Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 257 PM EST Thu Feb 23 2023 Valid 00Z Fri Feb 24 2023 - 00Z Mon Feb 27 2023 ...Powerful Winter Storm to Bring Significant Heavy Snow, Damaging Winds, & Exceptional Cold to the West Coast... ...Northeast... Day 1... The ongoing winter storm across the Great Lakes and Northeast will continue for another 12-24 hours, as low pressure currently over the Great Lakes slides eastward toward New England tonight. Another and north of the low pressure track, another round of locally heavy snowfall is expected across interior New England, particularly across Maine where an additional 4-6" of accumulation is expected based on the latest WPC probabilities (70-80% for 4", 20-40% for 6"). South of that area, another round of freezing rain and sleet is likely across the interior Northeast this evening and tonight as another surge of 850mb moisture overruns lingering sub-freezing temps over the region. Due to the multiple rounds of freezing rain, WPC PWPF shows parts of the Berkshires and Green Mountains with the highest probabilities (25-30%) for ice accumulations > tenth of an inch. WPC is continuing Key Messages for this storm system as it winds down over the next 12-24 hours, see additional details below in the Key Message bullets. ...Western U.S... Days 1-3... The deep longwave trough over the West will begin to pinch off a closed 500 mb low that drops southward along the California coast over the next few days. A steady stream of Pacific moisture will coincide with anomalously cold temperatures throughout the depth of the 850-500mb layer (NAEFS shows <1 climatological percentile at 850-700-500mb levels through tonight). As the upper trough continues to plunge south just off the CA coast into Friday, an increase in PVA and WAA ahead of the 850mb front will escort 40-50 kt 850-700mb winds out of the SW into CA. With temperatures plummeting in wake of the frontal passage, and snow levels rapidly falling, this combination of strong vertical ascent via both favorable upper level profiles and strong terrain-induced lift via upslope flow will result in periods of heavy snow from the Coastal Range and Sierra Nevada on south along the Transverse Range. The period of best ascent occurs Friday afternoon and into Friday night when the upper low taps into a richer source of Pacific moisture and winds within the 850-700mb layer are at their strongest. The intense vertical lift within the mid-upper levels results in heavy snow as far inland as southern Nevada thanks to southerly 850mb winds as strong as 35 knots ushering in additional moisture flux into the Lower Colorado River Valley. The upper low will continue to produce periods of heavy snow along the Sierra Nevada, the Transverse Range, and into the Santa Rosa and Laguna Mountains east of San Diego on Saturday, as well as the southern Great Basin on Saturday. As the next upper trough approaches the Pacific Northwest, the upper low will pick up the pace and track into the Desert Southwest Saturday night. This will place the best vertical motion and moisture fetch oriented towards the Mogollon Rim and southern UT where heavy snow is expected. Impressive snow totals are expected for much of the California given the impressive cold temperatures aloft, steady diet of Pacific moisture, and optimal 850-700mb winds aiding in the causing upslope snowfall rate enhancement. Given the highly anomalous setup, parts of the southern Great Basin can also expect this major winter storm to produce significant impacts. Snow will be measured in feet for many of CA's mountain ranges but the most anomalous snowfall resulting in extreme impacts are most likely to occur in the Transverse Range and on south to the Santa Rosa and Laguna Mountains. The latest Winter Storm Severity Index (WSSI) shows a large area of major to extreme impacts from the northwest CA ranges, through the Sierra and across the entire Transverse Ranges. The WSSI in particular stands out because in most cases, 1-2 categories will approach "extreme" at most (usually Snow Amount or Snow Load). In this case, not only do those two categories feature Extreme impacts, but so does the Blowing Snow component. Some winds in these higher ranges will see gusts exceed 60 mph, which combined with the heavy snowfall rates and weight of the snow will result in dangerous to impossible travel conditions, as well as the potential for tree damage and power outages. The experimental PWSSI showed Snow Load being the primary driver for Extreme impacts along the Transverse Range and into the San Bernadinos Friday into early Saturday. See additional details in the Key Message bullets below. By Saturday night into Sunday, another fast moving shortwave trough approaches the Pacific Northwest and combined with the colder low level temperatures in place and abundant moisture brushing the coast, another round of heavy mountain snow and potentially lower elevation snow is expected. The greatest totals look to be across the WA Olympics and the WA/OR Cascades where WPC probabilities for 8" are already above 40-60 percent on Day 3. Mullinax/Taylor ...Key Messages for Major Coast-to-Coast Winter Storm... -A last round of snow and freezing rain will progress through the Northeast overnight. Roads may be slippery as temperatures continue to fall. -Frigid temperatures over the Upper Midwest will move through the Great Lakes and into the Northeast by the start of the weekend. At times, wind chills will be well below zero over the Great Lakes and near zero into the Northeast. -Gusty winds this evening may create blowing and drifting snow with reduced visibility, especially in open and rural areas. ...Key Messages for California Winter Storm... -Significant mountain snows are likely tonight into Friday 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. -Windy conditions through the state 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.