Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 136 PM EST Sat Feb 8 2025 Valid 00Z Sun Feb 09 2025 - 00Z Wed Feb 12 2025 ...Pacific Northwest and Northern/Central Rockies... Day 1... Trough axis over eastern WA/OR and northern NV this afternoon will continue eastward through the northern Great Basin/Rockies, taking the last of any organized precipitation with it. Cold temperatures over the PacNW (700mb temp anomalies below the 10th percentile) will favor snow levels down below 500ft and to the inland valley floors, but precipitation will trend lighter overnight as heights begin to rise over WA/OR which will limit any additional and appreciable snow to the Cascades (and only a few inches at best). From the Blue Mountains eastward across northern ID into NW Montana, snow this evening may be moderate at times in advance of the trough axis and as additional PVA slips through from the NW. Total additional accumulations there may range from a few inches to perhaps >6" in the highest peaks. WPC probabilities for at least 4 inches after 00Z are highest above 3000ft in the OR Cascades and above 4000-6000ft in and around northern ID. Farther south, additional mid-level vorticity will slide through CO with some orographic enhancement across the central Rockies above 9000ft or so. ...Mid-Atlantic and Northeast... Days 1-3... Fast moving shortwave ejecting from the Great Lakes will remain progressive as it pushes east into New England early Sunday morning as it remains embedded within generally pinched/flat flow across the CONUS. This shortwave moving east will interact with a potent west-to-east oriented jet streak and a warm front draped from the Mid-Atlantic into the Mid-Mississippi Valley to spawn a low pressure off the coast of NJ by 06Z Sunday. This low will then deepen as it translates almost due east, crossing near the 40N/70W benchmark before pushing farther into the Atlantic. Increasing 290-295K isentropic ascent and accompanying WAA downstream of this wave will help surge moisture northward into New England and Upstate NY, resulting in an axis of mixed precipitation across the Mid-Atlantic, and heavy snow for Upstate NY and New England. As precipitation expands tonight across the Mid-Atlantic, it will be accompanied by a warm nose reaching +2 to +3C shifting as far north as Long Island. There remains uncertainty into how far north this will track due to the low pressure development offshore and its resultant return of cold air back to the south, but it appears areas from near the Mason-Dixon line northward through I-80 and as far as northern NJ will experience primarily a mix of sleet/freezing rain after a period of snow early. This will result in additional icing accretion, which has a high chance (70+%) of exceeding 0.1" from the MD Panhandle through the Laurel Highlands, with a 30% chance extending into northern MD and southern PA. Farther north, the setup is favorable for a short (12-18 hours) period of heavy snowfall. The impressive isentropic lift with mixing ratios exceeding 4g/kg will drive moisture northward, while the accompanying robust WAA produces impressive omega into the column as 850-700 fgen slopes strongly northward. This will effectively interact with the snow growth region, producing a swath of 1-2"/hr snowfall rates translating E/NE tonight through Sunday morning. Locally higher rates are possible as well as cross- sections indicate a threat for CSI as theta-e lapse rates collapse below 0C indicating instability near the DGZ. The duration of these intense snowfall rates may be shortened, however, as the guidance has become more insistent that a dry slot will pivot as far north as I-90 across New England, which would result in less cloud ice and potentially a cutoff of heavy snow with some freezing drizzle and much lower SLR. Where this occurs, amounts will be more limited, but some light icing on top of heavy snow will be just as impactful. Despite some uncertainty, there is still high confidence in a swath of 6-12" of snow across Upstate NY and southern/central New England, with variations in SLR (higher north, lower south) offsetting the lower QPF across central/northern New England. The latest WPC probabilities have increased due to a trend upward in QPF, and now exceed 90% for 6+ inches from near Albany, NY through the southern Greens, Berkshires, Worcester Hills, Monadnock region of NH, and towards far SW Maine. This includes the major metro areas of Albany, Worcester, Boston, and Portland. Locally, 10-12" of snow is possible, and major impacts are possible (>50%) according to the WSSI-P. 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 setup an environment favorable for LES E/SE of Lake Ontario. Shifting winds (more N/NW early, becoming more W late D2 into D3) will result in a shifting band of LES. For D2, the most significant accumulations are likely in the northern Finger Lakes region, where they have a 70-90% chance of exceeding 4" near the lake shore, with D3 probabilities for 4+ inches reaching 30-50% focused in the Tug Hill Plateau ...Central Plains through the Mid-Atlantic Days 2-3... The next in this parade of systems across the CONUS begins to develop late D2 /Monday evening/ as a shortwave trough ejects from near Baja and then moves east into the Southern Plains by Tuesday evening. This shortwave will remain embedded within a longwave trough axis as it moves east through its evolution, but guidance is insistent that it will weaken with time and remain a generally flat feature as it pivots to the east. This is reflected in 500mb height trends among the various ensemble systems showing a weaker and faster trough with stronger ridging downstream suppressing this impulse. Despite the modest mid-level ascent expected with this shortwave (through PVA and downstream divergence), upper diffluence will likely increase with time as a downstream jet streak intensifies towards 150kts and arcs sufficiently poleward while leaving its tail across the Central Plains and into the Lower MS VLY by Tuesday evening. This will provide additional deep layer ascent as the system organizes through D3. Then beginning early D3 /Monday night/ isentropic ascent downstream of this developing system becomes quite impressive, surging moisture out of the Gulf northward into the Lower MS VLY, TN VLY, and the Mid-Atlantic. 295-300K surfaces indicate that mixing ratios will reach values as high as 6-8 g/kg, indicative of an extremely moist environment, additionally reflected by NAEFS ensemble tables forecasting PWs approaching the 97th percentile within the CFSR database, highest along the Gulf Coast but stretching north into the colder airmass. While there is still quite a bit of uncertainty into the timing and track of the developing system, and the guidance has trended south this aftn, a large swath of precipitation will develop and expand bringing all precip-types Tuesday. The flat, fast, and suppressed flow will cause the moisture to translate more east than north, but this should result in an axis of at least moderate snow, with the greatest risk for significant snow from eastern KY through the Mid- Atlantic region. WPC probabilities at this time are highest for 4+ inches in the higher elevations of WV/VA where they peak above 70%, and locally 6+ inches of snow is possible. Elsewhere, WPC probabilities for 2" are 30-50% across much of this area. Additionally, light icing is probably in SW VA and far NW NC where up to 0.1" of ice may accrete. Weiss/Fracasso ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png