The following analysis of the Sunday, November 17, 2013 tornado outbreak in the Midwest by the National Weather Service has some very good points relating to the fact that A) late-fall tornadoes are rare but not unheard-of; and B) when they do form, they are usually powerful and thus, very dangerous.--Bryce
While severe thunderstorms are extremely complex, the ingredients they need to develop come down to a few simple ones: 1)warmth and moisture, which create instability 2) strong turning winds with height, creating wind shear and 3) a focus such as a cold front. The stronger these parameters are, especially wind shear, the greater potential for tornadoes.
In November, the environmental winds tend to be much stronger than in summer or even spring months. This leads to even greater wind shear. Normally though in late autumn and into winter, we do not have sufficient warmth and moisture for thunderstorm development.
(Last weekend) A powerful low pressure system across the Plains states had steered warmth and moisture northward for nearly two days, with moisture similar to summertime values (dew points in the mid-60s Fahrenheit). Given temperatures of around 70 deg F, there was more than enough instability to get thunderstorms to go. And, this instability was actually off the charts for November. Combine that with the high wind shear, and a cold front sweeping eastward, and you had all the ingredients you needed for thunderstorms, particularly supercells capable of tornadoes. With the parameters all extremely high for any time of the year, long-lived significant tornadoes became likely.
Below are listed the tornadoes that have occurred across the NWS Chicago CWA since 1950 prior to Sunday November 17, 2013. There had been a total of 12 before Sunday, in a total of six events (basically one such event every decade). Note that eight of the November tornadoes have been EF-2 or stronger, showing that when tornadoes do occur in November, there is a decent likelihood they will be significant (EF-2 or stronger). The primary reason for this is strong dynamics and wind field that tend to be present in the cool season (late autumn and winter), which add to a greater amount of wind shear. This is a key component to tornadoes, and the greater the wind shear the more likelihood for significant tornadoes.
November tornadoes to occur in the NWS Chicago County Warning Area (CWA) since 1950:
Date Time County Category
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11/13/1951 400 PM LAKE [IN] F2
11/26/1951 617 PM FORD F1
11/12/1965 248 PM DU PAGE F2
11/12/1965 345 PM COOK F2
11/12/1965 205 PM LA SALLE F2
11/12/1965 235 PM GRUNDY F2
11/12/1965 300 PM WILL F2
11/12/1965 453 PM LAKE [IN] F3
11/11/1971 1100 PM LIVINGSTON F1
11/10/1975 Unknown LAKE [IL] F1
11/09/1984 640 PM WILL F0
11/22/2010 304 PM WINNEBAGO EF2
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