12/19/2014

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Has Seattle been more 'thunderstormy' than usual?

Has Seattle been more 'thunderstormy' than usual?
Photo: Nitin Kansal Photography
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Ask a Seattleite to give their thoughts on the weather this summer that is in its final days and I'll bet the two most noteworthy items would be how many warm, sunny days we had... and the surprising number of thunderstorms.

While the numbers support that it has indeed been quite sunny and warmer than usual, you might be "shocked" to see this statistic:

Average number of days with thunderstorms per year in Seattle: 7

Number of days so far in 2013 with thunderstorms in Seattle: 8

Ahh, but the devil is in the details. Climate records only note if lightning was reported at the station. It doesn't say whether it was one lucky strike or an hours-long light show. And I would give puzzled Seattleites that many of those average 7 days with lightning a year are more along the lines of the "lucky strike" while 2013's tally has at least 3-4 days of "hours-long light show" with some storms reportedly putting out 500 lightning strikes an hour.

Seattle gets what few thunderstorms we have one of three ways:

1) The most common: Cold air from the Gulf of Alaska moves in behind a front, providing just enough difference in temperature aloft to allow air to rise into small thunderstorms. These are very weak and are what we call "One Hit Wonders" as in they might give a bolt or two of lightning-- and why much of the rest of the nation scoffs at us for considering them actual thunderstorms. These usually happen in the spring and fall when we're in the transition from warm to cold or vice versa.

2) A Puget Sound Convergence Zone sometimes gets enough energy from the colliding winds to create some lightning. They can sometimes get to about moderate strength but are limited to just the zone areas (which many times miss Sea-Tac). These usually happen anytime between September and April.

3) The raging line of thunderstorms that light up the skies for hours and make us wonder if more than the Sonics got moved to Oklahoma. These are typically the most rare and maybe 1-2 times a year.

But not this year, as the world found out watching the Seahawks game Sunday.

Why so many "Type 3" thunderstorms? Mother Nature found our chink in our armor.

The topography of the Pacific Northwest and our proximity to the Pacific Ocean provide a protective barrier against big thunderstorm development. Flow off the chilly waters of the Pacific Ocean keep our lower levels cool and that does two things -- cooler air on the ground doesn't give us the big vertical temperature differences to allow air to explode upward and create the towering thunderclouds, and that cooler air doesn't hold as much moisture so we don't have much fuel for the storms anyway.

Flow from the north is usually cool and dry; flow from the east is hot and dry so neither are conducive to thunderstorm development either. Those three scenarios typically make up about 95 percent of our year.

Ahh, but just like the Death Star has its one spot of vulnerability that the Rebel Fighters could exploit, Seattle has its own Achilles' Heel (yes, it's Double Metaphor Day in the weather blog) -- the southerly wind.

A ridge of high pressure has been parked off to our northwest for much of the year and has been credited with the sunny and warm-but-not-too-hot days. But in between, we've been plagued with a number of upper level lows to our southwest -- in the exact perfect spot to wrap around flow to where it's coming from our south/southeast, like the storm this weekend:



Southerly flow taps into warmer air from the south -- sometimes tropical air from warmer sections of the Pacific Ocean; sometimes monsoonal air from the Desert Southwest. That's been bringing the mugginess here, but also gives more moisture to fuel the storms.

In addition, many times this pattern brings what's called difluence where air is essentially getting pulled in different directions in the upper atmosphere. Best visual is like you're pulling taffy -- some of the air wants to keep spinning around the low but it's also getting tugged by other nearly low pressure centers aloft. The stretching effect essentially thins the atmosphere -- just like taffy thins as you pull it apart. The thinning creates lower pressure aloft (since you're "missing air" now) which helps pull up air from the ground and aids in creating lift -- that essential thunderstorm ingredient.

So now you've got warm, moist air for fuel, an air flow that is feeding in more warm, moist air plus providing lift in the upper levels - and toss in some pretty tall Cascade Mountains that trigger even more lift as the air flows over them, and south-southeast winds to steer all those storms blowing up over the mountains into Western Washington and voila! -- a perfect recipe for strong, towering thunderstorms with frequent lightning and heavy rain.

Now that we are heading into autumn, the thunderstorms should quiet down. I looked and last year, we didn't have any "thunderstormy" days at Sea-Tac between October and March (although that likely missed any Convergence Zone thunderstorms that were too far north) but suffice to say, future Seahawks games will likely be less eventful.

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