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Remembering the Derecho of late June 2012 and analyzing environments conducive to these destructive storms. N.

Remembering the Derecho of late June 2012 and analyzing environments conducive to these destructive storms. N.

Good afternoon!  On June 29, 2012, a powerful cluster of storms swept across portions of the Ohio Valley, initially forming as a few storms in Iowa, then continued to organize as it raced east southeast, really gaining strength once it hit Indiana, getting a boost from Lake Michigan.  A lake-breeze front (push of cooler, denser air from the lake) pushed out into northern Indiana, lifting the warm, moist air and helping enhance the cluster of storms.   An incredible amount of instability was in place given the exceptionally hot conditions, and pooled along a frontal boundary which was draped across northern parts of Illinois, Indiana into central Ohio.  Temperatures south of the storm complex were over 100º with dew points in the middle 70s!!!

The storms produced a long-lived swath nearly 700 miles of wind damage from the Ohio Valley to the mid-Atlantic.  Fort Wayne, Indiana saw a peak gust of 91mph, the highest wind seen anywhere along the line.  This event resulted in 22 deaths, and over 5 million people lost power, some over a week in the midst of an intense heat wave.  

The set-up featured an anomalously strong high pressure aloft with a record-breaking heat wave over the southern Plains, Ohio Valley, and southeastern U.S.  This strong high pressure was also pulling in a layer of dry/cool air in the mid-levels of the atmosphere, originating from the arid regions of the Rocky Mountains, and allowed this “Elevated Mixed Layer” to extend Far East into the Ohio Valley and eastern U.S.  Why is this important?  This air mass being injected into the mid-levels of the atmosphere typically features a sharp temperature drop with height.  This is running overtop the warm/moist air near the surface.   This creates more instability in the atmosphere and if storms can develop and break the “CAP” along the frontal boundary, they can erupt in a violent fashion given other ingredients are favorable for storms. 

At the surface, there was a frontal boundary draped across this zone on the northern fringe of this “heat ridge” aloft and “Elevated Mixed Layer” air.  This separated extremely hot/humid air from more stable/cooler air north of it. 

This boundary served as a mechanism to lift the air, in which one the developing storms and daytime heating allowed the atmospheric cap to erode, storms erupted violently as they released an incredible amount of instability, lifting from the extremely warm air near the surface to this layer of cool/dry air aloft.  Because the air parcels were MUCH warmer than the environment, they rose very rapidly.  This is the environment commonly found in the Great Plains, hence why severe weather is so prevalent there!  (Dry, cool air injected aloft from the Rockies and Mexican Plateau in conjunction with warm, moist air near the surface from the Gulf of Mexico).  

Derechos are a product of stronger Mesoscale Convective Systems – which again are systems with a collection of storms clustered together – typically a line of strong winds/bowing storms on the front side, then extensive stratiform rain behind the severe storms.  It turns out that the patterns ripe for derechos (stronger MCS’s essentially) are found on the edge of our more intense heat waves, as seen in late June, 2012.  On satellite, you can recognize an MCS by a HUGE blob of clouds which extend deep into the atmosphere.  Here is infrared satellite imagery of the June 29, 2012 derecho/MCS.  

MCS’s are most common from May to July.  Here is an interesting map showing the number of derechos from May through July from 1981-2001.  This helps you see where these are climatologically likely to occur.  You will notice the most favored area extends from northern Indiana to southern Minnesota, with another peak over the southern plains.

You will notice how there is a MUCH lower probability of a derecho to occur from the Appalachian mountains and east.  This is what made the 2012 event so unique – it kept going all the way to the Atlantic Ocean.  That is not common.  You don’t have to go too far back to find the last derecho  event.  It was not even 2 weeks ago.   A cluster of storms produced damaging wind from the lee of the Rockies to South Carolina, over 1200 miles!!!  This event killed a young girl from Jasper, Indiana, who was at a Girl Scout camp in southern Kentucky.  A tree fell on the girl.  Here are the storm reports from June 21, 2019:

It is troubling that these destructive events tend to occur in the midst of summer heat waves.  Extended loss of power can be dangerous.  (See: Memphis, TN Derecho July 2003: http://tennesseewx.com/index.php?topic=3434.0) Some people here lost power for over 2 weeks, when daily highs were well in the 90s.  

So what is required for a MCS to get deemed a “derecho”?  The swath of wind damage must extend for at least 250 miles, with gusts of at least 58mph along most of its length, and several, well separated gusts of at least 75mph.  

Hopefully this blog gave you a better understanding of summer-time MCS’s and derecho events!  

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