Weather in San Diego in September

A good month. September works. It is not the peak, but nothing about it fights you. Highs average 31.7 °C, lows 19.9 °C, and rain falls on 1 days — nothing in the month is working against you.

September ranks 10 of 12 — near the bottom of San Diego's year. May is the month that works, and the swing between the two is 25 points, which is a different holiday rather than a slightly better one. Worth keeping in perspective: September is the weakest month of San Diego's year and it is still a perfectly good one. The downside here is small.

Rain falls on only 1 days, so an itinerary with no wet-weather fallback is a reasonable risk. Pack for 31.7 °C days and 19.9 °C nights; the spread matters more here than any single number.

What the averages hide

Averages flatten a month, and this one hides a few things: 5 days clear 35 °C, so a serious heatwave is normal rather than notable. Across the twenty years measured, the hottest September day reached 41.5 °C and the coldest night fell to 13.8 °C.

September is dependably dry, and that reliability is the point: across the twenty years measured, the wettest September on record still only managed 22 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 17 mm. Rain simply is not the variable that decides this trip — heat, crowds and cost are.

Wind is light at 6.9 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 57%.

How September sits in the year

Temperatures hold steady either side: 32.7 °C in August, 31.7 here, 27.9 in October. Nothing about the timing within the month matters much.

Daylight in September

12.4 hours of daylight, losing 1 hours across the month. That is between San Diego's extremes of 14.3 and 10 hours, so days are long enough not to constrain the plan.

Sea temperature

The sea sits at about 20.3 °C in September — swimmable, but it takes a moment to commit. This is the warmest the water gets all year.

How busy it gets

Off season. The genuine off season: cheapest by a distance, with the caveat that some tours and restaurants close entirely.

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

September in San Diego versus nearby

If San Diego is one option among several, the nearby comparison is worth making. Irvine is a better bet in September — 8.3 °C cooler, 1 day less rain. Riverside is the weaker option in September — 3.8 °C warmer, the same number of rain days.

So: September is a compromise. May is 21 points better and is what San Diego is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in San Diego

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