Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 346 AM EST Sat Jan 06 2024 Valid 12Z Sat Jan 06 2024 - 12Z Tue Jan 09 2024 ...Southern Appalachians through New England... Days 1-2... Major winter storm to bring combination of freezing rain and heavy snow from the Southern Appalachians through New England. A positively tilted longwave trough enveloping much of the eastern CONUS will shed two shortwaves and accompanying vorticity maxima northeast through the weekend. The first of these will lift from the TN VLY this morning into New England tonight and will combine with the LFQ of an intensifying subtropical jet streak to drive cyclogenesis across the Southeast. This low will lift rapidly northeast as a Miller-A type system, with impressive moist advection ahead of it surging northward to produce an expanding shield of precipitation. As this low approaches New England, a second, more impressive, but still positively tilted, shortwave will again lift out of the Southeast, and interact with the surface low to stall and strengthen it more rapidly south of the New England coast Sunday morning. After a brief slowing, this low will eject eastward to become south of Nova Scotia by Monday morning. While in general this low will be fast moving and of modest intensity, the interaction of this second shortwave with the favorably positioned intense upper jet streak and a strong baroclinic gradient offshore will result in a rapid deepening of this low on Sunday, which should enhance snowfall across the interior northern Mid-Atlantic and New England. On D1, as the precipitation expands to the north, it will begin to overrun retreating high pressure on moist 290-295K isentropic upglide. Robust WAA aloft will surge a warm nose above 0C well north, but at the surface this cold high will remain entrenched and wedged against the Appalachians, with some enhancement of this wedge likely during precipitation onset. Weak ageostrophic drainage from the high pressure could additionally enhance the wedge and lengthen the period of a favorable environment for freezing rain, but eventually the WAA should overwhelm the airmass to change precip over to all rain. While a brief period of snow/sleet at onset is also possible, most of the p-type in the Southern/Central Appalachians and along the Blue Ridge should be freezing rain. The duration is modest, wet bulb temperatures are only around -1C, and precip rates may at times produce some runoff instead of accretion, but significant ice accumulations are likely. WPC probabilities for 0.1 inches are generally 40-60%, with probabilities for 0.25" as high as 20-30% across the Blue Ridge of VA. Late this afternoon and into tonight, as the precipitation expands northward, an axis of moderate to heavy snow driven by WAA leading to waves of robust 850-600mb fgen will develop from the Central Appalachians and across the interior Mid-Atlantic states. Snowfall rates just inland from I-95 from Washington, D.C. to NYC could reach 1-2"/hr in many areas, albeit very briefly closer to the fall line before the strong SE flow pushes 850mb temps above 0C all the way into central MD and eastern PA. SLRs will likely be below climo in the modest thermals, but an impressive DGZ depth noted by SREF probabilities for 100mb of depth reaching 50% in central MD/central PA coincident with the best fgen should still result in a few hours of heavy wet snow despite the progressive nature of the storm. Higher elevations from eastern WV through northern PA have a high chance (70-90%) for 4+ inches according to WPC probabilities, with snow winding down late D1. The most significant snowfall and related impacts from this event are expected from the Poconos and northwest NJ through the Catskills and into much of southern and central New England. While there is still some uncertainty into the R/S line along the south coast of New England, locations just inland, and especially across higher terrain, will likely receive a long duration of heavy snowfall. This will be aided by the developing TROWAL which will pivot back into New England, and the setup should support an intensifying deformation band which will lift through eastern PA/eastern Upstate NY and then may transition to a pivoting band across southern New England, and the presence of likely CSI in cross-sections suggests snowfall rates could be 2"/hr within this band despite the low SLR. The guidance has again trended a bit NW and warmer tonight, but the overall forecast remains on track with the greatest uncertainty along the south and east coasts of New England due to strong onshore flow advecting warmer air onshore. As the low occludes and shifts east on Sunday, the cold air may still collapse back to the southeast resulting in an axis of heavy snow dropping back into southeast MA late, although accumulations still should be modest there. Just NW of