Weather in Napa in September

A good month. September works. It is not the peak, but nothing about it fights you. Highs average 32.4 °C, lows 15.2 °C, and rain falls on 1 days — nothing in the month is working against you.

September ranks 6 of 12 in Napa: past the worst of the year but short of May, 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. Pack for 32.4 °C days and 15.2 °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: 9 days clear 35 °C, so a serious heatwave is normal rather than notable; the day swings 17.2 °C from low to high, which means one bag has to cover both. Across the twenty years measured, the hottest September day reached 46 °C and the coldest night fell to 8.7 °C.

September is dependably dry, and that reliability is the point: across the twenty years measured, the wettest September on record still only managed 16 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 13 mm. Rain simply is not the variable that decides this trip — heat, crowds and cost are.

Wind is light at 6 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 47%.

How September sits in the year

Temperatures hold steady either side: 33.8 °C in August, 32.4 here, 27 in October. Nothing about the timing within the month matters much.

Daylight in September

12.5 hours of daylight, losing 1.2 hours across the month. That is between Napa's extremes of 14.8 and 9.5 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 28.6 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. Within September itself the daily swing is 17.2 °C, so mornings and afternoons need different clothes.

How busy it gets

Shoulder. The value months — prices ease off and you get the place closer to itself.

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

September in Napa versus nearby

If Napa is one option among several, the nearby comparison is worth making. San Francisco is a better bet in September — 7.9 °C cooler, the same number of rain days. Sacramento is close enough to be a coin toss — within a degree on temperature, the same number of rain days.

So: September is a compromise. May is 20 points better and is what Napa 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 Napa

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