Weather in San Francisco in September

Prime time. If you get to pick your dates, September is the one. Highs average 24.5 °C, lows 13.4 °C, and rain falls on 1 days — nothing in the month is working against you.

September ranks 4 of 12 in San Francisco: past the worst of the year but short of July, which is the month to book if the dates are open.

Rain falls on only 1 days, so an itinerary with no wet-weather fallback is a reasonable risk. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: 4 days top 30 °C — hot spells arrive, but they break. Across the twenty years measured, the hottest September day reached 39.5 °C and the coldest night fell to 9.8 °C.

September is dependably dry, and that reliability is the point: across the twenty years measured, the wettest September on record still only managed 14 mm, and the driest managed 0 mm. There is no meaningful variability to plan around, which makes this one of the few months anywhere that you can build a fixed outdoor itinerary against without a fallback. The wettest single day in twenty years of Septembers brought 12 mm. Rain simply is not the variable that decides this trip — heat, crowds and cost are.

Wind is light at 7.4 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 70%.

How September sits in the year

This is a turning point in the year — August and October sit on opposite sides of it, so the month contains the change rather than following it.

Daylight in September

12.5 hours of daylight, losing 1.2 hours across the month. That is between San Francisco's extremes of 14.8 and 9.6 hours, so days are long enough not to constrain the plan.

Sea temperature

The sea sits at about 15.7 °C in September — cold — a quick dip rather than a swim, and a wetsuit if you are in for longer. This is the warmest the water gets all year.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in San Francisco versus nearby

If San Francisco is one option among several, the nearby comparison is worth making. Napa is the weaker option in September — 7.9 °C warmer, the same number of rain days. San Jose is the weaker option in September — 6.5 °C warmer, the same number of rain days.

So: September is effectively as good as it gets here — 2 points behind July, which is inside the margin where the weather stops being the deciding factor and price starts being it.

Every month in San Francisco

See all twelve months in San Francisco, or where else to go in September.