Oskaloosa.com

CNHI/Southeast Iowa

July 23, 2013

Heat burst hits Knoxville

KNOXVILLE — For 15 minutes last night, Knoxville's temperature rose by almost a degree per minute as the area was caught in a heat burst.

The National Weather Service says the event took place beginning at 10:55 p.m. Temperatures rose from 70 degrees to 84 degrees.

At the same time, the dew point plunged from 63 to 48 degrees as the warm, dry air spread.

Meteorologist Frank Boksa said that's pretty standard for the phenomenon.

There are a couple of aspects involved with a heat burst. One is the basic mechanics of a thunderstorm, which has both an updraft that feeds warm, moist air into the storm, and a downdraft that takes air out.

As that downdraft falls, it passes through different layers of air. The mid-level air can be warmer than air at the top of the thunderstorm or at the surface, and can also be drier.

“It generally is some warmer air, drier air at the mid levels,” Boksa said.

“Typically, warm air rises so you don't see that at the surface,” he continued. When that warmer level gets caught in a thunderstorm's downdraft, it falls. “You're taking that warmer air and pushing it to the surface.”

The result was evident last night. A wind gust with the storm hit 73 mph during the heat burst as the mid-level layer was slammed to the ground. Temperatures skyrocketed, but the air was much drier than what it was replacing.

As storm effects go, that's not the worst Iowa saw on Monday. Large hail hit a number of areas, with Newton reporting 0.88 hailstones. The largest may well have been in Stratford, northwest of Des Moines, where people posted photos of 1.5 inch hail.

Forecasters expect the weather to calm down considerably for the rest of the week. Temperatures are forecast to be below normal, with little chance of severe weather.

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