00:44 Waffle House Is Closed Ahead of Hurricane Michael When Waffle House shuts their doors before a storm, you know it's serious business.

At a Glance The "Waffle House Index" helps FEMA monitor impacts from hurricanes and tropical storms.

The index turns red when Waffle House closes, which FEMA says is rare.

Some locations along Florida's Gulf coast were closed due to Hurricane Michael.

While it may sound silly, one of the tools the Federal Emergency Management Agency informally uses to decide how bad a storm is and if it warrants evacuations is the status of Waffle House.

With 1,500 locations, mainly across the South and along the Gulf Coast, that are open 24 hours a day, 7 days a week, 365 days a year, it's a big deal when Waffle House closes its doors.

Given the southern locations of most Waffle House restaurants, they are vulnerable to hurricanes, tropical storms and flooding rains. For that reason, it can be a good indicator of a storm's path of destruction when Waffle House is closed for business.

(MORE: Hurricane Michael Forecast )

This so-called "Waffle House Index" was created by former FEMA administrator Craig Fugate . Here's how it works.

If Waffle House is open and serving its full menu, the index is green.

If the restaurant is open but only serving a limited menu, the index is yellow.

The index turns red when Waffle House is closed. FEMA noted it's rare for the index to turn red because "Waffle House is well-prepared for disasters ."

Waffle Houses in Destin, Fort Walton Beach, Panama City and Panama City Beach, Florida, were closed as of late Tuesday. However, restaurants in Gulf Breeze and Tallahassee, Florida, were open, and it was unclear if they were going to close at a later time.

The company, according to a 2011 Wall Street Journal article , has a "hurricane playbook" that helps locations re-open following a catastrophe by limiting locations' menus as needed. The playbook shows "what to serve if there is gas but no electricity, or a generator but no ice."

FEMA cautioned against taking the Waffle House Index too seriously, however.

"Look, I think it was used at a point in time," FEMA's Alex Amparo told reporters on a phone briefing obtained by ABC News . "There is no official use for it. Our focus is on more empirical data."

Michael was expected to make landfall along the Florida Panhandle Gulf coast Wednesday with life-threatening storm surge flooding, destructive winds and flooding rainfall.