Tornadoes in November: Unusual, Not Impossible
By Justin Gehrts, Meteorologist
CEDAR RAPIDS, Iowa - Sunday’s severe weather outbreak across the Midwest and Ohio Valley was proof that there is truly no “tornado season.” While most of the thunderstorms that produce tornadoes and damaging winds do happen in the spring and summer in the central U.S., unseasonably strong and dynamic weather systems can create severe weather even late in the year.
This was a situation in which there was relatively high confidence of a severe weather outbreak. It’s not unusual for severe weather to be conditional on a lot of ingredients coming together at just the right time but not being sure if they really will or not. For example, you could think of it as saying “if this happens and if this happens, then maybe that will happen.” Sunday was a different situation. In fact, the Storm Prediction Center had outlined a “moderate” risk of severe weather the day beforehand, which is somewhat unusual in general and downright rare in November. They upgraded that to a “high” risk with their Sunday morning update. The last time a high risk was issued in November was in 2005 and involved the Lower Mississippi River Valley – much farther south than Sunday’s severe weather.
So, why did this happen if November is such an unusual (but not impossible) time for this kind of severe weather? Severe storms don’t follow a calendar, so they use the same kinds of ingredients that they would require any other time of year. First, a powerful low pressure system was swinging through the region during the daytime hours when the air would be warmest. It drew relatively moist air ahead of it, too. As the cold front passed through Iowa, it began to produce a line of storms. A couple hours later, another boundary ahead of the cold front was the focus for the tornado-producing storms in central Illinois. Ahead of that boundary, winds were blowing out of the south. Moving up through the atmosphere, those winds changed direction to the west and also picked up in speed. This is called wind shear, and it was what allowed thunderstorms to begin to rotate. The strong winds aloft also made the storms move at 50 mph to even 70 mph, which is much faster than usual. That faster speed left people with less time to react to warnings.
There were a couple of other factors that are a bit more difficult to explain that were also important. The atmosphere has waves just like water does, and there was a large wave of energy moving into the Midwest on Sunday that provided a lot of lift to the air. Rising air is what storms need to grow, and this wave helped that process. There was also a very powerful jet stream punching into the region, which also helped allow storms to be able to grow large.
On November 12, 2005, eight tornadoes touched down in central Iowa. Sunday’s weather system had a flavor similar to the one that produced those tornadoes in Iowa, although it was substantially more powerful and covered a larger area. Even so, it was a similar set of circumstances – a very dynamic weather system, unusually warm and muggy air for the time of year, and winds turning and getting stronger with height.
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