Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 08 2025 18:36:23 FOUS11 KWBC 081836 QPFHSD 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...=20 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)=20 will favor snow levels down below 500ft and to the inland valley=20 floors, but precipitation will trend lighter overnight as heights=20 begin to rise over WA/OR which will limit any additional and=20 appreciable snow to the Cascades (and only a few inches at best).=20 =46rom the Blue Mountains eastward across northern ID into NW=20 Montana, snow this evening may be moderate at times in advance of=20 the trough axis and as additional PVA slips through from the NW.=20 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=20 above 4000-6000ft in and around northern ID. Farther south,=20 additional mid-level vorticity will slide through CO with some=20 orographic enhancement across the central Rockies above 9000ft or=20 so.=20 ....Mid-Atlantic and Northeast...=20 Days 1-3... Fast moving shortwave ejecting from the Great Lakes will remain=20 progressive as it pushes east into New England early Sunday morning as it remains embedded within generally pinched/flat flow across=20 the CONUS. This shortwave moving east will interact with a potent=20 west-to-east oriented jet streak and a warm front draped from the=20 Mid-Atlantic into the Mid-Mississippi Valley to spawn a low=20 pressure off the coast of NJ by 06Z Sunday. This low will then=20 deepen as it translates almost due east, crossing near the 40N/70W=20 benchmark before pushing farther into the Atlantic. Increasing=20 290-295K isentropic ascent and accompanying WAA downstream of this=20 wave will help surge moisture northward into New England and=20 Upstate NY, resulting in an axis of mixed precipitation across the=20 Mid-Atlantic, and heavy snow for Upstate NY and New England. As precipitation expands tonight across the Mid-Atlantic, it will=20 be accompanied by a warm nose reaching +2 to +3C shifting as far=20 north as Long Island. There remains uncertainty into how far north=20 this will track due to the low pressure development offshore and=20 its resultant return of cold air back to the south, but it appears=20 areas from near the Mason-Dixon line northward through I-80 and as=20 far as northern NJ will experience primarily a mix of=20 sleet/freezing rain after a period of snow early. This will result=20 in additional icing accretion, which has a high chance (70+%) of=20 exceeding 0.1" from the MD Panhandle through the Laurel Highlands,=20 with a 30% chance extending into northern MD and southern PA. Farther north, the setup is favorable for a short (12-18 hours)=20 period of heavy snowfall. The impressive isentropic lift with=20 mixing ratios exceeding 4g/kg will drive moisture northward, while=20 the accompanying robust WAA produces impressive omega into the=20 column as 850-700 fgen slopes strongly northward. This will=20 effectively interact with the snow growth region, producing a swath of 1-2"/hr snowfall rates translating E/NE tonight through Sunday=20 morning. Locally higher rates are possible as well as cross-=20 sections indicate a threat for CSI as theta-e lapse rates collapse=20 below 0C indicating instability near the DGZ. The duration of these intense snowfall rates may be shortened, however, as the guidance=20 has become more insistent that a dry slot will pivot as far north=20 as I-90 across New England, which would result in less cloud ice=20 and potentially a cutoff of heavy snow with some freezing drizzle=20 and much lower SLR. Where this occurs, amounts will be more=20 limited, but some light icing on top of heavy snow will be just as=20 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)=20 offsetting the lower QPF across central/northern New England. The=20 latest WPC probabilities have increased due to a trend upward in=20 QPF, and now exceed 90% for 6+ inches from near Albany, NY through=20 the southern Greens, Berkshires, Worcester Hills, Monadnock region=20 of NH, and towards far SW Maine. This includes the major metro=20 areas of Albany, Worcester, Boston, and Portland. Locally, 10-12"=20 of snow is possible, and major impacts are possible (>50%)=20 according to the WSSI-P. As this low pulls away Sunday afternoon and drying occurs rapidly=20 in its wake, snowfall will wane quickly late D2. However, renewed=20 CAA behind the accompanying cold front will setup an environment=20 favorable for LES E/SE of Lake Ontario. Shifting winds (more N/NW=20 early, becoming more W late D2 into D3) will result in a shifting=20 band of LES. For D2, the most significant accumulations are likely=20 in the northern Finger Lakes region, where they have a 70-90%=20 chance of exceeding 4" near the lake shore, with D3 probabilities=20 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=20 develop late D2 /Monday evening/ as a shortwave trough ejects from=20 near Baja and then moves east into the Southern Plains by Tuesday=20 evening. This shortwave will remain embedded within a longwave=20 trough axis as it moves east through its evolution, but guidance is insistent that it will weaken with time and remain a generally=20 flat feature as it pivots to the east. This is reflected in 500mb=20 height trends among the various ensemble systems showing a weaker=20 and faster trough with stronger ridging downstream suppressing this impulse. Despite the modest mid-level ascent expected with this=20 shortwave (through PVA and downstream divergence), upper diffluence will likely increase with time as a downstream jet streak=20 intensifies towards 150kts and arcs sufficiently poleward while=20 leaving its tail across the Central Plains and into the Lower MS=20 VLY by Tuesday evening. This will provide additional deep layer=20 ascent as the system organizes through D3. Then beginning early D3 /Monday night/ isentropic ascent=20 downstream of this developing system becomes quite impressive,=20 surging moisture out of the Gulf northward into the Lower MS VLY,=20 TN VLY, and the Mid-Atlantic. 295-300K surfaces indicate that=20 mixing ratios will reach values as high as 6-8 g/kg, indicative of=20 an extremely moist environment, additionally reflected by NAEFS=20 ensemble tables forecasting PWs approaching the 97th percentile=20 within the CFSR database, highest along the Gulf Coast but=20 stretching north into the colder airmass. While there is still=20 quite a bit of uncertainty into the timing and track of the=20 developing system, and the guidance has trended south this aftn, a=20 large swath of precipitation will develop and expand bringing all=20 precip-types Tuesday. The flat, fast, and suppressed flow will=20 cause the moisture to translate more east than north, but this=20 should result in an axis of at least moderate snow, with the=20 greatest risk for significant snow from eastern KY through the Mid- Atlantic region. WPC probabilities at this time are highest for 4+=20 inches in the higher elevations of WV/VA where they peak above 70%, and locally 6+ inches of snow is possible. Elsewhere, WPC=20 probabilities for 2" are 30-50% across much of this area.=20 Additionally, light icing is probably in SW VA and far NW NC where=20 up to 0.1" of ice may accrete. Weiss/Fracasso ....Winter Storm Key Messages are in effect. Please see current=20 Key Messages below... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!9FQuy6w8JcBb94PqkHxDXn9FxnzYlP0po11teqmEZLFAs= 8sQha50Xstk-4ty1tmntMruOafb6DTXwENF-tFHu2UJoN8$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .