Derecho. I never pronounce the word right, so I affectionately refer to them as big mean wind bags. It’s a term reserved for the most intense, long-lived, damaging wind producing thunderstorm clusters that sweep through the United States every year.
Derechos can often be found during the late spring and summer months in a corridor spanning from the Dakotas to to the Ohio Valley. As high pressure builds over Mexico and the Desert southwest, heat and humidity swells to boiling levels over the central U.S. Subtle disturbances in the jet stream running over the top of the ridge trigger thunderstorms over the high plains. As these storms feed on extreme levels of instability they grow more intense and develop big areas of rain cooled air, called a cold pool.
It might sound a little counterintuitive, but the same hot muggy air that feels heavy when you walk out the door is actually very light and buoyant compared to the very dense, rain-cooled air rushing out of the thunderstorms. The cold pool surges out of the storm, and the volatile, ready-to-erupt hot, humid air ahead of it rushes up and over it. The leading edge of this rain-cooled air is called the gust front, it’s within the gust front that winds can turn destructive. The cluster of storms begins to race forward feeding on the extreme levels of instability ahead of it and you develop a forward-propagating MCS. If conditions are volatile enough, the cluster of storms takes on a bowing shape as it surges ahead at forward speeds exceeding 60 mph, winds gusts over 100 mph blowing down everything in its path, and embedded circulations produce fast-moving, impossible-to-see tornadoes. While derechos don’t produce the same annual casualty numbers, a single derecho event can produce damage on the same levels of a landfalling hurricane or a tornado outbreak. Unlike a tornado which produces a lot of damage over a small area, it’s pretty hard to hide from a derecho.
As a meteorologist and storm chaser born and raised right here in the middle of derecho alley, I’ve got a growing list of derecho battle stories. One of my earliest memories of a specific storm growing up was the June 29th, 1998 Cornbelt Derecho. I’ve got some photos of the shelf cloud of this derecho sweeping into Urbana somewhere in a box in the basement that I’m sure I’ll uncover after uploading this video. As I was going back and looking at archived radar data from the June 29th 1998 derecho I noticed numerous embedded circulations, likely tornadoes along the leading edge of the storm which produced extreme wind damage across Iowa, Illinois, and Indiana. One particular area of tight rotation that passed near the airport south of Champaign-Urbana was associated with significant structural damage and a derailed train.
This isn’t about the science of derechos, it’s about gawking at the shear insanity of these storms, sometimes referred to as inland hurricanes.
These are my top 5 derecho encounters over the last 20 years.
August 13th, 2007. This was a classic case of a 20 year old effing around and finding out. I had quite a bit of young success in 2007 and kind of felt like I’d arrived on the scene. On August 13th, I was vacationing at our family cabin in nearby Wisconsin, but there was a moderate risk straddling the Minnesota Wisconsin border. The moderate risk was for wind damage potential, but there was a 5% tornado probability area and a non-zero chance of a couple of long-lived supercells. I didn’t have much experience chasing in Minnesota but I knew the terrain south of I-94 and the Minneapolis metro was pretty decent. If I could get a visible daytime supercell in that good terrain, why shouldn’t I be over there? I could handle that on my own. I marched my butt over the border and the cap south of 94 help. My only option was an incredibly intense supercell north of the warm front that was quickly accelerating into a missile headed for the Minneapolis metro. I didn’t come all this way not to see a storm, so I drove right up to the storm as the terrain began to transition into dense forests near Foley Minnesota. As darkness slowly overspread the area, the intense storm with a 75 dbz core swallowed me with blinding rain, severe winds, and eventually some hail. I probably should have lost a side window in my 94 Oldsmobile Achieva, but the storm gods let me off with some vegetation being plastered to the side of the vehicle. The storm subsided and I began the quiet, radio off drive home where you know you messed up today. It’s fine, I just look back at it with a little more experience and I’m a little nervous for 20 year old Andrew in an unreliable car in an area he’s never chased before and has no immediate support nearby if he gets into a bind. Alls well that ends well though, and I never learned my lesson so let’s move on to the next one.
