Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 335 PM EST Fri Feb 24 2023 Valid 00Z Sat Feb 25 2023 - 00Z Tue Feb 28 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... An unusually strong upper level low will move into Southern California on Saturday, bringing a period of extreme weather to the region. Both the mid-level heights and temperatures are below the CFSR climo period (1979-2009) for this time of year (-3.5 to -4 sigma), and lead-in temperatures are already cold. A modest moisture plume ahead of the upper low and surface cold front will drive a strong moisture flux into the Sierra and Transverse Ranges this evening into early Saturday, with IVT values 200-500 kg/m/s. South to SSW flow at 850-700mb (40-60kts between the levels) will maximize orographic ascent into the terrain with multi-inch liquid equivalent QPF and at rates of up to 2-4"/hr. Blizzard conditions are likely in areas that include some passes. Snow levels will rise ahead of the warm/moist surge then fall post-FROPA as the cold-core upper low moves in. Snow-to-liquid ratios will be low, likely below 10:1, resulting in high snow loads. These ingredients will drive prolific snow amounts for the terrain, even down to lower than usual levels, over Southern California that will translate eastward into Saturday. The upper low will weaken as it moves into the Four Corners D2, spreading light to modest snow over the Mogollon Rim and across southern NV/UT into the San Juans in CO as both the available moisture and upper dynamics wane. WPC probabilities of at least 8 inches of snow D1 are >50% above about 3000ft with several feet likely above 4500-5000ft outside Los Angeles. 1-2 ft is likely in the southern Sierra, with less farther north. To the east, WPC probabilities for at least 8 inches are >50% along the CA/NV border into the central/southern NV terrain. By D2, probabilities for at least 8 inches are >50% over the Mogollon Rim. WPC's Key Messages for this winter storm are below. ...Pacific Northwest & Northern California... Days 2-3... A pair of shortwaves will dive into the Pacific Northwest D2-3 as troughing remains favored in the West. First cold front will push inland Sunday morning and the next on Monday afternoon. Each will have only a modest moisture surge with PWs barely above average late-February values, but temperatures will remain below normal in the region, favoring lower than normal snow levels that will vary between 1000-2000ft and at times to near 500ft into D3 as colder air is reinforced. Upslope enhancement will favor the Cascades, Olympics, and Coastal Ranges as well as into Idaho as some moisture carries eastward. By D3, the focus will be farther south as height falls dig more through NorCal and also through the Great Basin, promoting modest snowfall for the Wasatch into the Tetons. Two-day totals will likely be above 12" in much of the higher terrain but with significant snow for many pass levels due to low snow levels. Some lowland snow is likely for many areas in the Northwest, some of which may accumulate a few inches. WPC probabilities for at least 4 inches of snow around 1000ft are around 30-50%. ...Upper Midwest/Great Lakes... Day 3... Intense upper low through SoCal D1 will lift into the Corn Belt by early Monday (D3) as a compact upper low. Surface cyclogenesis over southeastern CO will move quickly to the Great Lakes with a precip shield developing into and around low pressure as Gulf moisture gets drawn northward. WCB will bring light to moderate snow to eastern MN across much of central/northern WI and into the U.P. and northern Lower Michigan, just north of the warm front and on the north/northwest side of the surface low. Additional snow is likely into the Northeast D4. WPC probabilities of at least 4 inches of snow through 00Z/28 are low to moderate, generally <60%, in the Upper Midwest to northern Great Lakes. Fracasso ...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.