Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 322 AM EST Fri Feb 24 2023 Valid 12Z Fri Feb 24 2023 - 12Z Mon Feb 27 2023 ...Powerful Winter Storm to Bring Significant Heavy Snow, Damaging Winds, & Exceptional Cold to the West Coast.. ...California into Southern Nevada & Northern Arizona... Days 1-2... A potent upper level low located off the coast of California is the catalyst for what will be an extraordinary period of wicked winter weather in the Golden State and into southern Nevada today and into Saturday. NAEFS depicts 500mb heights off the California coast on Friday are progged to fall outside the lowest observed 500mb heights for the time of year based off CFSR climatology (1979-2009). That is also coinciding within similarly anomalous temperatures at mid-upper levels that will swing over Southern California by Saturday. Along with the deep upper low and anomalously cold temps aloft, there will be an impressive moisture fetch from Friday afternoon into the day on Saturday. NAEFS shows integrated vapor transport values >500 kg/m/s embedded within SW flow and aimed at the southern Sierra Nevada, the Transverse Range, and into the southern Great Basin. Expect 850mb winds will rage between 40-50 kts and 700mb winds as strong as 60 kts, winds that according to NAEFS through Friday night that are above the 90th climatological percentile. The southerly direction of the 850mb flow also favors strong orographic ascent into the southern Sierra Nevada and the Transverse Ranges, further maximizing precipitation rates in these mountain ranges. With such unusually cold temperatures, strong winds, and ample moisture to work with, the stage is set for prolific snowfall rates and blizzard conditions in the mountainous terrain of California and southern Nevada. In terms of impacts, snowfall amounts and rates combined with whipping winds in the mountain ranges will make for a treacherous trifecta of hazards both Friday and Saturday. Speaking to amounts, snow will be measured in feet along the Sierra Nevada, in southwest Nevada, along the Transverse Range, and as far south as the Laguna Mountains east of San Diego. This is also the case along the Coastal Range south of the Bay Area for areas with elevation >2,000 feet. It was telling to this WPC winter forecaster seeing the PWSSI showing >70% odds of Extreme impacts for Overall Winter storm Impacts, with the majority of this driven by the Snow Load and Snow Amount components of the product. While it is still experimental and relatively new, there have been very few instances with probabilities that high in the Extreme criteria. Snowfall rates will be intense with WPC's Snowband Probability Tracker, utilizing the 00Z HREF, showing all 10 model members depicting 2-4"/hr snowfall rates in the Sierra Nevada and Transverse Ranges Friday morning and into Friday night. Some members show 2"/hr snowfall rates in southwest Nevada as well Friday night in the higher terrain to the north of Death Valley. The winds cannot be understated either with some of the mountain tops seeing wind gusts over 60 mph in some cases in the southern Sierra Nevada, Transverse Ranges, and San Bernadino Mountains. Eventually by Saturday night, the upper trough will make its way over the Desert Southwest and direct its supply of moisture and cold temperatures aloft over southern Nevada, southern Utah, and northern Arizona where heavy snow is on tap along the Mogollon Rim, in the mountains north and west of Las Vegas, and as far east as the San Juans. All this information above is to say that there will be no shortage of significant impacts, even at lower elevations than normal given the much lower snow levels arriving Friday night and into Saturday. The WSSI shows an astonishing footprint of Extreme impacts that extend from the Shasta/Trinity mountains of northern California on south along the entire spine of the Sierra Nevada, throughout the higher terrain of the southern Great Basin, and along the Transverse Ranges of southern California. Blizzard/whiteout conditions contributing to zero visibility at times is expected in these areas which will inevitably lead to some road closures and impassable roadways. Strong wind gusts >60 mph will result in downed trees and power lines as well. There will also be an increased threat of avalanches in the highest elevations of these mountains ranges. Lastly, abnormally cold temperatures (some record cold temperatures in fact) will stick around through Saturday with the blustery winds aiding in causing bitterly cold wind chills for some in an area not accustomed to sub-freezing wind chills. In summary, this has all the makings of a historic winter storm for much of the mountainous terrain in southern California in particular with residual impacts (downed trees, power lines, road closures) that will likely extend into the upcoming weekend. WPC's Key Messages for this winter storm are below. ...Pacific Northwest & Northern California... Days 2-3... The next storm system in the northeast Pacific looks to slam into British Columbia this weekend and provide another surge of moisture into the Pacific Northwest. This storm system is being driven by another vigorous shortwave trough diving southeast from the Gulf of Alaska. This trough will track inland and bring snow to not only the Cascade Range but also the northern Rockies and northern California. As the cold front swings through Sunday morning, snow levels will drop and SLRs will rise, resulting in periods of heavy snow in these three regions during the day Sunday. Meanwhile, a second upper level disturbance will be hot on the initial shortwave trough's heels and become a more intense upper low off the Pacific Northwest coast early Monday morning. There remains some uncertainty on positioning and timing of the upper low, but it will provide yet another round of heavy snow to the Shasta/Trinity and Sierra Nevada early next week. WPC PWPF for Sunday shows a large swatch of 50-60% probabilities for snowfall >8" in the Olympics, the Cascade Range, the Coastal Range of southwest Oregon and northwest California, and along both the Shasta and northern Sierra Nevada. The best odds of >12" are in the Washington Cascades and northern Sierra Nevada, featuring as high as 60-70% odds according to WPC PWPF. Farther east, the Bitterroots and Sawtooth of Idaho, the Tetons of Wyoming, and the Wasatch of northern Utah all contain at least 40% chances for >6" of snowfall. Latest WPC WSSI does include Moderate impacts along the Cascade Range of Washington and Oregon, with some Major impacts highlighted in the Olympics. Expect treacherous travel conditions in these areas from both potentially snow covered roads and heavily reduced visibilities. Mullinax ...Key Messages for California Winter Storm... -Significant mountain snows are likely tonight into early 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 below freezing readings possible in some areas of Northern California.