- The Washington Times - Sunday, May 5, 2002

The tornado that devastated La Plata, Md., last Sunday was born out of a series of powerful spring thunderstorms that formed over the Midwest.
Meteorologists say the conditions over Southern Maryland were bound to spin a tornado especially in La Plata, which was caught between a cold front and a warm front.
"This storm was already inclined to produce a tornado, and twisters love to form along boundaries," said Barbara Watson, a warning-coordination meteorologist at the National Weather Service in Sterling, Va. "Hitting that front may have been what it needed to spin one. But we just don't understand [the formation of tornadoes] completely yet."
La Plata has been plagued by tornadoes in the past, but meteorologists can't say exactly why the twister struck the town, which measures approximately 5 square miles. In 1926, a tornado that followed a similar path tore through a school, killing 14 students and injuring 56 others. The town also has been hit by three less-destructive tornadoes.
The twister that struck La Plata last Sunday had been listed at F5 on the Fujita tornado scale, which rates tornadoes based on wind speed and damage. An F5 is the highest possible rating, and an F5 tornado indicates "incredible damage," with winds of between 261 and 318 mph.
The twister eventually grew to about 1 miles in width and was traveling up to 40 mph without lifting off the ground for at least 30 minutes. It was the strongest tornado on record in Maryland and the most damaging to strike an area in the United States since 1999, meteorologists said.
Dan McCarthy, a warning-coordination meteorologist at the National Storm Prediction Center in Norman, Okla., said the center saw dangerous weather forming over the Midwest on April 26 and issued alerts to forecasters throughout the mid-Atlantic.
The path of the storms began in the Ohio Valley, just as a cold front was forming along the Indiana and Ohio border. The jet stream last Sunday pushed the cold air over the warm, humid air in the region, creating a ripe environment for thunderstorms to form.
The storms and the cold front moved east, unleashing at least 35 twisters and spreading heavy rain, wind and hail from the Tennessee Valley to Maryland's Eastern Shore.
The first line of thunderstorms rolled through Tennessee and Kentucky on Sunday morning. The northeastern end of that system produced steady rain from Virginia into the New England states.
It was followed by a second line of violent weather that afternoon. There was wind damage in southern Illinois, hail the size of golf balls that smashed windows in southeast Ohio, and stronger thunderstorms in Kentucky, Tennessee, Indiana, West Virginia, Virginia and western sections of Pennsylvania and upstate New York.
Many of these storms were supercells powerful storms in which the whole cloud mass rotates, wrapping the storm in rain. The rotating cloud mass helps create a favorable environment for the formation of a tornado because of a low cloud base, said Steven Zubrick, a weather service science and operations officer.
He said another factor was strong wind shear, which occurs when winds blow in one direction at one level and another direction above, causing the air in between to rotate.
Locally, a warm front was moving north across the Chesapeake Bay region late Saturday night, triggering heavy rains Sunday. When that passed, it left a warm and humid air mass brewing under sunny skies, even as a cold front was heading down from the Appalachian Mountains, Mr. Zubrick said.
Moist, warm air is lighter than cold, dry air and as it rises, it cools. The moisture begins to condense, producing clouds and rain. When the warm, moist air rising from below reaches that rotating air, it tilts the rotation and the funnel clouds form, Mr. Zubrick said.
"It didn't take too much for this storm to spin something," he said.
Mr. Zubrick said the twister that hit La Plata formed over the Potomac River after the cloud mass traveled through several counties in Northern Virginia.
After absorbing moisture from the river, the tornado traveled full force into Southern Maryland, first touching down in Smallwood State Park in Charles County before heading into downtown La Plata, where two persons were killed.
The twister began to weaken over the Patuxent River and a second, less-severe tornado formed. Both hit Calvert County, where a woman in Prince Frederick died. The tornadoes eventually swept over the Chesapeake Bay and into Dorchester County.

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