Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 AM EST Thu Feb 23 2023 Valid 12Z Thu Feb 23 2023 - 12Z Sun Feb 26 2023 ...Major Winter Storm to Continue Causing Significant Impacts From the Great Lakes to the Northeast Today... ...Powerful Winter Storm to Bring Heavy Snow, Damaging Winds, & Exceptional Cold to the West Coast... ...Upper Midwest into the Northeast... Days 1-2... The ongoing winter storm across will continue to generate periods of heavy snow from the Upper Midwest to northern New England today. In the Midwest, the upper level shortwave trough swinging through the region is the primary feature driving the wave of low pressure responsible for the swatch of snow from the Dakotas to the northern Great Lakes. On the northern and western side of the 850mb low, snow will come down heavily at times while wind gusts up to 30-40 mph aid in producing blizzard conditions through Thursday morning in the eastern Dakotas and southern MN. This is also the case along the shores of the MN Arrowhead, and the coast of northern WI and the U.P. of MI where the easterly 850mb cold conveyor belt aloft will allow for some lake enhancement and winds coming off the friction-less surface supports similar wind gusts. The latest HREF does depict some modest probabilities (30-50%) for >1"/hr snowfall rates this morning in these areas. The surface low will track into the Great Lakes during the afternoon hours while the shortwave trough also begins to flatten, resulting in a weakening of the storm system. Despite this, the Arctic air-mass to the north with cyclonic ENE winds on the back side of the storm will keep period of heavy snow in the forecast across the northern Great Lakes. The winter storm will wind down over the Midwest past midday as drier air wraps around the system. The expectation is for the U.P. of MI to pick up another 6-12" of snowfall today with several more inches possible in the Upper Midwest before snow concludes there around midday. Farther east, a wave of low pressure tracking off the Northeast coast will develop along a frontal boundary separating exceptionally mild/moist air to the south and frigid/fry air to the north. Strong isentropic glide aloft and associated 850mb WAA fostered periods of heavy snow over much of northern New England. This burst of heavy snow will be short lived as a dry slot in the 700-300mb layer moves in later this morning and into the early afternoon hours, so snow is expected to wind down as the dry air aloft moves in and the coastal low races east into the north west Atlantic. The remnant 850mb low over the Midwest still must track through New England Thursday night, bringing one more round of snow to the region more it too races out to sea Friday morning. By the time it is all said and done, another 4-8" of snowfall is likely across far northern NH and much of ME with perhaps a burst of snow possible over southern New England Friday morning as the Arctic front moves through and potentially kicks up some ocean effect snow in the MA Capes. Along the frontal boundary, an icy wintry mix will continue to cause treacherous travel conditions. While not as much as observed late in the day on Wednesday, up to a tenth of an inch remains possible Thursday morning from central MI on east into the interior Northeast. Central MI will continues to contend with ice via freezing drizzle into the morning and midday timeframe thanks to persistent moist easterly flow. As the surface low moves through, a cold front will pass through and allow for any lingering precip to change over to light snow. This same wave of low pressure will bring another round of freezing rain and sleet to the interior Northeast Thursday evening 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 (50-70%) for ice accumulations > tenth of an inch. Wind gusts above 30 mph are expected in wake of the cold frontal passage (both across MI and the interior Northeast), so tree branches and power lines could be brought down due to the added stress from both the weight of ice and the added force from strong wind gusts. WPC is generating Key Messages for this major winter storm. See additional details in the Key Message bullets below. ...Western U.S... Days 1-3... A longwave trough over the West with a strong 150+ kt 250mb jet streak will continue to support periods of heavy snow along the Sierra Nevada and across the higher terrain of the Four Corners region today. Meanwhile, a ridge of high pressure breaking over the British Columbia will prompt an upper level disturbance rounding the western flank of the deep upper trough over the West to dive south along the West Coast to further amplify, forming a closed 500mb low along the OR coast. 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 Thursday night). As the upper trough continues to plunge south just off the CA coast Thursday night and 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. In terms of totals, impressive quantities of snowfall are expected for much of the Golden State's mountain ranges 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 WSSI Snow depicts a remarkable swath of Major impacts that stretches from southwest Oregon on all the way south along the Coastal Range of CA, the Sierra Nevada and Trinity/Shasta/, into southwest NV, southern UT, and northern AZ. The Extreme impacts include a large extent of the Sierra Nevada, the Coastal Range and Transverse Range between the Bay Area and north of the Los Angeles/San Diego metro areas. 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. Mullinax ...Key Messages for Major Coast-to-Coast Winter Storm... -Periods of heavy snow will continue this morning from the Midwest and northern Great Lakes to the Northeast. Snow will begin to wind down across the Upper Midwest by Thursday afternoon. -Heavy snow rates of 1"/hr and wind gusts of 30-40 mph this morning will create blizzard/whiteout conditions, nearly impossible travel, and may result in additional power outages and tree damage. -A stripe of freezing rain is also on tap across parts of the Upper Midwest, Great Lakes, and Northeast. Some localized ice accumulations greater than 0.25" are possible, especially in portions of western and central Massachusetts. This will lead to treacherous travel conditions and scattered power outages in the region of heaviest freezing rain. -Frigid temperatures will stick around in wake of the storm system as snow and ice both conclude by Friday morning. Wind chills at times will be below zero from the Upper Midwest to the interior Northeast both Friday and Saturday mornings. ...Key Messages for California Winter Storm... -A dynamic winter storm will bring in a surge of moisture through the end of this week as cold temperatures remain over the region. -Heavy rain and prolific mountain snow are likely today into Friday with significant impacts to travel and infrastructure. 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. Blowing dust is possible in desert areas. -Temperatures will be well below normal over much of the region with below freezing readings possible in some coastal areas in Northern California. -Strong rip currents and high surf will be hazardous to swimmers and beach goers.