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How Rare was the November Midwest Tornado Outbreak?

November 20, 2013; 7:48 AM

In the wake of the deadly Midwest tornado outbreak on Sunday, many people are wondering how rare tornadoes are during November.

The short answer is that tornadoes can occur in the Midwest during any month of the year. However, the number of tornadoes diminishes substantially during the cold-weather months.

There is a secondary severe weather season that occurs during October and November, which favors the Deep South.

While rare, tornadoes reaching as far north as the Midwest and mid-Atlantic are not unheard of during November. Occasionally, a small number of the tornadoes can be rather strong.

According to Harold Brooks, senior research scientist at the National Severe Storms Laboratory in Norman, Okla., "The peak of the secondary season for the United States as a whole is rather diffuse, but is centered over the middle of November."

The uptick in severe thunderstorms during October and November can be simply explained by the routine strengthening of storm systems during the autumn that are able to pull lingering warm and humid air northward from the Gulf of Mexico.

Brooks stated that this particular event had very strong winds aloft, which not only greatly increased the forward speed of the severe weather, but also added fuel to the individual storms.

According to Greg Carbin, warning coordination meteorologist with the National Weather Service's Storm Prediction Center, "Winds aloft over the region strengthened rapidly from 70 mph to 140 mph on Sunday."

A chain of events happened at just the right time over a concentrated area. Winds near the surface rapidly brought in moisture. During the midday Sunday, the sun came out and warmed the air near the ground as the strong winds aloft brought in dry, cooler air. The result was an extremely unstable atmosphere and a significant number of strong tornadoes.

Every decade as far back as the 1980s has brought multiple tornado outbreaks during November with a number of fatalities.

"During November there is a tornado outbreak about once every seven to eight years," Carbin stated.

"The most comparable event is probably Nov. 22, 1992, which had a large number of tornadoes in Indiana and Kentucky," Brooks said.

According to the Indianapolis National Weather Service office, the 1992 outbreak produced the largest number of November tornadoes [15] on a single day in Indiana on record.

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Other significant November outbreaks have occurred during the last 12 years. The last decade brought eight tornado outbreaks. The most significant of these for the Midwest occurred in 2001 and 2002.

During the Veterans Day Outbreak of Nov. 9 to 11, 2002, there were close to 80 tornadoes that took the lives of 36 people and injured more than 300 others.

In 2001, spanning Nov. 23 to 24, there were approximately five dozen tornadoes that killed 13 people and injured more than 200 others.

November tornadoes were very rare during the 1960s and 1970s.

The preliminary count of tornadoes through Nov. 17, 2013 is 886, which is well below the most recent eight-year annual average of 1,424 through mid-November.

As bad as the event was on Sunday, it could have been worse.

"If the storm system would have tracked over the lower Mississippi Valley, closer to the source of warm and humid air, instead of the Great Lakes, we would have likely had an even greater number of severe thunderstorms and tornadoes," Carbin said.

Moving forward through the end of the month, there will likely be a few more potent storm systems developing. However, the chance that all of the necessary ingredients will come together to produce a tornado outbreak for each and every system is quite low.

For people in the Midwest, the secondary tornado season is winding down through the latter half of November.

Odds are against a similar setup as far north as the last over the Midwest. However, as climatology suggests, the chances are higher for severe thunderstorms over the South.

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