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August 27, 2020 – Smoke and haze from wildfires will continue to impact air quality and temperatures over interior Northern California for the next several days. Overall dry weather this week but a few late day storms possible over the mountains today and Friday. Hot temperatures continue through next week.

Discussion

Early morning temperatures across most of interior NorCal are reading around 4 to 12 degrees cooler than this time yesterday thanks to the return of a healthy Delta Breeze. This is due to the weak upper level trough that continues to slowly move over northern California and increasing onshore flow. Along with cooler overnight temperatures, the Delta Breeze has also helped clear some smoke out of the Delta, southern Sacramento Valley, and northern San Joaquin Valley, though smoky conditions are expected to continue for the foothills and northern valley. Air quality is still expected to remain poor to unhealthy across interior NorCal as numerous wildfires continue to burn.

Near normal temperatures are expected today due to the influence of the weak upper trough with a continuation of a moderate Delta Breeze overnight. Heights are forecast to rise Friday, allowing temperatures to rise to around 5 to 10 degrees above normal, peaking in the upper 90s to 105 across the Valley. There is also a slight chance of mountain thunderstorms this afternoon and Friday afternoon over the Sierra and Coastal Range, though the moisture and forcing does not look great and isolated thunderstorms that do form are likely to be out of our forecast area.

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Ensembles continue to indicate the possibility of a weak upper level low developing off the Bay Area coast this weekend, lowering heights over NorCal and allowing temperatures to cool back to near normal. Onshore flow and associated Delta Breeze are expected to continue through the weekend, keeping overnight temperatures mild.

Extended Discussion (Monday through Thursday)

GEFS and EC ensemble in general agreement dropping a short-wave thru the northern Rockies as the eastern Pacific ridge builds early next week. Appears this feature will be too far to the east (even with the slightly stronger EC) to have much impact other than possibly bringing a relatively brief period of light to moderate north/northeast wind in the Sacramento Valley and surrounding terrain. Ensembles weaken the ridge slightly as it moves over NorCal next week, but another period of anomalously hot weather looks likely with temperatures potentially exceeding upper bounds of the NBM forecast.