Flood waters nearly touch the bridge on Langdon Street in Montpelier hours before the town flooded entirely on July 10 and 11. Photo by John Lazenby.
Vermont experts in climate and the environment continue to gather the countless threads of data that will explain how the storm that devastated Vermont this month happened. But many pieces of the puzzle are already clear — and experts believe it could happen again.
The July 10 storm was not the first of its magnitude in Vermont, nor even the first in recent memory. Tropical Storm Irene was devastating when it swept through the state in August 2011. This month’s storm, though, came about through a different series of colliding circumstances.
“There was a lot of water and a lot of heat that moved into the region,” noted Gillian Galford, a professor with the UVM Rubenstein School of Environment and Natural Resources. “We consider a day of heavy precipitation more than an inch of precipitation in a 24-hour period, and so when you start hearing about four inches, or six inches, in 24 hours, that’s really a lot of rain.”
The storm was a frontal system, with cold and warm air meeting to form rain clouds, explained Vermont State Climatologist and UVM Professor Lesley-Ann Dupigny-Giroux. A front can drop a few inches of rain, she noted. But the July 10 front stalled (in part) over Barre and Montpelier, and there it sat.
“If it stalls over a place — if you think about Barre and Montpelier, where the geography is really these valleys, which have really steep slopes on either side of them — when you have those steep slopes, it sort of helps to concentrate the water flowing into the North Branch and the Dog and into the Winooski itself, and so it concentrates the rainfall that’s falling in those water bodies into the main stem of the river itself,” Dupigny-Giroux said. “So when you look at that entire stretch along the Winooski there, you cannot separate or divorce what the surrounding floodplain looks like from where the storm itself was producing all that heavy rain.”
Heavy rainfall events like this one are growing more common because of climate change, noted Ned Swanberg, flood hazard mapping coordinator and regional floodplain manager with the Vermont Agency of Natural Resources. “Vermont has been suffering from increased levels of heavy precipitation,” he explained. This is “typically from what they call entrained thunderstorms, where it’s not just one cloud that goes by, but it’s a series, and that can really take a toll and destroy a number of roads and so forth — but not affect the next town.” He added, “You could be dry just a little further down the road.”
These highly localized pockets of extreme rain can leave a patchwork of devastation on the landscape. So while the Mad River valley, which was devastated during Irene, saw nothing close to that damage in this month’s storm, Montpelier, Barre and other nearby towns were completely inundated.
And such events are happening in a larger context, as a warming world sees changes in the jet stream, an air current that, according to NOAA, runs west to east about 30,000 feet above Earth’s surface, between the northern continental United States and the Arctic. With the Arctic warming, Dupigny-Giroux explained, “these really fast, intense changes in the Arctic regions make the difference between the Arctic and where we live in the mid-latitudes less.” This induces changes in the jet stream that make storms more likely.
While one piece of the puzzle is whooshing 30,000 feet overhead, another is drifting out of sight of Vermonters, far offshore. “(A) lot of the warming is being stored not just in the air, but is also being stored in the ocean,” state climatologist Dupigny-Giroux said. “(A)s your oceans warm, that means more moisture is going to evaporate a little bit more.” This warm air then holds more moisture as it makes its way inland to Vermont. All these factors, from hyperlocal to global, play roles in determining the weather that Vermont experiences.
Is this the new normal with climate change? That’s tough to say.
“We have no idea what’s ahead,” ANR’s Swanberg said. He pointed to a warming Arctic, a changing jet stream, El Niño, and other factors, which can combine to have the effect that “a lot of these weather patterns kind of lock into place, and they’re hard to budge. So you end up with hot or cold, wet or dry … (once) you get it, you get a lot of it. That seems to be what we’re seeing so far.”
Montpelier had already recorded its wettest July in 75 years by last Friday, according to local TV station WPTZ, well before the end of the month, and with a few days to go, the rain continues to fall.
“One thing that we know about climate change is that warm air can hold more water, which is why we see more precipitation here in the Northeast,” UVM’s Galford said, “but we before this had experienced that in terms of big nor’easters or hurricanes, tropical storms like Irene. But this was just an air mass with a lot of water and a lot of heat.”
It’s challenging — and perhaps not accurate — to try to attribute the cause of any one storm to climate change. Galford noted that “it’s more like every storm is a little bit exacerbated by climate change. It’s not that one storm happened or didn’t happen because of climate change, but that the intensity of every storm is enhanced because of changes we’ve experienced in climate globally.” She noted that such changes, although ultimately global in scale, are “really felt locally.”
Scientists make predictions about what to expect from a changing climate based on models, Galford pointed out, and these models are created using existing climate data. What happens, then, when the climate shifts into never-before-recorded territory? What happens when there is no data precedent?
“With climate change, one of the hard things is that we are starting to experience events that we haven’t historically experienced … scientific models, like climate models, are built on observations,” Galford said. “So we’re starting to experience things that we just don’t have the power to exactly predict. So it gets hard. It’s hard in a model to know — is it going to be two inches or four inches?”
What does seem clear is that this storm won’t be the last of its kind in Vermont. “It will happen again,” Swanberg said, noting that reconstruction regulations will help to make rebuilding safer, but many residents will “really need to make hard choices” when their homes fall somewhere between locations that are clearly out of the flood zone and those whose homes are no longer safe to occupy.
Vermonters may have felt somewhat buffered from climate change up until now, Swanberg noted, “… and one of the things is that Vermonters have felt that even though we’re really keenly aware of the weather, and keenly sensitive to changes in the sugaring season and the onset of fall and crops because of the short growing season, and we’re very much aware of climate change, but we also have this sense that we’re somewhat more protected than most places, but this is proving that we are not.”
What, then, can Vermonters do? “My personal answer is, we need to do everything,” Swanberg said. “We need to be adapting to these wildcards, but we also need to be stopping the drivers of chaos. And Vermont is trying to get its bearings on that, and Vermont needs global partners at federal (and) international levels to succeed at that. And it’s still possible if we are globally heroic as a species, as nations. And we’re right at that edge.”
