Over the next few days, you may hear the word “derecho” (deh-REY-cho), so to peel back all the fancy jargon and not have a million tabs open looking up what a derecho is and what causes one, this Weather 101 Article is sure to answer all your questions.
Derecho is Spanish for “direct” or “straight”. These types of storms produce intense winds straight out from the storm. Meteorologists typically refer to these winds as “straight-line winds.”
So what exactly is a derecho? Simply put it is an intense line of thunderstorms with strong damaging winds over a long distance.
How is a derecho formed?
Let’s start with the beginning. To form a thunderstorm you need 4 ingredients: butter, eggs, mil- ok sorry, but cake sounds really good right now. Meteorologists refer to the 4 ingredients as SLIM:
S-Shear (a fancy way of saying a change in wind direction and/or speed as you go higher up into the atmosphere.
L-Lift. This often comes in the form of frontal boundaries. You know cold and warm fronts.
I-Instability. The best way to describe this is a ball rolling down a hill. The steeper the hill the faster you go. What this looks like in the atmosphere is a cloud is our ball and the rate at which the column of air above it cools is our hill. If the cloud rises super fast, it’s got a lot of momentum, which means it can grow a lot taller.
M-Moisture. Think of this as the humidity in the air. Clouds grow at the expense of water vapor, so the more moisture the more “food” the cloud has. What grows this food? the heat from the atmosphere.
Put all of those together and we have a thunderstorm, folks!
Now let’s say we have a line of thunderstorms, each has rain coming off the backside of them. When the rain falls it hits the ground and spreads out in a puddle called a cold pool. This creates a ramp for the rear inflow jet, which is a strong wind created by the rain. The cold pool acts as a ramp because warm air is less dense than rain-cooled air. The rear inflow jet causes the storms to tilt with the leading edge further away from the cold pool, which in turn allows the storms to keep eating that warm moist air and grow more upscale, which causes more rain and this feedback loop is created. Because the middle cell was the initial cell it has gone through the loop more times, which leads the center to have slightly stronger winds. The shape the storms have now formed with their new speeds is a bow echo. It’s called that well because it looks like a bow.
This massive bow echo is now chugging along and strengthening that RIJ, leaving damage in its wake. Eventually, the RIJ builds up to speeds of at least 58 mph. At this point the RIJ is faster than the storm motion itself, it’s outrunning them. Once that happens, the intense winds continue to travel just out in front of the storm until the complex runs out of fuel. The finish line to be called a derecho is 240 miles.
Where do they happen?
Well, unfortunately for us, NWA is the derecho capital of the United States. The image below shows us in the bullseye for derechos. The last derecho in Arkansas? June 12th, 2009. Just nicking NWA.
Below is a series of radar scans over time showing the last derecho in Arkansas.
Check out some of the intense wind gusts that this derecho produced in this video! Followed by some pictures from the event in Iowa in 2020.
In summary, derechos are strong winds produced by a feedback loop within a line of severe thunderstorms.