May 18, 2021 – Dry weather with warmer temperatures today. Next weather system approaches Wednesday bringing cooler temperatures. Cool and unsettled weather expected Thursday into the weekend, then dry and milder weather returns early next week.

Temperature loop


Onshore flow persists across the region early this morning, but not as strong as yesterday as the surface pressure gradient begins to slacken a bit. Short-wave ridging has moved overhead working to suppress the marine layer and IR difference imagery indicates that stratus is much less extensive than early Monday with only patches noted in the Bay Area. Current temperatures are mostly a few degrees milder compared to 24 hours ago and are in the 50s to lower 60s across the Central Valley.

Dry weather with mild temperatures expected today as the onshore flow decreases. The low that brought the unsettled weather to NorCal over the weekend is now over Colorado and New Mexico, so its remaining influence has waned and we’re not expecting any mountain showers or thunderstorms today. The Delta, southern Sacramento Valley and northern San Joaquin Valley will see warmer temperatures today, but readings will be similar to Monday’s across much of the remainder of interior NorCal.

The next upstream trough developing off the BC coast will dig southward into NorCal after mid-week and will become the focus of the region’s weather late this week into the weekend as it brings a return of cool and unsettled weather. Dry weather is expected to hold through much of Wednesday, though temperatures will begin to trend down a bit as onshore flow picks up again.

Shower chances may develop as early as Wednesday evening across the mountains of Shasta County, but will become more likely across the northern mountains and northern Sierra late Wednesday night and Thursday as the deep cold closed low digs south into NorCal. SREF and GEFS plumes indicate potential for several inches of accumulating snow in the mountains, potentially as low as 4500 feet or so early Thursday. At this point, potential travel impacts to the higher mountain passes would be mainly from late Thursday morning into Thursday evening, but the recent warm weather may result in only slushy conditions across the higher passes.

Ensembles favor the low shifting east into the Great Basin on Friday with NorCal on its backside continuing a chance for a few late day mountain showers and thunderstorms. Temperatures are forecast to be a little milder with locally breezy northerly winds through the Central Valley.

Extended discussion (Saturday through Tuesday)

Ensembles and cluster analysis in good agreement depicting a closed upper low meandering over the NV/Great Basin region early in the extended period as ridging persists over the eastern U.S. This low may support shower and thunderstorm chances Saturday afternoon and evening over the Sierra and southern Cascades.

This system is expected to weaken and exit the region on Sunday, ending mountain shower threat. Potentially breezy north winds may linger into Saturday morning. High temperatures will rebound over the weekend into early next week. However, clusters hint at possible onshore flow late in the period with temperatures potentially trending cooler.