A Story from the Tornado Zone
Editor's Note: On Wednesday, April 27th, Science@NASA writer Dauna Coulter found herself near ground zero as a super-outbreak of tornadoes ripped through North Alabama. This is the story about the science of the event she wrote and submitted from within the disaster zone.
April 27, 2011: Tornado sirens wailed all day long. They'd sound off and then wind down for a little while, only to start up again a few minutes later as forecasters spotted yet another hook echo on their radars and adrenaline-revved storm spotters confirmed the twisters that dropped from seething skies one after another.
We lined our bathroom closet with blankets and pillows for my grandson. My husband kept going outside to look at the sky. If an atmosphere can become sentient, the one out there was a malevolent, living thing. This was not your usual storm. Even our Golden Retriever was pacing the floor.
In the late afternoon, we lost power. We scrambled to find our radio, batteries, flashlights, candles, and matches, as tornados continued to train across the area. The wind finally died down around 10 pm.
April 28, 2011: The power was still out when I woke up the morning after the storm. I made coffee on our camp stove in the back yard and sat in a lawn chair to listen to the radio. The news was worse than I'd expected. As the sun rose to begin an apologetically brilliant, clear day, I said a silent prayer for those who had lost their lives in the storms. They were, and still are, on my mind and in my heart. (More than a week later, all the missing have yet to be accounted for.)
I couldn't check on family or friends by cell phone until that evening, and even then phone service was spotty, as it would be for two more days. The people I finally reached said they'd been trying to call me. Science@NASA editor Tony Phillips was among them. When we finally talked, he made sure I was okay and then said something that blew my mind: "I want you to write a story about this."
With no lights, no computer, spotty cell phone coverage?
"Okay," I answered, and then began figuring out how to do it.
April 29, 2011: The next morning I located a friend who lived near the Tennessee border, 30 miles away, and had power and internet. The route to her house led through one of the "war zones." There was nothing left intact.
What had caused a storm of this magnitude?
I emailed prospective interviewees at NASA and the National Weather Service, hoping they'd have internet service – and some answers. I urged my contacts to call me as I had no computer access anywhere in Huntsville and was headed back there.
By mid-afternoon, no one had called back, so it was time to try another tack.
I drove to the National Space Science and Technology Center, a leading-edge severe weather research center housing investigators from NASA and the University of Alabama as well as the National Weather Service's Forecast Office in Huntsville. Ironically, the historic line of tornadoes had torn through the skies right above the center on the 27th, causing it to be briefly evacuated except for key weather personnel.1
I was glad to see a few cars in the parking lot and hear the hum of a generator. Using my badge to enter the building, I made my way through semi-dark hallways to the Weather Service Office. There I found Larry Burgett of their Public Service Unit along with forecaster Jennifer Lee. Both of them were part of a large team that had worked from 3:30 a.m. to 10 p.m. central time, nonstop, the day of the storm to track the event and alert the public.
"It was a day unlike any other," said Burgett. "We've never issued so many warnings in rapid succession. And we've never had so many damage reports in one day."
The field survey team2 examining the damage in north Alabama reported one twister as an EF-5, a ranking reserved for the most violent and destructive tornadoes. It had peak winds of 210 mph, was 1.25 miles wide in spots, and stayed on the ground for 132 miles.
"Not only was there a tremendous outbreak of tornadoes – but many of them ripped along the ground for a long time," said Lee. "It's very unusual for that many funnels to stay down that long."
Burgett described some unbelievable sights: "Some homes were hit not once but twice – by one tornado on the heels of another. That's unheard of. Maybe in a rare case you'll hear of a home being hit twice over several years, but not in the same day. And everywhere a tornado touched down – there was major damage."
I drove home, digesting and mourning all I'd learned that day, yet no one had explained to me how this century-class catastrophe had happened. How did this storm brew enough energy to transform itself into such a monstrous tornado-machine?
Later, NASA meteorologist Walt Petersen3 and UA-Huntsville meteorologist Tim Coleman called me with some facts.
"A deep cold batch of air in the central US seeped toward the southeast behind a cold front that trailed a ground-level low pressure system," explained Petersen. "Moist air from the Gulf of Mexico streamed in toward the lower-pressure area like water heading for an open drain. Above that moist air, we had very strong winds with lots of wind shear. That means that the wind turned and sped up with height, causing the updrafts in the storm to corkscrew as they ascended into the turning wind. Collectively, those ingredients turned deadly, causing storms to form and intensify, and at the same time rotate because of the wind shear."
Coleman added: "This kind of set up in the atmosphere is extremely rare. We have a number in meteorology called the EHI (Energy Helicity Index)4 that indicates the likelihood of tornadoes developing. With an EHI over 2 you can expect some tornadoes. Over 5 and you can expect some significant tornadoes. The EHI predicted by computer models the day before this event was 10, and that prediction was correct."
