Breakdown: What derechos are and why they can be dangerous

Breakdown: Why derechos can cause so much damage

MEMPHIS, Tenn. (WMC) - Memorial Day 2017 was a very humid day with a rather unstable airmass across the Mid-South. While most of the day stayed rather calm, a batch of storms moved into Randolph County, Arkansas, from the northwest.

This line of storms quickly pushed east across the Mid-South around 60 to 70 mph. The bow echo shaped radar signature continued towards Walnut Ridge, Jonesboro causing damage.

Derecho Event in Memphis 2017
Derecho Event in Memphis 2017 (Source: NWS Memphis)

As it moved into the Memphis metro, winds were reported around 61 to 67 mph at Memphis International, causing damage to North Memphis and Midtown Memphis, causing numerous downed trees and power lines.

The storm then caused damage along U.S. 78 and Interstate 22 in North Mississippi, knocking down trees from Olive Branch, and New Albany.

Fortunately, no deaths occurred, and just two injuries were reported. This widespread damaging wind event is called a derecho, and this one event spanned over 240 miles, across 4 states. It had winds as high as 105 mph at times and caused major power outages across Memphis.

So, what is a derecho and why was it so damaging? In this episode of the breakdown we will explain how this weather phenomenon forms in the atmosphere.

The winds in derechos can be as strong as those found in hurricanes or even tornadoes. Unlike hurricanes and tornadoes, these winds follow straight lines. Derecho stands for straight in Spanish.

The winds come from a downburst, this is when wet air in a thunderstorm meets drier air surrounding it, the water in the air will evaporate. When this occurs, it cools the air around it. Due to cool air being denser, it rapidly sinks to the ground and creates strong winds.

Derecho Keys
Derecho Keys (Source: WMC Action News 5)

This downburst can suck more dry air into the storm and make even stronger downbursts or downburst clusters. Derechos occur when the right conditions for downbursts occur over a wide area.

To be classified as a derecho, the whole event of the storm must have winds at least 58 mph and must produce a swatch of damage that is at least 240 miles long.

Derechos are most common in the Midwestern United States, but they are still fairly rare, it usually occurs ones a year, but they can happen in other areas.

Derecho Likelihood
Derecho Likelihood (Source: WMC Action News 5)

The best thing to do in the event of a derecho is to go someplace safe and protected, as we know high winds and falling trees can be quite dangerous.

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