September Weather in Washington, United States

Last updated: June 24, 2025

In September, the average temperature in Washington, United States is a pleasant 22°C (72°F), with fluctuations ranging from a cool 5°C (41°F) to a warm 37°C (99°F). During this month, you can expect approximately 67 mm (2.7 in) of precipitation over 6 days. With an average humidity of 71%, it's a time when the air feels comfortably balanced.

September Temperature in Washington

In September in Washington, the average temperature is 22°C (72°F). Maximums can reach 37°C (99°F), and minimums can be as low as 5°C (41°F). This range offers travelers the opportunity to enjoy both warm days and cooler evenings, making it a versatile time to explore.

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 22°C (72°F), with highs reaching up to 37°C (99°F) and lows dropping to around 5°C (41°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 67 mm (2.7 in) over the course of 6 days, indicating a moderate level of rainfall for the region during that time.

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 67 mm (2.7 inches) of precipitation spread over approximately 6 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 71%. With an average temperature of 22°C (72°F), this humidity feels like the air is thick and slightly stifling, making outdoor activities require frequent breaks to stay hydrated and cool.

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 71%, 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 9, which corresponds to a very high Exposure Category. The time to burn is just 15 minutes, so it's essential to take precautions. For 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 9, 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 (30°F)	28 mm (1.1 inches)	76%	moderate
February Weather	-1°C (30°F)	27 mm (1.1 inches)	77%	moderate
March Weather	7°C (45°F)	86 mm (3.4 inches)	70%	high
April Weather	12°C (53°F)	60 mm (2.4 inches)	73%	very high
May Weather	18°C (65°F)	201 mm (7.9 inches)	82%	very high
June Weather	25°C (78°F)	94 mm (3.7 inches)	82%	extreme
July Weather	27°C (81°F)	89 mm (3.5 inches)	81%	extreme
August Weather	25°C (77°F)	95 mm (3.7 inches)	78%	very high
September Weather	22°C (72°F)	67 mm (2.7 inches)	71%	very high
October Weather	13°C (55°F)	121 mm (4.8 inches)	71%	high
November Weather	6°C (43°F)	31 mm (1.2 inches)	81%	moderate
December Weather	2°C (35°F)	28 mm (1.1 inches)	84%	moderate