In south-central Vermont, especially around Rutland, a severe windstorm swept through last Friday, knocking down trees and leaving many without electricity.
The storm swept through quickly, and by Saturday it was all about cleaning up. That got us wondering whether this sort of weather event is an isolated incident or if there's something about the Rutland area that makes it more susceptible to damaging wind events.
VPR spoke with Scott Whittier from the National Weather Service to learn more.
This transcript has been edited for clarity and brevity. Listen to the full audio above.
VPR: What caused this windstorm?
Whittier: “The western slopes of the Green Mountains are prone to what we call downslope winds, what we call an inversion.
“In a normal standard atmosphere, temperatures decrease with height at about 5 degrees per 1,000 feet. And a stable or inversion atmosphere, you're actually increasing temperatures with height. It's kind of like putting a lid to a jar. So everything in that atmosphere that may flow from the mountains or down south, does not have any way to penetrate upward. It has to go down.
“So if you have an east or southeast wind along the western slopes of the mountains with the momentum of gravity and everything, you get these winds. But it's maybe every one or two years that you get something that will be 70 miles an hour and cause wind damage.”
You’re talking about the western slopes of the Greens. Does this make the Rutland area more susceptible to these kind of wind events?
“Yeah, it does. The Green Mountains are largely a south-to-north oriented mountain range, and if you end up having a very strong area of low pressure to your west or to your south and west, you could see strong easterly [winds] coming in off at the Atlantic.
"The winds will come at anywhere between 3,000 feet ... and 6,000 feet, travel over the mountains, and as they do they lift over the mountains, form what they call a 'mountain wave,' and then that mountain wave is kind of like an ocean wave, where it wants to crash.
"How far the winds crash down to the ground all depends on a certain set of conditions. The mountain wave has to be — or the inversion needs to be — at an exact certain height in order to get the best attenuation, and then the winds need to be at the right origin."
How is the National Weather Service going to follow up on this kind of event? What is the sort of thing you guys do after something like this happens?
"We're already doing it. Once we witnessed when reports, we were like, ‘OK, what's causing this?’ I mean, there were no thunderstorms in the area, so we can't put anything to thunderstorms. The question for us to reexamine is, what caused those winds to translate down in this case, where in other cases very similar [to it], it did not?
“So, we are looking at different aspects of it. One thing is it was at the back end of the rainfall, not during the rainfall. It was as soon as the rain went out. Is that enough to cause it? It shouldn't.
“But we looked at other features, and what was a unique feature that was witnessed about four o'clock, about an hour beforehand in the Berkshires, it wasn't noticed until after the fact, is people's home barometers, you would see a trace that was pretty much flatlined and then all of a sudden it took a dip down and a sudden dip back up. And that's indicative of some kind of pressure perturbation or a gravity wave ...
"Take an example of gravity wave: [It] would be the same thing as if you had an earthquake under the ocean. That little earthquake or shudder brings about some sort of wave, and it's not until you get to the shore where that wave all of a sudden crashes.
"So you had the atmosphere that already had these gravity or mountain waves taking place, and then that was staying at 2,000 or 3,000 feet. And then all of a sudden you get this perturbation that comes on, captures that higher wind and translates it down to the ground.
"And it's just now, trying to find out what's the best way to see those. To see [if that] is an outlying event, or is this actually real? And that's going to be our challenge in future."
If somebody happens to be outside during one of these kinds of events, what's the best approach for them to take for safety?
“When it comes to a windstorm like this ... the best solution is to go inside. If you know about these storms ahead of time or if did have a high wind warnings in effect, which we didn't ... one of the things we would tell people is, loose furniture or objects — your deck chairs, your umbrellas, your gas grills, garbage pail — bring them inside.
“What you can do preventively — maybe months if not years — is, if you've got an old dying maple or an oak that you see the roots are shallowed up, maybe you go to your home insurance and say, ‘Hey, I want to do a preventative measure, I think in a big windstorm this may fall on my house. Can the insurance help pay for me to remove this?' So that you're not paying tens, twenties of thousands of dollars for damage.
“So there's a lot of things you can actually do well ahead of things in your own property to help prevent some significant damage.”