With the hot air swelling and spreading the storm, rain continues to fall behind the derecho. This forms a cold pool behind the storm front -- "like a bubble or a dome of cooler air," according to Herzmann -- that, in turn, allows more downbursts to form and the cycle to repeat itself.
Looking at radar images of the derecho, the system appears as a kind of boomerang-shaped band of high-intensity winds with a circular, cooler mass of air behind it. That boomerang -- called a "bow echo" by meteorologists -- is in fact a long line of clustered downbursts, each 2 to 4 miles wide.
That makes the derecho easy to identify, although given the speed with which one can form, that doesn't necessarily give communities much time to prepare, said Herzmann.
"The problem is that, on any given day, you're going to see small weather events that have the potential to turn into a derecho," he said. "It's still really hard to know which ones are going to roll into the full-fledged derecho and which ones will just dissipate."
No one could say climate change was the direct cause of this storm, but rising temperatures and more powerful storms have been predicted by climate models for more than two decades.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500