Created by Roger Edwards, Storm Prediction Center
This storm hammered parts of Hidalgo County TX with softball size hail (4.5 inches in diameter), before moving south, with the gravity wave, across the Rio Grande and into Mexico. Balloon soundings from Brownsville and Corpus Christi (not shown) indicated a strong cap that morning, which was weakened during the day by heating of a moist layer underneath, near the surface. Apparently, that heating was still not enough to break the cap; it also required the combined lift of a strong gravity wave and a convergence boundary. No storms formed either elsewhere along the gravity waves, elsewhere along the cumulus line, along the Mexican mountains to the southwest, or along the cold front.
(Thanks to Dana Quinn, Ian Wittmeyer and Dave Blanchard for assembling these animations from SPC's 9-image set.)
These gravity waves act much like the waves you see after tossing a rock into a pool. Air is also a fluid; and gravity waves are actually quite common in the atmosphere. They don't often show up this well, though. In this case, over the Gulf of Mexico, there was enough moisture in the layer containing the waves, and enough lift along the front of each wave, to form cloud material. Then, on the back of each wave, the air sunk and dried, destroying the clouds. This process repeated many times as more waves followed. Over land, only the first wave was consistently strong enough to form a cloud arc. The cause of the waves in this case is unknown.