Flickr/Steve Snodgrass

Editor's Note: This post has been updated at 8:01 a.m. ET following a tweet from Waffle House indicating that several locations have been closed.

There are a bunch of unusual ways you can track what's happening in the economy.

And one of the more popular informal measures is the so-called "The Waffle House Index" — an indicator that is actually unofficially used by FEMA.

Here's the gist of it: Waffle House restaurants notoriously stay open during natural disasters. So if the diner closes during an event, that suggests it was a really bad natural disaster with devastating effects on the economy. And on the flip side, if it stays open and serving a full menu, damage was relatively limited.

As a historical reference point: after 2011's Hurricane Irene, 22 Waffle Houses lost power in North Carolina, Virginia, Maryland, and Delaware. Only one wasn't open by the Wednesday after the storm passed through — a particularly hard hit location in coastal Virginia.

"If you get [to a place where a disaster hit] and the Waffle House is closed? That's really bad," said FEMA administrator Craig Fugate back in 2011.

As of Friday, 8:01 a.m. ET, Waffle House's official twitter account tweeted that 25 restaurants had been closed across Florida, Georgia, and South Carolina amid Hurrice Matthew.

Waffle House had been planning to stay open during the height of the hurricane, which was expected to hit Florida as a Category 4 hurricane. Florida Gov. Rick Scott issued a statement on Thursday warning residents living in evacuation zones to leave immediately or risk death because of the impending hurricane.

“We’re a 24-hour restaurant, so oddly enough shutting down is a big deal for us,” Waffle House’s Vice President of Culture Pat Warner told FoxNews.com on Thursday.

“When it comes to making the final decision, we let our operations team on the ground, like individual restaurant managers, make the final decision based on local conditions. But our job [as corporate officials] is to give them all the support they need to stay open.”