Posted on July 11, 2013
The word “derecho” has been in the meteorology lexicon for over a century, but it has recently garnered broader attention through a notable derecho event on June 29, 2012. This marked increase in interest of the term can actually be seen in Google Trends data, of which there is a screen shot below. I used the phrases “derecho” and “storm” in the Google Trends search in a basic attempt to filter out more common searches for the word derecho in its language of origin – Spanish.
The first small peak in 2009 is related to the May 8, 2009 derecho that swept from Kansas and Missouri eastward into portions of Kentucky and Tennessee. However, it is clear that the term was not receiving many Google searches until late June and July 2012 when there is a significant peak.
Since that time, there have been a number of convectively-induced wind events which have generated discussion amongst meteorologists about whether they could legitimately be classified as a derecho. Comparisons to the June 29, 2012 derecho, however, create an extremely high standard to meet for subsequent events. The June 29th derecho traveled for approximately 700 miles and produced over 1000 wind damage or severe wind gust reports (SPC Page, link here) – including 26 measured wind gusts of at least 33 m/s (65 kt, 74 mph). 4 of those measured wind gusts were at least 38 m/s (74 kt, 85 mph). Millions of people were without power for up to a week.
I won’t be discussing classification of specific events in recent years in this blog post. What I will attempt to do is to provide an overview of the derecho “criteria” that exist in the scientific literature, and then let others sort out how exactly they wish to apply that criteria. Of course, this is mainly of interest to meteorologists; the average person probably doesn’t mind what sort of label one applies to a storm, but rather is most likely just interested in how the storm impacts him or her.
So what exactly is a derecho? The definition was first explored in depth by Johns and Hirt (1987), hereafter JH87. The full text of the paper is available on the SPC website by clicking here, and the basic answer to that question is answered in the title, “Derechos: Widespread Convectively Induced Windstorms”. The literature review in the introduction provides some historical context:
The remainder of the JH87 paper goes on to propose a specific criteria for derechos. Since that time, there have been a variety of papers that have discussed derechos and derecho climatology, but not all have used the same definition that was originally published in 1987. The reasons for that are described well in the first few pages of Coniglio and Stensrud (2004), hereafter CS04 – “Interpreting the Climatology of Derechos” – that you can find on the SPC website by clicking here. This probably contributes to the disagreement on whether or not certain damaging wind events can be classified as a derecho.
Areas of Agreement in Criteria
The following two JH87 criteria have remained basically unchanged in most publications since that paper was written, as described in CS04. These are quoted directly from JH87.
Although there is no specific requirement for number of reports within a given area, one should look for a “concentrated area of reports”. JH87 considered both measured wind gusts of at least 50 knots and reports of wind damage. Measured wind gusts are only possible at discrete points, and therefore do not necessarily reflect conditions across the entire parent mesoscale convective system (MCS). Small-scale features within the parent MCS like microbursts and mesovortices can produce concentrated areas of much stronger winds, and these may not always be sampled directly by an anemometer. As a result, equivalent wind damage reports are considered as well when identifying windstorms that may meet derecho criteria. JH87 specifies that the reports should progress along an axis that stretches at least 400 km, which converts to 250 miles. The next criterion is straightforward.
Nuances in Criteria
Although there are slight differences in how these are specifically defined, there is also agreement that the associated mesoscale convective system (MCS) must have spatial and temporal continuity, multiple swaths of damage must be part of the same MCS, and that no more than 2-3 hours (varies slightly) can elapse between successive wind damage reports. These minor distinctions are described in a table at the top of the second page in CS04 (link).
The main difference, therefore, appears to arise from the inclusion or exclusion of the JH87 requirement for “at least three reports of either F1 damage or wind gusts greater than 33 m/s (65 kt, 74 mph) separated by at least 64 km (40 miles) during the MCS stage of the event”.
Bentley and Mote (1998) did not include that criterion for 33 m/s wind gusts or equivalent damage (link), and therefore considered all wind damage swaths that extended at least 400 km, as long as they met the other criteria as described above. This is probably the broadest definition of derechos that exists in the literature, with JH87 applying a slightly more stringent requirement for significant wind gusts.
Recognizing the differences in how various derecho climatologies were constructed, CS04 parsed out derechos into three categories:
- Low-end: meeting only the Bentley and Mote (1998) criteria.
- Moderate: meets the JH87 requirement of 33 m/s wind gusts
- High-end: at least three reports of wind gusts greater than 38 m/s (75 kt, 85 mph) separated by at least 64 km (40 miles), with at least two reports occurring during the MCS stage of the event.
Applying the requirement for “high-end derechos” creates a derecho subset with only the most intense convectively induced windstorms. This subset includes the likes of the June 29, 2012 derecho, the May 8, 2009 “Super-Derecho”, and the May 30-31, 1998 derecho that swept across the southern Great Lakes and produced multiple instances of wind gusts in excess of 100 mph. The authors do suggest that in the absence of measured wind gusts in excess of 38 m/s, equivalent damage may be used based on damage descriptions in Storm Data:
Coniglio and Stensrud examined derecho cases for the years from 1986 to 2001, a total of 16 years. They separated the cases into the low-end, moderate, and high-end categories, and each derecho case was classified in only one of those categories. 244 cases in total were identified, for an average of approximately 15.3 per year. A total of 171 would have met the original JH87 criteria (classified as moderate or high-end), for an average of 10.7 per year.
Even applying the most stringent criteria as outlined in CS04, the authors identified 55 high-end derecho events, which would work out to an average of 3.4 events per year based on the 1986-2001 period of record. These statistics suggest that applying more stringent criteria to classify a damaging wind event as a derecho would still yield at least a few cases per year, on average.
Even though we are hearing the word “derecho” more than ever, the statistics suggest that one would expect to see it applied close to a dozen times per year on average using the original JH87 criteria. There will obviously be differences in how people wish to parse derechos. The simplest solution to confusion about what constitutes a derecho is to be transparent about what sort of criteria one is applying. No matter which set of criteria that someone chooses to apply, they will probably have a basis in the scientific literature to support that choice. I’m not going to advocate for any particular set of criteria; I am merely pointing out that there are different criteria that exist and that no matter which criteria one chooses, derechos are more common on average than a once-per-year type of event.
Perhaps more importantly, impacts of a convective system producing widespread wind damage should be emphasized first and foremost over the particular classification of the event. This is far easier for people to understand and process, and gets the key message across in a less confusing fashion. The term derecho is obviously nuanced and has been applied differently over the past few decades. Therefore, simply saying “this is a derecho” or “this is not a derecho” may not tell the entire story, and will at the very least require more explanation for a general audience. ∗
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