SAN FRANCISCO (KPIX) — So far this fire season, Bay Area skies have remained largely free of wildfire smoke. Persistent onshore winds — sometimes extending over 10,000 feet up in the atmosphere — have pushed the smoke from California’s fires to the northeast away from the region.

However, that could start to change this weekend.

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Winds in the mid-to-upper levels of the atmosphere are going to shift, beginning to blow east-to-west by late Sunday and continuing into Monday and Tuesday. That shift will allow smoke produced by the Dixie Fire and Tamarack Fire — as well as other fires burning in the state — to blow closer to the Bay Area.

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It’s a complicated scenario. While those upper-level winds will be blowing offshore, winds near ground level will still be onshore or at worst, blowing parallel to the coast. The onshore winds at the surface will try to improve the Bay Area’s air quality, while the upper-level smoke tries to settle down into the lower levels of the atmosphere.

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At this point, the forecast calls for “moderate” air quality for the entire Bay Area Monday and Tuesday. Inland parts of the East Bay and the Santa Clara Valley would be most likely to tip into the “unhealthy” categories if air quality deteriorates further.

Paul Heggen