Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 326 AM EST Sat Feb 25 2023 Valid 12Z Sat Feb 25 2023 - 12Z Tue Feb 28 2023 ...California into Southern Nevada & Northern Arizona... Days 1-2... The unusually deep and cold upper low (NAEFS shows <1st climatological percentile 500mb heights and 850-700-500mb temps through Saturday morning) off the southern California will continue to dawdle along the coast before finally picking up speed and moving into the Desert Southwest Saturday night. The corridor of best 850mb moisture and strongest 500mb PVA will move through southern California and the Lower Colorado River Valley into southern Nevada today, which also with the falling snow levels will support heavy snow not only in the usual areas >5,000 ft in the southern Sierra Nevada and Transverse Ranges, but as low in some areas as 3,000 ft. The strong southerly low level flow is ideal for strong topographic enhancement in the southern Sierra Nevada, the Transverse Range, as far south as the Laguna Mountains, and as far north as the mountains north and west of Las Vegas (such as Mt. Charleston). Winds will also remain quite intense with 850mb winds of 40-50 kts, not only aiding in the increased topographic enhancement of snowfall rates, but also resulting in blizzard conditions for some areas. The WPC Snowband Probability Tracker continues to suggest hourly snowfall rates of 2-3"/hr are possible in elevations >5,000 ft along the Transverse Range and in the southern Great Basin through the daytime hours on Saturday. Latest WPC experimental PWSSI depicts 60% chances for Major overall winter storm impacts on Saturday in the Transverse Range, portions of the southern Sierra Nevada, and in the higher terrain of southern Nevada. The components driving the PWSSI in these areas is a combination of Snow Amount and Snow Rate. Hazardous travel conditions, as well potential for downed trees and power lines will continue in these areas through Saturday. Conditions will improve by Saturday night and into Sunday. As the upper low moves east Sunday morning, the corridor of moist 850mb southerly flow will be directed at the Mogollon Rim, far northern Arizona, and into southern Utah. Latest WPC PWPF shows 60-80% probabilities for >6" in these areas with the Mogollon Rim lying on the higher side of the range mentioned. WPC experimental PWSSI shows similar 60-80% probabilities for Moderate impacts in these areas on Sunday. The upper low will take off east from the Four Corners region on Sunday and head for the Great Plains by Monday where it will become the next winter storm to impact the Midwest and Great Lakes. ...Upper Midwest/Great Lakes to the Northeast... Days 2-3... Speaking of the upper low, it will continue to maintain an anomalously deep signature as it enters the Nation's Heartland Monday morning. The 850mb low in particular is quite intense and is responsible for a deep fetch of 850mb moisture flux and a robust 60-70 kt LLJ over the MS River Valley. Meanwhile, there is a cold area of high pressure over southeast Canada, but it will continue to move east. This is leading to an air-mass that is considerably less conducive for widespread heavy snow than the prior winter storm in the Midwest. Instead, this storm will have to rely more on dynamic cooling aloft and residual low-level sub-freezing air in areas with fresh snow pack. The LLJ will already be a problem when it comes to precip type, as that strong of a LLJ and a departing Canadian high pressure system will undoubtedly lead to a deep warm nose at/above 850mb. Given the lingering snow pack over the Midwest, there is an opportunity for accumulating ice from the Twin Cities metro on east to central Michigan. Latest WPC PWPF does contain 60-70% probabilities for >0.1" of ice late Sunday night into the day on Monday in these areas, with even some 20-30% probabilities for >0.25" of ice accumulation in central Wisconsin. Looking at snowfall, the WPC WSE shows the footprint with 24-hour mean snowfall >4" on Monday encompasses the Minnesota Arrowhead and much of the northern Great Lakes, with the best odds for >6" amounts in the Michigan U.P.. These areas will reside favorably to the north of the the 850mb low track across the Great Lakes, which also supports northeast flow over Lakes Superior and Huron, resulting in some modest lake enhancement as well. The latest WSO does show some >30% probabilities for snowfall exceeding warning criteria. Farther east, the position of the cold high pressure area to the north is a little more supportive for a CAD signal over the Northeast. As the 850mb low approaches from the east on Monday, a coastal front looks to form along off the coast of Long Island and will eventually result in a secondary wave of low pressure by Tuesday morning somewhere near Nantucket. Strong WAA in the 850-700mb layer will run into a sub-freezing air-mass over the Northeast and foster periods of snow throughout much of the region. The typical interior areas, particularly the Adirondacks, Catskills, Berkshires, and the Green Mountains, have 40-60% odds of seeing snowfall >6" on Monday according to WPC PWPF. The most unclear area is along the southern New England coast where they will be more reliant upon the development and speed of the developing coastal low Monday night. The initial front-end thump of snow via WAA aloft can allow for several inches of snow to accumulate Monday evening. If the coastal low forms sooner and tracks slower along the coast, a steady CCB on the northern side of the low can boost snowfall totals late Monday night into Tuesday. If it forms too late or is too fast, then there will not be enough moisture to work with before a dry slot within the 700-300mb layer arrives Tuesday morning. These details will take a few more model cycles to be resolved on seeing which scenario is most plausible. ...Pacific Northwest to northern and central California... Days 2-3... A large upper level North Pacific ridge will carve out a longwave trough over the Pacific Northwest and eventually on south to California starting Sunday and lingering into next week. As the trough deepens over the West Coast, several impulses are set to bring more heavy mountain snow to the Cascade Range, the Olympics, the Coastal Range, and the Sierra Nevada beginning Saturday night and lasting into early Tuesday morning. The first storm system arrives Saturday night in western Washington delivering a slug of Pacific moisture and lowering snow levels across western Washington, western Oregon, and northern California by Sunday. Latest WPC PWPF does show high chances (at least 70% chances) for >12" of snowfall in the Olympics, Cascades, and as far south as the Trinity/Shasta and northern Sierra Nevada through Sunday. The cold frontal passage will also result in plummeting snow levels to the point where some light snow accumulations are possible along the I-5 corridor from Seattle on south to the Willamette Valley by Saturday night. Then, a second and more vigorous upper level disturbance arrives in the form of a closed 500mb low off the Pacific Northwest coast early Monday morning. This system will feature not only a healthier fetch of Pacific moisture, but also feature <10th climatological percentile heights (700-500-200mb) and temperatures (700-500mb) Monday into early Tuesday. This is a recipe for higher SLRs and heavier snowfall rates, particularly from the Oregon Cascades on south through the Siskiyou, Salmon, Trinity/Shasta, and down the spine of the Sierra Nevada. Snowfall will be measured in feet along these mountain ranges on Monday with 60-80% chances for snowfall amounts >18". The Day 3 WSSI is already sporting Major impacts in these ranges, indicating considerable disruptions to daily life are expected in these areas. Dangerous to impossible travel is possible in these areas with whiteout conditions possible due to a combination of >2"/hr snowfall rates and/or gusty winds. Mullinax ...Key Messages for California Winter Storm... -Significant mountain snows are likely into Saturday with major impacts to travel and infrastructure. Downed trees and power lines are possible. Lower snow levels may bring significant snowfall to areas where snow is unusual. -Strong winds will lead to blizzard conditions in some mountainous terrain and passes and may lead to power outages and tree damage into lower elevations. Blowing dust is possible in desert areas. -Heavy rain may lead to flooding of some rivers/streams and flood-prone areas. Strong rip currents and high surf will be hazardous to swimmers and beach goers. -Temperatures will be well below normal over much of the region with sub-freezing morning lows likely along the coasts of Oregon, northern California, and central California.