Chance of isolated thunderstorms this afternoon and evening in the mountains

June 23, 2020 – A prolonged period of hot weather is forecast this week. Moderate to very high heat risk is expected in the Valley and foothills, with little overnight relief. Slight chance of afternoon and evening thunderstorms over the mountains today and Wednesday.

Discussion

High pressure ridging will continue to dominate the weather pattern this week, bringing hot temperatures. The ridge axis shifts to the east today and Wednesday as a trough passes through the Pacific Northwest, with heights lowering a little over NorCal. Afternoon highs will be just a bit less hot than on Monday, by a degree or two. Delta breeze will bring some cooling the Delta into the southern Sacramento and northern San Joaquin valleys, especially this evening, but lows elsewhere in the Valley and the foothills will remain in the 70s. Temperatures rise again Thursday and Friday, with the northern half of the Sacramento Valley mostly in the 105-110 degree range. For the southern Sacramento and northern San Joaquin valleys, highs will range from 99-105. Both the above normal daytime temperatures and warm overnight temperatures will cause moderate to very high heat risk across the Valley and foothills, with most of the mountain areas in moderate heat risk as well. A Heat Advisory remains in effect until 8PM Saturday for the Central Valley and the foothills.

The upper trough and some instability will bring a slight chance of thunderstorms for this afternoon and evening over the Sierra Crest, mainly south of I-80, and over the mountains of far western Shasta County. On Wednesday, this potential includes the Lassen Park area and the Sierra Crest south of I-80. The ridge rebuilds Thursday, with dry weather returning.

Extended Discussion (Saturday through Tuesday)

Upper ridging weakens and shifts east Saturday as upper trough deepens along the West Coast. Upper low then drops through OR Sunday and lingers over the Intermountain West early next week. This pattern change will result in cooling. By Sunday, highs will be near normal with low 90s in the Central Valley and some breezy northerly wind. Little change in high temperatures Monday with a slight rise on Tuesday. Dry weather expected through the extended forecast period.