March 4, 2019

Deadly Alabama Tornado Tests The Boundaries Of Advanced Tornado Warnings


A significant tornado outbreak in the southeastern United States on Sunday killed at least 23 people despite hours of advanced warning that dangerous storms were on the way. Most of the fatalities occurred in Lee County, Alabama, likely making that tornado the country's deadliest in nearly six years. Both the strength of the tornado and the types of homes in the storm's path are a stark reminder that advanced tornado warnings can only go so far in keeping people safe.

The Storm Prediction Center recorded 38 tornado reports as of 12:00 AM Central Time on Monday. Many of those tornado reports were likely duplicates from the same long-track tornadoes as they moved across Alabama and Georgia. The worst tornado moved through Lee County, Alabama, around 2:00 PM CST on Sunday afternoon. The tornado killed at least 23 people, with officials warning that the death toll could climb as rescuers comb through the debris.



It may not seem like it given the high death toll, but Sunday's tornado outbreak was a well-predicted and well-warned event. The SPC issued an enhanced risk for severe weather with its update on Saturday morning, giving residents a full day to prepare for the risk for dangerous thunderstorms.



Sunday began with an enhanced risk for tornadoes across the hardest-hit areas. The black hatching within that 10% risk zone indicated the risk for strong, long-lived tornadoes. The amount of wind shear and instability across the region was ideal for the development of supercells capable of producing strong tornadoes.

The SPC issued a tornado watch for the region at 11:40 AM CST, about two hours before the fatal tornadoes touched down. The storms were hauling toward the east at highway speeds, but meteorologists were able to keep up and issue alerts and warnings well ahead of their arrival.



The above radar image is enough to put a knot in your stomach when you know the environment is ripe for severe weather. If you're a frequent reader of my posts, you'll know that one of my go-to lines ahead of a severe weather day is "the severe threat will come in two rounds—first in discrete storms, then with the squall line that follows." Discrete storms are more likely to produce tornadoes and large hail, while the predominant threats with a squall line are damaging winds and occasional tornadoes.

Discrete thunderstorms are able to fully engage with the environment in which they develop. If they're far enough away from other storms, they can take advantage of all the instability and wind shear they need to attain the maximum strength the environment will allow. We saw that happen in abundance on Sunday. There were dozens of tornado warnings in effect at one point across the southeast, with many meteorologists remarking on Twitter that they can't remember how long it's been since we saw so many confirmed tornadoes occurring at once.

That discrete-before-the-squall scenario doesn't always materialize during a severe weather event. Oftentimes we'll see sloppy storm modes that result in widespread, blobby masses of thunderstorms that are rough, sure, but not what they could've been given the amount of wind shear and instability.

That's what happened up near Memphis during last week's severe weather threat. All of the ingredients were there...it's just that only a couple of storms were able to take advantage of it. (It turns out that the rare storm that could fully engage produced an EF-3 tornado in Columbus, Mississippi.)



A mesoscale discussion—basically a localized heads-up from the SPC—issued a full hour before the lethal tornado in Lee County, Alabama, actually pinpointed the affected areas as having the maximum risk for a strong tornado as the storms moved through. "Given the ample buoyancy and intense shear profile in place, it appears tornadogenesis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring."



The NWS office in Birmingham issued a tornado warning for Lee County at 1:58 PM CST, continuing the warning polygon from the storm's previous track. The tornado was already on the ground by the time it entered Lee County's tornado warning. Radar imagery shows how quickly the storm wrapped up and produced an intense tornado. Forecasters soon upgraded the warning to a rare tornado emergency once it was clear that a destructive tornado was underway.



If you're familiar with radar imagery, you know this image. We've seen it too many times. This is a supercell like you'd see in a textbook. The tornado is the pendant at the end of the hook, right at the intersection of where the inflow of unstable air wraps in to meet the rear-flank downdraft pumping around the back of the system. The dark purple circle within the hook isn't just rain and hail—it's debris swirling around in the tornado. That's pieces of homes and trees and vehicles being picked up by the radar.

The long-track tornado appears to have followed the entire length of Lee County, continuing for a while across the Georgia border. The storm missed several dense population centers, including Auburn and Opelika to its north and Phenix City to its south.

Unfortunately, the storm missing population centers didn't keep the death toll down. A cursory look at satellite imagery along the tornado's path—confirmed by a scientist who study tornadoes in the southeast—shows that many of the homes damaged or destroyed in the storm were likely mobile homes. There are dozens of them on satellite imagery, some directly beneath the debris signature in the different radar images.

Meteorologists and other scientists are always working to lengthen the lead time ahead of a tornado. The longer ahead of time someone has a warning, the longer they have to get into a safe place and brace for impact. The SPC warned of the threat hours ahead of time and the NWS issued a tornado warning before the storm arrived. Local news channels showed the debris swirling around in the air on radar imagery as the tornado moved from town to town.

From beginning to end, this was a well-warned event.

We always focus on making sure as many people as possible hear a tornado warning the moment it's issued. But all of the advanced warning in the world can only go so far in preparing people for one of nature's strongest forces. Sunday's tornado was at least an EF-3 and probably stronger than that. The storm was moving at 60 MPH. Many of the residences in its path appear to have been mobile or modular homes. A human being simply cannot survive that kind of storm without being underground or deep within a much stronger structure. Many people in weaker homes out in the country simply have nowhere to go to ride out a storm like that. A tornado warning is only as good as your ability to act on it.

[Maps: me | Supercell Radar Images: Gibson Ridge]


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1 comment:

  1. Excellent writeup, Dennis!!!!!! Have shared on both my Twitter and FB accounts.

    ReplyDelete