Storm’s Worst Deluge Swamped the Mountains in the Northeast (2)

In the end, the storm made more waves in the mountains than it did along the shore.

Before Hurricane Irene’s arrival, there were fears of devastating storm surges along the Eastern Seaboard, from North Carolina to New England. But while its winds did lead to surges that produced tidal flooding, the worst floods were inland, especially in upstate New York and Vermont.

These floods had nothing to do with tides and little to do with wind, experts said. They were mostly about topography and the sheer size of the storm — not its intensity, but its geographical area.

Even though Irene weakened to barely hurricane force on Sunday, it was still an enormous storm, a spiral of warm, wet tropical air more than 500 miles wide. “It had a lot of moisture with it to begin with,” said Dave Radell, a meteorologist with the National Weather Service in Upton, N.Y.

When a hurricane hits land, it loses some of its moisture when the colder ground causes condensation and rainfall. That happened when Irene passed over coastal North Carolina on Saturday. But the storm was so big that even while part of it passed over land, much of it was still over water, gathering more moisture, said Frank Marks, director of the hurricane research division of the National Oceanic and Atmospheric Administration.

When Irene reached the New York City area, it started to move inland, its winds reduced to tropical storm level, less than 73 miles an hour. Then topography took over.

As the warm, moist air hit the Catskills and other mountains, it was forced upward, what meteorologists call upslope flow. That brought it into an area of higher, colder air that made the tropical air condense, producing heavy rainfall. “The air is forced to rise a bit and that wrings out the moisture,” Mr. Radell said.

It is a common atmospheric effect, often seen in the Rockies. In this case, it was enhanced by westerly winds from the west side of Irene, Mr. Radell said. In the Hudson Valley and Vermont, he said, “we had a good six to eight hours of heavy rainfall.”

And that meant rainfall of up to eight inches in Vermont, according to the National Weather Service office in Burlington. In New York, the most rainfall recorded in the 24 hours that ended Monday morning was more than eight and a half inches in Delanson, west of Schenectady.