Coleman admits that he was frightened by what he'd seen developing. And as part of a storm survey team sorting through the rubble the day after the mega-storm, he found that his fears had been realized.
"Brick homes were blown apart. Even above-ground walls of their basements were gone, and there were mountains of debris – including concrete blocks and large pieces of lumber – on basement floors. Some of these tornadoes were almost unsurviveable. Only in a well-built storm shelter would you make it through."
April 30, 2011: As time allowed over the next few days, I worked on this story outside under a shade tree in daytime and inside by candlelight at night. Though I stayed busy the rest of the time heating water to wash dishes, waiting in long lines to buy ice and gas from the few stores with generators, and checking on my elderly parents, life had, in a sense, slowed and quieted. No televisions blaring, no air conditioners humming, no weed eaters buzzing. (The extra gas was needed in cars.) Even the dogs were unusually silent. I've never slept better.
In any other circumstances, you'd think we were all enjoying an extended holiday. People were out all over town walking to the store, biking, visiting with one another. And they kept their good spirits and sense of humor through it all. One neighbor posted.
In the evenings, our neighbors brought the contents of their defunct refrigerators and freezers over to share, and we cooked out on grills and the camp stove. Communities all over town had similar gatherings. My 86 year old father, who cares and cooks for my semi-invalid mother, reported each day on what their neighbors had brought them for supper. "We've been eating better than we ever did with my cooking!" he said.
At night the city was so dark, it was like living in a different century. I rode my bike under the stars. Replacing the gaudy glare of incandescent lights, a soft glow of candlelight issued from the windows of neighbors' homes. Some families gathered around backyard fires. And with not even a moon in view, I saw stars I'd never seen before. There above me was the whole Milky Way!
May 2, 2011: It's been a bittersweet week.
I was sitting in the back yard last night as a wave of cheering and yelling started in the distance and seemed to roll toward me. It grew louder and louder, closer and closer, building to this crescendo: my son leaning out the back door and shouting, "We have power!"
People were cheering as the lights came on swiftly, street by street, across the neighborhood.
"Let there be light!" (That was me, joining the sound wave.)
I'll remember these experiences for a long, long time: Slow, halting drives through the city with no street lights working, waiting in long lines for a ration of ice or gas or to buy a few hotdogs for the grill, curfews, taking ice cold showers, fumbling around in the darkness for my toothbrush, stubbing my toes in dim rooms … and thinking of those who were not as fortunate.
1. The Huntsville Weather Service Office has a generator and large battery computer backup system for emergencies.
2. Survey teams conduct field surveys of tornado damage, examining the length and width of the tornadoes' paths and the severity of damage. The team uses these observations to determine where the twisters touched down, how long they stayed on the ground, their size, and the velocity of their winds.
3. Walt Petersen's personal account: "We ran the ARMOR Radar at the NSSTC throughout the event until the power went out. As the worst storms approached in the later afternoon, we decided to run low-level (that is, low elevation angles near the surface) scans back-to-back so we could get the most frequent updates on any tornado developments, because at that point public safety was a clear concern. In other words, we switched our thinking from trying to collect data over the full depth of the storms for research to continuous collections at low levels of the storms to enable rapid updating for monitoring of tornado circulation development. As the system continued to move into our area from the south and southwest around 4 p.m., we could see things that were looking potentially bad. For instance, we could see occasional debris signatures in the radar data, and all of this was moving right at us. I left at about 4:45 p.m. and headed home to be with my family. Once I got there, I monitored the ARMOR Radar at my home with my computer. As I watched, I saw one vortex that seemed to be heading for my neighborhood. Atmospheric scientist that I am, I had to take a look outside too. I did that until at one point I saw all the trees in my backyard twist, in unison, toward the northwest. I also heard a loud noise on my roof. I knew, from all this, that the circulation was nearby. That spooked me and I headed downstairs where my children were. When we were sure all was clear, we climbed the stairs in the darkness, assuming the house was okay. We did have a sizeable leak in the ceiling. The next morning I went out and saw a 4-inch tree limb stuck in the roof like a spear. Later that day I had to drive from Madison to Athens, and the landscape was just surreal."
4. Meteorologists calculate the EHI by combining the wind shear that flows in and makes a storm rotate with the instability that makes a storm grow: more.
Here are pictures of what I passed by on way to my friend's house to get on internet. The neighborhood is called Anderson Hills and it's in the outskirts of Huntsville.
Storm Survey Information from the north Alabama April 27 Super Outbreak
Satellite views of the tornado tracks