Unseasonably cool weather continues today. Scattered showers and a few thunderstorms will be possible across far northern California today. Warming trend returns by the weekend.
Discussion
Deep trough remains just off the coast with an extensive band of high clouds moving up from the south-southwest covering NorCal early this morning. Onshore surface pressure gradients have relaxed considerably since yesterday as cooler temperatures have spread inland across the region weakening the thermal gradient. Southwesterly winds are considerably lighter compared to 24 hours ago with only a few spots still reporting gusts in the teens to lower 20s mph.
Upper trough forecast to finally swing inland later today. Scattered showers and a few thunderstorms are expected across the northern third of the forecast area, and a few showers will be possible late this afternoon and evening across the northern Sierra as the strong vort at the base of the trough moves into central California.
High temperatures today will be similar to Tuesday’s, in the 70s again across the Central Valley (around 10-15 degrees below average). Southwesterly breezes will be picking up again later this afternoon and this evening, especially in the Delta and over the mountains.
Ensembles in good agreement that NorCal will be between troughs on Thursday, as current system moves into the Rockies with another trough approaching the West Coast from the Eastern Pacific.
A slight warm up to high temperatures near 80 degrees possible Thursday. Most of the energy with the Friday trough will shift north of the area over the Pacific Northwest, but the system will help keep onshore flow and cooler temperatures going another day before a larger warm up over the weekend.
Extended discussion (Sunday through Wednesday)
Confidence is high in the region being located between an eastern Pacific upper level trough and a 4-Corners upper level ridge. A shortwave passing into the Pacific Northwest will brush through northwestern California.
While most moisture will remain to the northwest, western Shasta and Tehama counties could see a slight chance of rain showers Sunday afternoon, then spreading across eastern Shasta County by Monday afternoon. Any rainfall is expected to be light. Dry conditions are expected elsewhere.
Onshore flow is forecast to continue under this synoptic pattern and will contribute to breezy conditions at times through the Delta into early next week. The upper trough is expected to move inland to the northeast, with the 4-Corners upper level ridge expanding westward. Above normal highs are expected, potentially reaching triple digits by mid week.