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September Weather in Walnut Creek, United States

Last updated: June 24, 2025

In September, the average temperature in Walnut Creek, United States hovers around 20°C (68°F). Expect a range from a cool 5°C (41°F) to a warm 34°C (93°F). During this time, precipitation amounts to 105 mm (4.2 in) spread over about 10 days, with an average humidity level of 77%. Perfect for those who enjoy a mild, albeit slightly wet, climate!

September Temperature in Walnut Creek

In September in Walnut Creek, the average temperature is 20°C (68°F). Maximums can reach 34°C (93°F) and minimums can be as low as 5°C (41°F). Travelers should be prepared for warm days and cool nights, making layering essentials for this time of year.

September Weather in Walnut Creek: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Walnut Creek, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Walnut Creek, September typically has an average temperature of 20°C (68°F), with highs reaching up to 34°C (93°F) and lows dropping to around 5°C (41°F).
JanFebMarAprMayJunJulAugSepOctNovDec303441516371757368574236
Data source: Temperature data for Walnut Creek 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 Walnut Creek

In Walnut Creek in September, the precipitation amount is 105 mm (4.2 in) over a span of 10 days.

September Precipitation Trends in Walnut Creek

The chart displays monthly precipitation patterns in Walnut Creek, 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 Walnut Creek, September typically receives around 105 mm (4.2 inches) of precipitation spread over approximately 10 days.
JanFebMarAprMayJunJulAugSepOctNovDec89106103129123126119861051099495
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Walnut Creek

In Walnut Creek in September, humidity is 77%. With an average temperature of 20°C (68°F), this humidity feels like a warm, sticky embrace, making it essential for travelers to stay hydrated and take breaks in shaded areas to avoid feeling sluggish in the heat.

September Average Humidity in Walnut Creek

This chart illustrates the average monthly humidity levels in Walnut Creek, 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 Walnut Creek is approximately 77%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec89%88%87%87%81%83%82%76%77%82%89%86%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Walnut Creek

In Walnut Creek in September, the maximum UV index is 8, which corresponds to a very high Exposure Category. This means that the time to burn is only 15 minutes. For more detailed information, you can refer to the Hourly UV Index in Walnut Creek.

Maximum UV Index Levels in Walnut Creek for September

The chart shows the monthly maximum UV Index (UVI) for Walnut Creek 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 Walnut Creek typically reaches 8, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec356910101098642
Data source: Walnut Creek 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 Walnut Creek Compares to Other Months

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

This table compares monthly weather conditions in Walnut Creek, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather-1°C (31°F)88 mm (3.5 inches)88%moderate
February Weather1°C (34°F)106 mm (4.2 inches)88%moderate
March Weather5°C (42°F)103 mm (4.1 inches)87%high
April Weather11°C (51°F)129 mm (5.1 inches)87%very high
May Weather17°C (63°F)122 mm (4.8 inches)81%very high
June Weather22°C (71°F)126 mm (5.0 inches)83%very high
July Weather24°C (76°F)118 mm (4.7 inches)82%very high
August Weather23°C (73°F)86 mm (3.4 inches)76%very high
September Weather20°C (68°F)105 mm (4.2 inches)77%very high
October Weather14°C (57°F)109 mm (4.3 inches)82%high
November Weather5°C (42°F)94 mm (3.7 inches)89%moderate
December Weather2°C (36°F)95 mm (3.8 inches)86%low

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