Photo: Dllu · CC BY-SA 4.0Mercer Island’s winters are milder and less snowy than most of the U.S.
A typical July afternoon reaches 77°, a January night drops to 37°, and about 4 inches of snow falls in a normal winter.
A year in Mercer Island at a glance
Each month’s typical high and low, and what it brings.
What stands out about Mercer Island’s climate
Compared with towns across the U.S., with Mercer Island’s place in Washington beside it.
What a year in Mercer Island feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
About 247 frost-free days a year
In half of years the last 32° night comes by March 13 and the first by November 17. Frost and day counts come from NOAA’s station at Seattle Boeing Fld, WA, 4 miles from the town’s center, at about the same height.
The 2023 USDA map puts Mercer Island’s center in zone 9a: the coldest night of a typical winter, averaged over 1991–2020, reaches about 21°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 4 inches of snow a winter
Snow, measured at Seattle Sand Pt Wsfo, WA, falls mostly in December. Precipitation (rain plus melted snow) adds up to 37 inches a year; November is the wettest month and July the driest. Rain or snow falls on about 163 days a year.
Rain + melted snow by month
Inches of water
Snowfall by month
Inches of snow
Heating and cooling a home in Mercer Island
94% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from October through April.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Seattle Boeing Fld, WA.
Living with Mercer Island’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
247 frost-free days in zone 9a. The last frost comes by Mar 13 in half of years, so many gardeners wait a week or two past that to set out tomatoes. The first fall frost comes by Nov 17.
Months with highs of 62–84° and lows of 40° or more: May, June, July, August and September. 18 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Mercer Island from natural hazards, earthquakes are the largest share (68%); heat waves, cold waves, winter storms and ice storms together are 1%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Mercer Island and its neighbors
Every town next door is within a degree of Mercer Island in January and July, closer than the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Mercer Island’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Mercer Island, WA | 37° | 77° | 37" | 3.8" |
| Scotts Valley, CA | 39° | 79° | 37" | 0.0" |
| Tigard, OR | 36° | 81° | 40" | 3.6" |
| Nantucket, MA | 26° | 76° | 44" | 25" |
| Brigantine, NJ | 28° | 83° | 45" | 10" |
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in Mercer Island averaged 1.1° warmer than the 1900s. The biggest change is in January (+2.4°).
A smaller shift than in 71% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Mercer Island compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Mercer Island’s place among Washington’s.
Mercer Island weather by month
Highs, lows and precipitation from NOAA’s grid at Mercer Island’s center; snow, wet days and snow on the ground from Seattle Sand Pt Wsfo, WA. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 48° | 37° | 5.1" | 0.9" | 19 | 1 | 38° – 47° |
| February | 51° | 37° | 3.5" | 1.0" | 16 | 1 | 37° – 49° |
| March | 55° | 39° | 4.0" | 0.0" | 18 | 0 | 43° – 52° |
| April | 60° | 42° | 3.0" | 0.0" | 16 | 0 | 47° – 57° |
| May | 67° | 48° | 2.0" | 0.0" | 12 | 0 | 53° – 61° |
| June | 71° | 53° | 1.6" | 0.0" | 11 | 0 | 59° – 68° |
| July | 77° | 56° | 0.7" | 0.0" | 5 | 0 | 62° – 71° |
| August | 78° | 57° | 1.0" | 0.0" | 6 | 0 | 63° – 70° |
| September | 72° | 52° | 1.6" | 0.0" | 9 | 0 | 59° – 66° |
| October | 61° | 46° | 3.7" | 0.0" | 15 | 0 | 50° – 59° |
| November | 52° | 40° | 5.6" | 0.2" | 19 | 0 | 42° – 51° |
| December | 47° | 36° | 5.3" | 1.7" | 20 | 1 | 37° – 45° |
| Year | 62° | 45° | 37.1" | 3.8" | 163 | 2 |
How much a month can swing
A normal is an average. February swings the most: from 1991 to 2020 its average ranged from 37° to 49°. September is the most reliable, staying within 6°.
Where Mercer Island’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Mercer Island’s center.
- Snow: Seattle Sand Pt Wsfo, WA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Seattle Boeing Fld, WA, 4 miles from the town’s center, at about the same height; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Mercer Island’s climate
What is the weather like in Mercer Island?
A typical July afternoon reaches 77°, a January night drops to 37°, and about 4 inches of snow falls in a normal winter. Milder January nights than 84% of U.S. towns; cooler July afternoons than 97% of U.S. towns.
How cold does it get in Mercer Island in winter?
December nights average 36°. About 30 nights a year reach 32° or below. In the coldest December of the last 30 years the month averaged 37°.
How much snow does Mercer Island get?
About 3.8 inches in a normal winter at Seattle Sand Pt Wsfo, WA, most of it in December, with snow on the ground about 2 days a year.
How hot does it get in Mercer Island?
July, with a typical high of 77° and a low of 56°. About 3 days a year reach 90° (Seattle Boeing Fld, WA).
When is the first and last frost in Mercer Island?
In half of years the last spring frost (32°) comes by March 13 and the first fall frost by November 17, about 247 frost-free days, at Seattle Boeing Fld, WA.
What growing zone is Mercer Island in?
Zone 9a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 21°.
What is the best time of year to visit Mercer Island?
By one simple rule (highs of 62–84° and lows of 40° or more), May, June, July, August and September. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Mercer Island getting warmer?
Yes: 1991–2020 averaged 1.1° warmer than 1901–2000 on NOAA’s grid. A smaller shift than 71% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Mercer Island.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.