Why recent storms have been so frequent and severe
There's been plenty of heat and humidity to spark storms, but unlike most summer days we've had other things happening in the atmosphere to make the storms severe and more frequent.
There's been plenty of heat and humidity to spark storms, but unlike most summer days, we've had other things happening in the atmosphere to make the storms severe and more frequent.
On typical summer days, high pressure over the Atlantic Ocean keeps major, organized weather systems away from the southern United States. This is what's known as the "Bermuda High." However, beginning late last week, this high pressure shifted a bit, allowing fronts to slip toward the Tennessee Valley. The fronts provide lift which leads to more storms.
Also, in the upper atmosphere, where planes typically fly, the wind flow has been sandwiched between high and low pressure. This has placed us in the path of several back-to-back ripples of energy called "short waves." They spark storms that often turn severe and can develop morning, noon, or night.
Downdrafts of cold, dense and heavy air rush from the top of the storm, sometimes at more than 40,000 feet, to the surface spreading across the ground. Think of it like water coming out of a faucet really fast. The column of water is the downdraft, and the water hitting the sink is the wind. Downdrafts can be strong enough to knock down power lines and uproot large, healthy trees.
The two types of wind events that are most common in our region are microbursts and straight-line winds. Microbursts occur very suddenly and usually last for only five to 10 minutes. They're accompanied by downpours, but in the western U.S., they're usually dry. Their damage spreads over a small area of two and a half miles or less. Straight-line winds typically last longer and cause damage across a larger area.
Much of the damage in the Chattanooga metro area the past several days is typical of microbursts. For example, think of downed trees that are across the street from each other facing opposite directions, sort of in a spiral pattern. However, the trees themselves aren't twisted like they would be by a tornado. However, downed trees from straight-line winds almost all face the same direction.
Most microbursts and straight-line winds produce winds of 60 to 80 miles per hour, but the most intense winds can be faster than 120 miles per hour. Microbursts develop so quickly that they're very difficult to predict. So, the National Weather Service doesn't issue warnings specifically for microbursts, but they do issue warnings for storms that might produce microbursts as well as damaging straight-line winds.