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Storm was part of vast, violent system
The most violent tornado ever to hit Maryland was the deadly climax of a severe storm system that formed late last week in the high plains and ignited a daylong barrage of extreme weather Sunday across the Eastern United States.
The historic F5 tornado in Charles County was born in a "supercell" thunderstorm that crossed Northern Virginia ahead of a cold front that also packed heavy rain, hail and high winds.
Twisters also spiked other thunderstorms that struck from Missouri to southern Virginia, and from western New York state to South Carolina.
At least six people were killed in three states, and many more were injured. High winds damaged or demolished thousands of vehicles, homes, businesses and other structures. Heavy snow fell in the upper Midwest.
The center of the storm system formed in southeastern Wyoming and Colorado. Saturday, it moved across the Central Plains, reaching the Great Lakes region by Sunday morning, according to Joe Schaefer, director of the federal Storm Prediction Center in Norman, Okla.
Air behind the front was cold and dry. The air in front of it was warm and heavy with moisture from the Gulf of Mexico.
It was the perfect setup for tornadoes: A difference in wind speed and direction between the colliding air masses creates a spinning effect. Rising warm, moist air within a thunderstorm cell tilts the rotating air into a rotating vertical column. And a tornado is born.
From tiny to terrors
The smallest twisters, rated F0 on the Fujita tornado damage scale, might last just a few minutes, damaging chimneys, awnings and signs, and breaking tree branches with winds of 40 to 74 mph. Most of Maryland's have been ranked F0 to F2, with winds up to 157 mph.
The fearsome F5s pack winds of 261 mph and more. They can last more than an hour and are defined as strong enough to lift well-built homes off their foundations, hurl automobiles hundreds of yards, and rip the bark off trees. That's what the damage teams found in La Plata.
Only 51 other F5 tornadoes have been recorded in the United States since 1952. This was just the second F5 ever seen in the weather service's eastern region.
"Historic and tragic," said Susan Weaver, spokeswoman for the Sterling, Va., forecast office.
Started in Missouri
Sunday's outbreaks began just after midnight in Missouri.
Schaefer said a 12-year-old boy was killed and several homes were damaged when a twister touched down near Marble Hill, Mo., shortly after midnight. Another fatality and dozens of injuries were reported in Dongola, Ill., an hour later.
Two strong twisters, with winds of 153 to 206 mph, touched down in Paducah, Ky., and Murfreesboro, Tenn. Nearly four dozen people were hospitalized, and more than 125 homes were damaged or destroyed.
More tornadoes, with damage and some injuries, struck Sunday in Stark County, Ohio; Erie and Allegany counties in New York; Blacksburg and Emporia, Va.; and Westminster, S.C.
Maryland got its first alert on the brewing storm in a tornado watch issued at 3:05 p.m. Sunday by the Storm Prediction Center.
The watch said that tornado formation was possible across most of Virginia and Maryland between 3:30 p.m. and 9 p.m.
Alerts and alarms
The alert triggered alarms on special weather radios, and it was broadcast on many local TV and radio stations as part of the Emergency Alert System. Anyone who catches the broadcasts is supposed to monitor TV and radio for further developments.
At 4:37 p.m., the National Weather Service issued a tornado warning for Shenandoah County in northwestern Virginia. The warning meant that a funnel cloud had been spotted or detected in the area.
Twenty-one minutes later, Schaefer said, a tornado (later rated an F2) touched down south of Mount Jackson. That twister cut a four-mile path across the county before expiring in the mountains of the George Washington National Forest.
At 5:57 p.m., as that same storm cell pushed eastward across Virginia at 35 to 40 mph, a second tornado watch was issued. It covered most of Virginia, Maryland, Delaware and southern New Jersey until 10 p.m.
Less than an hour later, at 6:45 p.m., forecasters issued a severe thunderstorm warning for Charles and Calvert counties. But then something happened to the storm.
About 7 p.m., as it crossed the Potomac River, the thunderstorm responded violently to the atmosphere over the water, said meteorologist Chris Strong, of the weather service's Sterling forecast office.
"It really just went extreme and just intensified very rapidly," he said. On radar, forecasters spotted an ominous "hook echo," a little curved appendage on the bottom of the storm's radar reflection that signals the development of tornadic winds.
Doppler radar confirmed it.
At 7:02 p.m. the Sterling office quickly issued a tornado warning for Charles and Calvert counties. Minutes later, certainly by 7:10 p.m., officials said, the funnel cloud touched down just west of La Plata.
La Plata had little time after that to react, Schaefer acknowledged.
"The horses were running out of the barn" by the time the warning was issued, he said. "For the people downstream it still does help. But obviously, we'd like to issue a tornado warning before it occurs."
Once the funnel cloud was on the ground, it carved a 30-minute trail of destruction that was a least 24 miles long and 400 yards wide. It crossed Charles County, leaped the Patuxent River and plowed some six miles into Calvert County.
On average, about 1,000 tornadoes are reported in the United States each year, resulting in 53 deaths and 1,500 injuries, according to the Storm Prediction Center.
The greatest loss of life from a Maryland tornado occurred in August 1818, when a twister crossed the Potomac River south of Indian Head, in Charles County. At least nine vessels were sunk, and more than 30 people drowned.
Maryland is not commonly regarded as a tornado-prone state. The weather service ranks it 36th among the states in the amount of property damage done annually -- an average of about $2.3 million each year between 1950 and 1999.
100 since 1990
But twisters, especially small ones, are surprisingly common here. More than 100 have been recorded just since 1990.
And, as Maryland's population grows and development intensifies, the threat to life and property rises every year.
The 1926 LaPlata tornado, for example, churned northeastward for 18 miles across Charles and Prince George's counties. But it was a poor, rural area then, and total property damage came to just $752,000 in inflation-adjusted 2002 dollars.
The Sept. 24, 2001, tornado that ripped through College Park, killing two people, was rated an F3 storm, downgraded to an F2 as it pushed north just east of Interstate 95 into Laurel.
Damage from that storm totaled more than $73 million, according to the weather service. Until this week, it was the costliest tornado in Maryland history.
For more information on tornadoes, go to http://www.spc.noaa.gov/faq/tornado/