Mercer Island Weather in September
Average highs run from 76°F on September 1 to 67°F on September 30, and lows from 57°F to 51°F. Normal precipitation for the month is 1.37 in over about 9 wet days.
September day by day
The normal high and low for each date, and the record, in °F.
- Normal low
- Normal high
- Record high
- Record low
September at a glance
- Average high
- 72°F
- Average low
- 54°F
- Precipitation
- 1.37 in
- Record high
- 92°FSep 16, 2025
- Record low
- 36°FSep 30, 1950
- Chance of a freeze
- 0%
- Daylight
- 13 h 21 min to 11 h 43 min
- Cloud cover
- 48%
- Humidity
- 76%
- Dew point
- 49°F
- Wind
- 3.4 mph
Every day of the month
A wet day has at least 0.01 inch of precipitation; snow means at least 0.1 inch of snowfall.
| Date | Normal high | Normal low | Record high | Record low | Chance wet | Chance of snow |
|---|---|---|---|---|---|---|
| Sep 1 | 76°F | 57°F | 85°F 2006 | 46°F 2021 | 20% | 0% |
| Sep 2 | 75°F | 57°F | 88°F 2017 | 43°F 1960 | 20% | 0% |
| Sep 3 | 75°F | 56°F | 87°F 2003 | 44°F 1956 | 21% | 0% |
| Sep 4 | 75°F | 56°F | 87°F 1963 | 45°F 1956 | 21% | 0% |
| Sep 5 | 75°F | 56°F | 89°F 2024 | 45°F 1950 | 22% | 0% |
| Sep 6 | 75°F | 56°F | 88°F 2014 | 46°F 1948 | 23% | 0% |
| Sep 7 | 74°F | 56°F | 87°F 2014 | 45°F 1960 | 23% | 0% |
| Sep 8 | 74°F | 56°F | 83°F 2011 | 45°F 1961 | 24% | 0% |
| Sep 9 | 74°F | 55°F | 90°F 2020 | 46°F 1952 | 24% | 0% |
| Sep 10 | 74°F | 55°F | 89°F 2020 | 43°F 1952 | 25% | 0% |
| Sep 11 | 73°F | 55°F | 89°F 2013 | 41°F 1952 | 26% | 0% |
| Sep 12 | 73°F | 55°F | 84°F 1961 | 41°F 1949 | 26% | 0% |
| Sep 13 | 73°F | 55°F | 85°F 1957 | 42°F 1952 | 27% | 0% |
| Sep 14 | 73°F | 54°F | 87°F 2000 | 48°F 1952 | 27% | 0% |
| Sep 15 | 72°F | 54°F | 87°F 2014 | 43°F 1952 | 28% | 0% |
| Sep 16 | 72°F | 54°F | 92°F 2025 | 42°F 1965 | 29% | 0% |
| Sep 17 | 72°F | 54°F | 78°F 2012 | 42°F 1965 | 29% | 0% |
| Sep 18 | 71°F | 54°F | 83°F 2009 | 41°F 1955 | 30% | 0% |
| Sep 19 | 71°F | 53°F | 82°F 1999 | 40°F 1957 | 31% | 0% |
| Sep 20 | 71°F | 53°F | 81°F 1963 | 41°F 1949 | 31% | 0% |
| Sep 21 | 70°F | 53°F | 88°F 1952 | 41°F 1955 | 32% | 0% |
| Sep 22 | 70°F | 53°F | 88°F 2009 | 40°F 1956 | 32% | 0% |
| Sep 23 | 70°F | 52°F | 85°F 1957 | 41°F 1948 | 33% | 0% |
| Sep 24 | 69°F | 52°F | 78°F 1957 | 38°F 1948 | 33% | 0% |
| Sep 25 | 69°F | 52°F | 78°F 1963 | 38°F 1948 | 34% | 0% |
| Sep 26 | 68°F | 52°F | 81°F 2022 | 43°F 1951 | 35% | 0% |
| Sep 27 | 68°F | 51°F | 83°F 1963 | 43°F 1950 | 35% | 0% |
| Sep 28 | 68°F | 51°F | 87°F 2017 | 40°F 1999 | 36% | 0% |
| Sep 29 | 67°F | 51°F | 77°F 2020 | 38°F 1948 | 36% | 0% |
| Sep 30 | 67°F | 51°F | 81°F 1957 | 36°F 1950 | 37% | 0% |
Chance of rain
- Chance of a wet day
- Chance of snowfall
Sunrise and sunset
Times for 2026, local time (PDT).
- September 1
- sunrise 6:28 AM, sunset 7:49 PM
- September 30
- sunrise 7:07 AM, sunset 6:50 PM
Other months
Sources
Normals: NOAA NCEI U.S. Climate Normals, 1991-2020, station USW00024234 (Seattle Boeing Fld), public domain. Records: NOAA GHCN-Daily, station USW00024234, every year on file, quality-flagged values removed. Cloud, humidity, wind and sunshine: NASA POWER (MERRA-2, CERES and GEWEX SRB), 1991-2020. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division: POWER Daily API v2, accessed 2026-09-24. Sun and moon: computed with the NOAA solar equations and Meeus.