Historical weather data, all-time temperature records, and monthly climate averages for Mercer Island, WA. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
April 4th on Mercer Island is classic shoulder-season coin-flip weather — nearly even odds of rain at 47%, with highs that have swung from a chilly 32° back in 1948 to a shirt-sleeve 74° in 1949. The average lands at a mild 57°/40°, so layers are your friend today.
Mercer Island's weather history spans some genuine extremes: a scorching 105° in July 2009 and a brutal 3° in January 1950 bookend the all-time range. October 2003 holds the single-day rain record at 4.04", and an impressive 7.8" of snow fell in a single day that same January of 1950.
Summer is the sweet spot here — the week of July 9–15 averages a pleasant 78° high, while mid-January bottoms out around 46°. If you're avoiding rain, steer clear of late November, the wettest stretch of the year with nearly 1.56" per week on average.