Weather Blog

Seattle: Home to 15 minutes of August rain

Seattle: Home to 15 minutes of August rain
Image courtesy: Puget Sound Clean Air Agency Visibility Camera

SEATTLE -- Our weeks-long dry streak has been getting all of the attention lately, but there is another record that is now sure to bite the dust, and it's one that's stood since Sea-Tac opened in 1945: The driest August on record.

So far, Seattle has only managed a trace of rainfall this month that fell as 9 minutes of rain on Aug. 6 and 6 minutes of rain on Aug. 21 -- 15 minutes total, and not enough to measure in the rain gauge.

Up to this point, the driest August on record was 0.01" set in 1974.

It won't be the first time we've gone through a month with just a trace -- August 2012 will join July 1958, July 1960, Sept. 1975 and Sept. 1991 as months that have not had measurable rain (all had traces of precipitation.) If you want to go way back, the Downtown Seattle Federal Building, which kept data from 1890-1972, reported zero rain twice -- July of 1896 and July of 1922. (In fact, 1922 was a bone dry summer -- with only a record low 0.03" in July.)

While this year will set a record, it's not that unusual for extremely dry Augusts. Just since 2000, we've had a an August with only 0.02" (2006) and 0.04" (2002). 2012 will be the 10th time overall August has recorded less than a quarter inch of rain.

This is the peak of the dry season -- August only averages 0.88" of rain a month anyway. Thus while a dry August makes for brown lawns and an increased risk of fire danger, climatologically speaking, it's not something that would have a long-lasting damaging effect on the Seattle area because we really don't expect to get rain this time of year.

Our water supply comes from the fall and winter mountain snows and we just withdraw from the "bank" as the summer goes on. And this past winter in particular had plenty of snowfall so no worries of any water shortages despite the rain that's gone into hibernation. If we were going to have a big dry streak, this was the summer to do it.

But put this dry streak in the middle of November or December and now we've got problems. Go 6 weeks without any rain or mountain snow and that could have some lasting effects. But dry streaks in the heart of summer are essentially guilt-free provided it hasn't been a theme of the entire year.

Speaking of dry streaks

Now, back to the dry streak, which stood at 39 days as of Thursday (tied for 5th place) and shows no signs of stopping anytime soon.

I was nervous that the streak was going to be knocked off by a rogue shower earlier this week as even just 0.01" of rain -- or drizzle -- will douse the streak.

In fact, we were lucky in that on July 28, Seattle reported 2 hours and 12 minutes of drizzle with a thick marine layer, but it didn't measure. (Seattle also reported some "rain" that got a trace on July 23 (Day 1 of the streak), but in looking at the statistics, that rain technically should not have even counted as a Trace. If you want the full story about why (it's complicated) E-mail me and I'll send you what happened. It didn't affect the streak though.)

So just like how Joe Dimaggio had a 16 game hitting streak after his 56 gamer ended, I wanted to see if any of our other long dry streaks ended on this 0.01" technicality or if it was a real rain that washed them out.:

Streak Dates Streak Ran: Rain Day Before Streak Rain Day After Streak
1) 51 July 7-Aug 26, 1951 0.06" 0.76"
2) 45 Sept. 1-Oct 15, 1991 0.37" 0.20"
3) 42 July 17-Aug. 27, 1986 0.40" 0.03"
4) 40 July 11 - Aug. 19, 1971 0.16" 0.01"
5) 39 Jul. 12 - Aug. 19, 1997 0.01" 0.11"
5) 39* July 22- Aug. 30, 2012 (Current) 0.04" ????

The data suggests all of the streaks ended pretty legitimately, especially the "big kahuna" streak of 1951. The 1971 streak, while it ended at 0.01" on Aug. 20, had 0.36" the next day. And the 1997 streak had an 0.32" rainy day the day before it recorded 0.01" on July 11.

So when this current streak ends, it better be a real rain -- none of this marine drizzle stuff. If we're going to go this long without rain in what might be the perfect summer to do it without any lasting issues, then might as well go for the gusto!