September is San Francisco's warmest month: the downtown station averages a high of 70.2°F (21.2°C) and a low of 55.6°F (13.1°C), with almost no rain. Fog is less frequent than in July and August, about 5 days reach 80°F, and occasional heat waves have pushed downtown past 100°F.
San Francisco's hottest day on record came in September: 106°F downtown on September 1, 2017. September brought the city's hottest day of the year in 14 of the 34 years with complete records from 1991 to 2025, more than any other month.
What Is September Weather Like in San Francisco?
September weather in San Francisco is the summer that July never delivers. The fog loosens its grip, the ocean reaches its warmest temperature of the year, and afternoons in much of the city run in the upper 60s and 70s. Summer arrives after Labor Day. It can also turn hot, and occasionally smoky, so the month rewards a quick look at the forecast.
| Average high / low | 70.2°F (21.2°C) / 55.6°F (13.1°C), the year's warmest highs |
|---|---|
| Days at 70°F / 80°F or warmer | About 14 / about 5 |
| Average rain | 0.10 in (2.5 mm); most Septembers are dry |
| Fog | Less than in July and August, still common at the coast in the morning |
| Record high | 106°F (Sept 1, 2017), the city's all-time record |
| Ocean temperature | About 58°F offshore, the warmest of the year |
| Sunset | About 7:39 PM on Sept 1, 6:54 PM on Sept 30 |
San Francisco Downtown station (USW00023272), NOAA 1991-2020 normals and records via RCC-ACIS. Fog from NOAA High-Resolution Rapid Refresh (HRRR) model analysis, September 2014 to April 2026. Ocean temperature from the NOAA NCEI Coastal Water Temperature Guide (buoy 46026). Sunset times computed for 2027.
Why Is September San Francisco's Warmest Month?
September is San Francisco's warmest month because the summer fog machine slows down. As the inland valleys cool from their July and August peak, the sea breeze weakens, upwelling eases and the ocean warms, so less fog is pulled through the Golden Gate. The downtown normal high peaks at 70.2°F in September, just ahead of October at 69.8°F and well ahead of July at 66.3°F.
For the full explanation, see why September is hotter than July and Bay Area Indian summer.
How Hot Does San Francisco Get in September?
San Francisco is usually warm in September and sometimes very hot. A typical September has about 5 days at 80°F or warmer downtown and about 1 at 90°F. Every September from 1991 to 2020 reached 80°F at least once, and nearly two in three reached 90°F.
The city's all-time record, 106°F, came on September 1, 2017, followed by 102°F the next day. Labor Day weekend 2020 hit 100°F on September 6, and September 6, 2022 reached 98°F. September brought the year's hottest day in 14 of 34 years from 1991 to 2025, and October was second with 8.
Heat days catch a cool-summer city off guard. When a heat wave is forecast, see Bay Area heat waves and heat domes for what to expect and how to stay safe, and the hottest temperatures on record for the history.
Is There Still Fog in San Francisco in September?
There is still fog in September, just less of it. In NOAA's HRRR model analysis from 2014 to 2026, fog covered the Outer Sunset in about 38% of 7 AM hours in September versus 54% in July, and in about 31% of 2 PM hours versus 50%. Downtown and the Mission are usually clear and warm by afternoon.
Some Septembers stay gray at the coast: in September 2025, the model showed fog on every Outer Sunset morning. As the inland valleys cool, the gap across the bay narrows to about 23 degrees at 3 PM between the Outer Sunset and Walnut Creek, down from about 28 in July. The bay shore also calms down: in HRRR analysis, the 2 PM wind at Crissy Field averages about 15 mph in September, and only about half of afternoon hours reach 15 mph, against 9 in 10 in July. See San Francisco weather by neighborhood.
September is also the best month for a beach day in the city. The ocean off San Francisco reaches its warmest point of the year, about 58.3°F, which still calls for a wetsuit for swimming, and in HRRR analysis the beach is foggy in fewer afternoon hours than in June, July or August. Ocean Beach keeps its strong rip currents year-round, so wade rather than swim. For more, see Bay Area water temperatures.
Can Wildfire Smoke Reach San Francisco in September?
Wildfire smoke can reach San Francisco in September, which falls in the heart of fire season. Smoke from distant fires can drift over the city even when no fire is nearby. On September 9, 2020, smoke from Northern California fires, including the August Complex in Mendocino County, settled over the fog layer and turned the sky dark orange all day.
The fog kept much of that smoke out of the air at ground level, according to the Bay Area Air District, while higher neighborhoods were more exposed. The marine layer that cools the summer also shapes the smoke. On smoky days, check the AQI before strenuous plans; see Bay Area fire season, wildfire smoke and air quality and the AQI explained.
What Is the Weather Like for Labor Day and September's Festivals?
Labor Day averages a high of 73.2°F and a low of 56.6°F downtown, reaches 70°F more than half the time, and has had no measurable rain since at least 1950. One Labor Day in six since 1950 has reached 80°F.
San Francisco Opera's free Opera in the Park concert usually brings afternoon crowds to Golden Gate Park in September. In HRRR analysis, the park averages about 65°F at 3 PM in September, about the same as downtown, but shade under the trees and the late-afternoon breeze turn cool, so bring a layer.
The Folsom Street Fair in SoMa on the last Sunday of September, and Dreamforce at Moscone Center when it falls in September, land on the east side of the city, which is usually clear by afternoon. On heat-wave days the pavement gets hot, so plan for sun, shade and water. Portola, on the last weekend of September, is at Pier 80, an exposed waterfront site where the breeze off the bay makes evenings feel cooler than the forecast high. Check the Dreamforce, Folsom Street Fair and Portola forecasts.
Is September the Best Time to Visit San Francisco?
For warm, clear city weather, September is usually the best time to visit San Francisco. It brings the warmest highs of the year, the least summer fog, an ocean near 58°F and sunsets that move from about 7:39 PM to 6:54 PM. Rain is rare: 43% of Septembers from 1991 to 2020 had none at all, though about 3 in 10 saw at least a tenth of an inch, and the wettest on record brought 5.07 inches in 1904. The main risks are a heat wave or a smoke day. Days shorten quickly, from about 13 hours on September 1 to 11.8 hours on September 30, so evenings cool sooner by the end of the month. Pack summer clothes for the afternoon and a light jacket for evenings and the coast. October is nearly as warm; compare it in San Francisco weather in October and the best month to visit.
In September, check the San Francisco forecast for heat, the Bay Area fog forecast for the coast and the AirNow AQI map on smoky days. For a beach day, compare Ocean Beach with downtown before you go.