I-95 however, the intense band and long duration of heavy snow will result in the greatest accumulations, reflected by WPC probabilities for more than 6 inches exceeding 80% from the Poconos and much of eastern Upstate NY through nearly all of SNE and into southwest ME. Locally, 12-15" of snow appears likely, with the best chance being in any higher elevations or across northeast MA due to pivoting band potential. ..Pacific Coast, Great Basin, Northern and Central Rockies... Days 1-3... A series of shortwaves, the last one being accompanied by an atmospheric river (AR) will spread widespread precipitation across the West into early next week. The first of these shortwaves will move onshore the WA/OR coast early today and then amplify as it drops southeast into the Great Basin by Sunday morning. This shortwave will rapidly deepen into a closed low over the Four Corners by Sunday night, with lingering vorticity streaming southward out of Canada through the weekend. The associated longwave trough which develops during this time will steepen lapse rates and combine with pockets of upper diffluence and an eastward advancing cold front to drive ascent supportive to wring out any available moisture from the Cascades through the Northern Rockies and southward into the Central Rockies. Although moisture will be modest, noted by PW anomalies from NAEFS that are generally normal to slightly below normal, the widespread and robust synoptic ascent will result in light to moderate snow across much of the West with snow levels falling from around 3000 ft to as low as 500 ft beneath the core of the upper trough and behind the surface cold front. The heaviest accumulations D1 are likely in the higher terrain of the Cascades and Sierra where WPC probabilities for more than 6 inches of snow exceed 80%, with pockets of >80% probabilities also extending into the Northern Rockies, and higher ranges of the Great Basin. In the Cascades and Sierra, due to periods of impressive upslope and higher moisture with onshore flow, locally 1-2 feet is likely in the higher terrain. During this time, light snow, generally less than 1 inch however, may occur into many of the western valleys as well. The most intense forcing drops southeast out of the West D2 with just some lingering moderate snow likely in the OR Cascades, before an even more significant shortwave approaches during D3. This shortwave may not actually reach the coast until just beyond this forecast period, but impressive confluence of the mid-level flow downstream of this trough will surge strong IVT eastward and onshore WA/OR characterized by high probabilities in both the EC and GFS for 250+ kg/ms. The best moisture will likely be acted upon by downstream mid-level divergence to drive ascent, with omega also being provided via upslope flow into the Cascades and more direct diffluence within the LFQ of a zonally oriented Pacific jet streak. WPC probabilities for more than 6 inches of snow on D3 exceed 80% in the WA Cascades once again, with widespread 50-80% probabilities appearing in the Olympics, OR Cascades, and much of the Northern Rockies, and locally more than 1 foot is likely in the higher terrain. With snow levels rising to around 4000-5000 ft by D3, the heaviest snow should remain above most of the area passes. ...Four Corners through the Upper Midwest... Days 2-3... An intensifying shortwave digging out of the Pacific Northwest today will close off over the Four Corners Sunday morning. This feature will continue to deepen as it progresses to near the TX Panhandle Monday morning, producing height anomalies at 500mb of near -4 sigma according to NAEFS. This extremely anomalous trough will drive impressive downstream ascent through height falls and divergence to the northeast, with intensifying upper jet streaks coupling efficiently overhead the greatest height falls to result in rapid cyclogenesis beginning late Sunday night over eastern NM. As this surface low deepens and tracks E/NE, it will drive increasing moisture advection downstream from the Gulf of Mexico noted by mixing ratios of 6 g/kg on the 300K isentropic surfaces surging northward into the Southern Plains Monday. This is also reflected by NAEFS PW anomalies reaching as high as +2 sigma Monday from TX into KS, with some of this theta-e ridge rotating cyclonically into a TROWAL back into the High Plains. The combination of moist mid-level ascent on confluent SW flow downstream of the primary closed low and this increasing low-level moisture, in a region of impressive synoptic ascent will likely result in an expansive area of heavy precipitation from the Four Corners through the Upper Midwest by the end of the forecast period. During D2 as the closed low digs across the Four Corners and interacts with the strengthening coupled jet structure, moisture within the mid-level SW flow will be wrung