September Weather in Washington, United States

Last updated: June 24, 2025

In September, the average temperature in Washington, United States hovers around 21°C (69°F), offering a pleasant climate for visitors. Temperatures can swing between a cool 3°C (38°F) and a warm 35°C (94°F), so it's wise to come prepared. Be aware that precipitation is common, with an average of 108 mm (4.3 in) over about 10 days, and expect a comfortable average humidity level of 68%.

September Temperature in Washington

In September in Washington, the average temperature is 21°C (69°F). Maximum can reach 35°C (94°F) and minimum can be as low as 3°C (38°F). This makes Washington a great destination for travelers looking to enjoy a variety of weather conditions, so pack accordingly!

September Weather in Washington: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Washington, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Washington, September typically has an average temperature of 21°C (69°F), with highs reaching up to 35°C (94°F) and lows dropping to around 3°C (38°F).

Data source: Temperature data for Washington generated using information from the Copernicus Climate Change Service.

September Precipitation in Washington

In Washington in September, the total precipitation amount is 108 mm (4.3 in) spread over a duration of 10 days.

September Precipitation Trends in Washington

The chart displays monthly precipitation patterns in Washington, with values for each month alongside September’s data, which is highlighted for easy comparison. It shows the amount of precipitation, providing insights into how September’s conditions compare to those of other months. In Washington, September typically receives around 108 mm (4.3 inches) of precipitation spread over approximately 10 days.

Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.

Choose your preferred precipitation unit

Millimeters (mm) Inches (in)

September Humidity in Washington

In Washington in September humidity is 68%. With average temperature 21°C (69°F), this humidity feels like a warm embrace, making it essential for travelers to stay hydrated and take breaks in the shade while exploring the local sights.

September Average Humidity in Washington

This chart illustrates the average monthly humidity levels in Washington, with September’s data highlighted for easy comparison. It shows the fluctuation in humidity throughout the year, helping you understand how conditions vary by season. In September, average humidity in Washington is approximately 68%, offering insight into how humid the weather typically is during this month.

Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Washington

In Washington in September, the maximum UV index is 8, which corresponds to a very high Exposure Category. This means that the time to burn is only 15 minutes. For more detailed information, you can refer to the Hourly UV Index in Washington.

Maximum UV Index Levels in Washington for September

The chart shows the monthly maximum UV Index (UVI) for Washington in September, representing the highest UV radiation level. The UVI measures the intensity of ultraviolet radiation from the sun, indicating the potential risk of sunburn and UV-related health issues. In September, the maximum UV index in Washington typically reaches 8, which falls under the very high exposure category.

Data source: Washington September UVI data generated using information from the Copernicus Climate Change Service.

UV Risk Categories

Extreme (11+): Avoid the sun, stay in shade.
Very High (8-10): Limit sun exposure.
High (6-7): Use SPF 30+ and protective clothing.
Moderate (3-5): Midday shade recommended.
Low (0-2): No protection needed.

How September Weather in Washington Compares to Other Months

Weather in Washington varies notably across the year, with each month offering distinct climate conditions. This page compares September’s weather to other months in Washington, focusing on differences in temperature, rainfall, humidity, and UV levels.

This table compares monthly weather conditions in Washington, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
	Temperature	Precipitation	Humidity	UV
January Weather	1°C (35°F)	59 mm (2.3 inches)	86%	moderate
February Weather	4°C (40°F)	90 mm (3.6 inches)	86%	moderate
March Weather	7°C (45°F)	76 mm (3.0 inches)	86%	high
April Weather	13°C (55°F)	105 mm (4.1 inches)	84%	very high
May Weather	18°C (65°F)	125 mm (4.9 inches)	78%	extreme
June Weather	22°C (73°F)	121 mm (4.8 inches)	79%	very high
July Weather	25°C (78°F)	103 mm (4.1 inches)	70%	very high
August Weather	24°C (75°F)	132 mm (5.2 inches)	59%	very high
September Weather	21°C (69°F)	108 mm (4.3 inches)	68%	very high
October Weather	15°C (60°F)	103 mm (4.1 inches)	80%	high
November Weather	7°C (45°F)	83 mm (3.3 inches)	86%	moderate
December Weather	4°C (39°F)	85 mm (3.4 inches)	85%	moderate