It turns out that derechos have an identity crisis. This article describes the opposing scientific viewpoints, presents a workable solution and discusses what’s at stake for the public interest.
The July 12-13 Midwest “derecho”
You’ll note my use of quotation marks regarding the official identification of this intense storm, because the jury is still out. The figure below shows the partial life history of what appears to be a classic derecho, traveling northwest to southeast nearly 900 miles over a 24-hour period. The radar appearance suggests several hallmarks of a derecho: a long-track bow echo (arc-shaped red region) that expands over time, travels briskly (40+ mph), traverses many states and is fueled by an exceptionally unstable air mass.
How did the storm rate in terms of wind damage?
The damage map is shown in the figure below. The diagram clearly delineates a long, broad swath of wind-damage reports (blue dots), stretching from Wisconsin to Appalachia. I didn’t count the dots, but there are 500 or so packed into that dense swath.
Does this event look like a derecho to you?
To be scientifically rigorous, let’s go down the list of necessary derecho characteristics, as used by the National Weather Service Storm Prediction Center (SPC).
The list reads: (1) Long-lived convective wind storm produced by a fast-moving line of thunderstorms? YES. (2) Total track length at least 240 miles? YES. (3) Wind gusts of at least 58 mph along most of its length? MAYBE. (4) Several, well-separated 75 mph or greater gusts. NO.
Uh-oh. It seems that we fell short on two criteria. I answered “maybe” to the criterion on severe wind gusts. Although there were widespread reports of trees down, including many broken limbs, according to SPC there were only TWO officially measured gusts exceeding 58 mph.
And unless new, official observations have come to light, the answer to criterion No. 4 is a resounding “no”. It quickly became apparent that the July 12-13 “derecho,” while impressive on radar, generated largely sub-severe wind. A derecho-wannabe? A sub-derecho? A marginal derecho? Derecho on a diet? What do you call this event?
More fundamentally, does a gust have to be officially recorded to deem a thunderstorm wind severe? Can’t we just go on extent and magnitude of the damage? Why isn’t 58 mph or more sufficient for a storm to be classified as a derecho, instead requiring a higher tier of gust intensity (75 mph or greater)?
With these questions, we start to unpeel the derecho identify debate.
Derecho definitions that don’t match up
The official Glossary of the American Meteorological Society states: “A derecho is a widespread convectively induced, straight-line wind storm.” Again, there is no specific mention of wind speed or damage path length.
The first scientific paper on derechos, penned by Gustavus Hinrichs in the late 1800s, made no mention of wind speed criteria. And in the corpus of present-day research literature, there is a subset of scientific papers that exclude SPC’s “75 mph or higher” gust criteria; regardless, these events are still classified as derechos.
Where does all this leave us? I think most scientists would unequivocally recognize a tornado when they see one, and the signature of a hurricane (no matter what intensity) is unmistakable from satellite.
The problem is, lines or arcs of fast-moving thunderstorms (called squall lines) are extremely common. Squall lines abound along fronts and congeal along mountain ranges. Plenty of convective lines have intense “red returns” on radar, and many are arc-shaped, but not all produce damaging wind gusts (some don’t even contain lightning — can they even be considered thunderstorms?). I have seem plenty of fast-moving bow echoes that look ominous but do little more than raise dust and scatter small branches.
It’s also surprisingly difficult to obtain accurate, reliable measurements of wind gusts — even in populated regions — because of the hit-or-miss nature of individual downbursts and microbursts that make up a derecho. Significant tracts of the Great Plains and Ohio Valley feature sparsely populated cropland and forest, compounding the sampling problem.
So fundamentally, we have a pattern-recognition and sampling issue, when it comes to accurately identifying derechos, lacking fairly stringent criteria.
What’s the solution to the derecho identification dilemma?
There is growing consensus among research and operational meteorologists that an improved and more universal definition of derecho is overdue, not just to avoid confusion in scientific studies but also to do a better job warning the public.
