September weather forecast and climate
Seattle, WA

Beautiful dusk skyline over the giant skyscrapers of Seattle, Washington
Beautiful dusk skyline over the giant skyscrapers of Seattle, Washington
September is transitional month and experiences summer weather only for the first few weeks. With a significant rise in humidity to 72.9%, temperatures in September cool to the range of 51.8°F (11°C) and 68°F (20°C) as autumn arrives. With the close of summer, rainfall sets to rain for nine days gathering moderately low precipitation of 2" (50.8mm).
Swimming in the 55.4°F (13°C) seawater remains rampant throughout September, as does sunbathe in the seven hours of warm sunshine.

What is the temperature in September in Seattle?

In September, in Seattle, the average high-temperature is 70.5°F (21.4°C), and the average low-temperature is 52.2°F (11.2°C).

How many days does it rain in September in Seattle?

In Seattle, in September, it is raining for 7.9 days, with typically 1.5" (38.1mm) of accumulated precipitation.

What is the water temperature in September in Seattle?

In September, in Seattle, Washington, the average sea temperature is 53.2°F (11.8°C). Swimming in 53.2°F (11.8°C) is considered life-threatening. Even a few minutes in 55.4°F (13°C) water is uncomfortable, and swimming below 50°F (10°C) may cause total loss of breathing control and cold shock, depending on a person's physique.
Average temperature in September - Seattle, WA
  • Average high temperature in September: 70.5°F

The warmest month (with the highest average high temperature) is August (76.3°F).
The month with the lowest average high temperature is December (45.7°F).

  • Average low temperature in September: 52.1°F

The month with the highest average low temperature is August (55.9°F).
The coldest month (with the lowest average low temperature) is December (35.6°F).

Average humidity in September - Seattle, WA
  • Average humidity in September: 73.2%

The month with the highest relative humidity is December (80.1%).
The month with the lowest relative humidity is July (65.4%).

Average rainfall in September - Seattle, WA
  • Average rainfall in September: 1.5"

The wettest month (with the highest rainfall) is November (6.6").
The driest month (with the lowest rainfall) is July (0.7").

Average rainfall days in September - Seattle, WA
  • Average rainfall days in September: 7.9 days

The month with the highest number of rainy days is November (18.4 days).
The month with the lowest number of rainy days is August (4.8 days).

Average snowfall in September - Seattle, WA
  • Average snowfall in September: 0"

Months with the highest snowfall are February and December (1.7").
Months with the lowest snowfall are April, May, June, July, August, September and October (0").

Average snowfall days in September - Seattle, WA
  • Average snowfall days in September: 0 days

The month with the highest number of snowfall days is December (1.6 days).
Months with the lowest number of snowfall days are April, May, June, July, August, September and October (0 days).

Average sea temperature in September - Seattle, WA
  • Average sea temperature in September: 53.3°F

The best month for swimming (with the highest average sea temperature) is August (53.5°F).
The coldest month (with the lowest average sea temperature) is February (45.3°F).

Average daylight in September / Average sunshine in September - Seattle, WA
  • Average daylight in September: 12.5h

The month with the longest days is June (Average daylight: 15.9h).
The month with shortest days is December (Average daylight: 8.5h).

  • Average sunshine in September: 7.4h

The month with most sunshine is July (Average sunshine: 10.1h).
The month with least sunshine is December (Average sunshine: 1.7h).

Average UV index in September - Seattle, WA
  • Average UV index in September: 5

Months with the highest UV index are June and July (UV index 7).
Months with the lowest UV index are January, November and December (UV index 1).

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