Late-season tornado outbreaks — like those that occurred in October in the Midlands and in November in the eastern United States — are rare but not as unusual as some might think.
October and November constitute a short, modest secondary peak in the number of tornadoes. The bump comes from the instability that accompanies the seasonal shift from summer into winter.
Brett Anderson, a meteorologist with AccuWeather, said late-season tornadoes, while less frequent, tend to be more powerful than those in the spring, the primary tornado season.
The October storms and Sunday's multi-state outbreak included the unusually strong EF4 category of tornadoes.
In early October, 12 to 18 tornadoes were reported in Nebraska, Iowa and South Dakota, Anderson said. One, a 19-mile-long tornado, reached EF4 strength as it churned through Wayne, Neb., according to the National Weather Service.
Extensively damaged were the city's airport and industrial park and area farmsteads. About 15 people were injured, but no one was killed.
Meteorologists still are trying to determine how many tornadoes occurred Sunday. Late Monday afternoon, the number stood at about 70, Anderson said. That number will drop as duplicate reports are eliminated. So far, though, at least one EF4 tornado has been confirmed, he said.
At least six people were killed Sunday, and many more were injured.
Similar weather conditions spawned the October and November storms, Anderson said:
— A strong jet stream.
— Abundant warm, moist air streaming northward from the Gulf of Mexico.
— Cold air surging southward that clashed with the warm, moist air.
“We had a very similar type of setup when we compare the two,” he said.
Key differences, however:
— Sunday's storms were more numerous and more powerful.
— The October storms occurred in a less populated area, which caused less damage and fewer injuries.
— The October outbreak resulted from two big storm cells, while Sunday's occurrence was produced by multiple storm cells.
Not surprising is that multiple tornadoes were reported in each case. Tornadoes tend to occur in clusters, because if the weather is optimal for one twister, it probably is for another.
Anderson said tornado season peaks in the spring and fall, because that's when the contrast is strongest between cold air to the north and warm air to the south. The season subside in the summer because the air is more uniformly warm across the continent.
Autumn tornadoes tend to be stronger than spring ones because the contrast between cold air from the north and warm air from the south is sharper.
Source: AccuWeather Inc., National Weather Service