The media can't survive without a scare term to make you tune in. This winter saw the not-at-all-new-but-scary-sounding polar vortex. Sandy wasn't really a "superstorm" but hey, let's just call it one anyway. And every summer for the last few years, we've seen the derecho. It sounds new and terrifying, but what is it?
What is a derecho?
A derecho is a long-lived line of severe thunderstorms that produces violent winds (at least 60 MPH) and consistent damage for at least 240 consecutive miles. If a line of severe thunderstorms produces damage and reported winds above 60 MPH for 239 miles, it isn't a derecho. It has to reach the magic (and arbitrary) 240 mile mark.
For example, here are all of the damage reports from the infamous June 29, 2012 derecho that tore a path from Chicago to Washington D.C. Every blue dot indicates a report of wind damage or recorded gusts above 60 MPH.
The term "derecho" likely derives from the Spanish word for "straight," alluding to the storm's straight-line winds — the winds in a derecho sweep across the landscape straight ahead like a wall rather than swirling around like in a tornado.
*Note: The above video shows straight-line winds similar to those found in a derecho. The event itself occurred on June 21, 2013 and did not produce a long enough trail of damage to be considered a derecho.
When do derechos occur?
Derechos can form during any time of the year, but they're most common during the summer months when the air is extremely hot, very moist, and instability is abundant. The worst derechos tend to form between June and August during or right at the end of a blockbuster heatwave.
Derechos are also prone to occur during the fall and spring months along cold fronts, allowing the systems to grow to over 1,000 miles long in the most extreme cases.
Where do derecho occur?
Mostly east of the Rocky Mountains in the United States, but there was one that moved through Utah and Wyoming back in 1994. They're also known to occur in Europe and South America. There's a whole Wikipedia page devoted to listing derecho events.
Two types of derechos
There are two types of derechos: progressive and serial. Progressive derechos are probably the most common and easily the most damaging of the two.
A progressive derecho is one that forms along a weak boundary such as a stationary front — the dividing line between two air masses that doesn't move one way or the other — during the extremely warm summer months. Progressive derechos are incredibly dangerous because they can produce damage over hundreds of miles with wind gusts over 100 MPH.
A serial derecho, on the other hand, doesn't form on its own. Serial derechos form along the leading edge of a strong cold front and can stretch along a north-south axis from Canada to the Gulf of Mexico. While usually not as strong as progressive derechos, they can create just as much, if not more, damage over a much larger area. A notable example of a serial derecho is the one associated with the "Chiclone" that occurred in October 2010.
Formation of progressive derechos
The formation of progressive derechos is fascinating from a weather geek perspective. The explanation gets pretty heavy on weather jargon, but it's easy to understand once you have the whole picture.
*Note: As you'll see, I can't draw. Bear with me...
The setup usually begins with an intense heat wave across much of the central and eastern part of the United States. The hot, moist air flows around a high pressure center through the Ohio Valley and into the Mid-Atlantic region.
A stationary front can set up between the heat wave and (slightly) cooler air to the north. A pool of moisture usually builds up along and around the stationary front, providing essentially a fuel tank ready to feed thunderstorm development.
As thunderstorms on the western edge of the stationary front develop, they can start to merge with each other and become a single complex of thunderstorms.
When the storms merge, the storms' cold pools (also called the outflow) — the bubble of stable, rain-cooled air that sinks to the surface underneath a thunderstorm — merge and turn into one large cold pool.
At this point, one of two things can happen:
A) The cold pool can either choke off the updraft (or inflow; the warm, moist air that fuels the storm) and the storm dissipates, or
B) The cold pool begins to race away from the storm and it tilts the updraft, allowing the storm to start to move along the surface while also maintaining its strength.
For a derecho to occur, the latter has to happen.
The line of storms is now similar in shape to the squall line (line of storms) that we're used to seeing on weather radar.
Structure of the derecho
The structure of the system is characterized by an intense squall line along the leading edge and a brief lull that's usually followed a few minutes later by a batch of steady rainfall called the "stratiform region."
The stratiform region is depicted by the yellow shading on the illustration above. It is an area of steady rain that occurs after the main part of the derecho moves through. This shield of precipitation occurs because the updraft is tilted towards the back of the storm. This causes heaviest raindrops fall in the squall line, while the lighter raindrops stay suspended well behind the heavy line of thunderstorms. The tiny suspended drops of water keep moving upstream until enough merge together that they're too heavy and fall to the surface, creating the rain in the stratiform region.
Rear inflow jet — the teeth of the derecho
Under the right set of atmospheric conditions, the cold pool develops a feature called a rear inflow jet.
The rear inflow jet is what gives the derecho its punch. It's a low-level jet of air that moves from the back of the derecho to the front. The jet can exceed 100 MPH by the time it reaches the front of the squall line and dives down to the surface.
The rear inflow jet maintains itself and gains strength through friction and a phenomenon called "bookend vortices."
The bookend vortices (seen above) occur on either end of the main squall line, where winds start to curl back and create vertical rotation. Think of the tiny swirls that form in the swimming pool when you quickly run your hand through the water — it's a similar principle behind the formation of the bookend vortices.
As the cold pool rushes along the surface, it creates a horizontal rolling motion due to friction against both the ground and the inflow above the cold pool. These horizontal rolls of air feed into the rear inflow jet to make it stronger.
These horizontal rolls and bookend vortices are critical for strengthening a rear inflow jet and helping them grow so powerful.
You can actually see the rear inflow jet on a vertical cross-section of wind velocity radar imagery. This July 2011 derecho in Iowa produced winds of over 100 MPH.
The red/pink/white colors show winds moving towards the front of the derecho on the right, while the green colors shows the warm, moist inflow moving left towards the back of the derecho. As shown by the arrows, the rear inflow jet strengthens as it moves towards the front of the derecho, and gets shoved down towards the surface as it comes in contact with the heavy thunderstorm activity, producing the intense damaging winds.
A slow but steady death
A derecho starts to die out when the cold pool moves too far ahead of the thunderstorm activity. The cold pool eventually marches out so far ahead of its parent thunderstorms that it cuts off the inflow of warm, moist air that keeps them alive. When this happens, the storms start to die out, as do the winds.
A perfect example of the cold pool killing off the storm activity is from the infamous Ohio-DC derecho on June 29, 2012. The northern end of the derecho mostly keeps up with the cold pool, so the storms were able to stay strong and continue producing winds of 70-90 MPH for the duration of its path. The southern end of the derecho, on the other hand, started to die off as is approached the Ohio River because the cold pool moved too quickly and killed the inflow that fed the storms their energy.
This is why this particular derecho tore up the Washington D.C./Baltimore area while much of central West Virginia was spared.
While I won't say that derechos are nothing to be afraid of (because they are dangerous), they're definitely nothing new. The term has a technical a definition that almost seems arbitrary. If a line of thunderstorms produces a path of damage that's 238 miles long, it's not a derecho. It has to reach 240.
It's important to remember not to focus on whether a line of storms is considered a derecho.
If a thunderstorm has severe winds, it's dangerous no matter what you call it. (Don't get any ideas, Weather Channel.)
[Images: Gibson Ridge / SPC / SPC / NWS / Gibson Ridge / Terrible illustrations by the author]