Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 253 AM EST Sat Feb 8 2025 Valid 12Z Sat Feb 08 2025 - 12Z Tue Feb 11 2025 ...Pacific Northwest and Northern/Central Rockies... Day 1... The end of this active wintry period is finally in sight across the mountainous West, but some lingering periods of heavy snowfall will continue today. An upper level trough will track through the Pacific NW with snow levels starting below 500ft in many cases on D1 before rising gradually. This would allow for some light snow accumulations along the I-5 corridor early this morning, while <10th climatological percentile 500-700mb heights could support steeper lapse rates that result in some snow squalls (particularly east of the Cascades) during the day. This upper trough is generally moisture- starved, however, which should limit snowfall amounts to lighter overall totals. WPC probabilities are medium (40-60%) for much of the Cascades for snowfall totals >6". Otherwise, most areas will likely witness snowfall totals of 1-4" through this afternoon with the lesser- end of that range most likely for communities along I-5. This upper trough will also produce some light snow tonight across portions of the northern Rockies, but totals will pale in comparison to the snowfall the region has witnessed over the past 5-7 days. Minor accumulations of generally 2-6" in the Blue, Bitterroot, and Lewis Ranges are forecast through today before snow tapers off on Sunday. Additionally, continuing light to moderate snowfall is expected early this morning across the CO Rockies as a cold front oozes southward across the Rockies and contributes to favorable upslope flow and low chances for an additional 6 inches of snow above 9000ft. ...Upper Midwest through the Great Lakes and Ohio Valley... Day 1... Ongoing storm system will bring a stripe of heavy snow to the Upper Midwest and Great Lakes region, with mixed precipitation including freezing rain impacting the Ohio Valley. Positively tilted shortwave crossing the northern Plains today is the driving force for an east-west swath of snow D1 from central MN and central WI through the northern L.P. of MI. This shortwave will traverse quickly to the east as flow across the CONUS remains predominantly flat and progressive. Despite the modest amplitude of this feature, it will work in tandem with the favorably placed left exit of a 150 kt jet streak to produce a brief period of intense ascent from west to east. Additionally, this jet streak will help surge IVT into the area, and combine with increasing 285-295K isentropic ascent to produce elevated PWs favorable for moderate to heavy snowfall. The WAA from the south accompanying this isentropic ascent downstream of the shortwave will help to deepen the DGZ, and the SREF probabilities are above 90% for 100mb of depth, which could support impressive snowfall rates approaching 1"/hr as progged by the WPC prototype snowband tool. With the wave being generally flat and fast, and the most impressive fgen lying at the top of the DGZ (around 700 mb), ascent within the best snow-growth region won't maximize efficiently for the most intense snow rates leading to heavy snowfall accumulations. However, SLRs should still generally be above climo in the cold column, and with rates around 1"/hr at times in the translating fgen band. WPC probabilities (beginning 12z Saturday) show a high chances (60-80%) of heavy snow exceeding 6" from central WI through the northern L.P. of MI. Farther to the southeast the WAA downstream of a developing surface low will push a warm nose into the Midwest and Ohio Valley. This will occur in conjunction with the expanding precipitation shield, leading to a stripe of mixed precipitation from central IL through eastern Ohio. There is high confidence in this mixed precipitation region, although some uncertainty continues in the exact p-type transition. Still, the mixture of sleet and freezing rain will create ice accumulations that could exceed 0.1" as reflected by WPC probabilities across northern OH and IN reaching 10-30%. ...Mid-Atlantic and Northeast... Days 1-2... After a day of modest CAA and surface high pressure shifting directly over the Northeast to start the D1 period, a significant winter storm begins to approach from the west as a potent shortwave embedded within generally zonal flow across the CONUS races towards the region. By this afternoon the shortwave will be moving across WI/MI/IL, accompanied by a 150kt upper jet streak. Some interaction of this upper level ascent with a warm front draped across the Mid- Mississippi Valley will help spawn a wave of low pressure in the lower Ohio Valley. As this low begins to consolidate, downstream WAA will intensify and manifest as 290-295K moist isentropic ascent surging northeast reflected by mixing ratios exceeding 4g/kg. This will produce PWs that may briefly exceed the 97.5th percentile of the CFSR climatology according to NAEFS, highest into the Central Appalachians and Mid- Atlantic. During this evolution, precipitation will rapidly expand as southern stream and northern stream