Mercer Island, WA Weather & Growing Conditions
Local forecast, frost dates, hardiness zone and planting guidance for Mercer Island, a city in King County, Washington.
Today
71°F
Partly Sunny
Partly sunny, with a high near 71. West southwest wind around 2 mph.
7-day forecast for Mercer Island
| When | Temp | Forecast | Wind | Precip |
|---|---|---|---|---|
| Today | 71°F | Partly Sunny | 2 mph WSW | 0% |
| Tonight | 56°F | Mostly Clear | 3 mph N | 0% |
| Sunday | 71°F | Mostly Sunny | 5 mph N | 0% |
| Sunday Night | 55°F | Mostly Clear | 5 mph NNE | 0% |
| Monday | 70°F | Mostly Sunny | 3 mph N | 0% |
| Monday Night | 56°F | Partly Cloudy | 3 mph NNE | 1% |
| Tuesday | 68°F | Partly Sunny | 2 mph N | 1% |
| Tuesday Night | 54°F | Mostly Cloudy | 2 mph E | 8% |
| Wednesday | 69°F | Partly Sunny then Slight Chance Light Rain | 3 mph SW | 16% |
| Wednesday Night | 54°F | Slight Chance Light Rain then Mostly Cloudy | 3 mph NNE | 16% |
| Thursday | 66°F | Mostly Cloudy then Slight Chance Light Rain | 2 to 6 mph NNE | 21% |
| Thursday Night | 53°F | Slight Chance Light Rain | 2 to 6 mph NNE | 21% |
| Friday | 65°F | Slight Chance Light Rain | 2 to 6 mph N | 15% |
| Friday Night | 51°F | Slight Chance Light Rain then Partly Cloudy | 5 mph NNE | 15% |
Forecast source: National Weather Service (api.weather.gov) for Mercer Island (47.564, -122.231). Updated through the day.
Growing snapshot for Mercer Island
Last spring frost
Mar 13
First fall frost
Nov 17
Growing season
247 days
Hardiness zone
9a
More local growing data for Mercer Island: nearby ZIP 98040 weather · frost dates · planting calendar · Washington frost dates
Nearby towns & communities
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Black Diamond
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Bryn Mawr-Skyway
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Carnation
Clyde Hill
Cottage Lake
Covington
Des Moines
Duvall
East Renton Highlands
Fairwood
Fall City
Federal Way
Greenwater
High Bridge
Hobart
Hunts Point
Index
Issaquah
Kenmore
Kent
Kirkland
Climate data from the nearest NOAA station, Seattle Boeing Fld, Wa (4 mi from Mercer Island). Frost dates are 1991–2020 averages, not a forecast. Page updated Sep 19, 2026.