675 FOUS11 KWBC 121919 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Wed Mar 12 2025 Valid 00Z Thu Mar 13 2025 - 00Z Sun Mar 16 2025 ...The West... Days 1-3... ...Storm system to produce impactful heavy snow for most California mountain ranges through Thursday, the southern Intermountain West terrain and Rockies late week, then emerge over the Central U.S. on Saturday with blizzard potential over parts of the northern Plains. Please see the latest Key Messages linked at the bottom... Potent upper-level trough continues to dig as it tracks south with the parent low drifting south from the Gulf of Alaska. The base of the trough and associated cold front approaches northern CA this afternoon, then digs south to the northern Baja while also pushing inland through Thursday. Enhanced moisture transport has raised snow levels to around 4000ft in WA, 4500ft in OR, 5000ft in northern CA and 6500ft on the Sierra Nevada. However, the cold frontal passage and falling heights aloft will result in snow levels generally around 3500ft along the Cascade Range on south to the Shasta/Siskiyou this evening and 2500ft late tonight as snow rates decrease after the upper trough passage. The Sierra Nevada will be the focus of the moisture plume which features >400 kg/m/s SWrly IVT (near the 99th climatological percentile via NAEFS SATs) just ahead of the cold frontal passage this evening. This is when snowfall rates will be at their heaviest per the 12z HREF and around 2-3"/hr for several hours between 00Z and 10Z Thursday down the length of the Sierra Nevada above about 6,000ft, which is where Day 1 PWPF for >24" snow is above 80%. Some upslope flow will linger into the Sierra Nevada Thursday morning, before a brief lull in the action arrives Thursday afternoon. By Friday morning, the next quick-hitting Pacific storm system arrives, producing more moderate snow rates for the Cascades and Sierra Nevada. However, snow levels are progged to remain low throughout this time period, around 1500ft in western WA, and 2500 to 3000ft OR through the Sierra Nevada. Eventually, the next progressive shortwave digging across the northern Pacific produces upstream ridging across the West Coast by Friday night and raises snow levels up to 4000ft across northern CA on Saturday. This sets the stage for the next Atmospheric River event progged to impact northern CA and southern OR primarily on Day 4. For the Day 2-3 time frame, PWPF for >8" is 60-90% for the OR Cascades and Shasta/Siskiyou. Farther south, the Transverse Ranges as well as the higher peaks of the Peninsular Ranges will see heavy snow late tonight through Thursday morning as the snow levels drop from 5500ft to 4000ft. Day 1 PWPF for >8" are 60-90% in the higher terrain. Overall, this is an ideal pattern for somewhat brief, but heavy snow for the Shasta/Siskiyou 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 and 3 feet at the higher pass levels such as on I-80. The WSSI retains Major to Extreme Impacts in most of these ranges with Snow Amount the primary impact, 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 Blue, Sawtooth, and Absaroka of the northern Rockies. The high moisture plume 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 Wasatch, Uinta, central Nevada ridges, the Kaibab Plateau and Mogollon Rim, and western Colorado Rockies. Days 1-2 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. ...Northern Plains... Day 3... The trough takes on a negative-tilt Thursday night over NM and closes in to a mid-level low over KS Friday morning. This low further deepens/strengthens as it tracks northeast over MN into Saturday. MSLP of this low is expected to drop into the 970s (mb) and drop below the 1st climatological percentile per the GEFS and ECENS. This extremely low pressure will be the driving force behind the widespread strong winds poised to be the primary weather hazard across the northern Plains. The southern stream wave crossing the southern Plains today limits Gulf moisture from streaming up the Plains until Thursday night, cutting off the moisture available moreso than if the wave weren't crossing today. While enough Pacific- sourced will be present to allow some wrap around banding on the low over the High Plains Friday that banding should be fairly sparse until Friday night when the activity should be over central Neb/SD. This is notable as the area is in drought and will be experiencing strong winds ahead of the system - in some areas of the High Plains there may be just a dry wind shift. Once those bands do develop as they head for the eastern Dakotas and MN, heavy snow banding is possible. Powerful winds with the system will immediately result in blowing snow and a potential brief blizzard. Marginal thermals at the onset should result in a rain to wintry mix to snow progression with some ice accretion potential most likely over the Red River of the North Basin along the ND/MN border. This transition occurring during the overnight period could limit snowfall totals as well. Latest WPC probabilities for at least 4" of snow are medium (40-60%) from the eastern Dakotas through far western and north- central Minnesota. WSSI-P values for moderate impacts are mainly associated with Blowing Snow and are within the 50-70% range for this region. Snell/Jackson ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png $$