As Vermonters last weekend eyed warnings of heavy incoming precipitation, many feared the forecast could resemble the torrential rains of July, which quickly turned nightmarish for thousands of people across the state.
The storm that struck on Sunday and Monday was much milder than the July deluge, with no reported injuries or deaths. Still, it caused widespread flooding, prompted 12 swiftwater rescues, closed 120 schools and flooded many basements.
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“I don’t think anyone anticipated the level (of flooding),” said Mike Leichliter, superintendent of Harwood Union Unified School District, which was hit particularly hard by the storm. “It just happened very quickly.”
Meteorologists say that type of December storm will likely become more common, driven by the warming effects of climate change. At the same time, such storms’ severity will remain trickier to forecast, made challenging by the difficulties in predicting snowmelt and the outsized impacts of temperature variations when doing so.
Over the weekend, those variables led to residents, superintendents and state officials anxiously watching and waiting, only for the storm to worsen throughout the day on Monday.
Leichliter said he looked at the forecast early in the morning and knew that “it could be a challenging day with the amount of water they were expecting.” At that point, the National Weather Service hadn’t issued warnings that would impact the Mad River Valley schools, he said.
Students arrived at Moretown Elementary School at 7:30 a.m., but just before 8 a.m., the principal called Leichliter to say that a river was forming near the school. They prepared to issue an alert to families, but before they could send it, water had started to seep into the school’s gymnasium. By 8:30 a.m., buses arrived at the school to take students home.
Other schools in the district were dismissed later that morning.
“The water was surging,” he said. While the buses did encounter some flooding while transporting students home, the flooding only worsened in the afternoon.
School leaders acted just in time “so that we didn’t have more challenges getting kids home safely,” he said.
This week’s storm was far from the first time in recent history that wet snow had sowed chaos.
Last December, several days before Christmas, a storm brought rain, wet snow and powerful winds that knocked out power for tens of thousands of Vermonters. From then until now, scientists have recorded six weather events that have featured wet snow, said Scott Whittier, warning coordination meteorologist with the National Weather Service in Burlington.
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“I’ve been here 30 years and I don’t recall more than two events a year,” he said.
In the case of this week’s storm, several variables collided to produce conditions that were somewhat more severe than the National Weather Service originally forecasted, Whittier said.
The weather service predicted that Vermont would get 1.5 to 2 inches of rain, but the area saw 1.5 to 2.5 inches, with some areas getting even more — up to 3.5 inches.
The rain was heavier than expected because it began off the Gulf coast of Florida, where warm ocean temperatures infused the air with moisture.
Warmer air has the capacity to carry more moisture, said Jonathan Winter, associate professor of geography at Dartmouth College, whose research focuses on extreme precipitation in the Northeast.
He thinks of the atmosphere like a bucket. The warmer the air, the bigger the bucket.
“When you have a larger bucket, when you get the right conditions, you can get a larger precipitation event,” he said, “because the atmosphere just has more fuel” — more water within a storm that is transferred to the landscape.
In Vermont this week, multiple areas of low atmospheric pressure areas also caused air to lift and add to the existing moisture, producing heavier rainfall.
Much of the state was already covered in snow, and predicting the extent to which snow will melt in a storm is difficult, Whittier said. Although meteorologists traverse the state to collect as much data as possible about the depth of the snowpack and the amount of moisture it’s holding, they’re generally relying on computer models for the big picture, “and sometimes they don’t always catch the texture of the snowpack or the consistency of the snowpack,” he said.
The storm was the fourth episode of heavy precipitation in four weeks, he said, which meant the snowpack was relatively moist. What’s more, forecasters expected temperatures would be in the 40-to-50-degree range, but they were even warmer — in the high 50s and low 60s in some spots.
The National Weather Service predicted that, when the snow melted, it would add another inch of water to the rain, but because of the warm weather, it contributed closer to 1.5 inches.
“So if you had an inch of rainfall more than expected, and a half inch or more of snowmelt than was expected, that’s an additional one and a half inches of rain, on top of two to three. So that will cause a little bit more issues,” Whittier said.
All of those factors together — rain, snowpack and warm temperatures — are a perfect recipe for quicker floods, Winter said.
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“Say you get like two inches from the sky, and you might also get two inches from the ground itself, like the snowpack itself, (then) you’re not really talking about a two-inch storm. You’re talking about a four-inch storm,” Winter said.
And a four-inch storm is much harder to handle than a two-inch storm, especially when the ground was already saturated from previous precipitation in the past few weeks.
“When we make our forecasts about flooding — you know, like flooding from rivers, like rivers coming up over their banks — there’s a lot of variables there. It becomes half art, half science,” Winter said.
Still, the state seemed ready, Whittier said. Officials deployed swiftwater teams, which were ready and able to assist with rescues. Business owners prepared for the storm with sandbags that many didn’t need to use. While schools had to close last-minute, everyone appeared to arrive home in time to be safe.
All told, if the same storm had shifted 50 miles to the west — where there were 4 to 6 inches of rain, and in some places, as much as 8 inches — Vermont could have seen July-level flooding along the stem of its main rivers, Whittier said.
“We were very fortunate,” he said. “We were always on that western edge.”
While scientists don’t typically tie a single event to climate change, Monday’s storm lines up with the types of storms that scientists expect to become more intense and frequent as climate change progresses, Winter said.
Lesley-Ann Dupigny-Giroux, a professor at the University of Vermont and Vermont’s state climatologist, said in an email that flooding “can occur in any season, winter, summer, fall and spring.”
“This week, it rained for almost 48 hours non-stop on 17-18 December,” she wrote. “This long duration event on unfrozen ground led to rivers rising to at/or above major flood stage.”
Vermont is also experiencing more frequent snowmelt events, said Whittier. While the melting season has started in February and lasted until April historically, Vermont has “seen a dramatic change in that due to climate change,” he said.
“We are witnessing snow melt situations happening even in December, like we just saw with this event,” he said. “We’ve had other events in December. We’ve had some in January.”
Winter said he could probably search the historical record and find a storm identical to this one, before human-caused greenhouse gas emissions entered the picture. This type of storm may not be new, but it’s likely to hit Vermont more often.
“It’s the frequency and intensity with which they happen,” he said, referring to the storm’s link to climate change. “I think it’s completely consistent with what we expect.”
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Category: WHY
