Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 456 AM EST Fri Jan 05 2024 Valid 12Z Fri Jan 05 2024 - 12Z Mon Jan 08 2024 ...Southern Appalachians through New England... Days 1-3... Major winter storm to bring combination of freezing rain and heavy snow from the Southern Appalachians through New England. A large trough across the center of the CONUS will expand to the east Friday night in response to a subtle vorticity impulse and accompanying weak shortwave trough rotating cyclonically through the flow and into the Mid-Atlantic. This will be followed quickly by a more substantial vorticity impulse streaking out of the Gulf Coast, and it is this secondary trough that will drive the more intense ascent and support cyclogenesis as a Miller-A type low develops in the Southeast and then moves northeast into the Mid-Atlantic and into New England. This shortwave will remain progressive at least into Sunday morning, but guidance has continued to become more aggressive with it tilting negatively near the New England coast, which could slow/stall the surface low, while yet a third piece of energy approaches from the Ohio Valley to re-invigorate ascent and lengthen the period of precipitation. At the same time, an intense subtropical jet streak will arc out of the TN VLY, with wind speeds approaching +4 sigma according to NAEFS, and the intense diffluent LFQ of this jet will overlap the best height falls sufficiently to help rapidly strengthen the low south of New England. While there is still some uncertainty into the exact track which will wreak havoc with the rain/snow line, there is increasing confidence that a stripe of heavy and wet snow with significant accumulations will occur just inland from the coast from WV through ME. Late D1 /Friday night into Saturday morning/ the expanding precipitation shield will begin to overrun cold Canadian air across the Appalachians. Moist isentropic upglide at 285K will feature impressive mixing ratios of 5-6 g/kg, suggesting a rapidly moistening column and potentially heavy precipitation. This overrunning will likely result in periods of snow changing to freezing rain in the foothills of the Appalachians where the surface wedge will at least briefly become reinforced by falling precipitation before slowly eroding to the northeast. Guidance tends to over-do the rate of this erosion, and with some weak dry-drainage on NE winds, it is possible some areas could receive around 0.25" of freezing rain. However, this system remains progressive into Saturday so precip will either wane or turn to rain quickly Saturday across VA/NC/SC, with the best freezing rain potential shifting northward along the Blue Ridge where WPC probabilities for more than 0.25" peak at 20-40%. North of the freezing rain/mixed zone, an area of heavy snow will develop early Saturday as the surface low tracks from near GA to off the NJ coast by Sunday morning. This low will be progressive during this time, but should be accompanied by strong WAA at onset which when combined with the impressive synoptic lift will result in some front end heavy snow most areas, before quickly changing over to rain along and east of I-95, with some sleet-to-rain farther inland. The WAA is impressive, 850mb flow is out of the S/SE, and the Canadian high is not in a favorable place to lock in cold air, so it is likely only elevated areas and regions well inland that will receive significant accumulations across the southern Mid-Atlantic states, but a burst of heavier snowfall rates within the WAA/fgen band could still result in hazardous travel for a time, and moderate accumulations as far south as the Laurel Highlands and Blue Ridge. However, the more significant snowfall is likely the latter half of D2 and into D3 as the low stalls south of New England and rapidly deepens in response to a strong baroclinic gradient offshore and better synoptic lift. During this time, onshore flow will maximize noted by U-wind anomalies approaching -3 sigma according to NAEFS, pushing impressive theta-e advection into New England and lifting into a TROWAL. While this will likely push warm air into the coast, locations just inland and in higher elevations could experience a longer duration of heavy snow as the setup seems to at least marginally support a pivoting band of heavy snow N/NW of the maturing and then occluding cyclone. During this time, likely late Saturday night into the first half of Sunday, the occlusion and isallobaric response to the strengthening low could cause the cold air to collapse back to the southeast, bringing heavy snow back towards the coast before the system winds down Sunday evening. Despite marginal low-level thermals, the strong ascent should result in heavy snow rates exceeding 1"/hr, especially in elevated inland areas. There remains some uncertainty as to where this impressive band will pivot, but WPC probabilities, in general, have increased and shifted a bit NW with this update, and now are above 50% for 6+ inches on D2 from the Poconos through the Catskills and into much of southern New England away from the immediate coast, shifting into northeast MA, southeast NH, and far southwest ME on D3. Locally more than 12 inches of snow is likely in some areas, especially in the higher terrain of the Catskills, Berkshires, and Worcester Hills. ...Southern Rockies and Four Corners... Days 1-3... A series of shortwave rotating through broad cyclonic flow encompassing much of the CONUS will bring waves of precipitation, primarily as snow, to the Four Corners and Southern Rockies each day. One impulse moving into the region late tonight into Saturday morning will bring a round of snow, generally to the higher terrain of UT/CO and the Sangre de Cristos, with NW flow providing favorable upslope into these regions. The shortwave is rather transient and moisture is modest, but above 3000 ft WPC probabilities for more than 4 inches reach 30-50% in these ranges. As this first shortwave ejects to the east and into the Plains Saturday, brief shortwave ridging will follow in its immediate wake, before another, stronger, impulse digs in from the Pacific Northwest. This next feature will amplify into a potent closed low centered over the eastern Great Basin Sunday evening and continue to deepen as it pivots into the Southern High Plains by the end of the forecast period. The result of this evolution will be a pronounced longwave trough which sets up over the Intermountain West, leaving both upstream and downstream jet streaks to amplify and support enhanced coupled divergence aloft. The overlap of this synoptic ascent will likely lead to surface cyclogenesis around the Four Corners Sunday evening, and this low