Here’s How An Intense Derecho Produced 100+ MPH Winds In Iowa On Monday

The term “derecho” has been around for a long time, but it gained public attention after the particularly intense storm that affected the Midwest and Mid-Atlantic in June 2012. The widespread damage and power outages caused by this storm in the Washington D.C. area, home to many national media outlets, forced this term into the American vocabulary and secured it a spot in the pantheon of scary, oft-misused weather terms.

While the term is often used incorrectly or just for hype, this was a derecho through and through, and it was a memorable one at that.

One of the greatest textbook examples of a derecho occurred in Iowa back on July 11, 2011. That derecho, which produced wind gusts as high as 100 MPH, was similar to the one that occurred today, although it was smaller and not as long-lived.

Here’s a look at the inner-workings of that derecho, which gives us a fantastic look at the structure that drives these storms.

Thunderstorms breathe in warm, unstable air and exhale cool, stable air. This outflow of stable air beneath a storm causes cold air to pool up at the surface. This cold pool spreads out from the parent thunderstorm like a ripple on a pond. The leading edge of a cold pool, called an outflow boundary, acts like a mini cold front as it scoops up unstable air and triggers new thunderstorm activity.

If thunderstorms form close enough together, their cold pools can merge into one entity. This merger causes the thunderstorms to become interconnected, moving and strengthening in unison. The thunderstorms latch on to the leading edge of that cold pool, forming into a line as they begin to race downwind.

The forward motion of the newly-minted line of storms causes their updrafts to tilt backwards, allowing unstable air to get scooped directly into the thunderstorms. This structure allows the storms to consume unstable air without getting choked off by the stable air beneath them.

Most squall lines only last for a little while, producing sporadic reports of wind damage across a few counties before they fall apart. Some, though, form in an environment with enough instability and favorable upper-level winds that they can produce incredibly strong winds and last for many hours before dissipating.

The radar cross-section above shows the winds throughout the 2011 derecho just as it started to produce 100 MPH straight-line winds. The image is oriented so that southwest is on the left and northeast is on the right. Des Moines is just off-screen to the left. The green colors show wind blowing southwest toward the radar, while the warmer colors show wind blowing northeast away from the radar.

You can see the tilted updraft on radar, clear as day. The cold pool at the surface is lifting up all that unstable air over central Iowa and feeding it right into those storms.

We wind up seeing lots of horizontal and vertical rolling motions within a squall line like this due to friction between the moving air, the calmer air around the storm, and the ground below. It’s kinda like the little whirls that form in a swimming pool when you run your hand through the water.

You can’t really see the horizontal rotation (diagrammed above) on radar, but the vertical rotation shows up as those little curly ends on the edges of squall lines. These curls are known as “bookend vortices.” This storm had a pretty hefty bookend vortex, but it’s hard to see on radar. These features are exceptionally pronounced in some squall lines, like the May 8, 2009, derecho depicted at the very top of this post.

These friction-induced areas of rotation create a feature known as a “rear inflow jet” that feeds air into the front of the storm. It’s the rear inflow jet that creates the powerful straight-line winds in a squall line. The rear inflow jet roars from the back of the storm to the front, getting shoved into the ground by the thunderstorms along the leading edge of the squall line. In stronger squall lines, the abrupt onset of violent winds is partially why people remember these storms so well.

Once the storms combined into one unit and the squall line starts chugging along, strong horizontal rotation within the structure of the squall line itself can create a rear-inflow jet (RIJ). The RIJ races from the back of the squall line to the front, giving the system its classic curvy bow shape on radar. When the RIJ hits the strong thunderstorms along the leading edge of the squall, it gets shoved into the surface by those storms’ downdrafts, creating the intense straight-line winds that caused all the problems today.

The derecho that occurred on Monday is similar to the example above, but it was much larger and longer-lived, producing widespread damage from eastern Nebraska through Indiana. 

The derecho began as a disorganized cluster of storms in South Dakota before sunrise on Monday:

The storms moved into northeastern Nebraska and organized into a squall line after sunrise: 

The squall line was already cranking by this point; a personal weather station near Bennington, Nebraska, recorded a 77 MPH wind gust as the storms moved over them. The squall line would continue to grow in size and strength as it moved into Iowa.

By the time it reached the Des Moines area, the squall line was easily recognizable to anyone familiar with radar imagery. The classic bowing shape to the storm reflects the intense jet of winds blowing from the back of the squall line to the front. The strongest winds occurred in and around the tip of that bowing segment:

A cross-section of the squall line just after it moved over Des Moines reveals a structure very similar to the examples shown above. 

This cross-section follows the center of the squall line from west (left) to east (right). The reds/oranges/pinks show wind blowing east, while the shades of green aloft show the tilted updraft blowing west.

The derecho reached maturity—and likely its peak strength—as it moved through eastern Iowa, resulting in a 112 MPH gust in Midway, 100 MPH gust in Hiawatha, and an 86 MPH gust in Davenport.

This is a classic appearance on radar.

– The heart of the squall line is healthy as it races through Cedar Rapids.

– A perpendicular line of severe thunderstorms developed on the north side of the squall line, a common feature that’s likely the result of southerly winds converging as air wraps into the north side of the squall.

– Meanwhile, the southern end of the squall line starts to fan out and look choppy as the outflow boundary outruns and chokes off the slower-moving storms that can’t keep up.

The storm’s structure remained healthy as it moved through northern Illinois toward Chicago and its suburbs:

Once the derecho moved into Michigan and Indiana, though, it started to lose some of its punch:

The squall line—which has grown many times its original size by now!—lost most of its steam as it moved into eastern Indiana. It’s more of a broken line of individual thunderstorms that share a common outflow boundary rather than a solid squall line like we’d seen for much of the day on Monday. The outflow boundary outran the storms by this point, disrupting that internal structure and forcing the storms to weaken. Some of the individual storms themselves still packed a punch with 60+ MPH wind gusts, but it’s nothing like what folks a few states to the west saw earlier in the day.

[Radar Images: Gibson Ridge | Damage Reports: SPC]

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