Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 PM EST Thu Dec 12 2024 Valid 00Z Fri Dec 13 2024 - 00Z Mon Dec 16 2024 ...Great Lakes.. Day 1... Surface high centered over central MN this afternoon will amplify and shift east over southern WI tonight and then to Lake Erie through Friday. Flow veers north from Wly and weakens as the high approaches which will cutoff the ideal LES conditions continuing over the Great Lakes. Banding in MI and off Lake Erie diminishes quickly after midnight tonight with Day 1 PWPF for >6" 40% or less in the typical Wly flow snow belts. However, potent single band LES persists through Friday morning for the Tug Hill area east of Lake Ontario. Day 1 PWPF for >12" here is around 70% with max additional around 18". ...Western U.S... An initial wave over CA/NV tonight weakens by Friday with a longer wave approaching the PacNW coast Friday night before sweeping over the northern Rockies through Sunday with heavy mountain snows expected. California... Days 1-2... A reinforcing trough currently over the southern OR Coast shifts south behind the main trough axis currently extending down the Sierra Nevada. This brings additional heavy snow to the northern Sierra Nevada 00Z-06Z tonight with snow levels around 4000ft. Snow rates above the snow level will top 1"/hr until dropping off quickly after 06Z per the 12Z HREF. Day 1 PWPF for >6" over the northern/central Sierra are 50-80%. At atmospheric river ahead of the next system arrives into the far northern CA coast midday Friday with snow levels quickly rising over 5000ft on the Trinity Alps/Klamath/Siskiyou. Heights begin to fall with the approaching trough Friday night with snow levels decreasing to 4000ft by 12Z Saturday under the trough axis. A potent plume of moisture surges over the length of the Sierra Nevada on Saturday which produces a multi-hour bout of heavy snow. Ridging then builds in a quickly ends heavy snow by Saturday evening. Day 2 PWPF for >12" is 50-80% for the length of the Sierra Crest along with the CA Cascades. Pacific Northwest through Northern Rockies... Days 2-3... Moisture streams over the PacNW coast later Friday with moderate to locally heavy snow above the 4000ft snow levels in the WA Cascades and 5000ft in the OR Cascades. These snow levels drop about 1000ft under the trough axis on Saturday. Day 2 PWPF for >8" are 40-80% for the higher Cascades. The moisture surge reaches the northern Rockies Friday night with heavy snow focusing mainly over central ID terrain through Saturday with snow levels 4000-5000ft. Day 2.5 PWPF for >8" are 50-90% over the Sawtooth and southern Salmon River Mtns as well as the Blues in OR. As the trough crosses the northern Rockies Saturday night into Sunday, the terrain from Yellowstone to the Wasatch sees heavy snow with the Day 3 PWPF for >8" is 50-80% with snow levels dropping from 6000ft to 5000ft through the event. ...Central Plains into the Upper Midwest... Day 2... A trough currently over the Sierra Nevada amplifies as it shifts east of the CO Rockies Friday night, closing over KS. Gulf-sourced moisture will stream over cold ground over eastern Neb/IA to produce a wintry mix late Friday into Saturday before shifting ENE to MI through Sunday. The combination of warm air advection over areas that had been under a cold 1040mb high makes for a notable freezing rain threat with a modest snow/sleet threat. Day 2 PWPF for >0.1" ice are above 20% from northeast Neb across IA into southern MN, as well as the Driftless region of southwest WI with a large area over 60% in central and eastern IA. ...Central Appalachians... Day 3... Cold air in the wake of the aforementioned 1040mb high lingers over the central Appalachians on Sunday with freezing rain threats. Day 3 PWPF for >0.1" ice are 20-30% in the Potomac Highlands of MD/WV into the Laurels of southwest PA. Jackson