Is Seattle's summer weather starting earlier?
"Forget the summer solstice, summer doesn't really begin until July 5th in Seattle."
It's an old saying around here that's about half tongue-in-cheek and half based in meteorological truth. Holiday weekends and sunshine usually don't go hand-in-hand much in Seattle -- heck, in 2010, it rained on every holiday!
The 4th of July in particular will probably conjure up some soggy memories for the long-time Seattleites, and sure enough the 4th of July is statistically the wettest day of July if you go back to our original records dating to the 1890s. Plus, long-term weather data does show a pretty stark drying trend as start to wade into July. So many probably figure you have to get past the holiday to really get into our summer, warm season.
"During all 5 of the decades from 1948 through 1997, it used to be that summers in western Washington didn't really start until the 5th of July," said research meteorologist Jason Phelps.
But lately, sun fans haven't had to be nearly as patient. For one, the Fourth of July holidays this century have actually fared quite well -- it hasn’t officially rained on the Fourth since the holiday-cursed 2010, and even then that was a morning rain. The only other year with measurable rain on the holiday since 2000 was in 2006 (0.02").
It's a trend that began in the late 1990s, Phelps says.
"If you look at the patterns for SeaTac Airport over the past 70 years, it seems like there has been a trend over the last 20 years for summers starting a little earlier than they used to," he said.
Let's start with the rain. Phelps says from 1948-1997, the average number of days with rainfall in Seattle during the July 1-10 period was about 40-44 percent, and it was consistently that way every decade.
"Even before the late 1940s, if you trace other stations back to the 1890s, it was that way all the way back to the late 1800s," he said.
However, in 1998-2007, only 29 percent of days in the July 1-10 period had rainfall on average, Phelps says, and in 2008-2016 it has been only 23 percent, on average!
"Yes, there will still be a wet summer every once in a while, like last year, but the overall trend seems to be that the rains are shutting off earlier than they used to," Phelps says.
So drier weather has been coming earlier of late; and so, apparently, is the warmer weather. Check out the average highs for June by decade:
The numbers this past decade are quite skewed by the past four years which have seen Junes with well-above average temperatures, especially June 2015's record heat that had its average a full 3 degrees above July's average temperature. But even this year is going to come in with a June having an average high temperature over 71 degrees. On the flip side, Phelps notes that June hasn't had a day with a high temperature below 60 degrees since 2012 -- the longest such stretch in Sea-Tac history.
But that isn't just a June trend. Summers overall have been pretty hot the past four years:
"If you look at the average number of 80+ days per year for each decade, there are some definite warming trends going on," Phelps says.
Decade, Number of 80+ days per year on average:
1948-57, 16.6 days
1958-67, 27.9 days
1968-77, 27.4 days
1978-87, 24.5 days
1988-97, 28.8 days
1998-07, 25.5 days
2008-16, 30.9 days (8 so far in 2017 through June 26)
"2008-17 has had the most 80+ days per year on average of any decade in the last 70 years at Sea-Tac Airport," Phelps said, although again those numbers are bumped up by the past four years which have seen summers with well-above average temperatures. Just the past four summers have averaged 44 days at 80 or warmer. That average is also impressive when you factor in that in 2011 and 2012, Seattle didn't get their first 80 degree day until July.
But his year has seen a marked cooling from the past few toasty years in the Northwest to conditions closer to what we typically average, and forecasts for July are leaning toward a fairly normal summer for temperatures so I'd suspect 2017's summer numbers won't be as lofty as the past four when all is said and done. And no telling if the next few years' "summers" might revert back to 2011/2012 versions and wait until July 5th or later to get going.
But at least for these past few years, whose very soggy winters and springs have tested the limits of sun fans' patience, the wait has been shorter for summer-like weather to begin!