Monday, July 25, 2016

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.

8 comments:

Bryan Black said...

That's why my wife and I got married on August 4. Statistically among the driest days of the year. I believe only 4 recorded instances of measurable rainfall in PDX on that day doing back to the mid/late 1800s.

MRT said...

Except that it looks like the jet stream is slightly south of Washington at this point, but interesting article nonetheless. I got married on June 28th and the day was perfect, although a little hot, 95 in Yakima. The next day we had a crazy hale and ice storm come out of nowhere which thankfully happened not on the day before.

Rod said...

This has definitely been a fine summer so far, Cliff. No complaints here. The temperatures have been amazingly consistent lately; between 70 and 80. I honestly think Seattle's summers cannot be beat by anyplace in the United States.

Kenna Wickman said...

Be nice if it wasn't so frigging humid! The Blob or what is left of it must go for the humidity to go away.

Cascadian Engineer said...

"During a magical few weeks"

The adjective I would use is miserable.

Bob Hall said...

Not that it has much to do with the content of this post, but I've never understood people who insist that it MUST NOT RAIN the day they get married. Resting your happiness on something you can't control is a recipe for disappointment. Remind me again the activities of a wedding that require dry conditions... ? Sunny skies actually make for worse photos, by the way.

Steve Simmons said...

The activities that require dry conditions would be all of them for an outside wedding.

iamlucky13 said...

"Drier than Arizona for instance. "

This isn't as good of a comparison as it sounds like. July is the wet season in Arizona, due to the monsoons they experience.

My wife and I are both from Western Washington, so the normal NW drizzle would have been no concern at our wedding, but it poured most of the day. We had already picked a backup photo location, and ceremony and reception were indoors. No worries. No regrets.

I don't really plan hikes around the weather unless it's severe. Temperatures much above 80 degrees are severe, in my book. July is usually one of the worst times for hiking.