Moisture, seasonal pattern change facilitate heavy rain in fall months

Hannah Ortega

Austin-Bergstrom International Airport, a weather data collection site, recorded 15.7 inches of rain during September and October. That marks the seventh-highest rainfall total for the two months since records began in 1942, state climatologist John Nielsen-Gammon said.

“In September and October, we had plenty of moisture coming in from the Gulf of Mexico, and the most unusual part was how much of that moisture ended up being deposited in Texas as rain,” Nielsen-Gammon said. “(From) preliminary numbers, Texas as a whole had its fourth-wettest month on record in September and its third-wettest month on record in October.”

Historically, the two wettest months for Texas were May 2015 and August 2017, when Hurricane Harvey hit. While Nielsen-Gammon said the state rarely lacks moisture, this year’s heavy concentration of water vapor and moisture in Texas was attributed to cold fronts in Central Texas and storms in Mexico. 

“We had several tropical cyclones make landfall over in western Mexico, and the moisture from those came across Mexico and into Texas,” Nielsen-Gammon said. “Although … most of our rain wasn’t from that mechanism, having a lot of moisture in the air makes it easier for thunderstorms to form without being killed by evaporation.”

Troy Kimmel, University and incident response meteorologist, said there have been several October and November floods over the past decade, making the recent rainfall not unusual but “outside what we would expect on an average basis.” Kimmel also said the rain came from a shift in seasonal patterns and the disappearance of upper-level high pressure, which ensnares heat and keeps rain at bay.

“Our weather was so dry just all summer long,” weather senior lecturer Kimmel said. “We didn’t get any rain, and we stayed that way. Then, all the sudden, the weather completely changed moods. A very drastic change occurred going into September and October and ended up being much wetter than usual, so when it comes down to it, (it’s) just a seasonal pattern change more than anything else.”

Cloud coverage from rain has created cooler weather in the Austin area. Kimmel said the temperature at the airport from Nov. 1 to Nov. 18 was about 2.5 degrees Fahrenheit below average, and the temperature for Camp Mabry, another weather data collection site, during the same time was about 4.4 degrees Fahrenheit below average.

Neuroscience sophomore Sydney Fischer said Austin residents are often ill-equipped for these cold temperatures.

“Maybe it’s because we aren’t marketed to buy winter clothes in this region, maybe it’s because we are so unsure of when the cold season starts here since it starts so suddenly, but I personally would love some kind of a cold weather countdown or month-ahead alert,” Fischer said.

Kimmel predicts this winter will be slightly wetter than usual, with a decent chance of ice and snow in Texas.

“The one thing that is kind of interesting for this year is the fact that we’ve got the Pacific where it’s a little bit warmer; we’re sort of in a little bit of a warm phase,” Kimmel said. “But there are a lot of other factors coming into play as well, which suggests that the winter overall will probably be a little wetter than normal … and near to slightly below average on temperatures.”