Over a Dozen Flash Floods in Two Weeks

By: Chris Dolce

Published: August 10,2013

 

 

 

Why So Many Floods?

Since late July, we've seen roughly two dozen flash flood events from the East to the Plains and Desert Southwest.
While severe thunderstorms can and do still rumble particularly across the nation's northern tier, mid-late summer is prime time for the threat of flash flooding for several reasons:

Trapped between cooler air to the north and hot, dry air to the south, areas from Kansas to Missouri, northern Arkansas into the Tennessee Valley have been soaked by flooding rain in late July and early August 2013.

1) Deep moisture:  The key is not only humid air near the surface, but also moisture through a deep layer in the atmosphere.
2) Light winds aloft:  In the summer months, weak winds aloft lead to slow-moving thunderstorms with a longer duration of heavy rain.
3) Stalled fronts:  When old fronts lose their temperature contrast, they can still serve as instigators for more thunderstorms, stalling over an area for days.
4) Thunderstorm clusters:  These "mesoscale convective systems" typically congeal during the late evening/overnight/early morning hours. When they move slowly, they can produce prodigious rainfall.
5) Tropical connection:  Any tropical cyclones moving inland can produce widespread flash flooding. That said, even a deep moisture tap from the tropics without a tropical cyclone is enough to set the table for flash flooding.
Let's list the "drenching dozen" recent local flash flood events.