Dry Weather on Schedule

One of the most extraordinary idiosyncrasies of Northwest weather is the profound drought during midsummer.   During a magical few weeks, generally including the last week of July and first week of August, the Pacific Northwest is usually the driest region in the nation.  Drier than Arizona for instance.  And the latest forecast charts suggest this year will be no exception.

Let's take a look at the precipitation climatology of Seattle Tacoma Airport, specifically the climatological probability of getting .01 inches of precipitation over a day.  The driest day is in late July (about 8% chance of rain), but late July and early August are right behind--only about a 10% chance of getting one hundredth of an inch, the definition of measurable precipitation.  The wettest period?  November.

What about a significant rain, like a tenth of an inch in a day?  Lower chances of course, and a very flat minimum from the second week of July to early August.

Really going for the gusto, how about .25 inches in a day?  Very low probability over June, July, and August.  November really stands out.  

Why is midsummer so dry?   With the jet stream heading north during the summer we get little rain from fronts and midlatitude cyclones.   And thunderstorms are infrequent west of the Cascade crest during midsummer due to lack of humidity, a relatively cool lower atmosphere (due to the ocean influence), and few upper level disturbances to give air parcels an upward kick.   

So if you are planning a wedding, hike, or outdoor activity:  do it now.  Our weather world will be very different in a month.