Wednesday was the eight-year anniversary of the Joplin, Mo., tornado; the nation’s deadliest in more than six decades. Wednesday night Missouri was struck again, when a large, violent tornado carved a path of destruction through Jefferson City, the state capital.
The tornado came as a surprise to some. To start the day, most concerns revolved around flooding, as the northern part of the city was ordered to evacuate by 5 p.m. as the Missouri River approached major flood stage.
In Tuesday’s outlook, the Storm Prediction Center listed Jefferson City as having a “marginal risk” of severe weather for Wednesday. By the morning, it had been upped to a slight risk. But around lunchtime, it became clear that a narrow corridor stretching from Oklahoma through parts of Kansas and Missouri could be dealing with a localized — but extremely dangerous — event. The Storm Prediction Center drew up a “moderate risk,” a 4 out of 5 on their risk scale. In their forecast, they described “an environment favorable for strong tornadoes.”
The first storms developed in Oklahoma about 4 p.m., with cells rapidly firing up along Interstate 44, stretching toward western Illinois. Tornadoes were sighted near Joplin, where three people were reportedly killed. Ultimately, wind damage would be reported as far north as Lake Erie.
Earlier Wednesday morning, the National Weather Service office in St. Louis had described “an interesting setup,” noting that a warm front would lift north and “eventually [become] stationary.” Stalled warm fronts are notorious for boosting the odds of tornado formation, enhancing low-level winds and increasing the twisting motion in the lower atmosphere. The front also serves as a focal mechanism, helping to initiate the storms.
If it seems things escalated quickly, they did. Some places in the southwest Oklahoma City metro area were given a “0 out of 5” severe thunderstorm risk in the morning, only to be placed under a “particularly dangerous situation” tornado watch by midafternoon.
The wild card was how long storms would remain isolated as they progressed into the evening. Lonely storms are the best at producing tornadoes. That is because they do not have to compete with neighboring cells for the warm, moist southerly flow that fuels them. Initially, it looked like storms would spend a few hours as these individual supercell storms before growing into clusters or lines. But that did not happen fast enough.
Instead, a number of supercells raged after dark — including the one on a crash course with Jefferson City. The city of 40,000 was placed under a tornado warning at 11:08 p.m., covering mainly its southern areas. At 11:21 p.m., a tornado was confirmed, and at 11:39 p.m., the National Weather Service warned it was a “large and extremely dangerous tornado” just outside Jefferson City.
What might have helped a strong tornado form after dark was the strengthening of the low-level jet stream. This near-ground jet is known for speeding up after the sun sets. That is because there is less mixing in the atmosphere, meaning pockets of air near the surface do not tap into its hefty momentum. It is like placing rocks in a river. With too many obstacles, the river’s flow speed will drop. But when left to surge unimpeded, the river — or the low-level jet — can rush by with fury. This additional wind energy helped maintain and even intensify storms after dark and make the destructive tornado possible.
The Weather Service alert described a “particularly dangerous situation” as the radar-confirmed funnel was bearing down on Jefferson City. Ordinarily, it is impossible to confirm tornadoes at night as they are happening, but the tornado was lifting debris so high it was visible on Doppler radar 84 miles away.
At 11:43 p.m., it was clear that a catastrophe was unfolding. A “tornado emergency” was hoisted, the most dire warning the National Weather Service issues. It warned that “complete destruction [was] possible,” echoing reports that would emerge from the storm-ravaged community as daylight dawned.
All told, nearly three dozen reports of tornadoes were received after Wednesday’s severe weather outbreak. It is a reminder of how things can change in a hurry and why Midwest residents never let their guard down in May.