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IRENE’S AFTERMATH | A FEROCIOUS HIT

Soggy summer intensified storm’s effects in Vt.

By Mark Arsenault
Globe Staff / August 31, 2011

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The storm was enormous and slow-moving.

Irene, once a hurricane, later a tropical storm, lumbered into Vermont on Sunday on an ideal track to deliver the heaviest deluge of rain.

“This particular storm lined up perfectly with a lot of the north-south-running valleys,’’ said Lesley-Ann Dupigny-Giroux, Vermont’s state climatologist. “When you have anything oriented just right, as this particular storm was, it kind of sits in place and that helps the rain come down over the same area for an extended period of time.’’

After a soggy summer, Vermont presented fertile conditions for last weekend’s tropical rainstorm to wreak widespread damage: The ground was already saturated, rivers had been running high, and the Green Mountain region has steep slopes that collect rainfall and accelerate the runoff, according to storm and climate experts.

Tropical Storm Irene was just the right storm to take advantage of those conditions. Irene destroyed homes, washed out bridges and roads, and caused at least three deaths in Vermont.

“Lots of things combined into an unfortunate recipe,’’ said Dupigny-Giroux, in an interview.

Mountains intensified the rain, she said, explaining that wet air moving onto a mountain is forced up toward the peak, and “every time the air is forced to rise, that enhances the precipitation that is wrung out of the storm system.’’

Portions of Vermont received 8 inches or more of rain in less than a day, which came on top of earlier flooding this summer. Culverts and tributaries could not handle the volume of water.

“If you’re already at flood stage and you get another 4, 6, or 8 inches of rain - that puts you over the top,’’ she said.

Although it is entirely landlocked, the Green Mountain State experienced some of the worst damage inflicted by Irene’s long trip up the Atlantic Coast. The storm, which skimmed the North Carolina coast, eventually tracked north through Western Massachusetts and then along the Vermont-New Hampshire border on its way into Canada.

Irene took a track similar to that of the infamous Hurricane of 1938, a devastating storm that barreled through the region with frighteningly high winds. Hurricane Irene, in contrast, had lost wind speed by the time it reached New Jersey and eventually plodded into New England as an enormous tropical storm with wide bands of rain, said Dennis Feltgen, a spokesman for the National Hurricane Center.

“All of that rain began pushing out well ahead of the core as it moved on to New England,’’ he said, which sent much of the rain falling over Vermont and New York state.

Vermont’s hilly terrain amplified the problem, said Bill Saunders, senior hydrologist at the Northeast River Forecast Center, part of the National Weather Service.

“Up in the Green Mountain area there are very steep slopes,’’ Saunders said. “And all of that water falling on steep terrain created an awful lot of energy.’’ The rain “fell fast and it ran off fast, and all of that energy contributed to the damage.’’

Rushing flood waters swept away historic covered bridges in Vermont, and the widespread flooding and erosion left some communities cut off from outside traffic. Emergency supplies were being delivered by National Guard helicopters.

Vermonters are used to flash floods after powerful rainstorms, but not on such a massive scale, said Conor Lahiff, a meteorologist with the National Weather Service in Burlington.

“We’ve seen destruction like this before, but more of a localized threat from intense thunderstorms that produce 4 inches of rain in a few hours,’’ said Lahiff. “The impressive thing about this storm was getting the same kind of [rainfall] numbers we’d get from a strong thunderstorm that produces flash flooding, but to [then] see it widespread across a wide area.’’

Bill McKibben, a Vermont environmental activist and the author of a dozen books on the environment, said climate change and a warming planet are producing wetter hurricanes.

“Warm air holds more water vapor than cold,’’ McKibben said. “The atmosphere is about 4 percent wetter on average than it was 40 years ago, and that loads the dice for this kind of event.’’

The loss of historic covered bridges is painful, but to be expected as the earth warms and storms pack more punch, he said.

“This rain is falling on a different planet than those bridges and roads were built on,’’ McKibben said. “Some of those covered bridges had stood there a couple hundred years, but they’re not capable of handling water in the volumes that we’re seeing now.

“This is the third huge flooding event of the year in Vermont; the number of floods around the country and around the world is off the charts, and the reason is obvious.’’

Mark Arsenault can be reached at marsenault@globe.com.