is progged to then strengthen as it shifts to the east on Monday. Downstream of this wave, at least modest moisture advection will emerge from the south and pivot into the Four Corners, with mid-level RH increasing dramatically late Sunday. Snow levels ahead of the wave may rise to as high as 4000-5000 ft within the WAA, but will fall quickly to as low as 1500 ft behind the accompanying cold front. The duration of this impressive ascent into a moistening column will result in areas of heavy snow beginning late D2 but especially on D3. WPC probabilities for more than 4 inches during this second wave reach above 80% from the Wasatch Front and Uintas along the Wasatch, southeast into the San Juans, and then much of the higher terrain from the Mogollon Rim into the Sangre de Cristos of northern AZ and NM. Locally more than 12 inches of snow is possible on D3, with the highest probability of that occurring in the White Mountains of AZ and the southern San Juan Range. ..Pacific Northwest/Great Basin/Sierra... Days 1-3... An active period of weather will be accompanied by lowering snow levels as an expansive trough amplifies across the West. The period begins with one shortwave impulse racing eastward across the inland Pacific Northwest and diving into the Great Basin/Northern Rockies. This will have limited impact to the sensible weather/precipitation, but will result in more zonal flow in its wake to surge additional moisture onshore. Within westerly flow, a shortwave will approach from British Columbia, pinching the flow to become more intense, while also producing downstream divergence ahead of the approaching impulse. This feature will then intensify as it drops southeast into the Pacific Northwest and northern CA Saturday evening, amplifying into a closed low over the Great Basin by Sunday morning before slowly pulling off to the east by the end of the forecast period. Broad but impressive synoptic lift will accompany this evolution, with lift being provided through substantial height falls, pockets of impressive PVA, and robust upper diffluence as the LFQ of a 150kt Pacific jet streak arcs down the coast and into the Intermountain West. Although IVT will remain modest during the duration of this event, generally AOB 300 kg/ms, the prolonged moisture fetch within the large scale ascent will result in heavy precipitation across much of the West, especially late D1 through early D3. The heaviest precipitation will spread southeast with time, starting in the Pacific Northwest (Cascades, Olympics) Saturday morning and then expanding and extending across most of the region through Sunday. For D1, the surface cold front will drop southeast through the PacNW with upslope flow enhancing snowfall across the Cascades, primarily above 2500 ft. WPC probabilities for more than 4 inches are 70-90%, with 1-2 feet likely in the highest terrain. During D2, WPC probabilities for more than 4 inches expand considerably as the front and waves of low pressure move east beneath the amplifying trough. The highest risk areas for more than 4 inches on D2 are again across the Cascades, but also extending south into the Northern CA ranges and along the Sierra, with additional high probabilities above 80% reaching the higher terrain of the Northern Rockies, Blue Mountains, and ranges in the Great Basin. Although the heaviest snowfall, which could exceed 1 foot, should be confined to the higher terrain of these areas, snow levels will be collapsing to as low as 1000 ft in WA and the Northern Rockies, and 2000-3000 ft elsewhere indicating an increasing potential for impactful snow even into lower elevations and many of the area passes to produce hazardous travel. By D3, the most intense ascent shifts southeast again into the Four Corners, but renewed precipitation is expected in the Cascades where WPC probabilities for more than 4 inches reach 20-40% in advance of yet another shortwave moving onshore. ...Northern Minnesota... Days 1-2... A cold front will waver across northern MN Friday night before returning north as a warm front by Saturday morning in response to a shortwave emerging from the Central Plains and lifting northward into the Upper Midwest. At the same time, a weak secondary shortwave will dig southeast from Saskatchewan, and the interaction of these features will drive weak cyclogenesis which will shift eastward, trailed by a cold front into the Great Lakes by Sunday morning. Increasing moist isentropic upglide on the 285K-290K surfaces concurrent with the warm front will result in an expanding shield of light to moderate snowfall, although modest ascent and low probabilities for even 50mb of DGZ depth from the SREF indicate rates should remain modest. The column will be cold, so efficient and fluffy SLRs are likely, and a long duration of this snow should result in moderate accumulations across the area. As the low shifts eastward Saturday into the Great Lakes, the trailing cold front may be accompanied by some enhanced 850-600mb fgen which could drive a narrow band of heavier snowfall across these same areas, adding light accumulations through D2. The heaviest snow this period will likely be across the Iron Ranges in the Arrowhead where some additional moisture from Lake Superior will contribute, and low-level flow with the warm front will upslope effectively to wring out additional moisture, but WPC probabilities for more than 4 inches D1.5 peak around 50% along the lake shore, with locally more than 6 inches possible. Elsewhere across northern MN WPC probabilities for more than 4 inches reach 20-40%. Weiss ***Key Messages for Jan 6-7 East Coast Winter Storm*** --Heavy Snow in the Northeast For the interior Mid-Atlantic and New England, heavy snow is expected from Saturday Afternoon through Sunday. Although there is uncertainty into where the rain-snow transition will occur, there is a high chance (50-90%) of exceeding 8 inches of snow from the Poconos eastward through much of interior southern New England. Locally more than 12 inches is possible at higher elevations. --Impacts from heavy, wet snow and wind The combination of heavy, wet snow and gusty winds from northeast Pennsylvania through much of southern and central New England will result in difficult travel, and may lead to some 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, particularly for the Sunday morning high tide cycle. --Ice Impacts to Travel in Southern Appalachians Along and east of the Southern Appalachians, accumulations of ice in excess of 0.1 inches, are likely (70-90%) with locally higher accumulations possible. This icing, along with some areas of sleet, may produce hazardous travel conditions on Friday Night and Saturday.