Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 313 PM EST Mon Jan 01 2024 Valid 00Z Tue Jan 02 2024 - 00Z Fri Jan 05 2024 ...California/Great Basin/Four Corners... Days 1-3... Deepening upper trough west of 130W this evening will split into two entities by Wednesday, with the norther portion rotating up toward Vancouver Island and the southern portion digging into/through NorCal and the Great Basin, then to the Four Corners region by early Thursday as a closed low. 500mb heights with the closed low may be as deep as -2 sigma, or below the 10th percentile. Moisture flux will initially be focused northward along the West Coast into southwestern B.C., but as the southern portion of the trough detaches, modest moisture/IVT will push its way through the NorCal ranges day 1 and into the Sierra by day 2. Snow levels will be around 5000-6000ft until the cold front moves through when they will drop to about 4000-5000ft from north to south. This will bring measurable snow to even the SoCal ranges outside Los Angeles around Big Bear. System will remain progressive, downstream of a fairly zonal/fast flow across the eastern North Pacific, carrying the system into the Great Basin and Four Corners region Wednesday afternoon into early Thursday. Snow will spread across Nevada driven mostly by upper divergence beneath a sharply-turned jet, PVA, and upslope enhancement as moisture levels will be near average to slightly above average (farther south into southern NV and especially AZ). By late day 3, accumulating snow will spread eastward to the northern NM ranges into the CO Foothills and plains as the system starts to weaken. WPC probabilities for at least 6 inches of snow are high (>70%) on day 2 over the NorCal ranges into the northern/central Sierra, generally above 6000ft or so. Into day 3, higher probabilities follow the higher/highest peaks in NV, northern AZ (esp. White Mountains), southern UT, southwest CO (San Juans), into northern NM (Jemez Mountains and southern Sangre de Cristos). In CA and NV, this will affect some higher mountain passes over the Sierra (above 5000ft or so). ...Great Lakes... Days 2-3... Canadian cold front will sink through the Great Lakes starting Tuesday night as several minor vort maxes rotate cyclonically underneath a larger/deeper upper low over northeastern Hudson Bay/Hudson Strait/northern Quebec. Relatively mild pre-frontal temperatures will cool through the column by late Tuesday into Wednesday, with T850 dropping to about -12C at best. Lake temperatures are well above normal with nearly no ice (0.2% coverage), allowing lake effect snow to increase once enough lapse rates become sufficient. Trailing mid-level trough by Wednesday will lower heights much more, resulting in more hefty NW flow streamers into the U.P. of Michigan and then east of Lakes Erie and Ontario as winds veer post-FROPA. Areas downwind of the Lake Ontario stand the highest chance of more substantial snow totals. There, WPC probabilities of at least 12 inches of snow for days 2-3 are above 70% just east of Lake Ontario. Elsewhere, WPC probabilities of at least 4 inches of snow are moderate (40-70%) or higher over portions of the U.P. of Michigan and into northern New York around the Tug Hill and into parts of the western Adirondacks. The probability of significant ice > 0.25" across the CONUS is less than 10%. Fracasso