Quick-moving weather system brushes across NorCal later today into early Tuesday. The system will bring scattered showers to the northern Sierra along with cooler temperatures and breezy south winds. Breezy north winds return Tuesday and Wednesday and may lead to locally elevated fire weather concerns.
Discussion
Weak system shearing out and bringing some high clouds to NorCal early this morning as it moves east ahead of the much stronger low dropping south off the BC coast. Surface pressure gradients have begun to reverse to onshore though remain generally light so far. Current temperatures are mainly in the 40s across the Central Valley, a little milder compared to 24 hours ago. Forecast remains pretty consistent with the strong low forecast to drop southward into NorCal by early Tuesday. Expect we’ll see showers beginning to develop across the northern mountains by late this afternoon, then spread south through the northern Sierra this evening and overnight.
This is a pretty robust system, so would not be surprised to see a clap of thunder or two overnight as the strong vort tracks south. The quick movement of the system and relative lack of deep moisture will limit overall QPF. Around a quarter inch is forecast over the higher elevations of the northern Sierra Nevada where an inch or two of snow accumulation will be possible along the crest.
Showers end Tuesday morning as the system continues south of the area. Winds will quickly turn to the north and east behind the trough and persist into Wednesday. This is forecast to be a moderate offshore event similar to last week’s with gusts up to 35 mph in the valley, and gusts of 35-45 mph (locally higher) across the wind-prone areas of the foothills and mountains.
After the record heat this past week and the lack of precipitation over the past month causing fuels to begin to dry, these winds may lead to elevated fire weather conditions over portions of the Valley and foothills.
Temperatures will be around 20 degrees cooler on Tuesday compared to Sunday, but will gradually recover after mid-week as the trough shifts east.
Extended Discussion (Friday through Monday)
Troughing will be lifting out of the four corners region as we sit on the eastern side of the Pacific ridge for the start of the extended forecast. This will bring the return of mild daytime highs and will keep locally breezy winds on Thursday especially over the Sierra. The ridge will flatten some over the weekend as troughing begins to move into BC and the PacNW.
Models diverge in solutions as we head into Sunday and early next week. Troughing is favored over NorCal but some of the members are slower with the digging trough and keep if further to the west. Either way moisture is limited and that will limit precip chances but maybe a few light showers over the mountains later Sunday into Monday.