digging south of CA into tonight allows broad low level flow off the Gulf of Mexico (along with Pacific moisture higher up) to lift across the the southeastern CONUS and over the surface boundary over the interior Southeast, setting up a prolonged freezing rain event from the Mid-South across the southern Ohio Valley. As the trough shifts east over the southern Rockies/northern Mexico Wednesday night/Thursday the Mid-South/TN Valley gets into the right entrance region of the backing jet streak aloft aiding lift and promotion surface cyclogenesis moving along the stationary boundary. As this low moves eastward, warm air advection will intensify downstream aiding lift atop the reinforced cold high pressure. 00Z guidance is in generally good agreement with another southern shift in the axis of freezing rain. The CMC remains an outlier with a more southern track/ice swath with the 00Z ECMWF/GFS/NAM preferred for a narrow axis of freezing rain with sleet and then snow to the north Wednesday through Thursday. Using the preferred blend, the heaviest freezing rain is currently most likely from the Mid-South across Kentucky into southwest West Virginia over a prolonged two day period. WPC probabilities for 0.25" or more are highest (40 to 60 percent) on Day 2.5 from northeast Arkansas through northwestern Tennessee, and the length of Kentucky. Day 2.5 probabilities for a half inch or more are 20 to 30 percent in northeast AR and around 10 percent along the swath through KY. Some locally backed flow and broad warm air advection over the cold surface allows to 20 to 40 percent ice probabilities for a tenth inch or more over eastern and central OK. ...Central Appalachians and Mid-Atlantic... Days 2-3... An approaching wave over the south-central CONUS will interact with strengthening upper diffluence in the right entrance of a jet streak departing the northeast produce a surface low along a stalled boundary over the Deep South Thursday. Prolonged warm air advection ahead of this feature will lift over a cold air damming wedge from a 1042mb surface high centered over northern Ontario and ridge that arcs down the eastern side of the Appalachians to upland SC by late Thursday. Precipitation spreads northeastward into the Mid-Atlantic Wednesday night and persists through Thursday night. While rates look more moderate than heavy, the prolonged nature of the event should bring about some significant wintry weather including snow for the central Appalachians and freezing rain for southwest to south-central Virginia. Day 2 snow probabilities for 6 or more inches are around 10 percent in Garrett Co MD. This expands to most of the Potomac Highlands on Day 3 as well as east across northern MD. Farther south, there should be transition zone from snow to sleet to freezing rain in the south-central Appalachians and adjacent foothills across central/SW Virginia. Day 2 WPC ice probabilities are 10 to 20 percent for a tenth inch of ice in the the Allegheny Highlands along the VA/WV border. Day 3 ice probabilities spread across southern VA into northwest NC with 40 percent risk of a tenth inch or more over across south-central VA and low chances for a quarter inch or more. ...The West... Days 1 and 3... The aforementioned trough digging south of CA tonight leads to confluent and moist mid-level flow converging on the west-central CO Rockies. This will lead to a few rounds of moderate to heavy snow tonight. Day 1 WPC snow probabilities are moderately high for 8 or more inches in the higher terrain of north-central CO. Increasingly zonal flow south of a reinforcing through over southwest CA sends Pacific moisture into the Pacific northwest Wednesday night with a cutoff low pushing into northern CA/southern OR Thursday night. Height falls under the backing jet over the northwest allows snow levels to drop into the metro areas (or sea level) of Seattle and Portland Wednesday night into Thursday with generally light precip in the Pacific Northwest until widespread precip pushes inland from the Pacific ahead of the low Thursday night. Moderate Day 3 snow probabilities are over the southern WA/northern OR Cascades, ranges of northeast OR, central ID to western WY ranges. Meanwhile Day 3 snow probabilities for 2 or more inches are 20 to 30 percent for lower elevations of western WA and northwestern OR including the Seattle and Portland metro areas. Jackson