Matthew Cappucci, Senior Meteorologist, MyRadar:
We knew that we had ample moisture in place, absolutely tropical moisture content in the atmosphere. Part of the reason for that was last Monday, Tamaulipas, Mexico got hit by Tropical Storm Barry.
Barry moved inland, dissipated, and then left this big blob of humidity that eventually wafted north into Texas. So we knew the antecedent atmosphere conditions favored very heavy rain, but we didn’t see much of a trigger. And so that’s why even more than 12, 24 hours out, yes, we thought there could be some isolated flooding, but we didn’t think we’d see anything more than like six, seven inches, never mind a foot-and-a-half of water.
What wound up happening, though, was that as we headed towards Wednesday and Thursday, dying thunderstorms in West Texas left something called an MCV, or a mesoscale convective vortex, like this leftover whirlpool in the atmosphere, this little, teeny eddy. And this invisible eddy, this little corridor of spinning air, parked over Texas Hill Country and did two things.
Number one, it helped focus moisture. It gathered storms and really served as like the local trigger to get storms going. And it wasn’t moving, so those storms anchored in place. And the other thing too, this thing was pulling in moisture from the south. So you had an uninterrupted supply, a fire hose of tropical moisture from the Gulf that was aimed directly into these stagnant storms.
And it’s July. The upper-level winds are very weak. The jet stream has retreated all the way to Canada. So there’s nothing to move these storms along either. And so you got downpours lasting six, seven, eight hours, dumping three inches per hour. Very quickly, some folks saw a foot to a foot-and-a-half worth of water.
