The storms formed over the Cascade Mountains just after 10 p.m. and with our easterly winds, those storms drifted west into the Cascade foothills.
The storms were so tall that the lightning could be seen several miles away, prompting e-mails from puzzled viewers from Everett to Kent and all points east, wondering why there were near-constant flashes of light but no thunder. But it was because those storms were so far away.
But as the evening progressed and those storms moved into the foothill areas, and then the metro areas; the distant lightning was replaced by close lightning and loud thunder, as well as heavy rains.
Matthew Moyer of Carnation sent in the above photo. Angela Morris of Lake Sammamish sent in the one on the right here.
So, What Happened?
We had just enough cool air move in at the higher levels last night with a very weak disturbance combined with our hot weather from the daylight hours from Thursday. Storms like to form when we have a wide temperature difference (since warm air rises, and with cold air moving in up high, it allowed that warm air to rise very high and result in towering thunderstorms.) Also a factor were the mountains, which helped provide lift to the air as our easterly winds ramped up the eastern slopes of the Cascades.
These type of thunderstorms aren't too uncommon -- just usually they blow into Eastern Washington with our predominately westerly winds (and are normally responsible for our lightning-caused wildfires). But since the winds were easterly thanks to our thermal trough offshore, they blew them to the west and over the Seattle area.
As to why there was so much lightning, it helps that the air was generally dry. Just like how when you scuff your socks on a carpet and then shock someone or something metal when you touch them, that shock is more intense when it's drier outside than when it's wetter.
Was That Heat Lightning?
We received some e-mails asking if this was "heat lightning". It's a common term in the Midwest and East Coast that people refer to for lightning that occurs on hot, summer days that doesn't have any thunder associated with it.
However, heat lightning really doesn't exist -- at least, not as a special kind of lightning.
The reason you would not hear the thunder is simply because the lightning strike was too distant for you to hear it.
In the initial stages of the storm when it was over the mountains, the storm was so tall (probably 20,000 feet or so) that the lightning could be seen for several miles. Light can travel much greater distances than sound, so if the lightning bolt is far enough away (say, over 10 miles), you'll see the flash, but the air will have long absorbed the sound of thunder before it reaches you. Areas with trees or city buildings can also help absorb the sound before it reaches 4-5 miles away.
Could It Happen Again?
We are sticking with a chance of thunderstorms again in the mountains Friday evening as the set up is somewhat similar. Especially since it looks like it's going to be as hot or even hotter than Thursday. Stay tuned.
Wow, It Was Toasty This Morning
And to top it all off, it's possible Seattle could set a daily record for highest low temperature for June 18, which is currently 64 degrees. The overnight low Friday morning was 68 degrees, and we have to see if it stays over 68 through 1 a.m. (since weather records are kept on Pacific Standard Time.) If so, it would be the second-warmest low temperature in Seattle history, just one degree shy of the all-time record of 69 set Sept. 2, 1974.
With the clouds moving in with the thunderstorm, they acted like a blanket to trap in all our hot air from the night before. It promises to be another warm night Friday night.
You can see a gallery of some of the images we received from viewers at this site.
You can find some lightning safety tips at www.lightningsafety.com. Also, if you're not reading this on a laptop and are connected to your in house telephone wire or power supply while the thunderstorm is overhead, it's best to shut it down.
If you get any photos and when it's OK to use your computer, e-mail them to us at firstname.lastname@example.org.