SIOUX CITY, Iowa (KCAU) — This past Sunday, September 22nd at exactly 7:23 AM CDT, officially marked the start of the fall season. This means the shift from shorts and tank tops to jeans and sweatshirts, pool days and lemonade to bonfires and pumpkin spice lattes.

However, as we see this changeover from the dog days of summer to the fireside nights of fall, what you may not realize is that we also enter into a secondary severe weather season.

While spring may be notoriously associated with everything from devastating tornadoes to significant hail and wind storms, fall can have equally as violent weather.

In fact, here in Siouxland, we've seen a handful of severe weather outbreaks during autumn.

Dry start to Meteorological Fall: What Siouxland can expect going forward

This includes the October 4th, 2023 Outbreak which featured multiple strong to violent tornadoes, including the Wayne, NE EF4 and another EF-4 tornado which impacted portions of Woodbury and Cherokee counties.

Other fall severe weather outbreaks in Siouxland include:

– October 23rd: 2015 NW Iowa Tornadoes

– October 11th, 2020: Hail & Wind

And it's not just Siouxland that sees these spikes in severe weather in the fall. Notable secondary severe weather season outbreaks include: November 21-23, 1992 , when 95 tornadoes touched down from Texas to Indiana. This included 6 F4 tornadoes, 15 F3, and 26 F2 strength which resulted in 26 fatalities, 641 injuries, and more than $3 million (nearly $7 million in 2024) in damage.

Another significant severe weather outbreak, a bit closer to home, was the November 17, 2013, in which 77 tornadoes touched down in Illinois, Indiana, Michigan, Kentucky, Tennessee, Ohio, and southeastern Missouri. The strongest two tornadoes of the day though, were in Illinois, one in/near Washington, Illinois and the other in/near New Minden, Illinois. Both were EF4, with the Washington one a high-end EF4 with estimated peak winds of 190 mph. The outbreak resulted in 8 tornado-related fatalities and 3 additional non-tornado related fatalities and hundreds of injuries.

But why exactly do we see this second severe weather season?

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KCAU 9 Morning Weather

For severe weather, you need a few key ingredients: moisture, lift, instability, and wind shear (change in speed/direction with height)

Fall is essentially a battleground between hot and humid summer and frigid cold winter as the polar and midlatitude jet streams both begin to drop southward.

So we essentially become wedged between two vastly different air masses. The stronger, cold, dry polar jetstream and the warmer, moister subtropical jetstream.

Think of it like this...fall is a transition season. So sometimes we see the polar jet dip a bit farther south and go from warm to cold, seemingly in the blink of an eye. And then we see the polar jet and its colder air retreat a farther north again and get back into warmer air. It's like a river that ebbs and flows. And as we get these ebbs, we sometimes get very sharp temperature gradients and where we see these temperature gradients, we also see sharp wind gradients.

These harsher temperature gradients result in stronger cold fronts and stronger weather systems. These stronger cold fronts equate to stronger lift available. And the large variance between our polar jet airmass and the subtropical jet airmass leads to large variations in wind sped and direction...both aloft and at the surface (also known as wind shear). Both wind shear and lift are key ingredients in the development of severe weather.

We also still still see that summertime moisture and heat which aids in, not just larger temperature gradients when we get cool/cold snaps, but also instability as we make the switch from summer, particularly in the early fall when we tend to see the biggest fluctuations in temperature and see those final jabs of steamy summer heat.

Abnormally dry, moderate drought conditions expand over Siouxland

This leads to a spike in severe weather as we see similar ingredients and setups to our spring severe weather season. And then as we finally settle into mid/late fall and head into winter, we finally see that polar and subtropical jets settle closer to their winter positions, our secondary severe weather season largely winds down.

So, to sum it up, because the jet stream is increasingly stronger in the fall, and because we see numerous fluctuations in temperatures, many significant, we see stronger weather systems, stronger frontal boundaries and larger changes in speed and direction of the winds both at the surface and aloft. This, paired with typically abundant moisture flow and the instability thanks to the two dueling air masses as we see our polar jet stream advance southward and the hotter more humid airmass still in place as we make the transition from summer to fall and winter, results in a brief spike in severe weather...a secondary severe weather season.

It's also important to note that, while spring is the most notoriously dangerous severe weather season, fall severe weather season brings its own unique dangers.

As we head into the fall months we lose daylight. So, in addition to a spike in severe weather, including tornadoes, we see more severe weather events occurring after sunset, one of the deadliest times for severe weather, especially tornadoes. And to make matters worse, many tend to let their guard down once we head into the fall season in regards to severe weather due to the days starting to get cooler and shorter and fall not being the typical season you'd think of for severe weather.

That being said, it's important to remain vigilant for severe weather at any time of year, because it can and does happen anytime, anywhere. But especially as we head into the first month or so of fall as we see this secondary rise in severe weat

And for the latest weather anytime in Siouxland, visit the KCAU 9 weather page .