For National Weather Service forecasters in areas stricken by last week's devastating tornadoes, the tension-filled business of tracking violent weather has given way to a more somber responsibility.
Many of them have been walking block by block through heavily damaged areas, interviewing survivors, recording the damage to homes and farms, in an attempt to to see what lessons the tragedies may hold for improved warnings, better building codes, and a better understanding of how the twisters and the storms that spawn them grow, mature, and fade.
In some cases, investigators also face the task of rebuilding -- forecasters whose homes were among the structures damaged or destroyed by the the April 25-28 outbreak.
By nearly every measure,"this was historic," says Jim Stefkovich, lead forecaster at the NWS's forecast office in Birmingham, Ala., which took a direct hit from an EF4 tornado that also took a swipe at Tuscaloosa, some 50 miles to the southeast. On what is known as the enhanced Fujita scale for categorizing twisters by the damage they inflict, EF4 is the second highest designation. An EF5 tornado, the most damaging category, powered through Huntsville County on April 27.
"We were predicting 24 to 48 hours in advance" that the area would face "strong to violent tornadoes" with long tracks, Mr. Stefkovich says. The storm system more than lived up to its billing.
Over the past 30 years, Alabama has averaged 37 tornadoes a year, he continues. In April 2011, the state experienced 72 tornadoes; 41 hit during a mid-April outbreak. The annual record for the state stands at 94 tornadoes, set in 2008.
The story is repeated, to varying degrees, throughout the eastern US, according to figures released today by the National Weather Service.
Some 305 confirmed tornadoes struck 14 states between April 25 and 28, the single largest outbreak in US history and more than twice the number of tornadoes that set the previous record – 148 twisters between April 3-4, 1974.
For the month as a whole, more than 600 tornadoes touched down – again, more than twice the previous record of 267, also set in 1974.
Yet for all the damage done – including an estimated 327 deaths so far attributed to last week's outbreak – the fatalities remain less than half the number attributed to an outbreak in March 1925, which killed 744 people in northeastern Missouri, southern Illinois, and Indiana.
No one downplays the tragedy surrounding last week's violent weather. Still, the lower death toll is noteworthy, specialists say, considering the disaster struck a region of the country that has seen significant population growth in the past several decades.
Many of last week's tornadoes moved through rural areas with houses built before communities began adopting codes aimed at improving a home's ability to withstand high winds, says John Ferree, who heads the severe-storms division of the NWS Office of Climate, Water, and Weather Service in Norman, Okla.
Had more of those homes been built to today's highest storm-resistance standards, the death toll might have been lower, he adds.
Still, he says, better forecasting, improved communications that allow people to see what's happening as it happens, and a deeper understanding of tornado-generating storms helped to reduce storm fatalities. The average lead time for warnings around the country during the outbreak was 24 minutes, according to initial NWS data.
"The general public knows more, so they take shelter" more often than they once did, he says.
Post-storm surveys – a kind of CSI: Tornado – provide a foundation for these improvements, he says.
"If we're issuing 30 tornado warnings and not verifying any of them, or only two tornadoes are occurring, people can become complacent," he says. "When big events like this one occur and we issue warnings, they may not take them very seriously. It's important we put some meat to the bone."
Still, he adds, the work "is physically hard and emotionally hard" as teams of at least two forecasters – often joined by civil engineers or meteorology professors from nearby universities, or forecasters and other specialists from forecast offices outside the affected region – spread out to record a tornado's aftermath.
From the air, field surveys try to map the start-to-finish path a twister takes and the damage it inflicts. Patterns in the orientation of debris on the ground –especially in forested areas – help distinguish between damage from the tornado itself and damage from intense downdrafts that can appear off to one side of a tornado. From the ground, the damage can look the same.
As happened last week, some of the more powerful funnel clouds spawned their own smaller twisters. Each of these siblings also leaves a distinctive signature along the main funnel cloud's path, although for reporting purposes, the collection still counts as one tornado.
In some cases, the evidence even from the ground is unambiguous. The twisters took out 90 tall, high-tension electrical towers in northern Alabama. These towers were designed to withstand winds of up to 150 miles per hour.
"I saw many of those taken all the way to the ground, some wrapped up like pretzels," Darden says. In other places, large oak trees had been flattened, their bark stripped, and the wood left sandpaper-smooth after scouring by the tornado-borne debris, he adds.
Investigators also try to track debris lofted from populated areas – from personal checks and other documents to full-size sedans.
Tracking light-weight debris – which can travel more than 200 miles – gives investigators an idea of the wind movement within the larger storm system.
The sedans – if they can be found – give additional measure of a storm's strength.
"We ran into a family in Oak Grove, Ala., who had a very well-built home, and fortunately they weren't home" when a twister struck, Darden recalls. "It was a two-story brick house, probably the best-built house I saw in four days of surveying."
Still, the home was totaled. Curiously, the father's Crown Victoria, which had been at the house at the time, was missing. No one has been able to find it.
"We don't know what happened to it," Darden says. "Did it get disintegrated by the tornado? Did it get sucked up into the vortex and get carried four or five miles away and we just can't find it?"
The owner said he had GPS in the car and half-jokingly wondered if it still worked, Darden says.
Ground surveys put researchers up close and personal with destroyed homes and people who have lost family and friends to the tornado outbreak.
Rigorous analysis can reveal how well homes fared whose walls were anchored firmly to their foundations or whose roofing timbers were reinforced with metal straps where they join walls.
It's also a boots-on-the-ground view that revealed a 25 foot section of asphalt in one neighborhood yanked up from the street and deposited on a house a half mile away.
It was in a neighborhood where many homes carried markings Darden says he hasn't seen before.
"This is where I kind of lost it," he says. He had asked about the 0/- painted on whatever was available at each home. "The guy said the one on the left is the number of survivors."
Investigators also confront the confounding effects of twisters. Credit cards embedded in steel refrigerator doors. A mobile home lifted from its foundation and deposited hundreds of yards away, while planters hugging the home's perimeter are still filled with vibrantly colored flowers, every flower undamaged, every nugget of bark mulch as neatly in place as if nothing had happened.
Survivors recount similar stories.
Darden describes a family of five who lived on a farm outside of Higdon, Ala., a small community in the northern part of the state. They had no storm shelter, but they did live in a home that he says was well-built.
On Saturday, Darden and a partner visited the family.
"The mother and three daughters were there at the time," he recalls. Looking at the wall-free ground floor -- all that remained of the home -- "I introduced myself and said: Thank God y'all were not home."
Her response? "Oh, we were here."
With no storm shelter and nothing but a slab foundation left, "I really thought she was joking," he continues. "I asked: Where were you at?"
She led the two men to a spot on the storm-swept slab, where nothing but a small patch of hardwood flooring and a scrap of carpeting remained – parts of each pulled up by the tornado. The rest of the flooring vanished into the vortex and hasn't been found. The patch is all that was left of the interior hallway in which the family huddled.
"They were not touched," he says, in a voice tinged with amazement. "They were not sucked up. They didn't have a scratch on them."