Weather in Seattle in September

Prime time. September is about as good as Seattle gets. Highs average 21 °C, lows 11.3 °C, and rain falls on 10 days — nothing in the month is working against you.

September ranks 3 of 12 in Seattle. Only July scores clearly higher, and the difference is small enough to be settled by price rather than weather.

Rain on 10 days, 1 of them heavy. Enough to need a plan B, not enough to need a plan around it. 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: it never really gets hot: not one day in an average September reaches 30 °C. Across the twenty years measured, the hottest September day reached 33.1 °C and the coldest night fell to 4.1 °C, and the wettest single day dropped 37 mm — roughly 64% of the month's rain in one day.

September is moderately variable: 5 mm in the driest year against 185 mm in the wettest, around an average of 58 mm. Roughly two Septembers in three land near that average, and the third does something else entirely.

Wind is light at 2.2 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 78%, high enough to notice on the warm days without dominating them.

How September sits in the year

The month is on the way down: 2.9 °C cooler than August, with October dropping another 5.5 °C. The first week and the last are not the same trip. Rain is the thing that moves: October brings 5 days more of it, so the earlier half of a flexible trip is the safer half.

Daylight in September

12.7 hours of daylight, losing 1.7 hours across the month. That is between Seattle's extremes of 16 and 8.5 hours, so days are long enough not to constrain the plan.

An inland climate

Despite being inland, the year stays fairly even: 21.9 °C between the year's warmest afternoons and coldest nights.

How busy it gets

Busy. Popular without being frantic; book the things you care about, improvise the rest.

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

September in Seattle versus nearby

If Seattle is one option among several, the nearby comparison is worth making. Maple Ridge is close enough to be a coin toss — within a degree on temperature, 2 days more rain. Vancouver is close enough to be a coin toss — within a degree on temperature, 2 days more rain.

So: September works, with July the better month by 11 points if the dates can move. If they cannot, nothing here should stop you booking.

Every month in Seattle

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