Derechos Hit Across US Heartland

I usually don’t comment on the weather threads & usually quit reading them once hard to pronounce 3 syllable words start to get thrown around but I thought this was interesting & might be entertaining to some of the bridge guys. I only experienced these offshore. They were frequent & severe in the southern parts of the Bay of Campeche. I think they hit during the spring months always going north. Control Marina usually issued a warning for everyone to untie, secure the decks & get away from offshore platforms. They seemed to last a hour or less & then it was like it never happened except everything was all clean.

Edit Reason: posted old link.

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This '18 gcaptain thread does a superior job of describing what a derecho is for any newcomers & those who missed it.

Experienced something like that while at a dock in Honduras. All I could do to get away from the berth with no assist. Funny thing, had a local news crew aboard interviewing the crew and the operation. We were doing work for Exxon at the time bringing in leaded gas from Houston. They couldn’t get off on the dock due to the severity of the weather in a very quick manner. The camera man got seasick while underway to anchor. I am still trying to remember the translation for being sick. Something that ended in O. Aroho or something like that. Had to wait at least 48 hrs before we went back alongside

Squall Lines

By definition, squall lines are clusters of thunderstorms that have a prominent, relatively linear signature on images of radar reflectivity. While squall lines can produce any kind of severe weather, they most prominently produce damaging straight-line winds. The convection in a squall line tends to be relatively narrow, while the characteristic length can range from approximately 50 miles to hundreds of miles. Squall Lines can be solid (thunderstorms “touch” each other)

Derechos

Formally, a derecho is a widespread, convectively-induced straight-line wind storm composed of numerous downbursts produced by a group of thunderstorms. Derecho is actually a Spanish word that, in this context, translates to “straight-ahead” or “direct.” There’s some disagreement in the meteorological community on what exact criteria should define a derecho, but we’ll use these as our criteria:

There must be a “concentrated area” of reports of wind damage and/or convective wind gusts greater than 50 knots that follows a chronological pattern consistent with a long-lived group of thunderstorms. This area of reports must be at least 400 kilometers (about 250 miles) long.

Within this area, there must be at least three reports of damage as severe as what could be inflicted by a weak tornado and/or wind gusts of 65 knots (75 miles per hour) or more. The reports must be sufficiently spread apart (at least 64 kliometers, or about 40 miles) to indicate that the thunderstorms maintained noteworthy intensity for a sufficiently long time.

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In Singapore these are known as “Sumatras”, since they originate over Sumatra:


Most frequent during the SW Monsoon season and a “nuisance” when doing complex Marine operations in Singapore waters

This is incorrect and it’s easy to check. A Sumatra is what is known as a squall line. This is what it says in the article you posted.

Maximum gusts of up to 50 knots have been recorded during the passage of a Sumatra squall (Gusts are temporary rise in wind speed).

50 kts is 57.5 mph.

On the other hand here is the NWS definition of a derecho.

By definition, 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.

I have seen gust of much more than 50 kts at timers during Sumatras.
Even steady wind of 52 kts. for 20 min. while holding a newbuilt Offshore Wind Construction vessel (Seafox 5) with tugs, waiting to load on a HLV at first light:


I used to arrange with Fugro Weather Service to keep an eye on the weather radar at Changi Airport when doing loadouts, or other weather sensitive operations in Singapore.That way we got abt. 20 min. warning of approaching squall lines.

PS> The last few years of doing this kind of operations we could watch the radar picture ourselves, since it was made available on the web.

The peak wind speed in gusts in only part of the definition.

By definition, 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.

This is from the linked article in the OP

The intense storms caused straight-line wind damage along a path that stretched for more than 600 miles, and in Nashville, the winds were clocked at up to 71 mph.

Those things are called “turbonadas”. They come off the Yucatan traveling more or less east to west. They present as a front line and blow like a bitch. 50+ knots. Duration: one to one and a half hours.
They typically show up July/August and run into September. They hit every evening in the season, until one day, they stop.

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A little to the west of there, Tehuantepecers blowing to the SW can get your attention.

A derecho is not simply a local name for a squall line.

Fundamentally similar weather phenomena can be categorized differently, for example single cell thunderstorms vs the so-called supercell thunderstorm. The same could be said of the difference between a tropical depression and a hurricane.

On the other hand Sumatra and the like are just local names for squall lines. Same as the German Foehn and the Chinook winds are the same phenomena but are given local names. Or the same typhoon called a hurricane in different area.

You might be right, I did hear the term, “turbonadas” used as well along with “derechos”. I’m not even to the level of novice on meteorology & suspect the bridge guys I was working with who used both terms weren’t too much farther advanced than me on the subject. It’s been over 15 years since I worked in that area but I do recall several of the blows coming from the south. I was working out of Pariaso/Dos Bocas for a few years & remember debris, trash & sometimes beach toys getting blown offshore from the south. When heading in it would be directly off the bow which wasn’t bad. I do recall them being over quickly, never up to 2 hours but never bothered to check the width of the fronts which I guess would be one of the determining factors of the correct terminology?

My understanding is poor but here’s what Wikipedia says:

, it has become apparent that the most damaging derechos are associated with particular types of mesoscale convective systems that are self-perpetuating (meaning that the convective systems are not strongly dependent on the larger-scale meteorological processes such as those associated with blizzard-producing winter storms and strong cold fronts). In addition, the term “derecho” sometimes is misapplied to convectively-generated wind events that are not particularly well-organized or long-lasting. For these reasons, a more precise, physically-based definition of “derecho” has been introduced within the meteorological community.[2

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more.

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Those Campeche things are definitely turbonadas. I don’t know anything about derechos.

You mentioned spring time and that got me thinking. There are similar events in the spring, but less violent I think. What’s weird is you get a daily cycle. Starting around noon to 1400 the sea would be flat. Then, sometime in the evening the squall would hit and blow the sea up to 5-7 ft. Lather, rinse repeat.

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From NWS: https://www.weather.gov/lmk/squallbow

A " squall line " refers to a linearly-oriented zone of convection (i.e., thunderstorms). Squall lines are common across the United States east of the Rockies, especially during the spring when the atmosphere is most "dynamic

Next is the bow echo:

" A " bow echo " or " bowing line segment " is an arched/bowed out line of thunderstorms, sometimes embedded within a squall line.

Bow echoes, most common in the spring and summer, usually are associated with an axis of enhanced winds that create straight-line wind damage at the surface.

image

From Wikipedia:

In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts.

In order for the derecho to “self-perpetuate” the conditions in the path have to be favorable

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Which is why they are rare:

image

From https://www.weather.gov/lmk/derecho

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