Great Smokies, Tuesday: I Will Call Them Mini Derechos

After last month's "Super Derecho" that raked Washington, D.C, a smaller wind storm killed two people and did damage in a much lesser-populated place: The Great Smoky Mountains. The video below details the damage and recovery:

Was it also a derecho? The official definition leaves it up to interpretation (more discussion about this below), but David Gaffin, Senior Forecaster at the NWS-Morristown, Tenn., office, said via email: "We feel that the July 5th widespread damaging wind event that killed two people in the Great Smoky Mountains National Park could be classified as a derecho. (More of his explanation why is shown below).

So that begs to question: Was yesterday's wind storm that caused damage from Minnesota to Tennessee a derecho as well? Meteorologists (even within our walls) disagreed, and there was much angst on social media. Here's a look at the storm at 2:30 p.m. yesterday:

The official definition on the Storm Prediction Center's website reads: "If the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho." Certainly there was a long wind damage path with Tuesday's storm:

I can understand why, Tuesday morning, meteorologists didn't sit down and measure this thing out on a map to make sure that it met the definition. Our job is forecasting and we had seen the model predictions showing a storm shaped like a derecho (HRRR short-range model shown at bottom; my understanding is that longer-range models, such as the NAM/NMM also showed this bowing structure). Then we came into work and saw increasing potentially deadly wind gusts between 70 and 80 mph southeast of Chicago.

But that's only part of the definition, we have to look at the wind gusts as well. It seems to me there are three possible ways to classify the path of winds from the >=58 mph LSR reports with yesterday's storm, which is the real sticking point here. Below are three possible solutions:

You either count the entire track from first report to last (475 miles, qualifies as a derecho), or if the breaks in data bother you (and you're confident that they aren't just because of lack of population or sparse weather stations), you count either first report to the break in Indiana (300 miles, qualifies), or first report northeast of Dubuque, Iowa, to the break (200 miles, does NOT qualify). Unless the NWS is willing to be more specific on the definition of "most" then this will be up to interpretation. There are some other great points on our news story. One that our Bernie Rayno mentioned to me was that he believes that derechos generally run around the "ring of fire" heat that often sets up this time of year, and this Tuesday's track did just that (map below). Capital Weather Gang links to the SPC definition, but then quotes things that aren't in the definition as proof that it was absolutely not a derecho).

And what do you call it when the wind gusts slow or stop? Former-derecho? Storm cluster? MCS? (which SPC called it late-morning, even though that system isn't typically present during the day). What if it later increases in strength again? Given the longer width of last month's derecho, Is there a width requirement (after all, a persistent thunderstorm could knock down trees over a 240-mile area that would only be a few miles wide, though this is rare the April 2011 Alabama tornado supercell comes to mind).

I think one of the biggest problem is this: Many meteorologists and reporters weren't familiar with the term before last month's Super Derecho (or the storm which held the previous name "Super Derecho" -- the "Inland Hurricane" of 2010). So now, we aren't sure how common they are, and if we start calling every small wind storm a derecho now, it will seem disingenuous, as if we are calling "wolf."

Which gets me thinking about the NWS again, and how they could help quantify Derechos. David went on to explain the storm survey he did on the July 5th Great Smoky Mountains Derecho and he is quoted below. This kind of in-depth, detailed discussion is normally required for tornado vs. straight-line wind events, which are small enough to be investigated by a local NWS office. This can't be done easily on a larger scale (for example: bigger than this small derecho) because it would involve coordination amongst offices. There is no requirement that someone official classify a derecho as of now, and if we really want to clear the air on this topic, that might be a path to take.

"The damage reports from July 5th indicate that wind damage covered more than 240 miles, and some observations sites in our area (see below) recorded winds of 50 kts. or greater." This band of thunderstorms intensified in West Virginia during the early afternoon hours of July 5th, and then quickly moved southwest across east Tennessee by mid- to late afternoon. The KMRX Doppler radar was estimating strong winds ahead of the storms between 60 to 65 knots (70 to 75 mph) around 5,000 ft MSL. While the entire momentum of these winds may not have completely descended down to the surface, there were several reports of winds in excess of 50 kts. (58 mph) measured at KTYS and several anemometers in the Great Smoky Mountains National Park (GSMNP). After looking at the extensive tree damage in the National Park near Laurel Creek and Cades Cove, it appeared like straight-line wind damage with pockets of damage mainly in areas that had a good open view to the northeast direction. The strong winds on July 5th were coming from the northeast direction down the Great Tennessee Valley, which is an unusual direction to observe strong winds in east Tennessee. Around the western side (Tennessee side) of the National Park, there are no mountain barriers to slow down northeasterly winds. Chilhowee Mountain usually blocks strong westerly winds from reaching the Laurel Creek and Cades Cove valleys, with Hannah Mountain likely slowing down any winds from the southwest (the predominant wind direction in this area). The strong northeasterly winds observed on July 5th likely flowed unimpeded into the northeast-to-southwest oriented valleys in the southwestern portion of GSMNP, and probably even accelerated in these valleys due to constricted flow. Thus, these winds might have accelerated to between 70 and 80 mph? Most of the trees that fell in the Laurel Creek and Cades Cove valleys were in the mid-slope region of the hillsides, with trees at the top of the hill remaining mostly intact. These fallen trees could have been the result of steeper slopes in the mid-slope hillside, but the fact that the winds came from an unusual direction could have also caused a lot of these trees to fall. It's likely that the tree roots are strongest in the direction of the prevailing winds, so these trees could have been more vulnerable to a blowdown from strong northeasterly winds?"

Report a Typo