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September Weather in Issaquah, United States

Last updated: June 24, 2025

In September, the average temperature in Issaquah, United States hovers around 15°C (60°F), with fluctuations between a cool 4°C (40°F) and a warm 32°C (90°F). Visitors can expect approximately 115 mm (4.5 in) of precipitation over about 12 days, accompanied by a moderate average humidity of 62%. Whether you enjoy mild weather or sunny days, there's something for everyone!

September Temperature in Issaquah

In September in Issaquah, the average temperature is 15°C (60°F). The maximum can reach 32°C (90°F) and the minimum can be as low as 4°C (40°F). This makes it an ideal time for travelers to enjoy a range of outdoor activities while being prepared for both warm and chilly moments.

September Weather in Issaquah: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Issaquah, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Issaquah, September typically has an average temperature of 15°C (60°F), with highs reaching up to 32°C (90°F) and lows dropping to around 4°C (40°F).
JanFebMarAprMayJunJulAugSepOctNovDec403844495560656560504540
Data source: Temperature data for Issaquah 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 Issaquah

In Issaquah in September, the total precipitation amount is 115 mm (4.5 in) over a span of 12 days.

September Precipitation Trends in Issaquah

The chart displays monthly precipitation patterns in Issaquah, 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 Issaquah, September typically receives around 115 mm (4.5 inches) of precipitation spread over approximately 12 days.
JanFebMarAprMayJunJulAugSepOctNovDec20023415214485782823115205217235
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Issaquah

In Issaquah in September, humidity is 62%. With an average temperature of 15°C (60°F), this humidity feels like a warm embrace, making it a bit muggy for traveling, yet pleasantly refreshing for leisurely strolls and outdoor exploration.

September Average Humidity in Issaquah

This chart illustrates the average monthly humidity levels in Issaquah, 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 Issaquah is approximately 62%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec86%86%87%84%81%82%79%63%62%83%89%88%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Issaquah

In Issaquah 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 Issaquah.

Maximum UV Index Levels in Issaquah for September

The chart shows the monthly maximum UV Index (UVI) for Issaquah 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 Issaquah typically reaches 7, which falls under the high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec235689987522
Data source: Issaquah 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 Issaquah Compares to Other Months

Weather in Issaquah varies notably across the year, with each month offering distinct climate conditions. This page compares September’s weather to other months in Issaquah, focusing on differences in temperature, rainfall, humidity, and UV levels.

This table compares monthly weather conditions in Issaquah, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather5°C (40°F)199 mm (7.9 inches)86%low
February Weather4°C (39°F)234 mm (9.2 inches)86%moderate
March Weather7°C (44°F)152 mm (6.0 inches)87%moderate
April Weather9°C (49°F)144 mm (5.7 inches)84%high
May Weather13°C (55°F)85 mm (3.3 inches)81%very high
June Weather16°C (60°F)78 mm (3.1 inches)82%very high
July Weather18°C (65°F)28 mm (1.1 inches)79%very high
August Weather18°C (65°F)23 mm (0.9 inches)63%very high
September Weather15°C (60°F)115 mm (4.5 inches)62%high
October Weather10°C (50°F)204 mm (8.1 inches)83%moderate
November Weather7°C (45°F)217 mm (8.5 inches)89%low
December Weather4°C (40°F)235 mm (9.3 inches)88%low

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