Last week, Austin, Texas, and surrounding communities found themselves flooded after a relentless thunderstorm dropped more than a foot of rain in just a few hours. This astounding rainfall event was the result of a phenomenon known as “training,” and as Austin saw, training can lead to devastating results.

Size Matters

While the distinctions don’t seem like much while we’re dodging hailstones and fleeing lightning strikes, there are three different types of thunderstorms, ranging from everyday occurrences to rare monsters.

The most prevalent type of thunderstorm we experience is known as a “single cell,” which are those small storms that pop up, pour for a few minutes, and then fizzle out, leaving behind clearing skies and stifling humidity.

The least common, on the other hand, is called a supercell, which features a tilted, rotating updraft that acts like an engine, allowing the storm to grow up to the size of an entire county and survive for many hours in the most extreme cases.

While they produce stunning landscapes (like the one above), supercells are also very dangerous; the violent processes within a supercell can generate hail larger than softballs, destructive winds, and tornadoes. The majority of supercells don’t produce tornadoes, but in the right environment, it’s this kind of storm that’s responsible for producing those EF-5s that can wipe a home clean from its foundation.

That brings us to the middle category, that in-between, kind of gray area where multiple thunderstorms develop and feed off of one another in an intricate process that can cause all sorts of problems. These storms, known as multicell storms, are probably most familiar to us as a squall line, or a line of connected thunderstorms that move in the same direction at the same speed. The strongest squall lines are called “derechos” when they produce destructive winds over a path hundreds of miles. The above radar image shows the June 29, 2012, derecho as it raced through Ohio on its way toward the Mid-Atlantic coast.

Battle Between Good and Evil

Last week, a cold front extending off of a low pressure system in the Atlantic Ocean dove south across the far southern extents of the United States. This boundary of drier, cooler air invading from Canada crashed into warm, humid air fed by the Gulf of Mexico to its south.

By the time the front reached Texas, it was several thousand miles away from its parent low pressure system, so it started to slow down and eventually stall out over central Texas. This created a stationary front, or a boundary between warm/humid and cool/dry that barely budges.

That front was pretty sharp—the dew point in San Antonio was 75°F, while the dew point was just 67°F a few miles north in Austin. That doesn’t seem like much—both are muggy—but it illustrates how dramatic the weather can change depending on which side of the front you’re on.

Meanwhile, widespread showers and thunderstorms broke out across Texas in this unstable environment, including some moving toward the two cities in the center of the state.

Breathe In, Breathe Out

A thunderstorm derives its energy from an updraft, a process generated when unstable air rapidly rises up through the atmosphere. When the updraft grows strong enough, it can sustain immense amounts of water up in the clouds, and the sheer weight of this suspended water will eventually overwhelm the updraft and begin to fall toward the ground.

One of the great things about a thunderstorm on a hot day is that it can cause the temperature to plummet once it starts raining. This natural air conditioning is called a downdraft, or rain-cooled air sinking from the thunderstorm to the ground. Since cooler air is more stable than warm air, this cooler air usually chokes off the storm’s updraft, starving it of energy and forcing it to dissipate.

That doesn’t always happen, though.

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Sometimes, this pool of cold air will radiate away from the storm like a cold front, producing a feature known as an outflow boundary. As this outflow boundary collides with unstable air ahead of it, it can force this air to rise and create more updrafts, creating new thunderstorms along its path. It’s very common to see this occur around the Gulf Coast—which is why places like Mobile and New Orleans can see a storm every afternoon for months on end—but this is also how squall lines develop.

Rarely, the outflow boundary will move in the opposite direction of the thunderstorm, causing the storm to “back-build” as if they’re being generated from a single spot (there’s a conspiracy theory for you!).

Imagine a day where there are stiff southerly winds blowing hot and humid air through your town. A thunderstorm bubbles up over your town and pours for a little while. The southerly winds blow that storm off to the north, but that storm’s outflow boundary moves back to the south. The outflow boundary creates a new thunderstorm that bubbles up right over your town (again!), and the process repeats for a couple of hours, dumping endless amounts of heavy rain that overflow your backyard rain gauge and turn your street into a river.

This process is called “training,” so called because the thunderstorms look like they’re train cars moving along railroad tracks. Training can continue for hours, forcing a few unlucky areas to get caught under this relentless heavy rain until the air stabilizes and the storm development stops.

The NWS office in Binghamton, New York, uploaded the above radar clip to YouTube a few months ago, showing a training complex of thunderstorms over Delaware County, New York, back in June 2007. The training occurs in the white circle, where six to eight inches of rain fell in just a couple of hours. The resulting floods killed two people, destroyed dozens of homes, washed out two bridges, and caused millions of dollars in damage.

The storms’ development is so sustained over one spot that it looks like they aren’t moving at all.

Austin Under Water

The training complex of thunderstorms that struck the Austin area wasn’t as photogenic as the radar loop above, but it was enough to cause significant problems, dropping more than a foot of rain in six hours.

Hey mobile users: the above .gif is 6.84 megabytes, so be mindful if you choose to load it.

Not only do outflow boundaries serve as a focus for thunderstorm development, but so too do large-scale fronts, like the stationary front that was draped across the region the morning of the flooding. The stationary front, along with outflow boundaries from nearby storms, allowed a large thunderstorm to persist over the Austin’s southern suburbs for most of the morning of Friday, October 30, producing more rain than natural and man-made drainage systems could handle.

The flooding was brutal. Six people died as a result of the rising waters, and neighborhoods that had never flooded before saw standing water from this event. The rain was so bad that even the weather nerds themselves weren’t safe—the weather station at Austin-Bergstrom International Airport was knocked offline after it became submerged, and the airport’s air traffic control facility is still closed because of flood damage.

While the flooding in Austin saw the brunt of the coverage from this event, they didn’t even see the greatest totals. Eastern Texas and most of Louisiana experienced even heavier rainfall from the same system, dropping up to 20 inches of rain in a couple of spots.

Training thunderstorms are often the cause of some of the worst flooding disasters possible, and believe it or not, the phenomenon isn’t all that uncommon. Training is usually much less intense, but it can still cause localized flooding even when it doesn’t drop an ocean of rain in just a couple of hours.

[Top Image: Getty Images | Supercell Image: Kelly DeLay via Flickr | Squall Line Radars: Gibson Ridge | Austin Radar/Precip Map: Author | Surface Analysis: WPC]

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