Dartmouth Study Finds Heavier Rainfall May Be Region’s ‘New Normal’

  • Penelope Wurr records drivers as they drive through the flooded section of Main Street, in Brattleboro, Vt., after a heavy rain causes flash flooding around the Brattleboro area on Monday, June 19, 2017. The National Weather Service has issued a flash flood watch for most of Vermont, New Hampshire and Maine. (Kristopher Radder/The Brattleboro Reformer via AP)

Valley News Staff Writer
Published: 6/21/2017 12:07:33 AM
Modified: 6/21/2017 12:11:27 AM

Hanover — Intense rainfall events, like the one that triggered flash floods throughout the region and a mudslide in southern Vermont on Monday, have become much more common in the last 20 years, according to researchers at Dartmouth College.

Though the jury is still out on whether climate change is behind the trend, the findings, which were published last month, do suggest that what we think of as 100-year flood events might actually be much more likely to happen than conventional wisdom suggests, said Jonathan Winter, who joined Dartmouth colleagues Huanping Huang and Erich Osterberg on the research team.

Winter found that intense rainfalls — generally thought of as 2 or more inches of precipitation in a 24-hour period — are 53 percent more likely to happen than they were before the mid-1990s.

And current conditions in the Upper Valley are likely to increase the impact of those heavy rains.

“One would expect it to have a bigger flood potential, because the soil’s already saturated,” he said on Monday, just as the National Weather Service was stepping up warnings about potential flash floods throughout much of the Northeast. “The river is already up, as opposed to if we had a drier spring.”

And by Monday night, the impact of the storm was obvious.

In Brattleboro, Vt., and surrounding areas, there were downed trees, several roads were closed due to high water, and a mudslide choked off Route 30.

A Windham County weather station recorded 4.3 inches of rain on Monday, while 1.51 inches were logged in Woodstock, according to the National Weather Service.

“We had a very warm, moist tropical air mass over the Northeast and a cold front approaching from the west, which triggered the storms,” National Weather Service meteorologist David Manning said. “All that hot, warm air provided the fuel for the heavy rainfall.”

Winters and his colleagues were surprised when they analyzed rainfall data at 116 weather stations throughout the Northeast between 1901 and 2014.

Instead of seeing a slow and steady increase in storm events, they found a dramatic change that happened all at once — in 1996.

The study was published in the American Meteorological Society’s Journal of Hydrometeorology by the team, which included Dartmouth’s departments of Earth Sciences and Geography, as well as faculty from the University of Vermont and Columbia University.

“From 1901 to 1995, we find there was no increase,” he said. “From 1996 to 2014, we’ve experienced 53 percent more extreme precipitation.”

The increase hasn’t happened equally across all seasons, either.

Spring and fall downpours have seen about an 84 percent increase, while winter and summer have seen more moderate increases of 45 and 27 percent, respectively.

If the study has indeed identified what Winter called a “new normal” for weather patterns, it could have significant consequences for development in flood zones, and for those who design and build stormwater systems.

The study certainly has their attention, according to Kevin Geiger, a planner at the Two Rivers-Ottauquechee Regional Commission in Woodstock.

“We recommend that the powers that be fit that data into their calculations about flooding,” he said. “That they’re not looking at the rain from the 1950s, but looking at the rain from the 2050s.”

Geiger said it’s much more cost-effective to build larger pipes into stormwater systems that are already under construction than it is to dig them up after finding that they haven’t kept pace with the real world.

“This study says that past data, which drives a lot of standards, or our memory from our childhood are wrong for the future,” Geiger said. “These frequent, intense storms are much more probable than they used to be.”

Geiger said that, in looking at the study’s data, he was struck by the fact that intense rainfall events happened more frequently later in the 20-year period than earlier.

“It’s not that they’ve increased, but they’re increasing,” he said. “So now I’ve got to look at, when I put this pipe in the ground, I need to look at 20 or 30 years from now, and what’s that storm going to be?”

Geiger said individual homeowners should be thinking about little things they can do to take the bite out of the peak flow during a heavy rain.

“If you can put a little more gravel someplace, find a little more lawn, that can shave off a bit of that flood peak. It all adds up,” he said.

The National Weather Service recommends that families living in flood-prone areas build an emergency kit, make a family communication plan, and elevate household fixtures like the furnace, water heater and electric panel.

Winter and his colleagues now are taking a closer look at the storms covered in their study, and plan to analyze the specific tropical storms, nor’easters and other weather patterns that triggered them. Winter said that will allow the team to draw more conclusions about whether the storms are linked to climate change, which in turn will help forecast whether such events are a 20-year anomaly, or likely to continue to escalate in the future.

He said they hope to publish the next phase of their research next summer.

Matt Hongoltz-Hetling can be reached at mhonghet@vnews.com or 603-727-3211.

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