Every year about 100,000 thunderstorms rumble through the United States, but only 10 percent of them are severe. Newschannel 6 went up to the National Weather Service (NWS) forecasting office in Norman, OK to find out how the NWS determines which storms pose the greatest threats.
Rick Smith, the Warning Coordination Meteorologist for the NWS in Norman, OK, said "According to the NWS, our criteria for what we call a severe thunderstorm is a storm that has hail 1 inch or bigger, about the size of a quarter, and or with winds of 58 mph or higher."
The first tool that comes to mind when analyzing thunderstorms is radar and the NWS has more than just your average radar.
"[NWS meteorologists] have the basic reflectivity. We also have velocity data which measures how the winds are behaving inside a storm," said Smith. "Doppler measurements of the winds can sometimes tell us that damaging winds could reach the surface soon," he said.
Many people might think that the only thing the NWS meteorologist would take into account when issuing a Severe Thunderstorm Warning would be radar, but that could not be further from the truth.
"Determining exactly what a storm is doing on the ground based just on radar is very difficult if not impossible," says Smith. "We still need humans. We still need storm spotters. We still need people who are observing the hail and giving us reports by calling the weather service, by calling the tv stations or by posting on social media and letting us know what is happening," he said.
Ground-truth, or what is actually happening around a storm, can actually be important in determining potential hazards. It can help the NWS meteorologist calibrate the radar.
Smith says "If we know that a storm that looks like this on radar produced quarter-sized hail in Iowa Park, [we know] if we see a storm like that again that the potential for quarter-sized hail continues."
NWS meteorologists also look at the environment surrounding the storm. They look at the temperatures, dew point temperatures and the winds' direction and speed to see if the environment is going to give the storm enough fuel to become severe.
Once a NWS meteorologist has decided a storm is producing 1 inch sized hail or 58 mph winds, it is time to issue a Severe Thunderstorm Warning.
"We use a computer program called WarnGen," said Smith. "We would put a marker where the severe thunderstorm or tornado would be, then it would draw a proposed warning for us. It would then be up to the meteorologist to adjust the warning based on the storm's motion, how fast it is moving and which direction it is moving," he said.
WarnGen has made issuing a Severe Thunderstorm Warning so much easier and faster. In the old days, a NWS meteorologist would have write it out in a word processor. Now, they usually only have to choose between a few already written statements that describe the storm's hazards and they are done.
In many cases, it takes less than a minute for a warning to go from the Norman forecasting office to your television at home.
A tornado warning is issued the exact same way as a Severe Thunderstorm Warning. The only difference is that there is not a set of criteria for a Tornado Warning. The NWS will issue a Tornado Warning any time they think a tornado is occurring or is likely to occur.