Bay Area heat wave brings temperatures up to 11°C (20°F) above normal, unhealthy air quality, and Major HeatRisk, California
A heat wave across the San Francisco Bay Area and Central Coast brought temperatures well above normal on September 9, 2026, with daytime highs and overnight lows forecast 6–11°C (10–20°F) above normal and widespread Moderate to Major HeatRisk across the region. The National Weather Service (NWS) said the heat would persist through September 10, with inland highs reaching approximately 32–41°C (90–105°F), while the Bay Area Air District issued a Spare the Air Alert as ground-level ozone was forecast to reach unhealthy levels in parts of the Bay Area.
HeatRisk map for California - September 9, 2026. Credit: NWS
The National Weather Service (NWS) San Francisco Bay Area office said temperatures across the Bay Area and Central Coast were running substantially above normal on September 9, with daytime highs and overnight lows forecast approximately 6–11°C (10–20°F) above normal as the regional heat wave reached its peak.
Widespread Moderate to Major HeatRisk was expected through September 10, with inland temperatures forecast to reach approximately 32–41°C (90–105°F), while locations along the immediate coastline were expected to remain cooler as an afternoon sea breeze developed.
San Francisco was also under a Heat Advisory through 22:00 PDT on September 10 (05:00 UTC on September 11), with a point forecast for the central part of the city calling for a high near 31°C (88°F) on September 9 and an overnight low near 18°C (65°F). Temperatures were forecast to ease on September 10, when the high was expected to reach about 24°C (76°F).
The heat was accompanied by deteriorating air quality, prompting the Bay Area Air District to issue a Spare the Air Alert for September 9, as high inland temperatures, light surface winds, and a delayed afternoon sea breeze were expected to increase ground-level ozone concentrations, particularly in the Eastern Zone and Santa Clara Valley.
Ground-level ozone develops when pollutants react in sunlight, and the combination of high temperatures, light surface winds, and a delayed afternoon sea breeze favored elevated ozone concentrations across parts of the region. The Air District warned that ozone can irritate the respiratory system, trigger asthma and aggravate existing lung conditions, particularly among children, older adults and people with respiratory or cardiovascular disease.
The heat was driven by strong high pressure and light offshore flow that weakened the marine layer and reduced the cooling influence normally supplied by air moving inland from the Pacific, allowing unusually high temperatures to extend across much of the Bay Area and Central Coast.
At the same time, mainly mid- and high-level tropical moisture associated with the remnants of Marie continued moving north across the forecast area. Although the lower atmosphere remained comparatively dry away from the immediate coastline, the cloud cover was expected to limit nighttime radiative cooling and contribute to unusually warm overnight temperatures.
The heat episode also coincided with a large, unplanned power outage in San Francisco on September 8, when the San Francisco Department of Emergency Management (SFDEM) reported that Pacific Gas and Electric Company customers in Hayes Valley, South of Market, the Mission, and adjacent areas had lost electricity.
A PG&E-sourced outage alert issued at 15:44 PDT (22:44 UTC) initially listed 3 269 affected customers, with the number falling to 446 by 17:14 PDT (00:14 UTC on September 9). The alert marked power restored at 19:41 PDT, and no connection between the outage and the heat had been established.
NWS warned that prolonged exposure to the heat could affect people without effective cooling or adequate hydration and could also produce impacts on health systems and heat-sensitive industries, particularly during the warmest part of the day and in locations with limited overnight relief.
Residents were advised to remain hydrated, wear lightweight clothing, limit strenuous outdoor activity to cooler parts of the day, and recognize the symptoms of heat exhaustion and heat stroke, while children and pets should never be left unattended in vehicles.
The regional heat was forecast to begin moderating along the coast on September 10, with temperatures across the broader Bay Area and Central Coast expected to move closer to seasonal levels from September 11 through the weekend.
References:
1 Heat Advisory for San Francisco and the Bay Area – NWS – issued September 9, 2026
2 Area Forecast Discussion – NWS – issued September 9, 2026
3 Spare the Air Alert for September 9 – Bay Area Air District – issued September 8, 2026
Rishav Kothari is Editor-in-Chief of The Watchers, where he runs the newsroom and covers earthquakes, volcanoes, severe weather and space weather.
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