February 2, 2021 – A winter storm will move across NorCal through Wednesday with mountain travel impacts. Dry weather returns Thursday through the weekend with periods of increased north to east winds.
Discussion
Broad trough continues to slowly move east from the eastern Pacific over the Western States. Ensemble guidance shows trough splitting with upper low fracturing off the SoCal coast and main trough establishing over NE California-NW Nevada Wednesday. Main moisture plume ahead of the trough is just beginning to impact the Sierra. Plume dissipated over the nrn Sacramento Valley during the evening, as flow was shadowed by Coast range and less than 0.10 inch has fallen at Redding. Plume then intensified from Lassen to Paradise, where 1-1.5 inches has fallen.
Main precip band will slowly drop south through the Sierra and Sacramento/Nrn San Joaquin valleys through the day. Winter storm warning continues into early Wednesday, expecting at least moderate winter storm impacts at pass level with 10 to 18 inches and locally 2 feet of snow for storm totals above 5000 feet. Snow levels are currently near 6000 feet, expecting them to fall to 4500 feet this afternoon. Sref ensembles are indicating storm total 3 to 5 inches at Blue Canyon(5280 feet) into this evening and that should around the lower elevation of significant snow accumulations.
Atmosphere will also turn a bit convective this afternoon, as indicated by 200-300j/kg of cape over the Sacramento Valley, slight thunderstorm chances look possible Redding to Sacramento. Although, inversion around 10,000 feet will limit storm growth and intensity, mesoscale CAMS are also indicating convergence convective band possible over Shasta county. Valley Rainfall amounts are expected to range from 0.50 to 1.00 inches with 1 to 2 inches over the foothills.
Confidence is also growing in secondary disturbance dropping south on the back side of trough Wednesday afternoon with potential lower snow down to 2500 feet, where 3 to 5 inches may fall. May have to expand and extend winter storm warning into Wednesday afternoon/evening if future runs continue this trend.
Ensembles and clusters are in agreement with trough shifting east into plains Thursday/Friday with eastern Pacific high building east into California. This will bring a drying and warming trend.
Extended discussion (Saturday through Tuesday)
Dry weather expected through the weekend as interior NorCal is under the influence of upper ridging. Locally gusty north to east wind possible on Saturday. Forecast uncertainty increases early next week as deterministic guidance suggesting potential for light precipitation later Monday into Tuesday. Ensembles continue enough ridging to maintain a dry forecast attm. High temperatures expected to be 5 to 10 degrees above normal.