Seattle Weather in September

September continues with much of the warm, dry and sunny weather that's typical for Seattle, Washington in summer.

This page gives detailed information on Seattle's usual September temperature, precipitation, sun, humidity, wind and storms for Seattle. The weather measurements are averages based on climate data gathered at Seattle — Tacoma International Airport, located south of the city center.

Temperature Weather iconTemperature

72 °F September average high 22 °C
54 °F September average low 12 °C
16 days 70 °F max. or more (21 °C)
29 days 60 °F min. or less (16 °C)

Nearly every September day warms to over 60 degrees Fahrenheit (16 degrees Celsius) in Seattle. The city averages 4 days this month when the thermometer reaches into the 80s °F (over 27 °C). Temperatures above 90 °F (32 °C) are unusual here during September.

The thermometer dips to 50 °F (10 °C) or below on an average of 8 nights. But September does not normally get as cold as 40 °F (4 °C).

Rainy Weather iconRain

1.6 in total precipitation 41 mm
8 days of rain

Half the time, Seattle receives 0.8 to 2.1 inches (20 to 54 mm) of rainfall in September. One in four years has drier weather, while another one in four is wetter.

Sunny Weather iconSun

53 % sunshine
9 days clear skies

The sun's rays reach the ground in Seattle during 53 percent of daylight hours on average in September, for a total of 199 hours in the month. Typically on 9 days this month the sky is mainly clear, with at most 30 percent cloud cover.

Sunlight in September at Seattle lasts for 12 hours and 31 minutes a day on average. The longest day of the month is the 1st with 13 hours and 21 minutes of daylight. By the end of the month, the days have shortened to 11:43 from sunrise to sunset. These durations can vary by a minute or two from year to year.

Humid Weather iconHumidity

73 % September average humidity

The relative humidity in Seattle during September usually peaks at 86 percent in the morning then drops to 57 percent by late afternoon.

Local Time1 am4 am7 am10 am1 pm4 pm7 pm10 pm
Humidity % 82 86 86 73 60 57 67 75

Windy Weather iconWind

8.0 mph September wind speed average 12.9 kph

September is less windy than most other months in Seattle.

Stormy Weather iconStorms

1 day rainfall of 0.5 inches (13 mm) or more
1 day with thunderstorms in September

Seattle gets a September thunderstorm on an average of about once every two years.


The weather data here are long-term averages provided by the US National Centers for Environmental Information (NCEI). Temperature and precipitation are 1991-2020 normals while humidity and thunderstorms are 1961-1990 normals.

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