Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 307 PM EDT Tue Mar 11 2025 Valid 12Z Tue Mar 11 2025 - 12Z Fri Mar 14 2025 ...The West... Days 1-3... ...Impactful heavy snow for most California mountain ranges Wednesday into Thursday, then mostly the southern Intermountain West and Rockies late week. Please see current Key Messages linked below... An upper-level trough in the northeast Pacific will dive south towards the West Coast and direct an atmospheric river at California starting Tuesday night and continuing into the second half of the week. The enhanced moisture transport combined with a cold frontal passage and falling heights aloft will result in plummeting snow levels along the Cascade Range on south to the Shasta/Siskiyou tonight, then along the Sierra Nevada through Wednesday and into Wednesday night. By 00Z Thursday, a >400 kg/m/s SWrly IVT (topping out near the 99th climatological percentile via ECMWF SATs) will be aimed at central California at the same time the cold front pushes through "The Golden State". This when snowfall rates will be at their heaviest; Wednesday afternoon into Wednesday night, along the Sierra Nevada. Snowfall rates of 2-3"/hr will be common from 6,000ft and up along the Sierra Nevada at the snow's peak intensity. Farther south, the Transverse Ranges as well as the peaks of the Peninsula Ranges are likely to see heavy snow above 5,000ft through Thursday morning. Some upslope flow will linger into the Sierra Nevada Thursday morning, before a brief lull in the action arrives Thursday afternoon. By Friday morning, yet another Pacific storm system arrives, but with notably higher snow levels compared to the Wed-Thurs storm system. Overall, this is an exceptionally snowy pattern for the Salmon/Siskiyou/Shasta of northern California and along the entire length of the Sierra Nevada. Snowfall will be measured in feet with as much as 4-5 feet forecast in the peaks of the Sierra Nevada. For elevations above 5,000ft in northern California, above 6,000ft in the central/southern Sierra Nevada, and above 6,000ft in the San Gabriel and San Bernadino of southern California, snowfall amounts are likely to range between 1-3 feet of snow. WPC's WSSI shows Major to Extreme Impacts in most of these ranges with Snow Amount the primary impact denoted in the algorithm, but some impacts as a result of Snow Load and Blowing Snow are expected as well. Dangerous to impossible travel are anticipated with the potential for some instances of power outages and tree damage as well. The highly anomalous IVT responsible for the barrage of heavy snow in the California mountain ranges will advect moisture well inland across much of the Intermountain West as well. Heavy mountain snow is expected in the more remote areas of the Blue, Sawtooth, and Absaroka of the northern Rockies. The same recipe that produced heavy snow in the Sierra Nevada (moisture advection, strong cold FROPA, upslope flow) will coincide with the added help of strong diffluent flow downstream of the upper trough to generate heavy snow from the Tetons on south through the Bear River, Wasatch, Uinta, central Nevada ridges, the Mogollon Rim, and Colorado Rockies. WPC probabilities show high chances (>70%) for snowfall totals >8" in all of these mountain ranges, but it is the Wasatch, Uinta, and Mogollon Rim most notably that could see snowfall totals between 1-2 feet through Friday. The probability of significant icing is less than 10 percent. Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png