Every hurricane that heads for the Gulf Coast brings the same side question: what are the Waffle Houses doing?

When Hurricane Isaias took aim at the coast from Mobile to Panama City as a major hurricane, the answer became part of how people read the storm. That's because FEMA has long had an unofficial gauge that comes with hash browns: the Waffle House Index.

The area under threat is big. A hurricane warning covers Ocean Springs, Mississippi, to the Bay/Gulf County line in Florida, including parts of southwest Alabama and the western Panhandle. A storm surge warning runs from the mouth of the Mississippi River to the Suwannee River. Rain could reach 15 inches locally, and winds of at least 40 mph could last through at least Saturday morning near the coast.

(MORE: Hurricane Isaias forecast)

One way to see how much of the coast is in the path is the Waffle House Tracker. It's an independent site, not run by Waffle House or FEMA, and it estimates which of the chain's locations fall inside National Weather Service alert areas:

• Threatened (red): inside an active warning area.
• At Risk (yellow): under a watch or advisory.
• Open (green): outside any alert area.

Before a storm hits, the tracker is a map of exposure, not a report of damage. But some locations are already closing. Closures before landfall tend to reflect precaution. The harder read comes after the storm, when a closed restaurant or a limited menu can point to damage, no power or stalled deliveries.

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Why a diner chain became a hurricane gauge

The tracker is built on the Waffle House Index, which FEMA's former administrator Craig Fugate came up with. The restaurants stay open around the clock and are known for reopening fast after storms, so a closure or a limited menu can point to a bigger problem nearby.

Green means open with the full menu, yellow means open with a limited menu, and red means closed, which is rare. The chain even keeps a hurricane playbook that sets what it serves depending on whether it has gas, electricity or a generator.

What to watch for after landfall

The index will matter more tomorrow than tonight. The biggest risk to daily life is power. Downed trees and power lines could cause outages that could last "for hours or multiple days."

If a restaurant in Mobile or Pensacola moves to a limited menu, that's a hint the lights are out or deliveries are stuck, which is worth knowing before you plan a drive, a generator run or a grocery trip.

It's still shorthand, though. FEMA has been clear that it isn't an official measure. For evacuation orders and warnings, go to your local emergency management office and the National Hurricane Center.

weather.com Content Development Manager Joy Kigin chases stories where weather, science and culture meet everyday life, the kind that spark conversations and, now and then, a smile.