Most of yesterday's storm damage, the National Weather Service says, probably was caused by straight-line winds of tropical-storm strength. In fact, no tornadoes were spotted on area radar screens.
But if you swear you saw circular winds like a mini-tornado clawing at your roof, you could be right too. The National Weather Service says they may have been a phenomenon recognized only recently in these parts -- "gustnadoes."
That's how the weather service now describes the gusty winds that blow in ahead of storm fronts. They spin up into a circular motion, but never get strong enough to qualify as tornadoes.
They can cause property damage, and to someone caught up in them, gustnadoes appear to be a sort of "small tornado." But in terms of their destructive power, they fall far short.
Questions about what sort of winds caused the worst damage yesterday would have to be settled by on-site surveys. The weather service said those would probably be launched today.
Jim Belville, meteorologist in charge at the Washington Forecast Center in Sterling, Va., said Doppler radar, which can spot tornadoes as they develop, saw none yesterday as the severe thunderstorms churned their way from northern Virginia across central Maryland.
"What Doppler showed was very strong winds along the leading edge of the storm," he said.
Because the damage occurred along a broad front from Westminster to Baltimore, he said, "that would be indicative of a large-scale gust front and winds of at least 70 mph and possibly stronger."
That's enough to damage barns and roofs, Mr. Belville said. Lines of row homes are particularly vulnerable.
"If one part of the roof fails, then you could lose a lot if they share a common roof," he said.
The term "gustnado" was coined about 10 years ago in the Plains states, Mr. Belville said. They are created along weather fronts where winds in one air mass meet the winds in another and set up a kind of short-lived eddy.
"They get a bit of rotation, but it's much weaker than a bona fide tornado," he said. "We didn't know we had that many until we got this Doppler radar [in 1991] and identified a bunch of them over the last several years. They are really fairly common in the mid-Atlantic area."
"We could have had some of those" in yesterday's storms, he said. "Doppler indicated some areas of weak circulation."
Others may have occurred that were too short-lived to be seen clearly, he said.
Thunderstorms like yesterday's are common where cold fronts shove their way into masses of warm, moist air, said Penn State meteorologist Jonathan Merritt. The instability sets the warm air rising, and it is replaced by downdrafts of cool, dry air from high altitudes. The downdrafts create powerful horizontal winds when they hit the ground.
When the storms occur close to the high-altitude wind currents called jet streams, they gather added energy. Yesterday, the jet stream was "right on top of us," Mr. Belville said. "I would say at 40,000 feet it was running at 125 to 150 mph."
Where the downdrafts are extremely intense, and spread out in all directions over a narrow field less than two miles in diameter, they are called microbursts. They have been blamed for a number of airline crashes and the sinking of the Pride of Baltimore in 1986.