There are big implications when you label everything that “looks, smells and feels” like a derecho, a derecho, when, in fact, not all deliver widespread, severe wind damage. There is danger in over-warning, a problem that erodes confidence and breeds complacency, one that plagues our experience with tornado warnings.
Some argue that the term “derecho” should be reserved for only the most severe, high-impact convective wind events — and use “derecho warning” sparingly — only when the predictive confidence is particularly high.
But the flip side is this: One might inadvertently exclude important, lower-end derecho events. These are the “marginal” categories that still create significant societal disruption (damage, injuries, lives lost) and are thus worthy of a derecho warning and title. Because of the watershed June 29, 2012, event across the Ohio Valley and Mid Atlantic, the term “derecho” is now feared in the East; any warning bearing the term WILL get noticed — for now. But we can’t abuse its usage.
Is there a compromise? I along with other meteorologists advocate developing a derecho intensity scale (“D-Scale,” analogous to tornado- and hurricane-intensity scales). This is neither my own idea nor a new idea. It does make sense to rank derechos across an intensity spectrum, from low-end to extreme impact, because that’s how most meteorological phenomena play out.
I think derechos deserve their own rank index; it’s now been demonstrated that a derecho can create the same societal havoc as a tornado outbreak or landfalling hurricane.
To illustrate the utility of such a scale, consider the spectrum of derechos recently experienced in the Washington-Baltimore region. Contrast the intense June 29, 2012, derecho (8.6 million power outages from Iowa to the Atlantic, 1100+ wind damage reports, 29 deaths) with the much weaker July 8, 2014, event (widespread strong winds, 200+ wind damage reports, many sub-severe, one fatality and several injuries).
Both can be called “derechos” because their structural, evolutionary and damage tracks share basic attributes espoused in the NWS and AMS definitions. But we would rank the 2014 event as “low impact,” that of 2012 “high” or even “extreme” impact. On a hypothetical D-scale ranging from Cat 1 to Cat 5, 2014 would be labeled a “Cat 1” while 2012 gets a “Cat 4” or even a “Cat 5” ranking.
Can there ever be a “super derecho”?
Now and then, the atmosphere amps things up, creating mega-, hyper- and super-size variants of ordinary phenomena — from extratropical cyclones, to hurricanes and even derechos. Many practicing meteorologists, including me, feel that there is utility in attaching these labels to carefully selected cases.
Some scientists have already made the case, in the scientific literature, that the May 8, 2009, Midwestern derecho (shown below) is worthy of super-status. Indeed, not only was it long-track and incredibly intense (many widespread reports of 75+ mph wind gusts), but it also contained a violent “bookend” vortex on its north end that created an unprecedented swath of extreme wind damage. This vortex was not a tornado, rather more like the mesocyclone (even larger) in a giant supercell that extended to the surface, creating an inflowing spiral of very high wind over a large area. The level of organization and complexity were unlike anything ever seen.
The May 8, 2009, event clearly was at the upper end in terms of extreme wind-generating intensity — easily a “Cat 5” on our hypothetical D-scale.
But the question should also be asked: Is the June 29, 2012 event a “superderecho”? One could construct an objective rating scale based not on wind intensity and areal coverage, but societal impact — built on metrics concerning injuries, property damage, fatalities and utility grid damage.
The June 29 event did not have nearly the sustained, high rotary winds of the May 8 storm, but it left behind a record-breaking trail of devastation. I cannot recall a single, summertime U.S. thunderstorm complex that ever produced 8.6 million power outages. In fact, this singular event made the term “derecho” a household word across the Eastern United States. In terms of depth and breadth of societal impact, the June 29 storm probably comes out on top.
These are all points to ponder. The scientific debate continues, perhaps with some clearer vision in the near-future. In 2015, the dreaded “D-word” of summer may have passed our region by, but a storm of differing viewpoints continues, regarding the very definition of derecho and how to most effectively warn for these violent summertime events.