When National Weather Service forecasters saw late last week the mix of atmospheric conditions that models were predicting would come together Monday, they knew it would be significant.
They saw a clash of two strong, conflicting air masses, the perfect mixture of winds at the surface and in the upper atmosphere and enough instability to quickly create devastating, long-track tornadoes — it was like a bull's-eye over central Oklahoma.
“Some of the models were accurate four to five days before it happened,” said Marc Austin, a forecaster with the National Weather Service in Norman.
What developed was an EF5 tornado that tore through a densely populated metropolitan area south of Oklahoma City. Winds howled at more than 200 mph at some points along the tornado's 17-mile path. At its widest, the tornado carved a scar 1.3 miles in width in the earth.
In a hazardous weather outlook issued Wednesday, May 15, the Norman forecast office warned that the perfect mix of conditions would exist over central and eastern Oklahoma to support severe storms Sunday and Monday. In another outlook issued Thursday, forecasters continued their warning of severe weather, adding that tornadoes would be possible.
What atmospheric ingredients forecasters were seeing were the typical setup for supercell thunderstorms, and that's just what occurred, Austin said.
The first ingredient was a cold front parked across the state, a line separating cooler air from warmer air. The next vital ingredient was a dryline that separated dry air from the warm, moist air that had formed across the state around a surface-level low-pressure system rotating counterclockwise, bringing in warm, moist air from the southeast and pushing cooler, drier air from the west.
Where those two air masses come together is what gives the storms the lift to grow quickly.
Third, the winds at the surface Monday interacted with winds in the upper atmosphere, which caused the supercell storms to rotate once they formed. Surface winds from the southeast mixed with strong upper winds from the southwest, creating high levels of wind shear.
“As that storm went up, it organized very rapidly,” Austin said. “We had a dryline and stalled frontal boundary. That storm formed right along the dryline.”
At 2:08 p.m. Monday, Norman forecasters said a storm near Bridge Creek — a few dozen miles southwest of the Oklahoma City metro area — that had developed quickly showed severe characteristics and issued a severe thunderstorm warning.
About 30 minutes later, forecasters began to see vertical rotation indications on radar data and issued the storm's first tornado warning.
The storm had gone from nothing to potentially developing a tornado along where forecasters expected it would in less than an hour.
At 2:45 p.m., five minutes after the initial tornado warning, the tornado touched down 4.4 miles west of Newcastle. It moved northeast relatively slowly but increased its wind speed rapidly. Within 10 minutes — before the twister reached I-44 north of Newcastle — it was producing damage rated EF4.
As it crossed the Canadian River at I-44, it turned east and set its sights on Moore and south Oklahoma City. The tornado was more than a mile wide as it approached Moore.
“The thing that really sets it apart from most tornadoes is the highly populated area it went through,” Austin said. “Since 1999, the Moore area has exploded in terms of population growth. The population density was much higher now as opposed to then,” when an F-5 tornado hit Moore on May 3, 1999.
At 3:01 p.m., forecasters recognized how powerful the storm was and where it was headed and issued a rare “tornado emergency.” The first tornado emergency ever issued was May 3, 1999, when that massive tornado devastated much of the same area.
Norman forecasters on social media were warning of the storm's potential.
“The tornado is so large you may not realize it's a tornado. If you are in Moore, go to shelter NOW!” the official Norman National Weather Service office said on Twitter at 3:22 p.m.
As the tornado crossed Interstate 35 near Fourth Street in Moore, its path narrowed significantly and it began to weaken, but it was still a strong and destructive tornado.
At 3:35 p.m., the tornado lifted 4.8 miles east of Moore, ending its destruction but leaving a path of devastation that is difficult to comprehend.
Austin said that although tornadic supercells — especially storms this violent — are rare, the birth, life and death of Monday's Moore tornado was generally typical of that kind of storm.
“That was a normal supercell process,” Austin said. “We're not surprised that it happened; we're just saddened that it happened in the metro area.”