August 8, 2020 – Warming trend into Sunday/Monday with moderate heat risk over northern and central portions of the Sacramento Valley and surrounding foothills. Afternoon thunderstorms possible over higher mountain terrain, mainly Sierra Nevada, today into Tuesday.

Discussion

EPAC ridging extends inland over NorCal as closed upper low resides west of Point Conception. Moderate flow through the Delta overnight, but slightly weaker than 24 hours earlier. Marine layer about 1700 feet deep attm per Fort Ord profiler with some coastal stratus extending into the Carquinez Strait. Increased subsidence over interior NorCal today will result in slight warming of the AMS. High temperatures forecast to be around 3 to 7 degrees above normal. 700-500 mb MU CAPE and modified TT’s increase along the Sierra Nevada crest this afternoon under mid- level southerly flow. This to suggest a threat of isolated afternoon thunderstorms over the higher Sierra terrain from about Lake Tahoe southward.

Only minor changes in synoptic pattern expected Sunday with a continued upward trend in temperatures. Triple digit heat forecast for much of the Northern and Central Sacramento Valley with moderate heat risk. Upper 90s expected in the Southern Sacramento and Northern San Joaquin Valleys. Closed upper low progged to retrograde slightly offshore Sunday with elevated instability limited to southern portions of our higher Sierra Nevada, south of highway 50.

Above normal temperatures continue Monday with a continued moderate heat risk for the foothills and Sacramento Valley, north of the Sacramento area. Favorable modified TT values for deep moist convection extend farther northward Monday afternoon through the higher elevations of Western Plumas into Eastern Shasta county.

Heights/thicknesses begin to lower over interior NorCal Tuesday with increased onshore flow as short wave trough progresses into PacNW. Increasing mid-level westerly flow will shift thunderstorm threat mainly east of the CWA Tuesday afternoon except for a slight chance over higher elevations of our Sierra, south of Highway 50.

Extended Discussion (Wednesday through Saturday)

Weak trough forecast to persist along the West Coast later next week. Dry weather expected with temperatures close to average for mid-August.