The monstrous storm of March 1993 -- more intense than some hurricanes, bigger than the legendary blizzard of March 1888 -- is a classic East Coast winter cyclone, or low-pressure system. But this type of cyclone has not been seen in a decade, and its scope and power have not been seen in a century or more.
"I guess it has happened, but not in modern history, and certainly nothing this late in the year, over such a wide area," said Paul G. Knight, a meteorologist at the Pennsylvania State University Weather Communications Group.
Combining the attributes of both hurricane and blizzard, the storm that lashed and buried a vast swatch of the eastern United States from the Gulf states to New England was born in the convergence of three innocuous little atmospheric disturbances that by themselves would have been little noted.
The major one, a cluster of thunderstorms, developed over the western Gulf of Mexico. Another, a mixture of rain and snow carried on winds from the Pacific, moved across Texas. The third, a band of light snow and gusty winds, dropped swiftly down across the country's midsection from the Arctic Circle. "Any two would produce a significant storm," Knight said. "But all three together produced a historic storm."
The three disturbances converged over the Southeast, turning up, as Mr. Knight put it, "three cherries on the slot machine."
The resulting storm is larger than the blizzard that paralyzed New YorkCity for two days in March 1888, Mr. Knight said. The blizzard 105 years ago affected a stretch from Virginia to central and western New England. This storm has roared across an area roughly twice as large.
With its hurricane-force wind gusts, this weekend's storm recalls the northeaster that hit the Atlantic Seaboard in December. But that storm saved its snow for interior regions while pelting the metropolitan areas with rain.
Further, Mr. Knight said, this storm is about twice as intense as the December one; the latter generated its destructive winds in combination with a big high-pressure system over eastern Canada. This storm generated its own steam.
The atmospheric pressure at the storm's center was lower than the extra-low pressures at the center of some hurricanes. By creating a contrast with surrounding air, the low pressures create high winds.
The storm is part of a continental weather "regime" that began in early February, Mr. Knight said. It looked a few days ago as if this pattern had ended and that a transition to a new pattern had begun, he said, but the February weather system was merely interrupted briefly.
In this persisting pattern, the general path of the jet stream -- the high-altitude river of air that bears storms across the country from west to east -- has run from northwestern Canada southeastward along the eastern slope of the Canadian Rockies, down through Minnesota to the Tennessee Valley and up along the Eastern Seaboard.
It was this jet that picked up the merged disturbances and bore the resulting superstorm northward.
There is no way of knowing, Mr. Knight said, how long the present weather system will persist.
Is the storm related in any way to broader climatic changes? The late 1980s saw the warmest average global temperatures on record, and big Northeastern snowstorms were relatively infrequent in that period.
But a global haze cast into the atmosphere by the June 1991 eruption of Mount Pinatubo in the Philippines reflected enough sunlight to reverse the warming trend.
While you can't blame any specific weather event on a relatively small global force, "what you can say is that the probability of such events may be influenced by global climate change," said Dr. James E. Hansen of NASA's Goddard Institute for Space Studies in New York City.