NORMAN — There is nothing casual about the decision-making that takes place before residents ever hear the words:
“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
“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?
A look at last May
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.
There were 55 tornadoes in Oklahoma on May 10.
“You just keep adding little bits of evidence to the pile, and at some point, you have enough things that have been observed that you conclude that the like
It's important to note that things change in the course of an investigation. The meteorologists will look at patterns and see what could happen in two days.
However many factors can change in 48 hours, “such that it could be worse or not as bad, or it could be exactly what we forecast, but we really don't know that until we get to that day,” Foster said.
Severe thunderstorm warnings are something we hear more frequently than tornado warnings.
A severe thunderstorm warning brings with it the potential for wind gusts greater than 58 mph and/or hail an inch in diameter or larger.
“It's possible to have winds in excess of 100 mph from the kinds of thunderstorms we have around here without a tornado,” Foster said.
“A 100-mile-an-hour wind starts tearing things apart and you've got
“And when a piece of sheet metal gets picked up and is thrown through the air, if that hits somebody, it doesn't matter whether it was thrown by a tornado or just a gust of wind. It's dangerous.
“The point is that a severe thunderstorm warning should stand on its own merits.”