Derechos are an interesting weather event that don’t get seen very often here in the United States with many people not knowing what they are until they experience one. They resemble a squall line on radar, as they are typically embedded in them when they do occur. A derecho is a type of severe thunderstorm that is a long-lived, widespread wind event of which is associated with a line of thunderstorms. In order to be classified as such, the event must span more than 240 miles and acquire wind speeds of 58 mph or greater throughout the entirety of the event. There must be consistent wind damage reports that span along the specified minimum length of track to verify that a derecho occurred. They can produce tornado-like damage; however, the damage will be aligned all in the same direction.
Derechos are rare because for them to thrive for such a long time with such intensity, they need consistent fuel (instability) and enough shear throughout the environment to support constant wind speeds of that nature. When a line of thunderstorms form along a front, it isn’t unlikely that storms will pop-up ahead of it; the issue with this is that those storms can eat up the fuel that the approaching line needs due to the strong downdrafts that the pop-up storms can produce. Also, timing has a significant influence on the development of derechos.
Fronts moving through at night have a harder time to keep the fuel going as the differential heating used to promote lift during the day is no longer there. So the environment typically becomes more stable and less likely to have the energy to propagate the storms with as much intensity as they would if it was a hot, sunny summer afternoon in the south. Derechos are a very interesting weather event that we are constantly researching to try and begin forecasting them ahead of time.
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©2019 Weather Forecaster Ashley Lennard