Cooling trend and a chance of thunderstorms

July 20, 2020 – Gradual cooling trend this week. A few late day thunderstorms are expected over the mountains. Keep an eye to the sky if in the mountains. Southwest breezes continue near the Delta with locally breezy conditions elsewhere.

Discussion

Skies are clear across interior NorCal early this morning other than some remnant clouds from Sunday afternoon’s deep convection over the Sierra Nevada. Profiler data indicate the marine layer remains around 2k ft deep with IR difference imagery indicating stratus extends through the Carquinez Strait. Surface pressure gradients are trending stronger onshore and a robust Delta Breeze has spread around 5 degrees of cooling up through the southern half of the Sacramento Valley.

Temperatures will remain pretty hot across the northern Sacramento Valley and surrounding terrain early this week while the Delta Breeze keeps temperatures close to average elsewhere across interior NorCal. Highs across the northern Sacramento Valley will remain in the 100-105 range today and Tuesday before gradually cooling during mid-week.

Mid level moisture/instability continues to spread a little further to the north and west beneath the ridge axis extending from the Four Corners region into the eastern Pacific. More diurnal activity is expected across the northern Sierra today with perhaps an isolated storm over the Coast Range to the west of the Sacramento Valley.

Weak closed low forecast to develop off the NorCal coast on Tuesday is expected to draw moisture/instability further to the northwest with isolated late day thunderstorms possible across many of the NorCal mountains. Thunderstorm threat forecast to retreat back to the Sierra south of Tahoe by mid-week as the low drops into central California.

Locally breezy south to southwest winds will be possible at times in the afternoons and evenings, mainly through the Delta and portions of the Central Valley.

Extended Discussion (Friday through Monday)

Upper troughing/low remains along the CA coast through the extended forecast period. AMS remains relatively dry with only a slight chance of deep moist convection over extreme southern portions of our Sierra Nevada Friday afternoon. Highs will be near normal Friday through Sunday then above normal Monday.