A large, dark shelf cloud is rolling through the Ohio Valley this morning ahead of a complex of thunderstorms that’s traveled through five states over the past 12 hours.
But it doesn’t take a severe-level storm to generate an amazing, ominous shelf cloud that spans dozens of counties across two states. Lots of photos streamed through social media as the storm clipped north of Louisville, but it has been going strong since early this morning as the storm blew across southern Indiana.
Shelf clouds are common ahead of thunderstorms that form into lines — things like squall lines, bow echoes or derechos — and are usually indicative of strong wind gusts. They develop as the cool, moist outflow gets sucked back up into the storm. Because of the relatively low temperature and high moisture content in this pool of air, it condenses to form a cloud at a lower level than the surrounding storm clouds.
Shelf clouds are not the same thing as wall clouds, though they are formed through a similar process. They also don’t mean a tornado has formed, though tornadoes can and do develop within a thunderstorm line, or even on the curled-up ends of bow echoes and derechos.
This storm complex will continue to push east toward the Mid-Atlantic over the next four to six hours and eventually reach the D.C. area this evening. We’ll have an update on the storm forecast later this afternoon.