How fall can serve as a secondary severe weather season
The spring season is known for the activity it brings, including thunderstorms and severe weather. It has been deemed the main severe weather season, but, of course, it is not the only time of year when severe weather can occur.
Although it is not as active, the fall season serves as a secondary severe weather season. This is due to similar characteristics that the spring season has when it comes to the transition from cold to warm weather. This "secondary season" peaks in October-November, when colder air begins to arrive in the Midwest.
One of the main drivers of this peak in activity is the shifting of air masses. These air masses can be characterized by the two main jet streams present in the United States, the polar and subtropical jet streams. These are fast-flowing "rivers" of air that flow through the upper levels of the atmosphere, and they are the main driver of why temperature differences are observed in the states. The polar jet stream, as hinted at by the name, makes its presence known with a colder air mass from the north. The subtropical jet stream is closer to the equator, so it brings a more tropical-like air mass to the area it flows over.
The polar jet stream resides more in the northern portion of the United States and southern Canada during the summer, while the subtropical jet stream flows through the southern half of the states. During the fall season, these both begin to shift to the south. This places the subtropical jet stream well into the tropics and the polar jet stream in the central/southern United States region. This means that a colder air mass is following behind it, leading to stronger winds overhead and greater temperature differences for the Midwest.
Stronger winds overhead are important for the development of severe weather because they lead to a greater amount of shear for storms that do develop to sustain themselves. If storms don't have shear, they are unable to continue growing and fueling themselves with the proper ingredients. This shear is the change in speed and direction of winds with height. Ideal shear conditions for storm growth would be winds that increase in speed and change direction in a clockwise manner with height.
Greater temperature differences are important for the development of severe weather because they are what help the unstable air get pushed into the upper levels of the atmosphere for storms to fuel themselves. An example would be a strong cold front that is typical for this time of year as the colder air mass begins to take over. This cold and more dense air moves into a region where warm and less dense air is present. Since the warm, moist air is not as dense, it is forced to move over the cold air associated with the front. The greater the temperature difference with this front, the more efficient this lifting becomes. Once lifted, this unstable air begins to expand, resulting in the emergence of thunderstorms in an area. All these factors combined are why severe storms tend to briefly re-emerge as the fall season begins to take over.
