Check out this satellite loop of a line of severe thunderstorms over eastern Oklahoma extending northeast into western Missouri. You can see the classic telltale signs of explosive thunderstorm growth here. Notice how at the outset, the line of clouds near the center of the frame are relatively thin, but then balloon out as time goes by. What’s happening here is that the cumulonimbus (thunderstorm) clouds along this line are growing so rapidly that they are hitting the top of the troposphere. Since water vapor has a difficult time penetrating into the stratosphere, it gets forced out laterally instead. Indeed, the Storm Prediction Center in Norman, OK has issued a mesoscale discussion, along with severe thunderstorm warnings and tornado watches for much of the area.