Monthly climate guide
Seattle weather in September
Expect mild weather in Seattle in September: afternoons reach about 21°C (69°F), while nights drop to around 11°C (52°F). Around 65 mm of rain falls over the month (some rain), and skies are often cloudy. Daytime highs are 3°C cooler than August and 5°C warmer than October.
What stands out in September
- Temperatures are falling — expect highs around 5°C lower by October.
- Seasonally, September counts as early autumn in Seattle.
- Seattle is usually about 4°C cooler than Portland, 230 km away, in September.
- September is Seattle's third warmest month.
Avg high
21°C
69°F
Avg low
11°C
52°F
Rainfall
65 mm
some rain
Humidity
79%
relative
Sky
55%
cloud · often cloudy
Seattle weather by month
Show the monthly averages as a table
| Month | Avg high | Avg low | Rain | Humidity | Cloud cover |
|---|---|---|---|---|---|
| January | 7°C / 45°F | 2°C / 35°F | 169 mm | 90% | 79% |
| February | 8°C / 46°F | 2°C / 35°F | 138 mm | 88% | 78% |
| March | 11°C / 51°F | 3°C / 38°F | 155 mm | 85% | 76% |
| April | 14°C / 56°F | 5°C / 41°F | 107 mm | 82% | 75% |
| May | 18°C / 64°F | 9°C / 47°F | 70 mm | 80% | 69% |
| June | 20°C / 68°F | 11°C / 51°F | 51 mm | 80% | 68% |
| July | 23°C / 73°F | 13°C / 56°F | 20 mm | 78% | 47% |
| August | 24°C / 75°F | 14°C / 56°F | 23 mm | 77% | 46% |
| September | 21°C / 69°F | 11°C / 52°F | 65 mm | 79% | 55% |
| October | 15°C / 60°F | 8°C / 46°F | 142 mm | 85% | 68% |
| November | 10°C / 50°F | 4°C / 39°F | 184 mm | 89% | 76% |
| December | 7°C / 44°F | 2°C / 35°F | 172 mm | 91% | 78% |
What to pack for September
- T-shirts plus a light layer for the evening
- Comfortable trainers or sandals for long walks
- A small travel umbrella, just in case
Is September a good time to visit Seattle?
Compare the months in the chart above to find your ideal window.
Best time to visit Seattle →Other United States cities in September
Cite or embed this data
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Seattle weather in September. WeatherCompare, 2026 (NASA POWER / ERA5, 2011–2020 monthly averages). https://weathercompare.eu/weather/usa/seattle/september
<a href="https://weathercompare.eu/weather/usa/seattle/september">Seattle weather in September</a> (WeatherCompare)
<iframe src="https://weathercompare.eu/embed/usa/seattle" width="300" height="160" style="border:0;border-radius:12px" loading="lazy" title="Seattle weather in September"></iframe> <p style="font-size:12px"><a href="https://weathercompare.eu/weather/usa/seattle/september">Seattle weather in September</a> – WeatherCompare</p>
Seattle in September: quick answers
Is September a good time to visit Seattle?
September is a reasonable time for Seattle: highs around 21°C and about 65 mm of rain, though not one of the three best months.
How warm is Seattle in September?
Average highs are about 21°C (69°F) and average lows about 11°C (52°F), based on 10 years of daily data.
Does it rain a lot in Seattle in September?
Seattle typically gets around 65 mm of rain in September — some rain.
Figures are long-term averages for the area around Seattle (NASA POWER / ERA5, 2011–2020), not a forecast for a specific year. For upcoming days, see the live multi-source forecast.