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September Weather in Washington, United States

Last updated: June 24, 2025

In September, the average temperature in Washington, United States hovers around 17°C (63°F). Visitors might experience a minimum chill of -1°C (30°F), while the warmth can peak at a pleasant 32°C (90°F). Expect about 78 mm (3.1 in) of rain spread over 8 days, contributing to an average humidity of 71%. Be prepared for a mix of weather during your stay!

September Temperature in Washington

In September in Washington, the average temperature is 17°C (63°F). Maximums can reach 32°C (90°F) and minimums can drop to as low as -1°C (30°F). Travelers should be prepared for a wide range of temperatures and pack accordingly for both warm days and cool nights.

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 17°C (63°F), with highs reaching up to 32°C (90°F) and lows dropping to around -1°C (30°F).
JanFebMarAprMayJunJulAugSepOctNovDec232632445666717062513727
Data source: Temperature data for Washington generated using information from the Copernicus Climate Change Service.
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The average of daily temperatures (24 hours) calculated across all days in the month.

September Precipitation in Washington

In Washington in September, the total precipitation amount is 78 mm (3.1 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 78 mm (3.1 inches) of precipitation spread over approximately 8 days.
JanFebMarAprMayJunJulAugSepOctNovDec71857812683901299478132105114
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Washington

In Washington in September, humidity is 71%. With an average temperature of 17°C (63°F), this humidity feels like a warm embrace, making outdoor activities enjoyable yet a bit sticky, so travelers should 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 71%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec86%84%82%80%72%72%67%66%71%79%84%81%
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.
JanFebMarAprMayJunJulAugSepOctNovDec3468910997632
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.
 TemperaturePrecipitationHumidityUV
January Weather-5°C (23°F)71 mm (2.8 inches)86%moderate
February Weather-3°C (26°F)85 mm (3.3 inches)84%moderate
March Weather0°C (32°F)78 mm (3.1 inches)82%high
April Weather7°C (44°F)126 mm (5.0 inches)80%very high
May Weather13°C (56°F)83 mm (3.3 inches)72%very high
June Weather19°C (66°F)90 mm (3.5 inches)72%very high
July Weather22°C (72°F)129 mm (5.1 inches)67%very high
August Weather21°C (70°F)94 mm (3.7 inches)66%very high
September Weather17°C (63°F)78 mm (3.1 inches)71%high
October Weather10°C (51°F)132 mm (5.2 inches)78%high
November Weather3°C (37°F)105 mm (4.1 inches)84%moderate
December Weather-3°C (27°F)114 mm (4.5 inches)81%low

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