Flash flooding in Texas: How and why it happens

AUSTIN, Texas - The flooding rains we saw last week were caused by thunderstorms. But why did these storms bring flash flooding, while others over the rest of the week didn't?

The thunderstorms that caused the flash flooding were tropical beasts. The amount of moisture available to these storms was nearly twice as much as what Austin normally sees. It was all due to a weak cold front and moisture from decaying tropical systems. Both created a very effective heavy rain machine throughout Texas.

Flooding like that needs 3 crucial things:

• It needs a lot of moisture which is usually provided by the Gulf of Mexico. We felt that moisture by the extremely humid weather that preceded the storms.
• It also needed some upper level system to create some instability. That instability is what powers the thunderstorms and allows them to make the rain.
• Finally, everything needs a forcing mechanism to really get going.

Forcing mechanisms can be a cold front or an outflow boundary. The slower moving the boundary the better chance for flooding to occur. Along the boundary the very humid air from the surface is lifted to about 1,000 feet. That push is all it needs to develop thunderstorms to form which then can dump huge amounts of rain on the state.

Crucially flash flood events are less likely to occur if there is not any kind of upper level system to provide energy. Without a large-scale system to feed the storms moisture and warm air, the storms will just collapse on themselves. That can cause really heavy rain, but nearly every time that occurs the heavy rain is done in less than 30 minutes.

In extreme cases the storms can form what's called a "rain train" where the very heavy rain pounds the same areas for a long time. In those cases very dangerous flooding is possible and truly insane amounts of rain can fall. 

The catastrophic flooding that affected Dallas earlier in the day Monday was from one of those rain train events.