Why the Greenfield tornado was so powerful, and a forecast for more storms

At least 20 tornadoes were reported Tuesday, mainly in southwest Iowa, but more across Minnesota and Wisconsin will probably be confirmed in the days ahead. The Weather Service’s Storm Prediction Center had drawn a Level 4 out of 5 risk zone across Iowa early Tuesday afternoon, cautioning that some tornadoes could be “strong” and remain on the ground for considerable distances. The Greenfield storm comes exactly 11 years and one day after a catastrophic tornado struck Moore, Okla. That was the last EF5 tornado on the 0-to-5 Enhanced Fujita scale...