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BY GEORGE: Heat Burst?

On Sunday, July 1, 2012, much of the southeastern United States was in the grip of a record-breaking heat wave. Temperatures had soared well above 100 degrees Friday, Saturday, and Sunday, and heat indices had reached nearly 120 degrees across much of the area. Eventually this heat had to break...and as is typical with significant heat waves, it broke in grand fashion with a severe weather outbreak.

During the evening of Sunday, July 1, 2012, a significant severe weather event occurred across southeast North Carolina and northeast South Carolina. Although widespread wind damage affected much of the area, with more than 60 reports of severe weather received by the National Weather Service (NWS) office in Wilmington, a more interesting, and possibly overlooked phenomenon occurred late that night: A heat burst.

Heat Bursts are relatively rare phenomena that occur mainly across the Midwest, and are considered quite rare in our area. A heat burst is a term used for an event in which temperatures rise dramatically, nearly always occurring very late at night, and are associated with very strong winds and rapidly lowering dewpoints (moisture levels). There are some cases in history of heat bursts causing temperatures to rise to well over 100F in the middle of the night, and others with wind speeds in excess of 80 mph!

Between the hours of 8 p.m. and 11 p.m., the temperature at Georgetown, SC rose dramatically. The most rapid warming occurred between 9 p.m. and 1030 p.m. when the temperature rose from 79F to 90F! This occurred at the same time the dewpoint fell from 59F to 45F, and wind speeds gusted to nearly 30 mph. All classic signatures of a heat burst.

The heat burst occurs in the vicinity of a decaying thunderstorm. When a thunderstorm is in its dissipating stage, it becomes downdraft dominated, meaning it has lost all of its updraft-related fuel. When this occurs in an environment characterized by very hot and dry air aloft, and cooler air near the ground, a heat burst can occur. What happens is that the moisture in the downdraft evaporates into the very dry air, which then causes the air parcel temperature to cool. This is known as evaporational cooling. As the air parcels within the downdraft cool, they become more negatively buoyant (move faster towards the ground). Once all the moisture has evaporated out of the downdraft, it's cooling stops, and compressional warming then begins as the parcel approaches the surface. If the air is moving fast enough, it will penetrate the lower layers of the atmosphere and bring the now very warm air all the way to the ground. The higher the thunderstorm base is from the ground, and the drier the mid-level air mass, the more significant the heat burst is likely to be.

 

By: George Elliott