When a protracted rumble jarred him awake last week, National Weather Service meteorologist Johnny Burg wondered for an instant whether he was back in Iowa.

"I haven't heard that kind of thunder since I left the Midwest," he said.

Thunderstorms powerful enough to make a heartlander take notice aren't something you see every day in the Pacific Northwest. Nor are the spectacular lightning bolts that accompanied the past week's weather displays.

Overnight on July 12, the National Weather Service counted more than 500 lightning strikes in Western Washington.

"This has been one of the best thunderstorm seasons in the Northwest," University of Washington meteorologist Cliff Mass said Tuesday. Minutes earlier, Mass heard the growl of thunder and punched up a website that tracks cloud-to-ground lightning. Sure enough, a new plus sign blinked over Puget Sound.

Burg said the storms are likely to continue through Thursday, concentrated mostly near the Cascades and south of Seattle.

"But it's very difficult to forecast thunderstorms here out beyond 48 hours," he said. "It can look like the conditions are ripe, then nothing happens. Then it looks like the conditions are wrong, and the thunderstorms form."

Conditions usually are wrong in the Northwest, which is why thunderstorms are rare. Moisture, a key ingredient, largely is lacking during summer. That's because the air blowing in off the ocean is cool, and cool air packs less water vapor than warm air.

"Thunderstorms use water vapor as fuel," Mass said. "It really revs them up."

As water vapor rises and condenses inside a cloud, it actually releases heat. That warms the air, making it more buoyant and leading to the growth of towering cumulus clouds.

Another damper on thunderstorm formation in the Pacific Northwest is the high-pressure systems that park off the coast during much of summer. Not only do they bring the warm, sunny weather so many people crave, the systems also suppress the atmospheric lift needed to kick-start thunderheads.

The middle of the United States is the world's best breeding ground for thunderstorms, with scorching surface temperatures and the influx of warm, moist air off the Gulf of Mexico. Storms in the Midwest can grow to over 50,000 feet tall, Mass said. In the Northwest, a thunderstorm 20,000 feet tall is considered big.

"We have some of the wimpiest thunderstorms in the U.S.," he said.

The recent lightning and thunder are being served up courtesy of a low-pressure system hovering high over the region. With a counterclockwise circulation, the low is pulling in warm, moist air from the south and the southeast.

"It's not feeding off the ocean, it's feeding off the land," Mass said. "So we're getting air from the south that has enough moisture and is unstable enough to get thunderstorms."

Many of our recent storms were born in Eastern Washington, where conditions generally are more favorable, Mass said. A higher rate of thunderstorms survived the transit over the Cascades in recent days, due to the diminished influx of marine air. Thunderstorms also can form at the mountain crest and drift over the lowlands.

To track the potential for thunderstorms, Mass refers to a measure called CAPE — convective available potential energy. The formula isn't something most nonmeteorologists would want to memorize, but the gist is simple: "It's a measure of how much juice the atmosphere has — how much potential for convection," Mass said.

On a scale that ranges from zero to 3,000, the Pacific Northwest usually clocks in at less than 100. So Tuesday's projection that values will reach 1,500 or so in the Cascades and Eastern Washington over the next couple of days is a good indication that more thunderstorms could be in store.

"It's not going to last," Mass said. "It looks like after Thursday, we go back to normal."

Sandi Doughton: 206-464-2491

or sdoughton@seattletimes.com