CEDAR RAPIDS, Iowa (KCRG) - One lone shower – we didn’t even see lightning detected with it – caused a lot of commotion on Sunday evening. Many people saw a funnel cloud northwest of Cedar Rapids, which made it more than halfway to the ground, but we haven’t gotten any information that indicates it definitively touched down.
A weak wind shift was draped across the area, which may have led to a weak area of rotating air near the ground. As the shower grew, it pulled that spin upward and stretched it out. This makes it rotate faster, just like how a figure skater who pulls her outstretched arms inward will spin faster. Because there was so much moisture in the air, the rotation also developed into a cloud: a funnel cloud.
This is different than what causes the “typical” tornado. Those come from “supercell” thunderstorms, which are themselves rotating! In that case, there is wind shear: air moving in different directions and speeds at two different heights. That makes the air spin horizontally, just like how a barrel rolls along the ground. The powerful upward-moving air inside the thunderstorm pulls that rotation upward, stretching it high up into the storm and turning it into an “engine” that is able to draw in warm, moist air in one spot and drop out rain-cooled air in another. Most supercells don’t produce tornadoes, though, and even ones that have really strong, tight rotation don’t always make a tornado.
In the case of a supercell, we can easily see the rotation on radar. We know that there is a threat and where it is. There are bigger-picture things that point to the potential for supercells to develop in the first place, too. But in the case of showers producing funnels (and rarely, a tornado), the ingredients happen on a much smaller scale and are very hard to detect.