September Weather in Washington, United States

Last updated: June 24, 2025

In Washington, United States, the average temperature in September hovers around 24°C (75°F), offering a pleasant climate. Be prepared for temperature fluctuations, as it can dip to a chilly 4°C (40°F) and soar up to a sizzling 40°C (104°F). Rainfall is minimal, with only 5 mm (0.2 in) expected over a single day, and the average humidity sits comfortably at 47%.

September Temperature in Washington

In September in Washington, the average temperature is 24°C (75°F). Maximum temperatures can reach 40°C (104°F), while minimums can dip as low as 4°C (40°F). Be prepared for a wide range of temperatures when traveling to Washington during this month, ensuring you pack accordingly for both warm days and cooler evenings.

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 24°C (75°F), with highs reaching up to 40°C (104°F) and lows dropping to around 4°C (40°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 5 mm (0.2 in) over a duration of 1 day.

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 5 mm (0.2 inches) of precipitation spread over approximately 1 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 47%. With an average temperature of 24°C (75°F), this humidity feels like a refreshing embrace, making outdoor activities enjoyable but also reminding travelers to stay hydrated and take breaks in the shade.

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 47%, 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	3°C (38°F)	48 mm (1.9 inches)	80%	moderate
February Weather	5°C (41°F)	42 mm (1.7 inches)	82%	high
March Weather	9°C (49°F)	48 mm (1.9 inches)	72%	very high
April Weather	15°C (59°F)	12 mm (0.5 inches)	62%	very high
May Weather	19°C (67°F)	16 mm (0.6 inches)	45%	extreme
June Weather	27°C (80°F)	1 mm (0.0 inches)	34%	extreme
July Weather	29°C (85°F)	9 mm (0.3 inches)	30%	extreme
August Weather	28°C (83°F)	10 mm (0.4 inches)	31%	extreme
September Weather	24°C (75°F)	5 mm (0.2 inches)	47%	very high
October Weather	15°C (59°F)	13 mm (0.5 inches)	62%	high
November Weather	9°C (49°F)	29 mm (1.1 inches)	77%	moderate
December Weather	4°C (39°F)	35 mm (1.4 inches)	80%	moderate