NORMAN — The instability in the atmosphere stirring Friday above central Oklahoma was of rare proportions, a meteorologist said.
There was a mix of incredible amounts of moisture and warm, muggy conditions, said Kevin Brown, a senior forecaster with the National Weather Service, Norman Forecast Office.
A large, slow-moving cluster of supercell thunderstorms moved across central and northern Oklahoma in the late afternoon and evening on Friday resulting in at least five tornadoes. Other storms developed to the west and moved over the same areas as the first storm.
This produced significant flooding in areas including the Oklahoma City metro.
“When the atmosphere becomes very unstable like yesterday,” Brown said Saturday, “then a lot of times when we get storms to develop, it's not just one supercell, it's a huge conglomerate of very intense storms. We had this massive area of severe weather across Canadian and Oklahoma counties.
“And when the atmosphere is that unstable they start to alter the environment around them.”
As a result, storms that developed over southwest Oklahoma dissipated as they approached central Oklahoma, Brown said.
On Saturday, the National Weather Service, Norman sent out three damage survey crews to various parts of the Oklahoma City area.
The first tornado developed around 5:55 p.m. Friday near El Reno. This storm then moved east to southeast into western and central portions of Oklahoma City, producing other tornadoes. The primary tornado that started near El Reno has been given a preliminary rating of EF3 while the other preliminary tornado ratings include two EF1s and two EF0s.
For the EF3, a primary damage indicator during Saturday's survey was a single family dwelling just east of U.S. 81 near El Reno, Brown said. Based on the damage, the peak wind speed was estimated at 156 mph. He said additional EF3 damage was found in other locations both east and west of U.S. 81 near El Reno.
And the floods resulted as four to eight inches of rains fell in areas of the Oklahoma City metro to the Oklahoma/Arkansas state line, Brown said. This occurred primarily in a span of three to four hours.
On Friday, 5.64 inches of rain was recorded at Will Rogers World Airport breaking Oklahoma City's daily record for May 31.
The previous mark was 2.14 inches on May 31, 1892. The 5.64 inches at Will Rogers Airport was the second highest precipitation total on record in May, behind only the 6.64 inches on May 8, 1993, according to the Oklahoma Climatological Survey. It is also the second highest daily precipitation total for Oklahoma City for any month. Data goes to 1890.
Besides Friday's daily record, another 1.12 inches of rain was recorded at Will Rogers World Airport after midnight.
Combined with March and April, Oklahoma City's spring total rainfall is 23.18 inches, besting the previous wettest spring mark of 20.31 inches from 1947.
“These were very slow moving,” Brown said. “The overall wind fields weren't extreme, so these big storms just kind of took over the environment.
“And they liked where they were at and they were getting the fuel that they needed so there really wasn't a whole lot of movement.”
Brown said that's a difference between these storms and those of May 20, which included an EF5 tornado. Although it was a long-track tornado May 20, those storms went ahead and moved off, he said. But, Brown added, the storms that occurred Friday “set up shop.”
Again, he said, the wind shear was “not real strong” Friday.
Wind shear is the rate at which wind velocity changes from point to point in a given direction, such as vertically, according to the National Weather Service.
“For really, really strong tornadoes the wind shear was better back on May 20,” Brown said. “There are probably several reasons, but one of the reasons we didn't have several big tornadoes like we did on May 19-20 had something to do with the amount of wind shear in the atmosphere.
“For violent and long-track type tornadoes, a delicate balance of atmospheric conditions have to be reached, which includes the wind fields and instability. Fortunately, it is rare for this balance to be reached.”
The National Weather Service will continue to gather information on Friday's storms.