June 23rd, 2010. I’m a college student at Northern Illinois University and decided to spend the summer of 2010 in dekalb getting a calculus course out of the way. Luckily I scheduled the class for the early morning, because the summer of 2010 was one of the most active for severe storms across northern Illinois in my lifetime. There was another moderate risk in place across portions of the Midwest on june 23rd, again for the high end wind damage potential for that day. Still, there was some early hope for observing tornadoes and I made a couple of early intercepts on supercells in northwest Illinois before storms began to congeal into a line that was rapidly moving east. I spent some time swiftly moving east on the interstate so that I could square up a particularly intense portion of the line that was surging toward Oglesby, Illinois. The storm showed a little bit of rotation along the leading edge as it crossed Interstate 39 and slammed us in Oglesby with lots of close cloud to ground lightning bolts. No harm no foul on this one, a little car wash and a trip back to DeKalb for some Portillo’s for dinner.
August 10th, 2020. This is the big one, though it’s not the last one on my list. The August 10th 2020 derecho is also referred to as the cornbelt derecho, as it brought extreme levels of damage to a corridor across the I-states of Iowa, Illinois, and Indiana. Even though it was perhaps the worst derecho of all-time, this cluster of storms was not forecast well in advance. There was only a marginal risk for severe storms across the impacted area on the early morning day one outlook, and that’s no slight to the experts at the SPC. Weather models in the days leading up to August 10th didn’t pick up on a cluster of thunderstorms that would develop over the Dakotas on the 9th, that would steamroll through the night into western Iowa where it met an increasingly unstable air mass that ran unimpeded through the Midwest. By late morning, 100 mph winds were tearing through central and eastern Iowa as I made my way north to pick off the derecho in between Morris and Yorkville. This thing was clearly riddled with tornadoes, so I more or less found somewhere between swirls where I was pretty sure I wouldn’t get hit by a tornado and braced for impact. I’m not sure I’ll ever experience a louder, more angry thunderstorm. The other crazy thing about this storm was the entire storm system of it all - the rear inflow jet coming in from behind the derecho kept blustery winds gusting 40 to 50 mph in place for hours after the storm as I surveyed tornado damage in the Yorkville and Dwight areas.
June 29th 2023. Drought had been cooking the Midwest during the spring and summer of 2023, but weather forecast models were beginning to hint at the drought breaking in a rather violent way. Meteorologists were able to get an early read on a potentially intense cluster of thunderstorms developing during the day on the 29th, and an early enhanced risk was upgraded to a moderate risk across central Illinois. I initially rolled into Monticello, Illinois, just looking for some intense winds, but like a moth to a flame an intense and obvious circulation in the line headed for the Farmer City area made me flip a u-turn and head north. I could see a couple of vague kinks in the line as I raced it into Farmer City before tucking myself between some buildings downtown. While I initially wondered if Farmer City was hit by a tornado, I’m pretty sure it was subject to what Dr. Tetsuya Fujita called burst swaths, the narrowest of the downburst family.
July 15th, 2024. While it wasn’t my best executed derecho intercept, the event on the 15th set a new state record for most tornadoes in one day for the state of Illinois. That along drives home the magnitude of these storms for me… imagine a 200 mile wide storm that is producing severe winds of 80 mph across the entire thing, AND it’s setting a new record for tornadoes in the state at the same time. You just tornado warn the entire thing and hope folks find somewhere sturdy. My somewhere sturdy was my Subaru Forester on Interstate 39 after an early run-in with the storm in Geneseo. It was tough to document this storm well, and it frustrated me further by beating me home and prompting the tornado sirens to sound back home in Champaign-Urbana before knocking out power and toppling trees across the area. I didn’t have power at home, and after a couple weeks of heavy rains the loss of power meant my sump pump was backing up and flooding my basement. My wife and a couple friends from the neighborhood helped bail water out of our basement while I ran to a neighboring town to borrow a backup gas generator to run the sump pump. It was a long day when you factor in waking up early to warn folks and put out extra forecast material the morning of the event, covering the event, and then not getting to come home and sleep. But daylight revealed our neighborhood had been hit hard and many of our friends had trees on their homes or vehicles. These things are big, and they’re powerful. They intimidate me in a way that chasing supercells and tornadoes doesn’t. Maybe that doesn’t make sense, but I feel increasingly that way with every derecho scar. When the forecast begins to show those familiar patterns, I begin to envision days without power, and entire swaths of the area potentially being altered forever.