Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 406 AM EST Wed Mar 01 2023 Valid 12Z Wed Mar 01 2023 - 12Z Sat Mar 04 2023 ...California through the Four Corners region... Days 1-2... The onslaught of heavy snow across much of California will soon conclude, but not before one more day of heavy snow today from the southern Sierra Nevada and Transverse Ranges due to an amplifying upper trough over the southwestern U.S. It is a classic setup for copious amounts of impactful snowfall from the southern Great Basin to the Four Corners region: barreling and highly anomalous 500mb low (NAEFS ensemble tables shows heights below the CFSR climatology over the Lower CO River Valley 00Z Thursday) and strong 250mb jet streak dynamics combined with an IVT that above the 90th climatological percentile aimed at the Four Corner region today. In addition, temperatures at the 500-700-850mb high levels are also approaching climatological minimums and while 35-50 kts of 850-300mb mean southwesterly winds provide additional upslope enhancement along many mountain ranges. This is a recipe for a major winter storm in the higher terrain of the Southwest. Latest WPC PWPF shows high probabilities (70-80%) for >12" of snowfall in the San Bernadino Mountains, around Zion National Park in southwest UT, the Mogollon Rim and Kaibab Plateau of northern AZ, and the San Juans of southwest CO. The footprint of >8" probabilities (50-70% odds) grow to the southern Sierra Nevada and San Jacinto Mountains of southern CA, the Wasatch, more of the higher elevations of western CO and northern NM. The Day 1 WSSI shows a large footprint for Major impacts across many of these regions, with some Extreme impacts depicted over the ranges of southern CA, Zion National Park, the Mogollon Rim, and the San Juans. The WSSI is suggesting Snow Amount, Snow Load, and Blowing Snow as the primary drivers, the latter of which is also due to wind gusts as strong as 35-50 mph. These winds combined with hourly snowfall rates of 1-2"/hr on Wednesday will cause whiteout and even blizzard conditions at times. As the cold front swings through the Desert Southwest, lower elevation snowfall is also possible Wednesday evening. This is especially the case in southeast AZ where some mountains could see 6-12" of snow, and lighter accumulations at lower elevations. Lapse rates along the approaching cold front will be steep enough to support instability out ahead of the front. CIPS snow squall parameters are quite high across the Southwest, so there is the potential for an extended snow squall line that could traverse southern UT, much of AZ, and into western NM. Visibilities could drop to zero as soon as the snow squall arrives and whipping wind gusts would cause whiteout conditions and drifting snow on roadways. Motorists in these portions of the Southwest should be prepared for hazardous travel conditions as the snow squall moves through. As the upper low passes through AZ and eventually into NM early Thursday morning, snow will breakout across the San Juans of northern NM and along the Sangre de Cristo of northern NM and southern CO. Snowfall totals there are forecast to approach a foot and the WSSI does contain some Moderate impacts along these ranges through early Thursday morning. WPC is still creating Key Messages for the winter storm in the Southwest and can be found at the bottom of this discussion. Snow will continue to fall Thursday AM as upslope flow via ENE flow reaches the Sangre de Cristo, but will finally diminish by Thursday afternoon. ...Upper Midwest to Maine... Days 1-2... A potent wave of low pressure associated with a closed 500mb low traversing the Upper Midwest will produce heavy snow from the eastern Dakotas and northern MN to northern WI and the Michigan U.P.. The area of snowfall will occur to the north of an 850mb front where sub-freezing temperatures are present. Sufficient moisture supplied by southeasterly 850mb flow will allow for periods of snow to occur with snowfall totals of 3-6" expected. The upper low will open up and the troughing pattern aloft will flatten out, leading to a diminishing precipitation shield as it reaches the northern Great Lakes. WPC PWPF does suggest there are 40-60% probabilities for >4" of snowfall, but totals overall are low enough that the WSSI is keeping impacts topped out at Minor for Day 1. Farther east, a fast moving 250-500mb shortwave trough will eject over the Mid-Atlantic and help to spawn a wave of low pressure off the Northeast coast. This feature will have both Atlantic moisture and residual moisture from the Upper Midwest system to work with, but will be progressive. Periods of snow will track over northern New England and lead to a plowable snow there. In fact, the WPC PWPF puts the odds of receiving >4" of snowfall over northern ME at 30-50%, with some low end probabilities (5-10%) for >6". Should this wave of low pressure form up sooner or linger along the coast of ME longer, there could be more widespread amounts of >6" possible through Thursday evening. ...Pacific Northwest... Days 2-3... After a brief break in the busy winter pattern on Wednesday, the next frontal system associated with another upper trough in the northeast Pacific delivers the next round of heavy snow to the Pacific Northwest. 