Much precedes issuance of tornado warning

“The National Weather Service in Norman has issued a tornado warning for ... ”

What goes into that statement? How does the Norman Forecast Office reach the point where it issues a tornado warning?

It's the result of a weather investigation, said Mike Foster, meteorologist in charge in Norman. His office is responsible for coverage of 48 counties in Oklahoma. Other offices covering portions of Oklahoma are Tulsa, Amarillo, Texas, and Shreveport, La.

“It's not that one thing is seen or observed and immediately generates a tornado warning,” Foster said.

“It usually starts with a large-scale atmosphere flow that is something that is going to create conditions for the kinds of storms that would have tornadoes.

“You start building this collection of evidence.”

As storms develop, meteorologists are watching not only that system but also the weather conditions around the storm. What are the winds doing, what's happening with the temperature?

If the factors that contribute to a tornado aren't present now, will they come together at some point that day or at night?

Last year, the National Weather Service first indicated on May 4 that severe weather was possible on May 10.

The factors continued to build.

By noon May 10, the Norman office was warning that tornadoes were likely around rush hour in and around the Oklahoma City metro area. That proved true.