November 17, 2020 – Rain, high elevation snow, and gusty winds thru Wednesday with drier weather returning towards the end of the week.
Discussion
Mid and high clouds are increasing across NorCal early this morning as the trough approaches. Southerly winds are also beginning to increase across portions of the region, and local downslope from the Sierra is keeping temperatures in the upper 40s and 50s along the eastern edge of the Sacramento Valley and the northern San Joaquin Valley.
Radar is beginning to pickup precipitation associated with the frontal band off the northwest coast of California. Hi-res models time the north-south oriented band of precipitation with the front into Butte/western Plumas Counties around noon (where the flash flood watch for the North Complex burn scar is in effect), and south to around the I-80 corridor late this afternoon (just in time for the evening commute in the Sacramento area).
HREF is indicating high probability (over 80 percent) of hourly rainfall rates exceeding 1/2 inch across the North Complex burn scar during the mid to late afternoon hours as the N-S orientation will prolong heavier rainfall potential. Additionally, 24-36 hour QPF for that area is around 2.5 to 4.5 inches, so ash flows from the recent wildfires are a distinct possibility. The heaviest precipitation is expected to taper off in this area this evening, but several more incoming short-waves will continue showers, some heavy at times, through Wednesday.
Significant snowfall is forecast across the higher mountain elevations later today into Wednesday with several feet of accumulation expected near the higher northern Sierra passes and across the peaks. Strong southwest winds will also be possible into tonight. As this is a milder, wetter storm, snow levels will be higher around 6500 to 7500 feet, lowering to 5500 to 6500 feet on Wednesday. Heavy snowfall will likely hold off across Donner Pass on I-80 until late this afternoon.
Unstable post-frontal airmass will support continuing showers over the mountains on Wednesday. A few lines of showers/thunderstorms may organize over the Central Valley on Wednesday as a couple stronger short-waves move across the region, but QPF likely won’t be as high as today given the faster movement of storms along with less deep available moisture. Shower chances expected to taper off Wednesday night and Thursday as the trough begins to weaken and lift out.
Extended discussion (Saturday through Tuesday)
Dry Saturday with highs slightly below normal as upper ridging moves through. Pacific frontal system progged to drop into northern portions of the CWA Sunday and weaken. Best chances for precipitation will be over the coastal and northern mountains. Threat of overrunning light precipitation will continue in the northern portions of the forecast area Monday into Tuesday.