Texas tornadoes: How much warning time was possible?

Powerful Texas tornadoes that struck the Dallas-Fort Worth area Tuesday were as noteworthy for what didn't happen as for the damage the powerful twisters inflicted – no deaths reported so far, and only a handful of injuries in a heavily populated area.

For many, the remarkably low casualty rate is a testament to several factors: the twisters struck during the day instead of at night, warnings were timely, cooperation between the local media and the region's National Weather Service forecast office was good, and the Texas public is well attuned to tornado hazards.

Between 1981 and 2010, Texas saw more tornadoes, on average, than any other state – 150 a year, versus 78 for Kansas and 62 for Florida, according to records kept at the National Weather Service's Storm Prediction Center in Norman, Okla.

Now, federal and university researchers are developing an approach to tornado warnings that could extend the lead time from 13 minutes or less today to at least 40 minutes, and perhaps longer.

Typically, forecasters issue a tornado warning if:

• Spotters report a funnel on the ground.
• Storm clouds that are rotating.
• Weather radar picks up the characteristic hook-like signature of storm rotation.

It's virtually a warn-when-you-see-it approach.

Dallas's brief warning

The idea behind the new approach – warn on forecast – is to provide local forecast offices with the tools to predict a thunderstorm's development, direction of travel, and any tornado's likely path well in advance of the storm's arrival. That forecast would be updated every five minutes to keep tabs on the storm's progress and allow for rapid updates as conditions change.

Initial tests with the system using data from two past storms yielded 40-minute lead times for warnings, notes David Stensrud, a researcher at the National Severe Storms Laboratory in Norman, Okla.

“We'd love to get to 60 minutes,” he says.

Residents of the Dallas-Fort Worth area had but a fraction of that. In some cases the warning that hit home was a last-minute phone call.

Forney, Texas, resident Sherry Enochs told ABC news that she spotted a large tornado heading toward her daughter's house not far away. Ms. Enochs recounted how she called her daughter, noted the twister, and at her daughter's request, hung up and sought safety in a bath tub, where the she clung to her young grandson and another toddler as the tornado hit her house. All three survived.

As many as nine twisters were thought to have cut across the region, in one case striking a truck depot and hurling long-haul trailers more than 100 feet into the air like an angry 5-year-old tossing Tonka trucks.

At Dallas-Fort Worth International Airport, where airport officials canceled hundreds of flights, more than 100 aircraft reportedly sustained some form hail damage.

According to the Red Cross, at least 650 homes in the area have been damaged – in some cases cleaved in half, with the surviving half looking as though it came through unscathed.

The value of a few extra minutes

Given few injuries and no fatalities, what would extra lead time from a warn-on-forecast approach buy a community?

The National Severe Storms Laboratory's Dr. Stensrud recalls a conversation with an emergency manager in Oklahoma in which Stenrud asked how much lead time the official would find useful.

“He actually gave me an answer of two hours,” Stensrud says. That lead time would allow officials to plan their responses in a more effective way, even if they ultimately don't act until much closer to a tornado's projected arrival time. Hospitals would have more time to move patients into hallways and away from windows. Sport stadiums would have more time to evacuate the tens of thousands of people filling the stands. And airports – no strangers to lots of large glass windows – would have more time to curtail operations and help passengers reach the safety of shelters.

Much of the technology such forecasts require already exists. Some of the biggest challenges involve integrating information from a range of sources – commercial aircraft, satellites, weather radar among them – and combining and displaying them in ways most meaningful to forecasters. Computer horsepower is another limiting factor, although Stensrud says researchers have had some success using computer processors designed for heavy duty, graphics-intensive use.

That computing power will become all the more important as a type of radar known as phased-array radar becomes a tool for weather forecasters.

Unlike current weather radar, which produces a complete sweep of the sky ever four to five minutes, phased-array radar completes its sweep in one minute – because it's radar beam is steered electronically from a fixed antenna array, rather than having an antenna turn mechanically.

When monitoring a storm, “a lot can happen in five minutes,” Stensrud says.

Testing the model

Up to now, the research team, which includes scientists from other National Oceanic and Atmospheric Administration labs and the University of Oklahoma, has tested its new approach against two major storms – a 2007 tornado that destroyed some 90 percent of Greensburg, Kan., and 2003 tornadoes that struck Oklahoma City.

The team fed the forecast model they developed with weather information available prior to the storms. And while the model can't reproduce features as small as a tornado itself, it did provide a forecast that yielded a high probability of a storm with rotation at low altitudes – often seen as a surrogate for a tornado.

For Greensburg, the modeled path the rotating segment took moved pretty much along the path the real tornado took, including its pass over Greensburg, Stensrud explains.

“That would have been on the order of a 40-minute advanced warning,” he says.

The team saw similar results for its “hindcast” of the Oklahoma events. The region of the modeled storm with the highest probability of triggering tornadoes closely matched the paths the real twisters took.

It would come as little surprise if Tuesday's twisters in the Dallas-Fort Worth area provide a further test for the new forecasting approach.

“Early indications are that it looks like we can do it,” he says of achieving the program's goals. But, he cautions, “I just don't know how reliably we can do it – for every event, every different type of event we'd like to handle – and make sure it's a robust system.”

It is likely to take a couple more years, he says, to reach a point where the team can use the model experimentally in real time, side-by-side with current forecasting approaches as severe-weather happens. That represents the final trial before a decision is made on whether to add the high-tech arrow to forecasters' severe-weather-prediction quiver.