850mb moisture will originally be ushered in due to southwesterly flow ahead of the front Wednesday night, but in wake of the frontal passage, a steady diet of WNW 850mb moisture flux will be oriented at the Pacific Northwest much of the day on Thursday. Pacific moisture will spill over into the Northern Rockies, more specifically the Bitterroots and Lewis Range where WPC PWPF shows 30-50% probabilities for >8" of snowfall through Thursday night. Even as the first initial storm system that brought snow to the region races into the Canadian Prairies Thursday night, another Pacific trough will follow hot on its heels on Friday. This trough will be deeper than the Thursday system with colder temperatures at upper levels approaching by Saturday morning. Throughout this 3-day stretch, it will be the Olympics and the Cascade Range that are getting the brunt of the heavy snow. For Thursday, WPC PWPF depicts 70-90% probabilities for >12" of snowfall in the Olympics and WA Cascades. By Friday, there are 50-70% probabilities on average for >8" in the Olympics and the Washington Cascades with 10-30% probabilities in the northern OR Cascades. The WSSI depicts another expansive area of Major impacts along these ranges through Friday evening some treacherous travel conditions in some passes possible. ...Central Plains, Upper Midwest into the Great Lakes and Northeast... Days 2-3... By early Thursday morning, the upper low in the Four Corners region will be making its way into the southern High Plains. The 500mb low is astonishingly deep for March 1 as NAEFS by 12Z Thursday features heights below the CFSR climatology. In response to lee cyclogenesis over northeast NM, easterly 850mb moisture flux and 850-700mb WAA will occur over southern KS and northern OK as a by product of both vertical ascent through mesoscale dynamics and upslope flow into the High Plains and the Sangre De Cristo Mountains. Latest WPC PWPF does show 30-50% probabilities for >4" of snowfall in the high plains of southeast CO, northwest KS, and southwest NE with 40-60% probabilities for >8" in the Sangre De Cristo of southern CO and northern NM through Thursday. As the upper low and strong vertical ascent from a robust 150 knot jet streak transpire in the Southern Plains, it will generate a rapidly strengthening cyclone over the ArkLaTex Thursday evening. The 850-700-500mb heights will deepen quickly and colder temperatures will unfold at these levels via both strong vertical ascent and increasing precipitation rates into Friday morning. Precipitation is able to develop due to a strong warm conveyor belt of moisture in the 850-700mb layer intersecting the warm front and wrapping around the northwest side of the 850-700mb low, forming a classic TROWAL over eastern OK/KS and into the Midwest. One factor complicating the snowfall forecast is the lack of adequate sub-freezing temps in the boundary layer due to a retreating area of Canadian high pressure over Quebec. This makes snowfall accumulations highly dependent upon both snowfall rates within the atmospheric column and also during time of day, as the month flipping over to March brings about the importance of increasing solar input from a higher sun angle. Utilizing the WPC super ensemble (WSE), current guidance has been keying on an area from southern and eastern IA to southern WI, northern IL, and central MI for heavy snowfall. These areas feature boundary layer temperatures that are colder than their neighbors in the Central Plains, while still being favorably located north and west of the 850mb low where the TROWAL is likely to pass over. The WSE mean shows 4-6" worth of snow there with some totals in the 6-8" range over central MI through Friday. As the upper low tracks into the Ohio Valley Friday afternoon, strong 290K isentropic ascent and intense 850mb WAA over the Mid-Atlantic will prompt heavy precipitation to break out over the northern Mid-Atlantic and the Northeast. Unlike their neighbors in the Midwest, the high over Canada is in a more optimal position for a CAD wedge to form. Despite this, the storm track favors strong SW flow though the column that will lead to a burgeoning warm nose aloft, likely causing any snow over northern PA and Upstate NY to change over to a wintry mix late Friday. From there, snow potential in the Northeast hinges on when and where a coastal low forms along the Northeast coast Saturday morning. Should it form sooner and track south of the southern New England coastline, snow has a chance to remain the dominant precip type through much of the event. The region with the best odds of staying cold enough for snow for the event is central and northern New England. This is where WPC PWPF features the best odds for >8" of snowfall on Friday and into Saturday (roughly 60-80% probabilities for >8", 30-50% for >12" of snow). Given the amount of moisture and the sufficient cold present from the Great Lakes to the Interior Northeast, the experimental PWSSI shows 50-70% probabilities for Moderate impacts Friday into Saturday. WPC has begun Key Messages for this impending winter storm from the Midwest to the Northeast and are listed below. Mullinax ...Key Messages for Southwest Winter Storm... --A winter storm system will produce heavy snow from the southern Sierra Nevada and Southern California ranges to the mountain ranges of the Southwest U.S. today into Thursday. --Snowfall rates of 1 to 2 inches per hour are expected for the southern Sierra Nevada and Southern California ranges through this afternoon. Blizzard conditions will make for dangerous to impossible travel. --Heavy snow with rates of 1 to 3 inches per hour pushes across the high terrain of the southern Great Basin and southern Rockies through Thursday, where hazardous travel conditions are likely. There is the potential for snow squalls and avalanches in some mountain ranges. --If you plan to travel through any mountain passes in California or the Southwest through Wednesday, be prepared for rapidly changing conditions and have winter driving supplies on hand. ...Key Messages for Plains to Northeast Winter Storm... --Confidence continues to increase that a potent winter storm will move from the Southern Plains Thursday to the Great Lakes Friday and then the Northeast on Saturday. --A swath of heavy snow will likely accompany this storm, and it is probable that significant accumulations will occur north of the track of this low pressure. --The combination of heavy snow and strong winds may result in hazardous travel as well as impacts to infrastructure late this week into the weekend. --A corridor of significant mixed precipitation, including sleet and freezing rain, is possible south of the heaviest snow. --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. Keep up to date with the latest forecasts as this storm evolves.