It’s not often the National Weather Service tweets something that goes viral. Of course, we as meteorologists are more than willing to nerd out on upper-level soundings and surface boundary analysis, but that doesn’t always translate to the wider, non-weather-obsessed population.
This loop of a storm over Memphis, though, is universally awesome.
Wednesday was a very hot and humid day across the Mississippi Valley. Heat advisories covered at least portions of several states, including west Tennessee. Memphis hit 91 degrees around noon, and the dew point was 75, leading to oppressive humidity. The atmosphere was loaded with moisture.
In this kind of environment, with extremely hot temperatures and very high water vapor, it doesn’t take much for thunderstorms to quickly grow into large, towering cumulonimbus capable of squeezing out lots of rain.
Like many metropolitan areas, Memphis experiences a strong heat-island effect, where air over urban, concrete-covered areas heats up much more quickly than the rest of the region. Once the temperature got high enough, the air rose quickly. It drew in the surrounding hot air and a thunderstorm developed, fueled by heat and humidity. You can actually see it pull the air into the center of the storm.
As Newton says, for every action there is an opposite and equal reaction.
Once a storm has exhausted its energy source, it comes crashing down. In this case, the storm wasn’t moving anywhere, so it all fell on one spot — Memphis. Rain, hail and cold air hits the ground and spreads across the landscape in something called a downburst. The leading edge of these winds creates an outflow boundary, which you can see on this radar loop as it spreads in all directions away from Memphis.
What makes this boundary unique is how perfectly circular it is. Usually, they’re twisted and deformed as they interact with low-level wind and terrain. But it was a relatively calm day around Memphis, which allowed this downburst to create a boundary in textbook fashion.