The Answer To “What Is A Derecho?”

Today is one of the rarest of days for forecasters and meteorologists, exciting yet at the same time incredibly worrisome.

The Mid-Atlantic is in the bulls-eye for a ‘derecho’ event today. In this loop you can see the thunderstorm complex that ripped through the Northern Ohio Valley last night and looks to be forming a classic case for a potential ‘serial’ derecho.

A derecho is an organized complex of storms that merge together to form a continuous line of storms that can travel hundreds of miles and cover hundreds of miles, with devastating effect.

A serial derecho is one that lies within a larger line of stormy weather covering hundreds of miles. There are a few other types of derecho events, but I’ll focus on this one so your eyes don’t gloss in reading this.

The derecho cycle depends on thermodynamics and the ability for the parent storm complex to sustain itself with the new formation of thunderstorms to replace those that reach their ‘dissipating stage’ and die off. All thunderstorms undergo a cycle of life and death, but only those supported with just the right amount of Mother Nature’s best ingredients can stand the test of time.

In a derecho, storms form, mature and die off within the line as the line itself ‘bows’ outward, and pushes its downdraft gust front out ahead of it. The gust front is typically that first really strong gust of cool wind blowing outward from the storm, that signifies the arrival of any strong thunderstorm, but think of this as the super-sized version!

A typical thunderstorm that reaches severe limits is supported by upper level winds that allow the rising column of cumulonimbus to tilt (usually forward) and thus create an environment where the updraft column of warmer air feeding the growth of the storm is somewhat preserved. Once any storm loses that energy source (several factors can cause it, but in most of our thunderstorms in the Northeast it is either colder air drawn in from the cooler ocean water or simply the storm raining itself death by cooling the ground below with it’s own rain).

A derecho event has the uncommon ability to tilt itself toward the back vertically thanks to a rear-inflow jet, and then draw on air being rapidly forced up just ahead of the super-strong gust front to provide its inflow of warmer air energy. As long as this dynamic remains in place, the derecho can maintain its cyclic existence.

The threat in a derecho is two-fold. The gust front winds can easily reach 60, 70 or even 80 mph, and in covering such a wide area they can cause considerably widespread damage. The second threat is obviously the rain, and lots of it! Just what anyone in the northeastern quarter of the country doesn’t want to hear right!

http://radar.weather.gov/Conus/northeast_loop.php

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