A day before Hurricane Matthew battered the southeast, Georgia-based fast food chain Waffle House tweeted that “all restaurants on 1-95 between Titusville, FL and Fort Pierce, FL are closed.”

Although the restaurant chain’s Twitter account only has around 66,900 followers and is hardly retweeted, 1,980 people retweeted that post.

Every hurricane season, lore about Waffle House’s relationship with hurricane preparedness and disaster relief emerges, including the “Waffle House Index,” which legend maintains is a proxy used by the Federal Emergency Management Agency (FEMA) for how bad things are. “Green” is full operations, “yellow” is a partial menu with generator power, and “red” is the apocalypse.

According to FEMA, a lot of this is actually true.

“It’s an informal relationship,” Philip Strouse, FEMA’s Private Sector Liaison, told Yahoo Finance. “Waffle House stays on when the wind’s blowing—they never close. They have a small footprint, they’re easy, and if these little stores are going out when it only takes a few people to staff…that’s bad.”

So resistant to closure is this chain – which has about 2,100 locations, mostly in the South – that in the aftermath of Hurricane Katrina in 2005, Strouse said some Waffle House managers actually had a hard time finding the keys to the doors because they close so infrequently.

According to Strouse, the very unofficial index all started when FEMA Administrator Craig Fugate was director of the Florida Emergency Management Division. Noticing Waffle House’s incredible resilience in the face of natural disaster, Fugate coined the index in May 2011 after the Joplin, Mo., tornado. “He was saying if the Waffle House closes, it’s really bad and you know to go to work there first.” So when he came to FEMA in 2009, Fugate brought his barometer to the new position.

Many people misunderstand how the index works, however—a fact that’s partially represented by the response to that Waffle House tweet on Wednesday and articles like this. As an emergency management agency, FEMA handles the aftermath more than the run-up, funneling federal assets to damaged areas that need it most. “We really support the state and the state supports the local community,” said Strouse.

This means you should pay closer attention to the index post-hurricane. “The index was for aftermath, for damage assessments,” said Strouse. The chain’s ability to function on essentially nothing also makes it a prime location for first responders, who know they can get something to eat—almost no matter what.