energy begins to interact across the Mid-Atlantic states. This will provide increased ascent across the region, and a plume of snow, sleet, and freezing rain will overspread from the Ohio Valley and Central Appalachians northeastward. The very impressive accompanying WAA will push a warm nose steadily northward through 06Z Sunday, so even though many areas, especially north of the Mason Dixon line, will begin as a burst of snow, p-type should transition pretty rapidly to sleet and then freezing rain, eventually changing to rain Saturday night in lower elevations south of I-80. Snowfall rates are likely to be very impressive before transition from central/eastern PA to northern NJ and Long Island. This is depicted well by the 00z HREF, with non-zero chances for snowfall rates to exceed 2" per hour in the 00z-06z Sun timeframe across from northern NJ to southern CT and Long Island, including NYC. Icing could be just as problematic in the Mid- Atlantic as reflected by WPC probabilities for 0.1+" of ice that reach 50-90% in the Central Appalachians D1, extending northeast at 30-50% D2 as far as central PA and in the vicinity of Washington, D.C. northern suburbs. Probabilities for at least 0.25" of ice reach 50-60% across western MD and the Laurel Highlands of PA, where the greatest potential for tree damage and power outages exist. As the parent shortwave continues to track progressively to the east, the accompanying forcing and moisture will also pivot with it, and secondary low pressure development is likely off of the NJ coast late Saturday night into Sunday morning. This low will become dominant and intensify as zonal but modestly coupled jet structures interact with a baroclinic zone offshore. As this low deepens and moves almost due east south of New England, it will cross near the Benchmark (40N/70W), leading to a substantial snow event for Upstate NY, the Poconos, and at least Southern New England (SNE), although there remains uncertainty into how far north (or south) the heaviest snow will spread. As the low deepens and pulls away to the east, a renewed surge of cold air both through the ageostrophic flow into the low and the subsequent CAA in its wake will limit the progress of the warm nose, while dry air to the north noted in regional soundings inhibits PWs from aggressively lifting towards Canada. In between, an axis of heavy snow is likely as strong WAA produces impressive 850-600mb fgen, producing strong ascent into the DGZ. The overlap of this strong fgen with some theta-e lapse rates approaching 0C/km suggests banding potential from Upstate NY into SNE, and with elevated SLRs, snowfall rates of 1-2"/hr appear likely. Total snowfall will be limited by the speed of this system as it will only snow in most areas for 12-18 hours, but this will still result in heavy accumulations. WPC probabilities for more than 6 inches of snow are high (>70%) from the Mohawk Valley of NY eastward into much of MA, southern VT, southern NH, and the southeastern tip of ME. Locally 8-12" of snow is possible in some areas. As this low pulls away Sunday afternoon and drying occurs rapidly in its wake, snowfall will wane quickly late D2. However, renewed CAA behind the accompanying cold front will again setup an environment favorable for LES E/SE of Lake Ontario. Shifting winds (more N early, becoming more W D3) will likely cause multi- bands along and south of Lake Ontario into the Finger Lakes region, before organizing into a more significant single band late in the forecast period back into the Tug Hill Region. Confidence in the exact location is low at this time, but WPC probabilities do feature a high risk (70-90%) for 4+ inches of snow southeast of Lake Ontario. ...Central Rockies/Plains through the Ohio Valley and Central Appalachians... Day 3... Next widespread wintry weather event to spread across the country is expected to begin late D3 (Monday night) as shortwave energy at the base of a longwave trough off the coast of southern CA gets ejected eastward into the fast zonal flow dominating the upper pattern across CONUS. In response, a strengthening 160kt jet streak is forecast to stretch zonally from the southern Rockies to the Northeast and shift eastward with time, placing areas from the central Plains to the central Appalachians in the favorable right- entrance region for enhanced upper divergence and lift. Current WPC probabilities only go through 12z Tuesday, so values are low (20-40%) for at least 2" of snow for the central Rockies, central Plains, and central Appalachians, highest across the central Rockies and central Plains. There is also likely to be a ribbon of mixed ptype in the form of freezing rain/sleet, but not expected to have large geographic coverage given weak relatively weak WAA and not the coldest airmass in place at the surface. Current WPC probabilities for at least 0.01" of freezing rain are low (10-30%) and extend from north-central OK to the central/southern Appalachians where CAD could elevate the freezing rain potential. Snell/Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png