TROY, Ala. -- A tornado partially collapsed the roof of a southeast Alabama Wal-Mart, injuring at least six people but none seriously, forecasters said Friday.

A strong storm cell passed through Troy, about 50 miles southeast of Montgomery, late Thursday night. It caused significant damage to the Wal-Mart roof and tore out the wall of a nearby sporting goods store that was closed, according to Troy Mayor Jason Reeves.

An 18-wheel truck in the Wal-Mart lot also was overturned. It's not clear how many people were in the Wal-Mart when the storm hit. Photos on social media showed merchandise strewn in aisles and the ceiling sagging.

Employee Sarah Brookins told the Troy Messenger the storm came as a surprise.

"We heard a loud noise," she said. "All of a sudden the wind was blowing in the store. We saw dust blowing through the door."

Charlene Faulkner told WSFA-TV she and her husband had come to the store to take advantage of the tax-free holiday. When the lights flickered, her husband checked the radar on a phone.

"He said 'Charlene it's a tornado', and he threw me down to the floor and covered me up," she said. "Glass started going everywhere, the ceiling started collapsing."

Reeves said the fact that the tornado hit at night was fortunate.

"In Alabama, you never know when a tornado might form. The weather is strange," he said.

Kevin Laws, chief scientist at the National Weather Service in Birmingham, said a survey team traveled to Troy on Friday morning and determined the storm was a tornado. The group still is studying the storm's size and strength.

Laws said tornadoes are "very rare" for southeast Alabama in August. Only 65 have occurred there since 1950.

Forecasters issued two advisories for the area, one at 9:30 p.m. and another an hour later. A tornado warning was issued at 10:39 p.m., Laws said.

"Severe weather can occur anytime, anyplace," he said. "That's why we have to be very vigilant, especially down in the Southeast."

Wal-Mart spokesman Brian Nick said emergency and company officials are assessing the damage to determine what works needs to be done. Store employees can work at other locations in the area if they choose, he said.

Stores were anticipating a busy weekend as Alabama offers a tax-free period for back-to-school shopping before elementary, high school and college courses resume, he said.