Microburst winds can be a summer threat

Published 7:00 am Tuesday, June 12, 2018

By Skip Rigney

High winds associated with a thunderstorm on Saturday afternoon snapped and uprooted trees and lifted the roof off a barn in the Ceasar community in southeastern Pearl River County.

The damaging winds were almost certainly associated with a phenomenon known as a “wet microburst.”

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You don’t have to spend many summer days in the southeastern United States before you experience one of the many thunderstorms that form due to the daytime heating of moist, high humidity air, which is almost always in place in the lower levels of the atmosphere during the months of June, July and August.

As it develops, each thunderstorm has an updraft of rising warm air. This updraft tends to lift, or at least suspend, the raindrops that are forming in the thunderhead, or as meteorologists call it, the cumulonimbus cloud. But, at some point those drops grow so large and heavy that they begin to fall, dragging the cooler air that is aloft toward the surface and producing a downdraft that hits the ground and spreads out around the cumulonimbus cloud.

For most summer thunderstorms, the gusty winds accompanying the downdraft as it spreads out beneath the cloud are noticeable, but not damaging, often being in the 15-30 mph range.

However, sometimes the downdrafts can become violently intense, producing surface winds of 60 to even 100 mph. That’s strong enough to do the same damage as a weak tornado.

These particularly intense downdrafts are referred to as microbursts. “Micro” refers to the fact that damaging winds generally don’t affect more than a few square miles. The winds, rather than rotating as they do in a tornado, spread out in straight lines from the area where the downdraft hits the ground.

When microbursts are associated with rain reaching the ground, which is almost always the case in the humid, eastern United States, they are called “wet” microbursts.

Wet microbursts form when the air below the cloud bases is very humid, but there is dry air aloft that contributes to the strength of the downdraft by being sucked into the cumulonimbus cloud and evaporating some of the raindrops. The evaporation further cools the air, making it more dense, and thus contributing to the downward rush of the air.

It is not uncommon for those conditions to exist during the summer over the southeastern United States, so, it is not surprising that a number of wet microbursts occur in isolated locations across Mississippi every summer.

The National Weather Service estimates that for every 1 tornado there are approximately 10 microburst damage reports.

On Saturday, forecasters at the National Weather Service in Slidell were concerned that dry air aloft might help form wet microbursts. The measurements made by the weather balloon they launched on Saturday morning revealed a thick layer of dry air from one to four miles high. They noted in their Saturday forecast discussion that for “any storms that do develop this afternoon, especially over coastal Mississippi, microbursts nearing severe limits will be the main concern.”

It appears that is what happened in the Ceaser community on Saturday afternoon.

The threat of wet microbursts will continue today and Wednesday but is forecast to wane later this week as the air above us becomes more humid, matching the high humidity in which we will continue to be immersed down here at the surface.