September Weather in Washington, United States

Last updated: June 24, 2025

In September, the average temperature in Washington, United States hovers around 15°C (60°F). Be prepared for chilly nights, as temperatures can drop to a minimum of -4°C (25°F), while daytime highs can reach up to 31°C (88°F). Expect about 71 mm (2.8 in) of precipitation over 8 days, accompanied by an average humidity of 72%.

September Temperature in Washington

In September in Washington, the average temperature is 15°C (60°F). The maximum can reach 31°C (88°F) and the minimum can be as low as -4°C (25°F). Travelers should be prepared for a wide range of temperatures during this time, ensuring they pack accordingly for both warm and cool weather.

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 15°C (60°F), with highs reaching up to 31°C (88°F) and lows dropping to around -4°C (25°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 precipitation amount is 71 mm (2.8 in) over the course of 8 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 71 mm (2.8 inches) of precipitation spread over approximately 8 days.

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

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Millimeters (mm) Inches (in)

September Humidity in Washington

In Washington in September humidity is 72%. With an average temperature of 15°C (60°F), this humidity feels like a warm embrace, creating a slightly muggy atmosphere that can make strolling around a bit sticky. Travelers might find themselves longing for a refreshing breeze, and it’s advisable to stay hydrated while exploring 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 72%, 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 7, which corresponds to the high Exposure Category. This means that the time to burn is approximately 25 minutes. For more detailed information, you can check 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 7, which falls under the 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	-7°C (19°F)	77 mm (3.0 inches)	89%	low
February Weather	-6°C (22°F)	80 mm (3.2 inches)	88%	moderate
March Weather	-2°C (29°F)	71 mm (2.8 inches)	86%	moderate
April Weather	5°C (42°F)	117 mm (4.6 inches)	84%	very high
May Weather	12°C (55°F)	96 mm (3.8 inches)	76%	very high
June Weather	17°C (64°F)	97 mm (3.8 inches)	72%	very high
July Weather	20°C (69°F)	140 mm (5.5 inches)	66%	very high
August Weather	19°C (67°F)	98 mm (3.9 inches)	64%	very high
September Weather	15°C (60°F)	71 mm (2.8 inches)	72%	high
October Weather	9°C (48°F)	141 mm (5.6 inches)	81%	moderate
November Weather	1°C (33°F)	104 mm (4.1 inches)	87%	moderate
December Weather	-5°C (24°F)	88 mm (3.4 inches)	84%	low