A swarm of tornadoes that raked the Chesapeake Bay region Wednesday, some as strong as the strongest twisters ever to hit Maryland, sprang from a weather pat tern more typical of the Midwest's infamous "Tornado Alley" in May than of the Tidewater in July.
Meteorologists said yesterday that a mass of cool, dry air setting low-temperature records in the Midwest stalled in the eastern foothills of the Appalachians and clashed with warm, moist marine air along the Eastern Seaboard.
This volatile brew, brought to a boil by the heat of a July afternoon, produced a complex of unusually strong thunderstorms.
High winds at upper altitudes set those storms rotating, slowly, transforming them into the so-called mesocyclones that are often the launching pads for tornadoes.
This is not an everyday occurrence," said Jim Henderson, deputy director of the National Severe Storm Forecast Center in Kansas City.
"Your more likely scenario is where you get these real strong thunderstorms that quickly move out into the Atlantic."
Like a formation of heavy bombers, the storms moved slowly from southwest to northeast.
PD About 3 p.m., the first funnel cloud sprang from the base of one
storm and touched down near Lynchburg, Va. Nine hours later, the last of the twisters killed three people in Limerick Township, Pa., south of Philadelphia.
In between, the violent storms snapped trees, shatterebuildings, flooded roads and tossed a barn onto a road.
A tornado that hit Ironsides in Charles County, was classified as a "Force 3" on a scale that runs from 0 to 5.
That means it had winds of 158 mph to 206 mph, capable of inflicting severe damage.
That's the same rating estimated for a 1961 twister in Anne Arundel County that is counted as the strongest on record in Maryland, Mr. Henderson said.
Tornadoes strike infrequently in Maryland, where 127 have been sighted since nationwide recordkeeping began in 1952. Tornadoes have killed 23 people in the state during that time.
Across the United States, about 1,000 tornadoes strike each year, killing an average of 60 people and causing $1 billion in property damage.
As recently as August 1992, eight twisters hit Maryland in thwake of Hurricane Andrew. But they were relatively small and did not cause widespread damage.
The largest tornadoes, typically in the Midwest's "Tornado Alley" -- roughly the nation's Great Plains -- can spin at up to 300 mph, cut a swath two miles wide, travel 200 miles and last seven hours.
They are classified as "Force 5."
National Weather Service forecasters counted 10 tornadoes -- six in Maryland and two each in Virginia and Pennsylvania -- Wednesday.
But tornadoes are typically hard to document.
Weather Service meteorologists inspected fields, forests and subdivisions yesterday looking for evidence of additional tornadoes.
Jonathan Bles, a meteorologist who tracked the storms for the Weather Service, said the low death toll Wednesday probably resulted in part from the Weather Service's NEXRAD Doppler radar equipment, which has been installed in this region in the past several years.
"They allowed us to issue reports every five to 10 minutes, telling people where the tornado was and how it was moving," he said. "That thing saved some lives last night."
In the 1960s, forecasters were able to give people living in the path of such storms only about four minutes' warning.
Today, meteorologists using the NEXRAD radar routinely issue accurate warnings up to 20 minutes in advance.
Yesterday's tornadoes narrowly missed Washington and Baltimore and stopped short of Philadelphia. Some researchers think the heat generated by a metropolis acts as a barrier to the killer storms, which rarely hit large cities.
Mr. Henderson of the severe storms forecast center doesn't buy this theory.
"My own personal feeling is that . . . eventually urban areas will get hit," he said. "It's just a matter of time, 100 years or so, before it happens everywhere."