The monstrous, swift-moving flood that swept through the Hill Country of Texas killing at least 27 people and leaving many more missing, was a flash flood, the nation’s top storm-related killer

The region is also known as “Flash Flood Alley,” because of its propensity for high levels of rainfall and the runoff on thin soil.

Meteorologists say incredible amounts of moisture in the air fueled a storm that barely moved over central Texas, creating conditions for fatal flash floods.