‘Heat dome’ makes much of country feel like a steam bath

As much of the country continues to swelter under a relentless heat wave, the Pacific Northwest remains locked in a mostly cool, cloudy weather pattern.

Neither seems willing to budge, because a strong ridge of high pressure camped out over the Midwest won’t allow it.

“What that does is, it kind of makes the progression of the weather patterns kind of stagnant,” said Tyree Wilde, a meteorologist with the National Weather Service in Portland.

The pattern has created a low-pressure trough over the eastern Pacific Ocean, sending unseasonably cool temperatures toward the Northwest. That trough won’t be able to move farther east until the high-pressure ridge in that direction breaks up, Wilde said.

Even before the heat wave to the east arrived, the Northwest saw plenty of unusually cool and wet weather. The average high temperature in Vancouver is 75.7 degrees for the month of July so far, according to Wilde. That’s more than two degrees below the normal high of 78 for the month.

Warmer, sunny weather is on tap for this weekend, with temperatures in Vancouver expected to land in the low- to mid-80s both Saturday and Sunday, according to the weather service. Then it’s back to a slight chance of showers for the early part of next week.

CHICAGO — If the extreme heat and humidity lingering over much of the nation feels like a steam bath, it’s because the same principles are at work in the atmosphere.

Vast amounts of warmth and moisture have become trapped under a huge “heat dome,” bringing record-breaking temperatures and thick, topical air to scores of cities from the Plains to the Ohio Valley. Now the system is moving east to spread the misery to some of the country’s most densely populated areas through the weekend.

With temperatures hovering around 100, Jeff Grembocki and other construction workers prepared Wednesday to pour concrete for a walkway improvement project near downtown Kansas City. Empty Gatorade bottles lay strewn across their job site.

Grembocki said the heat saps his energy so much that he falls asleep soon after getting home. He only rouses for a couple of hours to watch TV before going back to bed.

“The air conditioning, when it hits you, it’s all you can do to stay awake,” he said.

The heat dome forms when a high pressure system develops in the upper atmosphere, causing the air below it to sink and compress because there’s more weight on top. That raises temperatures in the lower atmosphere, said Eli Jacks, a meteorologist with the National Weather Service in Silver Spring, Md.

The dome of high pressure also pushes the jet stream and its drier, cooler air, farther north — it’s now well into Canada — while hot, humid air from the Gulf of Mexico circulates clockwise around the dome, traveling farther inland than normal.

Combined with generally clear skies and the sun’s higher summertime angle, “it gets really hot,” Jacks said.

The cruel result: eye-popping heat index readings measuring temperature combined with humidity. In Newton, Iowa, it was 98 degrees Wednesday with a heat index that made it feel like 115. A day earlier, Newton’s heat index hit 129 degrees.

In Indianapolis, the thermometer read 98 degrees but it felt like almost 114. Chicago’s Midway Airport recorded a high of 99 degrees, which felt like 108. Humidity levels in some of the hottest cities ranged from 40 to 60 percent.

“In places where the highest temperature you ever expect is in the 80s and you’re at 102, there are big health concerns,” because fewer people have air conditioning or fans,” Jacks said. “Heat is the No. 1 killer out of all weather hazards.”

Humidity makes the weather feel far hotter because the body, which cools itself by perspiring, has to work harder when the air is already moist.

Although heat domes are not rare, this one is unusually large and long-lasting. It began three days ago and may persist for seven to 10 days in some locations, meteorologists said.

On Wednesday, it had begun moving out of Texas and the Dakotas, headed east and northeast. By Thursday, temperatures in Washington were forecast to hit 100, and the heat could linger for days along the Atlantic seaboard.