Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 AM EST Tue Dec 07 2021 Valid 12Z Tue Dec 07 2021 - 12Z Fri Dec 10 2021 ...A Winter Storm Develops over the Southwest Thursday and Tracks Northeast to the Great Lakes Through Friday Night... ...California...Southwest...Southern Rockies...to the Central High Plains... Days 2/3... An amplifying trough shifts east into northern CA Wednesday night, digging to 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 CO Rockies and rapid lee-side surface cyclogenesis that ejects northeast across the Plains on Friday. Models are in decent agreement with the timing, development, and placement of this low with the GFS a bit quicker and the ECM a bit slower than consensus. Confidence is increasing on heavy mountain snows mainly east of the Sierra Nevada over the Great Basin and UT/CO terrain with heavy snow spilling onto the central High Plains Thursday night in strong convergent banding forming north of the low over eastern WY/western Neb/SD. Brief heavy snow is likely over the Sierra Nevada Wednesday night into Thursday morning with Day 2.5 snow probabilities for 6 or more inches moderately high down much of the High Sierra. Then Day 3 probabilities for 6 or more inches are moderately high over the central NV ranges and eastern WY and over Pine Ridge in the northern Neb Panhandle with moderately high probs for 8 or more inches over the Wasatch and western CO ranges. ...Great Lakes... Day 1... LES in westerly flow continues for the northern UP and northwestern LP of MI with flow becoming southwesterly ahead of an upper trough axis that crosses Lake Superior and WI tonight with lake enhanced snow over the western LP tonight. Day 1 WPC snow probs are moderate for 4 or more additional inches over the Keweenaw Peninsula, the northern LP of MI, and over the Tug Hill. ...Northeast... Days 2/3... 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 looks to develop over the interior Northeast late Wednesday afternoon as surface low pressure develops off the Mid-Atlantic coast. Currently the main heavy snow threat is from 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 WA Cascades with Day 1 snow probabilities for 6 or more inches moderate, expanding down to the northern OR Cascades for Day 2 (with high probabilities in the WA Cascades), then moderately high probabilities spanning the WA and OR Cascades on Day 3. For Days 1-3, the probability of ice accretion over 0.1" is less than 10 percent. Jackson