There's a reason the big Seafair hydro race and Blue Angels performances are the first weekend in August -- they've done their homework.
This time of year is statistically the driest time, with the best odds of not getting rain. (Of course, this summer, you could use a calendar taped to a dartboard to find a dry day.)
But while the two weeks from late July to early August is pretty good, the gold standard goes to August 4th, which is the top dog for driest day of the year. Since 1893, it has just rained 9 times on this date -- for a 7.8 percent chance it'll rain on any given year.
(For the record, July 30th also has this distinction, but only because it rained on Aug. 4 a few years ago.)
The worst day? November 19th, which has rained 86 times in 116 years.
You can see the entire chart at this link.
So if you must have dry, get your picnic reservations in early for next year :)