Panic! At the Derecho

Prologue

Monday, June 10, I attended the funeral of a close friend. Four hours later, I came into the office to relieve Meteorologist Lyndsay Tapases for the 11pm newscast.

“It’s already started,” she said.

“What’s that?”

“Derecho rumors for Thursday.”

I had not looked closely at weather data for four days, my mind elsewhere with the sudden and unexpected passing of my friend.  Upon looking at the data, I noticed the classic weather setup for a large mesoscale convective complex (MCC) in the Midwest and Ohio Valley on Wednesday.  A cold front would then come through Virginia on Thursday.

I thought to myself: Sure… a derecho is possible on Thursday here, but scattered severe thunderstorms are more likely… and perhaps a severe squall line.

Semantics

The word derecho puts a very specific event in the mind of the public.  While the term has been around for about 30 years, the general public in the East Coast states has only one event that correlates that word to a storm: June 29, 2012.

So to speak of another derecho suggests to (most of) the public that a repeat of the June 2012 storm was coming. Using the term derecho, given its limited history in the minds of the public, begins to stir up weather fear. In turn, people scour the internet for information and latch on to the questionable nuggets running through cyberspace.

While I felt a repeat of the June 2012 storm was plausible, I believed it was an extremely low probability event.  So, I was tasked with dialing back the snowballing public weather fear, while at the same time, reiterating that there was still a very legitimate threat of thunderstorms with damaging winds.

More problematic, there is no standard meteorological definition for a derecho that is agreed upon by the entire meteorological community.  Sure, there are a couple of papers written with specific criteria, but they have not been universally accepted.

Consider the definition from the AMS Glossary of Meteorology:

Derecho – A widespread convectively induced straight-line windstormSpecifically, the term is defined as any family of downburst clusters produced by an extratropical mesoscale convective system. Derechos may or may not be accompanied by tornadoes. Such events were first recognized in the Corn Belt region of the United States, but have since been observed in many other areas of the midlatitudes.

Using the above definition, we could argue that we get one or two of these a year, not one every 5-10 years (like the June 2012 event).

The Forecast Problem

On Wednesday, I played the Thursday forecast as scattered thunderstorms with damaging wind gusts, hopeful to convey there was a significant threat without feeding the derecho beast. I posted the following paragraph to my professional facebook site Wednesday. The end user will have to judge if they felt they were prepared:

We still believe national and internet reports of a derecho are overstated. However, damaging winds are a threat with individual thunderstorms late tonight and then again tomorrow afternoon. Would suggest checking the weather situation every 3 hours or so between this afternoon and tomorrow evening. Only trying to raise awareness here so that you are not caught off guard, but this does not appear to be as epic as the situation last June.

My biggest forecast failure from Wednesday (June 12) was the prediction of a squall line. As of late Wednesday, I was not convinced that a full line of storms would form.  I thought a broken line or individual cells was a better guess.

Why? We expected strong west winds on Thursday before the storms developed.  The downsloping effect of those winds along the east side of the Appalachians often prevents the storms from surviving their trip eastward, out of West Virginia, and into the first 2-3 counties east of the Blue Ridge Parkway.

However, thunderstorms have been known to rapidly redevelop farther downstream, intensify, and race eastward.  We have seen this often, with Lynchburg getting missed, while areas from Appomattox, Brookneal, Gretna, and Martinsville catch the storms as they refire.

Walking the Line

The line between raising awareness and freaking people out is a thin one.  I have walked on the wrong side of that line more than once.  All of us in this line of work have.  But I will continue to do my best to convey the threats and their risks to the best of my ability.  That is true whether you think I’m a serious student of the atmosphere or a clueless television clown.

And yes, Virginia, putting out someone else’s fire is frustrating.  Losing control of the weather message is a new challenge broadcast meteorologists have to face with the democratization of weather data.  Rumors run rampant. People point me to blogs I have never heard of and want to know if what they read is true.

Here’s a good rule of thumb: Unless a blogger attaches his/her name to the weather blog, there is no way to cross check how credible the blog is.  As mentioned in previous posts, I have meteorology colleagues out of the television business that run strong blogs/sites/pages.  But can the general public tell the difference? Caveat emptor. Let the buyer beware.

Another axiom comes to mind. You get what you pay for. My name, my likeness, my reputation, and in the end, my job are on the line when I post.  Can you say that about all the weather blogs?

I know there are people out there… looking for deeper information… Those television guys aren’t telling us everything!

And you are correct.  I am not going to jump up and down every time I see the pattern for an MCC develop. It happens numerous times every year, and most of the time, the MCC dies a quick death as it moves eastward across the Appalachians.

I do not want to be the guy who cries wolf.

It’s Hard to Stop a Train

The west wind was not picking up Thursday morning as I expected. New data came in later that morning, pointing to a line of intense thunderstorms coming out of southern West Virginia and thriving as it marched away from the Blue Ridge.   So I did my best, from my home, to raise awareness, tweeting:

Damaging t-storm threat for this afternoon remains. Best guess for Lynchburg is between 1:30-4pm. Window about 1hr later Southside.

Sure enough, a dramatic squall line filled in east of the Blue Ridge, hitting various parts of Lynchburg a few minutes either side of 3pm.  The official observation at 3:11pm:

2013-06-13 METAR KLYH 131911Z VRB04G22KT 1 3/4SM R04/5000VP6000FT +TSRA SCT038 BKN055 OVC085 20/17 A2973 RMK AO2 PK WND 26038/1858 RAB00 P0001

Brief translation: Thunderstorm with heavy rain. Peak wind gust of 38kts (44mph) from the west at 2:58pm.

Apples and Oranges

While difficult to compare the event from Thursday (June 13) to the one in June 2012, it is clear that the amount of damage, when considered over the entire viewing area, was dramatically lower last week.

According to Appalachian Power (AEP), 104,432 Virginia customers were without power at the conclusion of the storms Thursday (website data from AEP at 6:30pm).

Now consider the map below. It displays power outages more than 24 hours after the June 2012 derecho:

Percentage of Virginia municipalities without power on July 1, 2012. Recall the derecho hit on the night of June 29. Click to enlarge.

More than 80% of Lynchburg City was without power more than 24 hours after the June 2012 storm ended. The Roanoke Valley still had 40-60% without power at this time. Huge sections of Amherst, Bedford, Botetourt, Campbell, and Rockbridge Counties had more than 60% without power.

Add the populations of those above municipalities and you get 474,000. Conservatively assuming half of those were without power gives 237,000, which is more than twice that of last week’s event.

(Having said that, it is important to remember that AEP is reporting customers without power. There certainly will be more than one person per home, but AEP also counts businesses as customers, where few people put their heads at night.)

In 2012, I remember watching the power crews coming up from Alabama to help with restoration efforts.  None of that this time.

The End Game

Last week was nowhere near the event of 2012.  That much, I was absolutely correct about.  That is why I played down the derecho talk.

Could you say the storm last week was a low-end derecho? Sure, I don’t have a big problem with that, but now we are playing word games.  With no objective standard for the term acceptable to the entire meteorological community, I can’t put up a big argument.  I would rather concentrate on real world effects: wind damage, wind damage, wind damage.

Last Thursday brought a nasty, damaging squall line.  If you feel better calling it a derecho, go wild.

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About seansublette

Meteorologist at Climate Central. Broadcast meteorologist in Virginia from 1995 to 2015. Born and raised in Richmond, VA. Penn State alumnus. Loves baseball and the rock band Rush. Views are independent of my employer. Past performance does not guarantee future results.
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