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Four years ago today, one of the more infamous derechoes in recorded meteorological history formed. This formed across northern Illinois and northern Indiana, and raced southeastward across the northern fringe of an intense surface high pressure system.
This was an absolutely classic derecho, and was one of the more destructive events in recent memory, and the main swath of damage traveled nearly 850 miles from eastern Iowa through the Mid Atlantic. The synoptic features that led to the initial forcing of a cluster of thunderstorms aren’t initially obvious. However, a few minutes of analysis have led me to a few things.
First of all, the ridge that helped to steer the system was incredibly intense. The ridge was centered across southern KY and central TN, and the Nashville sounding had 500 mb heights at 595 decameters, which is well above the 90th percentile for 500 mb heights for the date.
This allowed for an intense heat wave to form across the Ohio Valley, with many areas seeing record heat. This set the stage for a very intense cold pool to develop behind the initial derecho. Behind the derecho, temps dropped into the mid and upper 60s, and ahead of it they were nearly 100ºF. This created a 30-35 degree temperature differential across a very small spatial frame, which, in combination with a rear-inflow jet, created an intense gust front along the front of the derecho.
Secondly, there was a strong boundary present from eastern Iowa into the Mid Atlantic, helping to steer the system. However, more importantly, there was a very deep and moist boundary layer along and south of the boundary. The boundary was pseudo-warm frontal/stationary, and the two air masses that were separated weren’t much different; however, they had different temperature/moisture contents, and that allowed this boundary to be present.
The boundary layer to the south of this boundary contained deep moisture from the surface through 850 mb (the top of the boundary layer). Moisture at 850 mb was event anomalously high, with the Wilmington, Ohio setting their record dewpoint temperature of 21°C. This lends credit to the fact that this was a highly anomalous boundary layer in terms of moisture to the south of the boundary across the northern Ohio Valley.
What this did, in combination with the very high temperatures, was create likely one of the most unstable atmosphere ever sampled across the region. The SPC had Wilmington, OH release a sounding at 18z (2 PM EDT), and the surface based CAPE from that sounding was the highest on record according to the SPC Soundings Climatology page.
This was a historically anomalous environment present across the lower Ohio Valley, and because the derecho took a track directly through this, it was able to be as consistently intense as it was throughout its lifetime. Additionally, around 30 knots of shear was present ahead of this system for much of its beginning and mid life, and this allow for the derecho to remain organized through its lifetime.
Lastly, while the synoptic forcing wasn’t exactly the most obvious thing in the world, all the storms needed were to be initiated. With the environment present out ahead of it, a cold pool could be easily developed and maintained as it moved southeastward. The real forcing mechanism that forced the initial convective initiation looks to have been strong warm air advection at 850 mb. This is one of the main terms in my favorite theory, QG Theory, which discusses vertical motion. Between 12z and 14z that morning, warm air advection intensified rapidly across central and eastern Iowa, and the cluster of thunderstorms really intensified around the same time.
Once this occurred, the storms were able to begin organizing into the derecho that they would eventually become. Additionally, as time went on, a vort max at 500 mb developed behind the system, and continued to favor upward vertical motion within the system. This vort max keeping positive vorticity advection along and ahead of the derecho was one of the more important aspects of it being sustained as long as it was.
Once the derecho crossed the Appalachians, the vort max intensified on the downsloping side of the mountains. Vorticity tends to stretch on the downslope of mountains such as these, and while the Appalachians aren’t too tall, this action strengthened the vorticity, and allowed for the derecho to re-intensify on its path through the Mid-Atlantic states. This is evident when the radar is paired with the gif of the 700-400 mb differential vorticity advection. The vort max is a bit behind the main derecho, but that is what you want it in this case.
The cold pool began to develop, and with significant temperature differentials with respect to both horizontal and vertical axis, a strong rear inflow jet developed and helped to form an intense gust front along the leading edge of the storms.
These are what allowed for the derecho to form, and become one of the most intense meteorological entities in the past decade. This storm system forced the term “derecho” into popular lingo, and was just incredible. The outflow boundary extended from southern Pennsylvania all the way back to Louisville at one point.
This outflow boundary would remain strong enough to produce wind damage across large parts of Kentucky, western Virginia and into North Carolina even without rain falling. It was truly an incredible storm system. These videos really speak for themselves.
The SPC did an excellent job staying ahead of this forecast, as they had a great forecast early on during the day across the lower Ohio Valley. As the event unfolded, they issued a Mesoscale Discussion highlighting the need for a Moderate Risk upgrade, and they upgraded it where it needed to be upgraded.
This was a well forecast event during the event, and I will praise the SPC and the National Weather Service office’s for how well each did their own jobs during the event. They both likely saved many lives that day.
The societal impact of this event was interesting. The intense ridge didn’t really go anywhere afterwards, and so the derecho had a fairly significant impact on the communities along its path. The derecho knocked out power to five million people along its path, and this had significant ramifications. Because the heat wave continued across the hardest hit areas after the event, many people didn’t have adequate access to air conditioning. This led to 34 deaths after the event because of the lack of air conditioning in the heat.
This event was the first real derecho to become household talk, and it inserted the term “Derecho” into common weather lingo. This has had various impacts since the event within the public. The reason this was inserted into common lingo the way it was is largely due to the cities that were impacted by this storm system.
It struck Chicago, Indianapolis, Dayton, Cincinnati, Columbus, Ohio, Washington D.C, and Baltimore. This amount of population is going to take notice to something of this magnitude, and the media in those cities are much more likely to insert the term, “Derecho” into their news stories than it would be elsewhere. It was really the perfect combination of circumstances.
Since then, the term has come under scrutiny, and there have been discussions on on revising it. I tend to agree with those that argue for revision, as the term has more or less become a buzzword in meteorology since the event. We have had large scale events deemed as likely to produce derechoes even before the event occurs. I understand the theory behind it, but we don’t tell people before a tornado outbreak that they may likely see an EF-4 tornado with the day’s activity. Similarly, we should be more vague in talking about set ups for derechos, I think that saying, “…a strong MCS…” is much better terminology, and doesn’t get people caught up in specific or technicalities afterwards.
Much like with tornadoes, the term derecho likely should only be used in classification after an event has occurred, and I think that the storm needs to be assessed systematically and objectively before being termed a derecho. Because the public largely has its idea of derecho based upon the June 29th storm, every time the word is mentioned, it is likely to get a sort of hyped response for the system, even if it isn’t worthy of that hype. It is a doubled edged sword that meteorologists have to deal with going forward, and it is a term that needs addressing and careful usage.
None of that takes away from the June 29, 2012 derecho, though. This was one of the more destructive storms we have seen on a large scale in quite sometime. It is because of this system that derechoes have been more analyzed, and studied more. I think that is a positive that we can draw form an event such as this.