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Recently, I read a published piece out of the SPC title, “A Proposed Revision To The Definition of ‘Derecho'”, and it was fantastic. A derecho is a long lived, convectively induced windstorm. It can often be violent, and must have continuous swaths of wind damage. Currently defined as, “the swath of wind damage extends for more than 250 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and also includes several, well-separated 75 mph (121 km/h) or greater gusts, then the event may be classified as a derecho“, the term has come under scrutiny recently as either being too vague, or being too inclusive.
I love when the meteorological world reanalyzes their own terms and definitions in an attempt to take control of a term that may be overused or hyped up. That is what is being done with this paper, and it is just one example of where the meteorological community needs to reassess the way they define atmospheric phenomena.
Their newly proposed definition is as follows: “Derecho: A family of damaging downburst clusters associated with a forward-propagating, mesoscale convective system (MCS) that, during part of its existence, displays evidence of one or more sustained bow echoes with mesoscale vortices and/or rear-inflow jets. The damage swath must be nearly continuous, at least 100 km (~60 mi or about 1° latitude) wide along most of its extent, and 650 km (~400 mi) long. The damage also must occur after any preliminary storms have organized into a coldpool-driven MCS.” At the end of the paper, the authors asked for criticism and/or praise from people in the field, and while I’m not in the field yet, I do have opinions on it. Here are my three biggest opinion points.
Why take significant wind reports out? Within their newly proposed definition, they remove the necessity for significant wind reports (74 mph gust and greater). This is one of my biggest criticisms of their ideas presented in the paper. They make the argument that removing this creates a definition that tries to eliminate non-meteorological factors, but still allow for the term ‘Derecho’ to have a standout classification.
I don’t necessarily think that is feasible. While the 400 mile track requirement allows for the system to be separated by itself, I believe that the significant wind reports are what separate the kind of derechoes they are classifying from regular MCS’s and other convective systems. Because many MCS’ can be fed by strong rear inflow jets and deep cold pools, many can produce wind damage. However, to separate the term ‘Derecho’ into another class like ‘Violent Tornado’ is, the significant wind reports have to be in there.
How will the continuous path be determined? Even given my stance on the significant wind reports, I love the idea of trying to eliminate non-meteorological factors from the definition as much as possible, but that is one of the main reasons we classify these different meteorological phenomena. If there wasn’t a human impact, then we wouldn’t really care about classifying this stuff; a derecho would just stay an MCS.
Additionally, I have seen several instances where the SPC has derechoes classified where the damage reports are fairly sparse across a region. As in, the reports are far apart and not necessarily consistently grouped together. This leads this into an issue of meteorological judgement, which can vary from person to person as to what is acceptable and what isn’t.
This leads me to another problem within this: what happens when a derecho crosses a sparsely populated area in the US? This isn’t the most common occurrence, but it can and will happen at some point. The damage reports will be fairly sparse, and it will make it difficult to determine whether or not something was a derecho. Do we base it on scientific measures by taking radar data as greater than actual damage reports? This is exactly why this was put in place, but for the majority of derechoes occur in the eastern US, where reports will be commonplace.
May-August 18-yr derecho climatology (Guastini and Bosart 2016) shows MN/IA to OH has 1 to 2 derechos every year. pic.twitter.com/Ir2mYh6NoG
— Chad G (@DrWxologist) June 21, 2016
Therefore, this likely needs a solution. An easy one would be to implement a temporal wind report aspect, but I would imagine a spatial necessity would be more effective. Maybe a report is necessary every 20 to 25 miles? Something like that would settle that issue easily.
I am happy that they separated QLCS’ from derechoes. Thank you. A QLCS is a QLCS and not a ‘serial derecho’. As is mentioned in the paper, the dynamics that form the two couldn’t be more different. QLCS’ are developed with much more synoptic help than most derechoes are, and form generally on a frontal boundary.
They have widespread synoptic forcing and atmospheric ascent, where as typical derechoes don’t have that luxury. The dynamics that drive and develop bow echo type derechoes are substantial different from those that develop and drive QLCS’. They shouldn’t be organized as subclasses under the term ‘Derecho’. Separating these two is something that will simplify the term, and allow it to have much more meaning.
While I could go on about this, I think that the ideas presented in the paper are fantastic, and I hope they are truly considered and changed. Derecho and MCS meteorology is awesome stuff, and I am glad that they have presented this to the SPC. However, you should go read the paper too. It is educational and not to complex! Check it out, and let us know what you think of it. Do you agree with the authors? The paper is found here.