Surprise Thunderstorms Cap Off A Record Heat Wave

I’m writing from the front porch of an old fishing cabin in Sekiu, a small fishing village along the Strait of Juan de Fuca approximately 15 miles ESE of Cape Flattery. I’m being treated to an incredible – and unexpected – lightning show from some strong thunderstorms to our north over the Strait and Southern Vancouver Island.

I’m on a fishing trip up here with my family this week and just arrived tonight, so between packing in the morning and traveling in the afternoon/evening, I hadn’t paid much attention to the evolving weather forecast. I knew that today was going to be hot and that there was the potential for a few thunderstorms drifting into the Puget Sound lowlands, but I never imagined that we’d see anything as spectacular as the show we’ve seen tonight.

These thunderstorms occurred because we had a decent amount of low and mid-level instability ahead of a developing upper-level shortwave trough. This shortwave trough provided the dynamic lift to initiate these storms, and the hot, moist unstable atmosphere contained approximately 500-1000 J/kg of elevated CAPE (convective available potential energy) to fuel these storms.

Quite the electric evening across parts of Western Washington. Here is a picture taken by one of our off duty forecasters over the Cascade foothills this evening. If you hear thunder, seek shelter indoors, & watch out for fire starts! #wawx pic.twitter.com/6eCuEGCb7M

— NWS Seattle (@NWSSeattle) August 17, 2020

Although the surprise lightning stole the show this evening, the extraordinary warmth experienced throughout the Pacific Northwest (especially over Western Washington) was no less impressive. Sea-Tac measured a high of 98 degrees, its warmest reading since the all-time record high of 103 recorded back on 7/29/2009. And our moist, southerly flow only made things feel hotter. On the other hand, Portland and the Willamette Valley “only” hit the mid 90s today instead of the upper 90s/low 100s that were forecast due to smoke from fires over Northern California reducing the amount of solar heating during the day.

Monday and Tuesday will see light onshore flow, so we’ll remain much warmer-than-average but will no longer be in record territory. Onshore flow strengthens Wednesday/Thursday, and a weak system may even bring a few showers to the coast on Friday.

Hope you enjoyed the lightning show as much as I did!
Charlie