Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 PM EST Sat Feb 11 2023 Valid 00Z Sun Feb 12 2023 - 00Z Wed Feb 15 2023 ...Southern & Central Appalachians... Days 1-2... An anomalous and potent 500mb low traversing the Mid-South will work in tandem with a pair of 250mb jet streaks to support ample upper level divergence over the Southeast. The result is a deepening surface cyclone over southern Georgia this evening that will occlude and form a secondary wave of low pressure on the triple point along the Carolina coast tonight and into Sunday morning. The storm system and its track that more often than not, with sufficient sub-freezing wet-bulb temperatures present, would be a significant winter storm for much of the Mid-South and into the Mid-Atlantic. However, this is not the case as boundary layer temperatures are unusually mild for mid-February, making this particular winter storm setup have more March-April like characteristics. This is the say that the more elevated areas of the southern and central Appalachians are favored to receive heavy snow, while most low lying/valley areas witness a cold rain (although some snow mixing in cannot be ruled out). The 850mb low track will be just south and east of the Southern Appalachians with a strong 50 knot easterly LLJ ushering in a rich combination of the Gulf and Atlantic moisture. Between both the strong easterlies at 850mb (NAEFS shows <1 climatological percentile 850mb zonal wind between 06-12Z Sun) and the cold temperatures within the 500-700mb layer beneath the upper low, temperatures will be sufficiently cold to produce heavy snow in portions of the Southern Appalachians, more specifically areas residing above 2,500ft in western NC and southwest VA. Snowfall rates will also be quite heavy with a good chance for >1"/hr snowfall rates. 12Z HREF showed probabilities for >1"/hr snowfall rates as high as 60-70% stemming from western NC and southeast VA between late Saturday night and into Sunday morning, then up the spine of the Blue Ridge and central Appalachians by Sunday afternoon. Latest WPC PWPF showed moderate chances (30-50%) for snowfall totals >4" tonight and into Sunday. The latest experimental WPC PWSSI showed up to a 60% chance for Moderate WSSI impacts in these areas with Snow Load being the primary driver for potential impacts. This makes sense given the meager SLRs in place and the exceptional moisture content aloft, resulting in a heavy/wet snow. Expect treacherous travel conditions in the southern Appalachians with some spotty power outages possible where the weight from the heavy/wet snow potentially weighs down on tree branches and power lines. Upper level lows like this and during this time of year can cause some surprise areas of snowfall and ice. Current forecast calls for up to maybe a couple of inches in the Cumberland Plateau and as far north as the western VA Blue Ridge, but should the subfreezing temps stick around longer or dynamic cooling be more proficient here, these areas could see some locally heavier amounts. In terms of ice, WPC PWPF is depicting moderate chances (40-60%) for >0.1" of ice accumulation in southwest VA and into the central Appalachians of western VA and eastern WV. Slick travel conditions are possible in these areas, although road temperatures have warmed in recent days, which should keep impacts to more localized areas. ...Four Corners region... Days 2-3.. An upper low is set to approach the region from the southwest as it tracks east from northern Baja California. Out ahead of the upper low, strong vertical ascent combined with an influx of 850-700mb moisture and sufficiently cold temperatures in the mountain ranges will support periods of snow Monday and into Monday night. Latest WPC PWPF shows >60% probabilities for snowfall >6" in the San Juans of southeast CO and northern NM, the Sangre De Cristo of northern NM, and the White Mountains of east-central AZ. The experimental WPC PWSSI shows the highest probabilities (roughly 40-60%) for Moderate impacts focused in the San Juans with snow rate being the primary driver in potential impacts. By Day 3, an amplifying upper trough diving south through the Pacific Northwest acts to provide exceptional lift within the column over the Great Basin and into UT, western CO, and northern AZ. Timing differences remain amongst deterministic guidance, but all guidance agrees on a strong cold front traversing the Four Corners region by Tuesday afternoon, leading to plummeting snow levels and rising SLRs. The experimental WPC PWSSI is showing probabilities ranging between 30-50% for Moderate impacts Tuesday afternoon and evening from the mountain ranges of southwest UT, north-central AZ, and the San Juans of southwest CO. These areas are also sporting >60% probabilities for snowfall >6" according to the WPC PWPF. Expect hazardous travel conditions in the higher terrain of the Four Corners region Tuesday night and into Wednesday morning as the upper trough makes its way overhead. ...Pacific Northwest, Northern & Central Rockies... Days 2-3... ...Strong upper trough to kickoff periods of heavy snow from the Northwest to the Four Corners Region early week... A deepening upper trough is forecast to track into the Pacific Northwest early Monday morning with crashing snow levels in wake of a strong cold frontal passage. Snowfall in the Cascades and Olympics will initially start from the cold front, but then post-frontal westerly flow containing a steady diet of 850mb moisture flux will keep upslope-enhanced snowfall in the forecast into Monday evening. The cold front and the rich plume of Pacific moisture trailing it continues their push south and east, tracking through northern ID, the Great Basin, and into the Northern Rockies Monday night into Tuesday morning. The cold temperatures in the 500-700mb layer are quite anomalous for mid-February as depicted by NAEFS which showed some temperatures <1% climatological percentile. With sufficient moisture present, strong vertical ascent in the form of PVA out in front of the trough, and the cold front racing through, heavy snowfall rates with SLRs in the 14-18:1 range are expected. Latest WPC PWPF shows high probabilities (>70%) for snowfall totals >12" in the WA and OR Cascades on Monday. Moderate chances (40-60%) for snowfall >6" extended as far south as the Siskiyou Mountains of southwest OR and the Salmon mountains of northern CA. Farther east, WPC PWPF probabilities for >8" are highest in the Bitterroots of northern ID, the Absaroka of southern MT, and both the Little Belt and Big Snowy mountains of MT where probabilities are as high as 70%. In terms of impacts, the experimental WPC PWSSI showed a widespread swath of >80% probabilities for Moderate impacts from southwest OR on north along the Cascade Range on Monday, including the more traveled passes of the WA Cascades. There are also some embedded 40-60% probabilities for Major impacts in the central OR Cascades. It is here where the worst impacts could become more extensive due to the amount of snowfall forecast (>20" possible). This storm system, which will dive south into the Four Corners on Tuesday, ejects into the Nation's Heartland by mid-week where it will become a disruptive winter storm from the high plains of CO to the Great Lakes. WPC has initiated Key Messages for this potentially hazardous winter storm given the growing probabilities in the WWO in recent forecast cycles. The Key Messages can be accessed on our home page and bullet points can be found below. Mullinax ...Key Messages for Feb 14-17 Winter Storm... --Confidence is increasing that a low pressure area developing near the Four Corners on Tuesday will spread a swath of heavy snow northeast into the Upper Midwest by Thursday. This system may continue into the Great Lakes on Friday. --A corridor of mixed precipitation, including sleet and freezing rain, is also possible along this storm track. --The combination of heavy snow and mixed precipitation, especially where combined with strong winds, could result in hazardous travel conditions and impacts to infrastructure. --Uncertainty remains with the timing and location of the storm track, which determines where the axis of heaviest precipitation and most significant impacts will occur. --Much colder temperatures, falling to well below normal values, will follow in the wake of this system for the middle to latter part of next week.