How do experts come up with ratings for tornadoes?

MILWAUKEE (WITI) -- The National Weather Service rated the Moore, Oklahoma tornado as an EF-5 -- its most severe tornado classification. But how do they come up with that rating?

Issuing a watch or warning has come a long way since the 1950s.

"We had some idea where the severe storms were going to be, but back then, we didn't have Doppler Radar," NWS Meteorologist Rusty Kapela said.

Kapela says it was computers and Doppler Radar that have made all the difference.

"Now we're able to look inside the thunderstorm and identify rotations and say 'hey this thunderstorm has the potential to produce a tornado,'" Kapela said.

Technology has allowed meteorologists to issue warnings before a tornado hits the ground.

After a tornado strikes, meteorologists use what they call "The Bible," or the Enhanced Fujita Scale.

"We're going to look at the home damage, the building damage, whatever may exist out there," Kapela said.

In cases like Moore, Oklahoma, crews from the NWS with permission from the county, will look at the damage of a building on site and compare it to pictures of previous storm damage.

Each building has a different grading system because not all buildings are made the same way. Mobile homes are not as structurally sound as a condo or a hotel.

"The stronger the wind and the more structural connectors the tougher it is to get damaged," Kapela said.

Kapela admits the system is not perfect.

"The better evidence you have, the better documentation you have, the more indicators you have, the better chances are that you're going to be able to zero in on that wind speed itself," Kapela said.

The Enhanced Fujita Scale was adopted by the National Weather Service in 2007.

Kapela says experts are constantly looking at enhancing the guide.