September Weather in Washington, United States

Last updated: June 24, 2025

In September in Washington, United States, the average temperature is a delightful 26°C (80°F). Visitors can expect a minimum of 13°C (55°F) and a maximum of 37°C (99°F), making it a warm month overall. However, keep an umbrella handy, as precipitation averages 121 mm (4.8 in) over 14 days. Plus, with an average humidity of 79%, it’s wise to stay hydrated and enjoy the tropical vibes!

September Temperature in Washington

In September in Washington, the average temperature is 26°C (80°F), with maximums reaching up to 37°C (99°F) and minimums dropping as low as 13°C (55°F). It's an ideal time for travelers seeking warm weather but be prepared for a range of temperatures during your visit.

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 26°C (80°F), with highs reaching up to 37°C (99°F) and lows dropping to around 13°C (55°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 121 mm (4.8 in) over a span of 14 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 121 mm (4.8 inches) of precipitation spread over approximately 14 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 79%. With an average temperature of 26°C (80°F), this humidity feels like an enveloping warm blanket, making travel feel slightly sluggish and prompting frequent hydration breaks to stay comfortable while exploring the 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 79%, 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 reaches 10, which corresponds to a very high Exposure Category. With a time to burn of just 15 minutes, it's crucial to take precautions. 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 10, 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	11°C (53°F)	109 mm (4.3 inches)	83%	moderate
February Weather	14°C (58°F)	108 mm (4.3 inches)	83%	very high
March Weather	18°C (65°F)	82 mm (3.2 inches)	80%	very high
April Weather	20°C (68°F)	155 mm (6.1 inches)	81%	extreme
May Weather	25°C (77°F)	157 mm (6.2 inches)	81%	extreme
June Weather	27°C (81°F)	148 mm (5.8 inches)	78%	extreme
July Weather	28°C (83°F)	178 mm (7.0 inches)	79%	extreme
August Weather	28°C (82°F)	170 mm (6.7 inches)	79%	extreme
September Weather	26°C (80°F)	121 mm (4.8 inches)	79%	very high
October Weather	22°C (72°F)	116 mm (4.5 inches)	77%	very high
November Weather	15°C (60°F)	78 mm (3.1 inches)	85%	high
December Weather	13°C (56°F)	113 mm (4.5 inches)	88%	moderate