Cause of severe thunderstorms explained

dark clouds and lightningThunderstorms hammered much of Washington and Oregon last weekend with an intensity uncommon even for the storm-prone east slopes of the Cascades.

Lightning ignited wildfires throughout the region, baked dry by an unusually arid spell. Even temperate Seattle went the entire month of July without measureable rainfall.

Friday night, Aug. 9, my wife and I arrived at our cabin near Cle Elum and popped a movie into the Blu-ray player. Soon we heard booming sounds outside and muted the volume. Thunder! Opening the blinds, bright flashes pierced the darkness.

I unplugged the TV and Blu-ray to guard against a lightning-strike power surge, and we went upstairs to step outside on our covered deck, where our senses soaked up the spectacle: the clatter of heavy rain pelting our metal roof. The sweet smell of a summer cloudburst drenching the pines. Blinding, jagged spikes stabbing the horizon. Thunder rolling from all corners of the compass, punctuated by explosions directly overhead.

We sat on our porch swing and watched the show for a while, then lounged on our bed in the dark and gazed out the window. Nonstop lightning flashed over the Cascades to the west, Mt. Stuart to the north, and Mission Ridge to the northeast.

Lightning flashed so bright, it lit up the landscape like it was midday.

We worried about lightning strikes igniting forest fires, if not our own parched trees. Eventually we fell asleep and woke around 2 a.m. to find the storm still in progress.

Cliff Mass, atmospheric sciences professor at the University of Washington and a renowned weather expert, explained in his blog that Friday through Sunday, much of Washington and Oregon was covered by moist, unstable air from an upper-level low-pressure cell that slowly crept northward up the coast.  It caused a southeasterly flow in the lower atmosphere that spawned widespread thunderstorms.

Indeed, what we thought was a large but local Kittitas County storm actually covered much of both states, both west and east of the Cascades.

cumulus clouds Mass explained that as moist air approached the Cascades from the southeast, the mountains deflected that air upward, causing instability, intense rain, and widespread lightning. A particularly intense thunderstorm crossed the North Cascades on Saturday night that radar measured at 40,000 feet high, quite extreme for our region.

Mudslides soon flowed in several areas, including the North Cascades Highway, which lay under 25 feet of rock, trees, and mud west of Rainy Pass.

While at the Seafair hydroplane races Aug. 4, I shot photos of what I presumed were thunderheads – cumulonimbus clouds – towering over the Cascades. Mass confirmed that my first shot (above) shows “towering cumulus, and the biggest was probably a thunderstorm.”

Of my second photo (below), Mass said, “the middle tall one is clearly a cumulonimbus cloud; it even has a nice anvil.”

cumulonimbus cloud with anvil Textbooks describe cumulonimbus as vertical clouds formed by water vapor carried upward by powerful currents. They can have towering, rounded heads, and well-developed cumulonimbus have flat, anvil-shaped tops caused by wind shear. They commonly produce lightning and thunder, wind, and sometimes hail. Hence the nickname, “thunderheads.”

In his blog, Mass wrote, “The eastern slopes of the Cascades is a very beautiful and bountiful place (fragrant pine forests, apples, wind power and more), but it also has its vulnerabilities, including intense thunderstorms, powerful downslope winds, and lightning-induced fires.”

To stay abreast of road-clearing repairs to the North Cascades Highway, check WSDOT’s travel alerts webpage.

by  Jon Osterberg

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