An unseasonably strong low pressure system will bring cooler temperatures, breezy onshore winds and shower and thunderstorm chances this weekend. Warmer and drier next week with temperatures returning to near normal around mid-week.
Discussion
The unseasonably strong closed upper low is over northwest California early this morning. Radar shows showers continuing near the low center, and a large band of showers with some embedded thunder covering the northern Sierra in the frontal zone ahead of the low. Rainfall amounts the past 24 hours have been pretty impressive – around 2-3 inches in spots around the Redding area, and 1-2 inches over portions of the northern Sierra.
Current temperatures are much cooler (5-15 degrees) compared to 24 hours ago and are in the 40s and 50s in the mountains, and lower to mid 60s across the Central Valley. Cool and unsettled weather will persist across the region over the holiday weekend as the low slowly drifts across NorCal before exiting to the east later Sunday. Most showers and thunderstorms will occur over the mountains, but the valley won’t be completely immune either. Heaviest QPF will likely occur over the northern Sierra where another 1-1.5 inches of rain will be possible through Sunday.
Rainfall rates will likely be more tame than what we saw Friday across the northern Sacramento Valley, but some of the heavier showers may approach half inch/hr rates in the Sierra today per latest HREF exceedance forecasts.
A few showers may linger into Labor Day across the mountains as another short-wave drops down the backside of the exiting trough.
Otherwise dry and milder temperatures are expected across most of the region for the holiday and Tuesday as a weaker trough lingers. High temperatures will remain below average.
Extended Discussion (Wednesday through Saturday)
Weak upper troughing moves through midweek then interior NorCal sandwiched between upper troughing to the NW and upper high to the SE.
Dry southwesterly flow aloft will prevail over the CWA through the extended forecast period.
High temperatures expected to be near to slightly below normal Wednesday through Saturday.