(CNN) The folks watching the weather radar in Ohio this week spotted something that wasn't in the forecast -- swarming dragonflies.

The National Weather Service's Cleveland Office tweeted radar images on Tuesday that showed the hordes of small creatures. It looked like pulsating storm clouds over Ohio, Indiana and Pennsylvania, but the office said it wasn't rain.

While we are not biological experts, we have determined (through input from our followers) that it's most likely dragonflies mixed with other insects/birds! https://t.co/5MeJXj37zq — NWS Cleveland (@NWSCLE) September 10, 2019

Ohio State University Entomology Professor Norman Johnson said the dragonflies were likely Green Darners, which migrate south in the fall

He said the dragonflies don't usually travel in swarms like this, but local weather conditions can cause them to bunch up.

"The big swarms have been recorded a lot over the years, but they're not regular," he said.

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