So what exactly is a derecho? It depends ...

June 29, 2012, put “derecho” in everyone’s vocabulary around here.

Now, hopefully June 13, 2013, will spur the powers-that-be to define what a derecho actually is.

In the midst of much debate on social media by meteorologists and weather enthusiasts, the Storm Prediction Center eventually declared with some ambivalence that two squall lines on Wednesday evening and Thursday were low-end derechos, including one that pushed through the New River and Roanoke valleys.

As Thursday’s squall line moved across our region, it was definitely not a mature derecho. Most of the wind damage occurred from a couple of stronger cells within the squall line, rather than a wall of wind preceding the squall line across the entire area. Also, the storm line had not yet taken on the “bow echo” shape on radar, as it would later in North Carolina, that is often a signal of derechos.

I’ve come to call it “Son of Derecho.” For some, it was worse, even much worse, than last year’s event. For most, it wasn’t close.

Last week was a bizarre and somewhat frustrating week in severe weather forecasting when various entities went back and forth for four days about the possibility of a derecho similar to the June 29, 2012, event occurring, often at the exclusion of noting other modes of severe storms in the central and eastern U.S.

Then, after the fact, the debate went back and forth about whether a derecho or derechos had in fact occurred.

Much of this confusion stems from the lack of a standardized definition of what constitutes a derecho. Various academic papers written about derechos over the years have differently defined the term.

Without getting into a huge amount of technical detail, the differences are mainly about whether a derecho merely requires severe-level winds (58 mph or greater) over most or all of a 240-mile stretch, or whether 75 mph winds are required at certain intervals.

There are also questions about whether surface observations, storm damage reports or radar observations are the most important in determining these wind speeds, and also about whether the entire path of a squall line becomes a derecho if a given 240-mile section of it achieves the criteria.

This should all seem like a pointless academic debate if your power went out for days or you’re figuring out how to replace the hole in your roof caused by a fallen tree.

It’s akin to the oft-recurring tornado vs. straight-line wind debate when houses are damaged in a thunderstorm, and the hurricane vs. post-tropical cyclone debate that surrounded the landfall of Superstorm Sandy.

Once upon a time, it was fine to let this term be a little squishy for weather geeks and meteorologists to use, but the frightening and widespread impact of the winds on June 29, 2012, has forever attached a great deal of emotional and mental anguish to the term “derecho” for millions of Americans.

It deserves a proper and specifically detailed definition, so the public, media and various forecast entities can be sure they are talking about the same thing if there is ever a need to mention a possible derecho.

“Agree we need standard definition. Not sure whose to use,” the Storm Prediction Center tweeted to me on Friday evening as discussion roiled on Twitter.

Then come up with one, SPC. You are the authority on the matter. Don’t leave us flopping in the wind.

Weather Journal runs on Wednesdays.

Weather Journal

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