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The rainfall that hit parts of Vermont on Monday had less than a 1-in-100 chance of occurring in any given year, according to federal flood data. And yet, for some residents, it brought a sense of déjà vu — it rivaled the devastation of another so-called 100-year storm, Hurricane Irene, from a dozen years ago.
The chances of two such deluges hitting the Green Mountain State in such a short span? Just 0.6 percent, said Art DeGaetano, a professor at Cornell University and director of the National Oceanic and Atmospheric Administration’s Northeast Regional Climate Center.
There was more than chance at play, though, as the planet’s steady warming increases how much moisture the air can hold. Climate scientists have for decades predicted that would translate to extreme bursts of rain in places like New England. While other factors contributed to the floods’ destructive impact — Vermont’s mountainous terrain and preceding weeks of rainy weather — it is a likely sign that those forecasts are becoming reality.
“This, to me, is almost as classic a signal of climate change as warm temperatures,” DeGaetano said. “In a warmer world, this is what you would expect.”
Many of the areas inundated by the storms now face far more serious threats than current federal flood maps suggest, according to new data released by the nonprofit First Street Foundation. In Washington County, where Montpelier is, what was once considered a 1-in-100 year storm is now likely to occur about every 63 years. In nearby Orleans County, such events could happen three times as often as currently estimated by federal flood maps.
The intense and widespread rainfall that occurred from Sunday to Monday had less than a 1 percent chance of occurring in that part of the country, according to NOAA. Widespread rainfall totals exceeding 8 inches were reported from the ski resort town of Ludlow to Montpelier, the state capital.
NOAA estimates the likelihood of extreme rainfall based on past rainfall observations, and doesn’t take climate change into account.
The moisture was wrung from a storm system that remained parked over Vermont for hours, if not days, said Lesley-Ann Dupigny-Giroux, the Vermont state climatologist. While the state’s mountainous terrain always provides lift for rainfall — pushing moisture higher into the atmosphere, cooling and condensing it into precipitation — the weather systems that were steering the storm and slowing its movement were also serving to push the moisture skyward, she said.
And the sheer amount of moisture was unusual. Though the system didn’t start as a tropical storm or hurricane like Irene, it was fueled by a “plume” of moisture wafting in from the Atlantic — similar to the “Pineapple Express” storms that pummeled California with tropical rains this winter. Much of the Atlantic is experiencing record warmth, which means more evaporation to fuel heavier rain.
Places such as Montpelier and Ludlow, where flooding hit hardest, demonstrate why heavy rain can be so devastating in Vermont: all of its steep, rocky slopes send water coursing into river valleys, where people live.
Roads were historically built along streams and rivers because of the terrain, and that means when floodwaters surge, damage to transportation infrastructure can be common and severe, Dupigny-Giroux added.
