There are situations when weather emergencies sneak up on people. Tropical Storm Barry didn’t last very long. Once it came ashore in Mexico it unraveled into the general humid murk of southern Mexico, but it still retained enough integrity to be identifiable as it slowly moved uphill, and north. There are many cases in history where post-tropical flooding can be catastrophic. (Thinking Cyclone Yasi in Australia a few years ago.) Still, I can’t think of a situation where the onset of danger was as surprising as here. The middle of night on a holiday. Getting word to people in time and a place where flash flood warnings are somewhat common and thus easily ignored.
So you had the remnants of a tropical storm. Because of this, you had abundant moisture coming from that storm's source region in the Gulf. You had strong moisture transport coming northward as well with a strong low-level jet stream (a common feature in Texas located about 5,000 feet above the surface). But the jet was oriented to allow for maximum upsloping, aimed right at Hill Country. You had plentiful instability in the atmosphere as well. So, tallying all that together: A remnant tropical system, moisture levels in the 99th percentile or higher, forced upward motion due to geography and wind direction, and plentiful instability. That's a recipe for flash flooding.
So how do you go from flash flooding to catastrophic flash flooding, because the difference is clearly enormous. When you put those parameters in concert with a weather pattern that allows for maximum efficiency of rainfall, a monsoonal pattern, and slow movement, as well as geography that allows for rapid build up of water on dry ground and riverbeds that "funnel" that through an area, that's when you flip from ordinary to potentially tragic.
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