As communities struck by last week's tornado outbreak echo with the rumble of front-end loaders and the metallic thud of debris hitting dumpsters, weather forecasters are calling it the most intense week for twisters in US history.
The week-long onslaught was, perhaps, the tornado-watcher's equivalent of "The Perfect Storm." The set of conditions that led to outbreak are " not likely to happen not likely to happen very often in our lifetime," says Harold Brooks, a researcher at the National Severe Storms laboratory in Norman, Okla. "This was pretty high-end, even for severe-storm fans."
Yet for all its destruction, the outbreak also appears to be vindicating the country's investment in research and hardware to improve forecasts and warnings.
During the 1990s, the National Weather Service (NWS) spent $4.5 billion upgrading everything from its weather radar to computer models that help forecasters. The "improved radar data and improved model data really helped us as far as last week was concerned," says Dan McCarthy, a meteorologist with the NWS Storm Prediction Center in Norman, Okla.
"Everyone is comparing last week to 1974," when a "super" tornado outbreak April 3-4 triggered 148 tornadoes that carved a path of devastation 2,500 miles long, says Mr. McCarthy.
That outbreak killed 330 people and injured more than 5,000. Another 10 tornadoes would form before that week was out.
By contrast, 384 tornadoes struck 19 states last week, killing an estimated 43 people. The dramatic shift in fatality rates may be of little solace to those who lost friends or relatives in last week's storms. Still, the low casualty figure for the number of twisters last week remains "an incredible ratio," Mr. McCarthy says.
The reason for that rests, in part, on the particular circumstances of last week's storms and the paths that the twisters took. Yet efforts to further reduce fatalities from tornadoes may have played a role too.
One promising approach, NWS officials say, is the agency's storm-ready program, under which communities and counties undergo training, set up storm-spotter networks, and implement other procedures that improve readiness and communications among civil defense and other agencies to deal with a storm and its aftermath.
The program was started in 1999. Today, 563 cities, towns, and counties have been awarded a plaque to certify their readiness. Each community must be recertified every three years, which can lead to discounts on premiums for federal disaster insurance, notes Stephan Kuhl, a meteorologist at NWS headquarters in Silver Spring, Md.
He notes that on May 3, 1999, an F-5 tornado - the most destructive category - plowed through the city of Moore, Okla., killing 38 people and injuring several hundred. In June 2001, Moore earned its storm-ready rating. Last week, an F-4 twister, followed nearly the same path during roughly the same time of day. One person was killed and nine were injured.
As for giving longer lead times on tornado warnings, researchers have seen some interesting clues from a field-research campaign in 1994 and 1995 called Vortex. It might be dubbed the mystery of the "blob."
Recent analyses of Vortex radar data have shown that before the formation of an infamous hook-like radar echo indicating a tornado, a blob appears that looks to be associated with the hook. If it proves to be a telltale sign, it would give forecasters an earlier indication that they need to issue a warning. It may take another field campaign, however, currently being discussed for 2007 and 2008, to solve the mystery.