The calendar says October, but with severe storms and flash flooding in the forecast this week, you may be wondering what gives.

While the frequency and severity of severe storms that produce large hail, damaging winds, tornadoes and flash flooding peak in the spring, very similar setups occur in the fall.

These setups are large low pressure systems that bring cold, dry air toward Central Texas from the west. Ahead of these systems, large influxes of warm, moist air from the Gulf of Mexico stream in at the surface. The boundary between these two air masses are cold fronts that act as focal points for the lift needed for thunderstorm development.

One element in the fall that exacerbates the risk for flash flooding that doesn’t occur in the spring is tropical influence.

Heading out of winter and into spring, the sea surface temperatures (along with other factors) are too cold for tropical cyclone development.

But entering the fall season is actually when hurricane season is most active because of the months of sea surface temperatures warming.

In Texas, we’re always on the lookout for storms in the Gulf, but we also are on the lookout for the Eastern Pacific. This area is highly active and can see landfalling hurricanes on the west coast of Mexico that traverse the country and move into Central Texas.

The remnants of these storms act as additional lift and moisture needed for severe storms and flash flooding. This is also one of the reasons why October is our second wettest month on average behind, you guessed it, May.

Historically, some of Central Texas’ worst flood events have occurred in the fall. Halloween 2015 and Llano River Floods of 2018 are some of the most recent examples. Both events occurring in the month of October.