The Dry Days

Are you planning a wedding or outdoor event in the Puget Sound area this summer?  Do you want to know which days are the best bets for a rain-free event?  Today we rank the 25 driest days in Seattle, based on analysis of historic weather records going back to Jan. 1, 1893.

The tallies of dry days were originally published by the US Weather Service in 1997, but have been kept up-to-date by Scott Sistek, meteorologist at KOMO-TV and KOMOnews.com, who pushed the stats into their 121st year.  We sorted through the data for the top 25 below.

We also noticed a happy statistical fact in the data.  Beginning April 20, every day for the next six months has a better than 50% chance of being free of precipitation.  That’s not a generalization.  Going back to 1893, every individual day between April 20 and October 28 has recorded more dry days than wet.  We put together the accompanying calendar chart to visualize that Mostly Dry season (orange).  The red subset on the calendar represents the driest of the dry in which each day has a better than 75% no-rain history.  Is it any wonder that Seafair weekend is scheduled the first weekend of August?  “They’ve done their homework!” Sistek said.

The blue, orange, and red portions on the chart are streaks of consistent wet or dry histories.  The two portions of white are days of inconsistency.  The two yellow dots?  Those are the historically driest days of all: July 30 and August 4.  They have been wet only nine times each in the last 120 years.

THE MOST CONSISTENTLY RAIN-FREE DAYS IN SEATTLE

SOURCE: The National Weather Service originally tabulated the data in 1997, but meteorologist Scott Sistek (KOMO-TV and KOMOnews.com) has maintained and updated it since then.  He also kindly shared the data (http://www.komonews.com/weather/faq/4308877.html) we used to find the right days and discover the dry and wet seasons.

CHART: Steve Campion

April 20th, 2013