Very hot through Thursday with a chance of thunderstorms in the northern Sierra

July 24, 2018 – Hotter weather today through Thursday with a slight chance of afternoon and evening thunderstorms over the northern Sierra. A heat advisory is in place for the northern/Central Sacramento valley and surrounding foothills as well as portions of the San Joaquin valley this afternoon through Thursday. Cooler temperatures Friday through early next week but still a few to several degrees above normal.

Discussion

Upper level high pressure ridge over the southwest U.S will bring another hot day today with forecast daytime highs from 5 to 10 degrees above normal. Upper ridge retrogrades slightly westward today bringing a slightly warmer airmass so highs should nudge up just a bit over Monday.

Airmass warms just a bit more on Wednesday with temperatures expected to level off Thursday. Combination of hot daytime highs and warm overnight lows have prompted issuance of a heat advisory for the northern and central Sacramento valley and surrounding foothills as well as portions of the northern San Joaquin valley this afternoon through Thursday.

A moderately strong delta breeze is expected to continue through this time which should moderate temperatures in delta influenced areas slightly. Much of NorCal will remain under upper level southwest flow the next few days which should keep afternoon thunderstorm threat north and east of the region.

Exception will be the northern Sierra where a slightly more southerly flow could bring enough monsoon moisture into the region for a slight threat of afternoon and evening thunderstorms.

Upper ridge begins to break down just a bit on Friday as a weak trough begins to form over the eastern Pacific. This will bring a slight cooling trend although daytime highs will remain several degrees above normal. A more southwest to west flow aloft should end the northern Sierra thunderstorm threat as monsoon moisture is shunted eastward.

EXTENDED DISCUSSION (Saturday THROUGH Tuesday)

High pressure will bring above average temperatures across interior NorCal through the extended period. Forecast guidance hints at an amplification of the pattern toward the weekend into next week with the ridge building into the Great Basin. This could result in stronger onshore flow through the Bay Area, and modest cooling in Delta Breeze influenced locations. Daytime temperatures across the Valley will be in the 90s to lower 100s, with upper 70s to 90s over the mountains. Dry conditions are expected through the period.