Warm weather continues today. A cooling trend is forecast beginning Friday with below normal temperatures expected for the weekend. A weather system could bring widespread rain shower chances this weekend, mainly for the northern Sacramento Valley, mountains, and foothills north of Highway 50. No snow is expected. Warming and drying trend returns next week.
Satellite imagery is showing high to mid-level clouds continuing to streak over Northern California early this morning as a shortwave associated with broad upper level troughing approaches the coast. These clouds are helping to keep temperatures from cooling down as many locations in the northern Sacramento Valley and adjacent foothill thermal belt areas are still reporting temperatures in the 70s as of 1 am.
A moderate Delta Breeze has helped temperatures cool more in Delta influenced areas, including the Sacramento and Stockton regions. One more hot day is in store for interior NorCal today with temperatures peaking in the low 90s in the Valley, 80s in the foothills, and 60s to 70s in the mountains. The shortwave will continue to move through the PacNW and graze northern California. This will produce a chance of isolated showers and thunderstorms to northern Shasta County, though the better chances will be north of Shasta County.
Ensemble guidance indicates the shortwave will effectively push the current upper level ridge to the east by Friday, with the broad scale troughing taking place in the Eastern Pacific through the weekend. The main impact on Friday will be cooling temperatures and locally breezy onshore (south to west) winds.
Cooling trend continues into the weekend with weekend temperatures 15 to 25 degrees cooler than today. This pattern will also bring the return of precipitation to much of Northern California.
Guidance is coming to better agreement on timing. Another weak shortwave will bring a chance of light showers late Friday into Saturday. Then, the main weather system is now expected to begin moving into the Coastal Range and northern counties Saturday afternoon, spreading south Saturday night with the majority of precipitation overnight Saturday through Sunday morning. IVT values are forecast around 250-500 kg/ms, providing a good moisture plume, especially for June.
QPF amounts have increased some, though the best chances of rain remain in the northern Sacramento Valley and the mountains and foothills north of Highway 50. QPF amounts for these areas are 0.5 to 1 inch for the foothills and mountains, locally up to 1.5 over the western Shasta county mountains, and 0.25 to 0.5 inch for Valley areas in and north of Glenn County.
Elsewhere, QPF amounts are much lighter and rain could be scattered to non-existent. Snow levels still remain well above pass levels for this system, so no snow impacts are expected. Finally, confidence is increasing in isolated thunderstorm chances Sunday afternoon for the mountains, foothills, and northern Sacramento County.
Extended Discussion (Monday through Thursday)
Lingering mountain showers over Shasta county possible Monday morning then drier weather as upper ridging builds inland. Some gusty north to east wind at times. Warming trend through the extended with near to slightly below normal highs Monday, rising to about 10 to 15 degrees above normal by Thursday.