Tuesday, July 15, 2014

Microbursts produce wind damage in Shelby County on Monday night

Severe thunderstorms rolled through the Memphis metro on Monday late afternoon and evening ahead of a strong cold front of Canadian origin. A pre-frontal trough plowed into a very unstable airmass featuring temperatures in the mid 90s, high dewpoints, and high total atmospheric moisture that contributed to torrential rain in some storms, even though it generally didn't last very long.

One result of the early storms that formed in the late afternoon heat were strong downbursts of wind, called microbursts, that resulted in straight-line wind damage in some areas, including trees and branches down and even some power infrastructure damage. Microbursts, or downbursts, occur when a mature thunderstorm collapses (part of the normal life cycle of a storm). When this happens, the air that is forced up in thunderstorms (updrafts) quickly falls to the ground, hits the earth, and spreads out in all directions, like ripples in a pond spreading out when a rock is tossed in. Microbursts have been known to produce a great deal of damage due to wind exceeding 100 mph! The wind that spreads out creates outflow boundaries, or gust fronts. Often if the wind is strong enough (58 mph or higher), a Severe Thunderstorm Warning will be issued for these phenomena and it's best to seek shelter when the tell-tale outflow boundary, or shelf cloud, is approaching!

Schematic of a micrburst, in which a thunderstorm downdraft hits the ground and the wind spreads out in all directions.

It is important to know that Doppler radar only senses wind moving towards or away from the radar, not side to side. So while the wind near the surface spreads out in all directions when a microburst occurs, the radar only "sees" the components of the wind moving along the radar beam - towards or away from the radar.

Two radar images from late Monday afternoon show the microbursts as they were occurring. The first image below shows the Doppler data (wind direction and speed) for a microburst between Germantown and Collierville. The NEXRAD is northwest (up and left) of this location, so the green colors are showing wind blowing towards the radar and the red colors are showing wind blowing away from the radar. In between is where the downdraft wind hit the ground, then spread out. Doppler estimates are about 45 mph in the green area and 35 mph in the red area. Therefore, the wind that was collapsing down from the thunderstorm was traveling much faster than either of those values, resulting in the most damage directly under the microburst.

Doppler velocity data indicates the presence of a microburst near Collierville on July 14 with strong NW wind over Collierville and stronger SE wind on the east side of Germantown.

The second microburst image came from the Bartlett area about 20 minutes later. Once again, the NEXRAD is northwest (up and left) of this location, so the green colors are showing wind blowing towards the radar and the red colors are showing wind blowing away from the radar. This microburst (at least at this time) was not as strong as the Collierville one, but nevertheless produced very gusty wind over the east side of Bartlett in particular.

Doppler velocity data indicates another microburst near I-40 and Sycamore View with strong southeast wind over Bartlett and NW wind heading into Cordova.
When microbursts hit the ground, wind can seemingly come from all directions or "swirl" as the air moves chaotically in that localized area. Some people report this swirling wind as a tornado, but the microburst is a very different wind phenomena from a tornado. Typically, those surveying storm damage will see a wind pattern that spreads out from a central point in the case of microbursts, which make it fairly obvious that a straight-line wind event occurred, rather than a tornado. Your tree or the power pole really doesn't care what hit it when it's laying on the ground though! It just knows it got hit by a wind it couldn't stand up to!

Do you have any questions about microbursts or other wind phenomena? Leave them in the comments and we'll be sure to answer them!

Erik Proseus
MWN Meteorologist

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