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September 15, 2010 / Dave Gorham

Fall Severe Weather Season is Approaching

Fall’s approaching fast (next Wednesday to be exact) and it’s time to start thinking about our second severe weather season. It’s true that tornadoes are most common during the spring months. However, both the spring and fall experience peaks of severe activity since strong winds, wind shear and atmospheric instability are present.

During the fall, tornadoes are often generated from landfalling hurricanes and tropical storms. In the springtime, Tornado Alley (which is an area between the Rockies and Appalachian Mountains) is the region where tornadoes are most common in the late spring and early summer months. In the fall, a “second severe weather season” occurs mainly across the Southeastern U.S. For instance, in my home state of Alabama the fall severe weather season is typically most active in November and early December. On the other hand in Texas, October and November are more active months in terms of severe weather. This is the time of year when there’s still plenty of warm, moist air in place and when strong cold fronts start dropping out of Canada and this often results in widespread severe weather.

Summary of recorded tornado activity across the U.S. The darker colors represent Tornado Alley. Image: FEMA

Why’s there an increase in tornadoes during the fall months? It has to do with the combination of fast jet stream winds and strong frontal systems during this time of year. We all know thunderstorms are common throughout the summer (they always seem to spoil your day at the beach or your time spent at the pool). However, tornadoes aren’t as common during the late summer months as the winds in the atmosphere typically aren’t strong enough to allow them to form. The opposite is true in the spring and fall as we have a strong gradient in temperatures across the hemisphere and this drives powerful jet stream winds and stronger fronts across the area. This initiates thunderstorm development and since the winds are stronger, we tend to see another “peak” in tornadoes during the fall. With that being said, April through July are still considered to be the heart of tornado season. Tornadoes can occur anytime during the year but they are less common during the winter months.

This image depicts the severe weather reports throughout 2007. Notice a peak in severe weather from April through July and the second peak in October. Image: NOAA

So get ready for not only the fall foliage and cooler temperatures ahead, but be prepared for the second severe weather season (especially those of you living in the Southeast)!


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