A trio of tornadoes formed over Lake Pontchartrain on Tuesday as frightened drivers came to a standstill on the 24-mile causeway that spans massive lake.

The tornadoes developed over water just south of Mandeville, La., just after 12 p.m. central time. Traffic on the causeway stopped as the tornado formed close to the north end of the bridge.

When the video starts, the main tornado can be seen on the left, with what looks like a satellite tornado on the right. The main tornado funnel darkens rapidly as it meets the water, and a second satellite tornado drops from the clouds.

Satellite tornadoes are secondary tornadoes that rotate around the main funnel. They are considered to be separate tornadoes, not to be confused with multiple vortices. Researchers still don’t understand what causes satellite tornadoes to form, though they tend to develop in the strongest supercells.

The Lake Pontchartrain storm developed in an environment ripe for severe weather this afternoon along the Southeast coast. The Storm Prediction Center has issued a moderate risk for severe thunderstorms across a large portion of the Gulf Coast. Tornado watches have been issued, and schools are dismissing early because of the severe weather threat.

The Storm Prediction Center is warning that severe thunderstorms capable of producing strong tornadoes could last well into the overnight hours.

(Storm Prediction Center)