The tornado that killed six people and wrecked much of La Plata last week has lost its ranking as Maryland's first F5 storm - the most powerful category.
After a four-day inspection of damage along the storm's 68-mile track, a team of experts assembled by the National Weather Service gave it a final rating yesterday of 4 on the Fujita Tornado Damage Scale, with top winds of 260 mph in the La Plata area. A preliminary assessment of the storm's power, released the day after it struck, had rated it an F5, with winds of 261 mph or more.
"Just because we're calling it an F4 instead of an F5 does not mean it wasn't a bad tornado," said John E. Ogren, meteorologist in charge at the weather service's Indianapolis forecast office, who led the assessment team. "It was a terrible tornado.
"F4 and F5 tornadoes are only 2 percent of tornadoes, and that 2 percent of tornadoes cause 70 percent of the fatalities," he said.
Maryland has recorded two other F4s. The first was the storm Nov. 9, 1926, in La Plata that killed 17 people, all but three of them schoolchildren.
The second was a twister June 2, 1998, in Frostburg. It destroyed 30 homes and businesses and caused $5 million in damage, with five injuries.
Ogren's team included a Fort Worth, Texas, forecaster with extensive tornado experience, and a wind engineer with expertise in storm damage.
A characteristic that distinguishes an F5 tornado from an F4 is that F5s will lift even well-built houses from their foundations and carry them away. The first assessment team sent to La Plata, the day after the storm, based its F5 ranking in part on the observation that "at least six homes were completely wiped off their foundation."
On closer inspection, Ogren's team found that those houses weren't well-fastened. "So, they could be destroyed at lesser wind speeds than other homes that are well-connected," he said. The team found that some of the most severe structural damage was caused by flying debris, and not by wind alone.