out across the terrain of the Southwest, especially from the Wasatch through the San Juans and southward along the Mogollon Rim. WAA downstream of the surface low will surge snow levels to around 4000 ft in NM/CO, but these will quickly drop back to just around 500 ft behind the accompanying cold front as the low advects eastward. Most of the precipitation should wane behind the cold front so the heaviest snow is still likely above 3000-4000 ft, which is reflected by WPC probabilities of 70+% for more than 6 inches across parts of the Wasatch and southeast to include much of the Mogollon Rim, the White Mountains of AZ, and across the San Juans. Locally more than 12 inches is possible in the highest terrain, and with the falling snow levels as the event pushes east, light accumulations of generally less than 1 inch are possible through most of the valleys as well. As the evolution becomes more intense into D3, the shield of precipitation will become both more expansive and intense. Synoptic forcing remains robust across the region, but begins to interact with strong mesoscale ascent as WAA overlaps the best mid-level divergence, and waves of fgen develop both downstream of the low as well as upstream as a comma-head band tries to develop. Within the WAA snow downstream of the surface low, DGZ depths according to the SREF probabilities and through evaluation of regional soundings are modest, which could result in more moderate snowfall intensity. However, a long duration of snowfall is likely which could still accumulate significantly, and WPC probabilities for more than 6 inches have increased, reaching 50-60%, highest across eastern NE. Farther to the west, as the low consolidates and moves east, there is potential for a strong deformation band developing to the NW near the TX/OK Panhandles Monday morning, aided by the fgen response to the favorably placed upper jet streak. This could result in more intense snowfall rates combined with stronger winds nearer the low center. WPC probabilities at this time are still relatively modest for more than 4 inches in this area, at 30-40%, but have increased with this iteration. Additionally, the probabilistic WSSI has shown an increase in possible impacts due to blowing snow and snowfall, especially in the upslope regions around the Raton Mesa and where this band may develop near the TX/OK Panhandles. Weiss ***Key Messages for Jan 6-7 East Coast Winter Storm*** --Heavy Snow in the Mid-Atlantic and New England Heavy snow will overspread the Mid-Atlantic this afternoon and then lift into New England tonight. Snow will end from west to east by Monday morning. This will result in widespread snowfall accumulations of 6-12 inches, with locally as much as 15 inches possible in higher elevations. --Impacts from Heavy Snow and Wind The combination of heavy snow rates exceeding 1"/hr, and gusty winds will lead to snow covered roads and limited visibility to create dangerous travel. In some areas, especially southern New England, the snow may be heavy and wet which could cause isolated power outages and tree damage. --Minor Coastal Flooding Possible in the Northeast Gusty onshore winds may lead to minor flooding along the Mid-Atlantic and southern New England coasts during the Sunday morning high tide cycle. --Ice Impacts to Travel in the Southern Appalachians The probability for ice accumulations in excess of 0.25 inches along the Blue Ridge of North Carolina and Virginia are 30-50%. This icing, along with some areas of sleet, may produce hazardous travel conditions today. ***Key Messages for Jan 6-11 Major Winter Storm*** --Weather Impacts From Coast-to-Coast-- The next winter storm shifts down the West Coast Saturday crossing the Desert Southwest Sunday before rapidly strengthening over the southern Rockies/Plains Sunday night/Monday and then shifting through the Midwest/Northeast Tuesday/Wednesday. --Heavy Snow Expected; Blizzard Conditions Possible-- Expect heavy snow in the higher elevations near the West Coast and the Four Corners states this weekend before a swath of heavy snow develops over parts of the Southern/Central Plains Monday. Heavy snow then shifts through the Midwest Monday night and Tuesday. Given the intensity of the storm, strong winds may create areas of blizzard conditions over the Plains into the Midwest. --High Winds Ahead of the Storm-- Gusty winds on the warm side of the storm may exceed 50 MPH and cause damage over the Appalachians to the East Coast. Organized severe thunderstorms are possible along the Gulf Coast and Southeast Monday and Tuesday. --Flooding and Coastal Flooding Risk in the Eastern U.S.-- Heavy rain Tuesday into Wednesday is likely to lead to river and possibly flash flooding from the central Gulf Coast through the Northeast. Powerful onshore winds are also likely to lead to coastal flooding along much of the East Coast.