As Hurricane Ian barrels into South Carolina after leaving behind a trail of destruction in Florida, people are keeping a close eye on something that might surprise you: Waffle House.

The well-known restaurant chain currently has 10 locations closed in the hardest hit areas of Florida, a number that will "fluctuate rapidly" as staff works to re-open and aid community members, Waffle House spokesperson Njeri Boss tells CNBC Make It.

Such closures are incredibly rare for the Norcross, Georgia-based chain, which has roughly 2,000 locations spread mostly across the southeast U.S. And they mean more than just a community being waffle-less: A local Waffle House's status actually serves as unofficial shorthand for the Federal Emergency Management Agency (FEMA) to track the severity of a weather disaster, and how much damage it's done to an area.

It's known as the "Waffle House Index," created by former FEMA administrator Craig Fugate after he and his colleagues discovered the last places that stayed open during Hurricane Charley in 2004 were Waffle House locations.

"If you get there and the Waffle House is closed?" Fugate wrote in a blog post. "That's really bad. That's where you go to work."

The index has three levels, based on the extent of operations at a Waffle House location during a storm: 

• Green means the restaurant is serving a full menu, a sign that damage in the area is limited and lights are on 
• Yellow means there's a limited menu at the restaurant, signaling that there's low supplies and power is coming from a generator 
• Red means the restaurant is closed, which indicates that there is severe damage in the area or "unsafe conditions"

Currently, the 10 closed Florida locations include two in Naples, a few in Port Charlotte and several in Fort Myers, Boss says.

In a interview with NBC News on Thursday, Fugate acknowledged that the index "doesn't tell you everything." But he says restaurants labeled red are still a "pretty good gauge" of which areas of a community are impacted the most and need critical attention first.

Waffle House had no role in creating the index, but it values the "the goodwill gained from being open when customers are most desperate," its website says. Boss adds that the restaurant chain "takes pride" in always being among the first businesses to reopen once it's safe to enter an area devastated by a disaster.

"We know how important it is for a community to start the recovery process as quickly as possible," Boss says. "The fact that the index is being used, we're proud that our name is attached to it."

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