Mountain thunderstorms and above average temperatures

August 10, 2020 – Moderate heat risk persists today over the northern half of the Sacramento Valley. Afternoon thunderstorms possible over higher terrain, mainly Sierra Nevada, through Tuesday. Near normal temperatures midweek, then warming up again by next weekend.

Discussion

Skies are clear across interior NorCal early this morning other than some lingering remnant high clouds left over from Sunday afternoon’s Sierra thunderstorms. IR difference imagery shows stratus has returned to much of the NorCal coast, and profiler data indicates the marine layer has deepened slightly. The Delta Breeze is stronger at Travis AFB compared to 24 hours ago, but cooling has yet to be realized. Elsewhere, current temperatures are running slightly warmer across the region compared to early Sunday, and generally range from the lower 50s in the colder mountain valleys to the mid 80s across milder portions of the Sacramento Valley.

Little overall change expected today as mid/upper COL lingers across NorCal. Enough elevated moisture/instability remains within this area of weak flow for a few thunderstorms again later today over the mountains as daytime heating triggers deep convection. Isolated storms may be possible further to the west and north today, over the northern Coast Range and extreme southern Cascade Range.

High temperatures today will be a mixed bag as the return of the Delta Breeze spreads cooling into the Delta region, and slightly cooling into the Sacramento and Stockton areas. Elsewhere, hot weather will continue today with highs similar to Sunday’s.

Weak trough develops along the West Coast on Tuesday and Wednesday, pushing most of the late day thunderstorm activity back well to the east of the forecast area by Wednesday. The trough will also result in strengthening onshore flow by this evening which will bring cooler temperatures (back to around average for early to mid-August) for a couple of days.
Trough forecast to weaken by Thursday leading to slight warming as onshore flow slackens.

Extended Discussion (Friday through Monday)

Upper COL over interior NorCal Friday retrogrades offshore as upper ridging from the Desert SW builds over the CWA through the weekend into early next week. Dry weather expected with a gradual warming trend. Greatest departure from normals, upwards of 10 plus degrees by next Monday, will be over the southern half of the CWA under stronger subsidence.