Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 329 PM EST Tue Dec 07 2021 Valid 00Z Wed Dec 08 2021 - 00Z Sat Dec 11 2021 ...A Winter Storm Develops over the Southwest Thursday and Tracks Northeast to the Upper Midwest Through Friday Night... ...California...Southwest...Southern Rockies...to the Central Plains... Days 2/3... An amplifying trough shifts east into northern CA Wednesday night, digging into interior southern CA on Thursday. Low pressure currently moving south off the CA coast stalls off Baja CA Wednesday, allowing Pacific moisture to stream across the Desert Southwest ahead of the Wednesday night trough. Further amplification from a reinforcing trough shifting southeast from the Pacific Northwest Thursday night, allows low development over the Colorado Rockies and rapid lee-side surface cyclogenesis that ejects northeast across the Plains on Friday. Models remain in overall good agreement with the timing and development of the system. WPC probabilities suggest that heavy snow is likely for portions of the Sierra Nevada, the Great Basin and Utah and Colorado terrain, with heavy snow spilling onto the central High Plains Thursday night in strong convergent banding forming north of the low from eastern Wyoming eastward across southern South Dakota/northern Nebraska into southern Minnesota and northern Iowa. While the models are in generally good agreement, especially for a Day 3 period, there was a noteworthy shift south across across the Plains in some of the guidance, including the GFS and ECMWF. Areas impacted through late Thursday include the High Sierra, the central and eastern Nevada mountains, the Utah mountains, the western Colorado, and the south-central Wyoming ranges. WPC probabilities indicate a likely threat for at least localized amounts of 8 inches or more through Day 2. From late Thursday into Friday, snows are expected to diminish across the Great Basin, with heavy snows likely to impact a larger portion of the Colorado Rockies into northern New Mexico. WPC probabilities indicate that additional accumulation of 6 inches or more are likely from the Sierra Madre and Medicine Bow mountains in south-central Wyoming to the San Juan Mountains extending down into north-central New Mexico. Meanwhile as noted, heavy snows will extend east from the mountains into the central Plains, with WPC probabilities showing a significant threat for 6 inches or more from southeastern Wyoming into far southwestern South Dakota, and the Nebraska Panhandle and Sandhills. By Friday afternoon into the evening, snows will likely begin to accumulate farther east, with WPC probabilities indicating a significant threat for accumulation of 4 inches or more as far east as southwestern Minnesota and northwestern Iowa. ...Northeast... Days 2... An upper level feature rounding a low centered near James Bay tracks east from the Ohio Valley Wednesday afternoon before pushing off the New England coast Wednesday night. Any heavy snow threat is expected to be largely confined and attributed to an expected TROWAL development along the Maine coast Wednesday night into Thursday morning that could produce some persistent snow bands. Day 2 WPC snow probabilities for 4 or more inches are moderate along the length of the Maine coast. ...Pacific Northwest... Days 1-3... The next shortwave trough and accompanying Pacific jet streak reaches the WA coast tonight which begins a multi-day period of onshore flow and increasing likelihood of snow for the Cascades. Snow begins late tonight for the Washington Cascades with Day 1 snow probabilities for 6 or more inches moderate, expanding down to the northern Oregon Cascades for Day 2. For Days 1-3, the probability of ice accretion over 0.1" is less than 10 percent. Pereira/Jackson