NASA:

Those giant waves—”undular bore waves”—were photographed Oct. 3rd flowing across the skies of Des Moines, Iowa. (Credit: KCCI-TV Des Moines and Iowa Environmental Mesonet SchoolNet8 Webcam.)
“Wow, that was a good one!” says atmospheric scientist Tim Coleman of the National Space Science and Technology Center (NSSTC) in Huntsville, Alabama. Coleman is an expert in atmospheric wave phenomena and he believes bores are more common and more important than previously thought.