One of the inside jokes of the meteorological profession is that summer in the Northwest begins on July 12. And usually that is about right. June often is cloudy and gloomy (but not this year!), and sometimes showers darken the skies around the fourth of July. But in the later portion of July the climatological probability of precipitation plummets to around 10% in the lowlands with many locations receiving less than an inch. Strangely, the lowlands of the Northwest are some of the driest locations in the entire U.S. in July and August. Drier even than the desert southwest, where "monsoonal" rainfall hits during the summer.
Take a look at the plot of the probability at Sea-Tac Airport of at least .01 inch of rainfall (.01 is the minimum required to call precip "measureable"). During July the probability drops from near 30% in the beginning of the month to LESS THAN 10% during the last few days. This is really extraordinary. The driest day of the year? July 29 gets the least on average.... .004 inches. For many meteorologists there is a tradition of having a "dry sky" party during that day or the closest weekend, with suitable libations that I won't discuss in this family-oriented blog. Want to plan a wedding? That is the weekend. Some wise people picked the first weekend of August for Sea Fair.
And you can see why July 12 is often suggested as the start of summer --this is just before the rapid drop off to dry conditions.
Talking about the forecast...a band of showers are now over NW Washington and the coast...and are headed for Puget Sound. But conditions should improve towards the weekend.
