When the main US federal emergency agency arrives at the scene of a disaster-hit area, one of the first places it turns to is the local Waffle House – and not just for its officials to grab a quick bite.

Craig Fugate, the head of the Federal Emergency Management Agency, came up with the idea of the "Waffle House index" as an informal way of measuring the impact of a disaster. The chain, which has a large number of branches in tornado-prone areas, has a robust emergency management plan.

The index has three levels. If the local Waffle House is up and running, serving a full menu, a disaster is classed as green. If it is running with an emergency generator and serving only a limited menu, it is a yellow. If it is closed, badly damaged or totally destroyed, as during hurricane Katrina, it is a red.

There is only one Waffle House in Moore, the suburb worst hit by the tornadoes. The restaurant, located at 316 SW 19th Street and which normally offers a southern-tinged menu that includes grits, hash browns, and sausage and egg biscuits as well as hamburgers, was closed on Tuesday.

But the Moore tornado was classed as a yellow on the Waffle House index because managers were hoping to get it up and running soon. "It is a yellow because we are hoping to get a generator," said Kelly Thrasher, a spokeswoman for the Atlanta-based restaurant chain. "Once we have the generator, we will be able to serve a limited menu, maybe a full one."

She said the branch had survived the storm intact. "There is no damage to the building. Power is out and there is no gas or water but we do not have any physical damage. We are trying to get it open so we can serve first responders and the community," she said.

President Barack Obama on Tuesday issued a disaster declaration for Oklahoma and promised to provide the state with all the federal support it needs. Fugate has been despatched to the state.

Fema was heavily criticised for its slowness to respond to Katrina in 2005, a disaster that damaged the reputation of George W Bush as much as the 2003 Iraq invasion. But it received praise for the help it provided last year after hurricane Sandy. It has been the target of criticism by conservatives across the US as an example of an unnecessary federal agency, with calls for its abolition, though such carping is often dropped when the critics' home states are victims of catastrophe.

The bulk of rescue and repair work is done by state bodies and Fema only becomes involved when a governor asks the president for help, as Mary Fallin did on Sunday night after the first tornadoes hit. Obama ordered Fema into action.

Its initial deployments were modest. It sent a liaison officer to Oklahoma on Sunday to work in the state's emergency operations centre in anticipation of severe weather. On Monday, it deployed an incident management assistance team to work with local and state rescue teams. Also deployed were its urban search-and-rescue teams and a mobile emergency response team to help with communications and logistics.

By the afternoon, Fema activated its national response co-ordination centre in Washington DC which pulls together all requests for help.

As the weeks go by, Fema will play a bigger role, dealing with requests for federal aid to help with rebuilding. Fema has distributed at least $961m to help rebuild schools, hospitals and other buildings in the six months since Sandy hit but some areas complained the money is slow to arrive.

Fema became part of the Department of Homeland Security after 9/11, a move that was heavily criticised at the time and which was partly blamed for its poor response to Katrina: the criticism was that by being integrated into the DHS the focus shifted to post-terrorism attacks rather than natural disasters.

Like other federal agencies, Fema has been hit by the sequester budget cuts, with $1bn cut from its budget. It has $13bn left for the remainder of the fiscal year up until September.

The head of Fema at the time of Katrina, Michael Brown, resigned amid widespread criticism and is now a radio talk-show host in Denver. Fugate took over in 2009.

Fugate is said to have come up with the Waffle House Index as a way of humanising disaster and an easy way of describing the impact on communities. Another version is that agency officials arriving in New Orleans and elsewhere on the Gulf coast after Katrina saw so many Waffle Houses closed or destroyed and only then did the full scale of the disaster began to dawn on them. Fugate first used the phrase in public in 2011, quoted in the Wall Street Journal. "If you get there and the Waffle House is closed? That's really bad," he said.

Fugate's Index is a compliment to the restaurant chain, which is often cited along with three other companies – Walmart, Lowe's and The Home Depot – for their planning for natural disasters and the speed with which they rebound.

Waffle House, which has 1,600 restaurants in 25 states, stretching from Florida to Arizona and north to Pennsylvania, takes pride in providing meals for local communities in time of trouble. It began its post-disaster planning in earnest after Katrina.

Katrina forced the company to close 100 restaurants, with seven completely destroyed and a few others partially destroyed. Within a day, 60 were reopened, Thrasher said.

Because of the level of preparedness for natural disasters, hitting a red is increasingly rare. None of the restaurants were closed after the tornado that hit Joplin, Missouri, in 2011 in spite of the extensive damage caused. The tornado left 162 dead.

After Sandy, Thrasher said some Waffle Houses suffered from power outages in Virginia but no buildings were damaged and they were all up and running after generators were provided.

There is no emergency generator at the restaurant in Moore but was being brought in from a nearby location, and a team had been sent from Dallas, Texas, to help. The Waffle Index has been good PR for the 58-year-old restaurant chain. "Fema coined The Waffle Index," Thrasher said. "For us, it is helping the community and getting them back to normal."