June 16, 2018 – Cooling trend this weekend with below normal temperatures and a chance of mountain showers and thunderstorms. Temperatures warm above normal next week.
Cooling trend continues today as an upper low currently over the Pacific Northwest drops southward into Northern California. Temperatures this morning are quite a bit cooler this morning than yesterday at this time over much of the area, generally 5 to 10 degrees lower. In addition to a cooler airmass, the Delta breeze is bringing cool marine air into the Delta, the southern/central Sacramento Valley and the northern San Joaquin Valley. Some scattered morning stratus are also possible through the Delta into the Sacramento metro area. Temperatures over the northern Sacramento Valley are the exception and are similar to yesterday, and will again reach around 90 this afternoon. Other Valley locations will be in the 80s.
The upper low will bring an increase chance of mainly mountain showers and a few thunderstorms over the northern mountains, starting early this evening. Some rain showers may extend into the northern Sacramento Valley. A fairly strong inversion cap should limit convective development. More extensive showers and thunderstorms are expected Sunday, spreading southward into the Coastal Range and the Sierra. Sunday highs will be unseasonably cool, in the upper 70s to around 80 for the Valley, upper 60s to lower 70s for the foothills, and 60s for the mountains.
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Monday looks warmer and drier as upper level ridging develops and the upper low shifts eastward into Nevada. Shower and thunderstorm chances are expected to be limited to the Lassen National Park/Western Plumas County area. Highs will increase by about 10 degrees compared to Sunday, reaching around normal levels for this time of year. Tuesday highs rise another 5 to 10 degrees, with some triple digit highs possible in the Redding/Red Bluff area.
Extended discussion (Wednesday through Saturday)
Models showing upper level high pressure over the west coast on Wednesday for dry conditions and above normal daytime highs. GFS and ECMWF both bring a shortwave over this ridge but differ on timing. GFS brings shortwave through far NorCal Wednesday night, Thursday for ECMWF. Either way, looks like any precipitation will remain north of the CWA and temperatures will likely remain above normal. Upper ridge rebuilds over the west coast bringing warming for the end of next week with central valley highs pushing up around 100 again.