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July 17, 2020 – Near, or slightly below, average temperatures expected today, then hotter weather returns for the weekend into early next week. A few late day thunderstorms are expected along the Sierra crest the next several days. Keep an eye to the sky if in the mountains. Cooling returns middle of next week.

Discussion

Clear skies cover the region early this morning. Surface pressure gradients are trending gradually more onshore, and the result is a little cooler temperatures and slightly stronger breezes across much of the area compared to 24 hours ago. IR difference imagery shows extensive stratus along the coast extending well inland again into the Carquinez Strait. Profiler data indicate the marine layer is a little more shallow compared to Thursday with its depth presently around 1500-2000 ft.

Cooling will continue to spread inland today with much of the northern half of the forecast area expected to see highs around 3-6 degrees lower compared to Thursday.

The remainder of the region will see similar temperatures to Thursday. Isolated late day thunderstorms are expected again over portions of the northern Sierra Nevada, but will be mainly confined to areas south of Lake Tahoe on the west slopes today.

Hotter weather is forecast to return to the region over the weekend into early next week as high pressure builds northwestward. This will suppress the marine layer, but enough onshore flow is expected into the Central Valley to help moderate temperatures a bit.

Highs across the northern Sacramento Valley will increase to around 105 returning heat risk to the moderate category, while the southern Sacramento Valley and northern San Joaquin Valley will likely see highs in the mid to upper 90s.

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Diurnal Sierra thunderstorms will likely remain south of Lake Tahoe through the weekend.

Extended Discussion (Tuesday through Friday)

Current ensemble runs continue to indicate a weak upper low approaching NorCal early next week and pushing the Four Corners high back to the east. Longwave troughing is expected to sit over the West Coast Wednesday through Friday with multiple small, weak perturbations moving through NorCal along the flow.

Temperatures will fall to and hover right around seasonal normals Tuesday through Friday, except for the northern Sacramento Valley will still be around 100 to 105 degrees Tuesday.

A decent onshore flow and associated Delta Breeze is expected to continue, keeping overnight temperatures mild in the Valley and foothills.