Chance of mountain thunderstorms again, slightly cooler temperatures

August 11, 2020 – Cooling temperatures today with afternoon and evening thunderstorms possible over Sierra Nevada. Average temperatures expected for mid-week, then warming up again by next weekend.

Discussion

Some mid and high clouds remain over portions of interior NorCal early this morning (the remnants from Monday afternoon’s thunderstorms over the mountains), otherwise skies are clear. Surface pressure gradients continue to trend onshore and Travis AFB is gusting over 30 kts while the marine layer is gradually deepening to 1500-2000 ft. Current temperatures are running around 3-7 degrees cooler compared to 24 hours ago across most of the Central Valley while elsewhere readings are similar to early Monday.

Weak trough develops over the region today. This will allow for cooling to spread across the area as onshore flow persists, and will push the thunderstorm threat back to the east (mainly over the Sierra Nevada today).

Trough weakens slightly Wednesday, but moisture/instability is pushed well east of the forecast area. Onshore flow will slacked and temperatures begin to rebound.

Trend continues toward hotter weather returning at the end of the week as ridging builds northwestward from the Desert Southwest and the trough retreats offshore. Most of the Central Valley is expected to see triple digit heat by Friday with moderate to high heat risk.

Extended Discussion (Saturday through Tuesday)

Upper high from the Desert SW builds northwest over interior NorCal through the extended forecast period with 5h heights climbing into the mid 590s DM. This will result in a period of increased heat risk with triple digit highs throughout the Central Valley, extending into portions of the foothills and Delta area.

Max temperatures expected around the 100 to 105 degree range with overnight lows in the upper 60s and 70s. These values are around 10 to 15 degrees above normal in the Southern Sacramento and Northern San Joaquin Valley. Model differences exist with progression of upper trough in EPAC, which results to differences in orientation of upper high.

This in turn leads to uncertainties in location of monsoonal flow and thus mountain thunderstorm threat. Will need to continue to monitor, but for now forecast remains dry through the extended period.