By ANDY KIRKALDY
ADDISON COUNTY — An uncooperative and mysterious shift in the jet stream, the major west-to-east airflow across North America, lies behind this summer’s steady diet of wet weather, according to a National Weather Service meteorologist in Burlington.
The NWS’s Brooke Taber said that the jet stream, a high-speed upper atmosphere wind that he likened to a river of air flowing toward the East Coast, normally crosses well to the north of Vermont and New England at this time of the year.
But since early June the jet stream has sagged down across the northern U.S., where it soars between humid tropical air to the south and cooler air to the north.
The result, Taber said, is that Vermont and its neighbors are caught in a “battle zone” between the conflicting air masses that usually collide over central Canada.
The result Taber described probably goes without saying.
“This contrast of air masses has produced numerous showers and thundershowers across our area,” Taber said.
What has caused the summer without much sun is hard to say, he said, although experts have been able to rule out a couple of the usual suspects — changes in the surface of the Pacific Ocean.
“It’s a long wave pattern that has been very persistent across the U.S.,” Taber said. “It’s not El Niño or La Niña.”
When the pattern occurs, the showers and thundershowers follow; that much is known, Taber said. But why the pattern has persisted as well as why it started both remain unknown.
“That’s the question: Why has it hung in for several months now?” he said.
Although at times it may not seem like it, there was a point this spring it did not rain excessively: May saw just 1.94 inches of rain in Burlington, more than an inch below normal.
But then with June the jet stream lowered the boom on Vermont, Taber said.
“The pattern changed in the end of May and has continued in June, July and August,” he said.
In Burlington, 15.01 inches of rain had fallen between June 1 and Monday, 6.19 inches more than typical — that’s a 70 percent increase. Some areas, including Addison County, have been hit harder, although official weather service rainfall information is limited by a lack of local observers.
“There are parts of Addison County that have received up to seven inches of rain in one storm,” Taber said.
The data for the county that are available from the weather service through Aug. 11 are as follows:
• At one South Lincoln site, 15.56 inches of rain had fallen since June 1: 7.52 in June, 5.4 in July, and 2.64 in August.
• At one Salisbury site, 18.22 inches of rain had fallen since June 1: 6.2 in June, 6.48 in July, and 5.44 in August.
• At on Cornwall location, 8.53 inches fell in June.
Also, Taber offered data for Hanksville, in Huntington on the Starksboro border: 6.9 inches of rain fell in June, and 6.91 in August at one site.
And yes, about like Olympic swimmer Michael Phelps after the fourth turn in a 200-meter race, the rainfall in Vermont is on a record pace. Taber said the highest recorded annual rainfall in Burlington is 50.42 inches in 1998 (an average year sees about 36-40 inches of rain in Burlington and as much as 12 inches more in the Green Mountains). As of Monday, 28.74 inches had fallen in Burlington, with more in the forecast.
Still, on Tuesday, Taber offered some hope for the next six to 10 days.
“We do see the temperatures warming to near normal, and the threat of showers and thunderstorms decreasing,” he said.
But another branch of the weather service, the Climate Prediction Center, recommends that Vermonters should perhaps enjoy the next week while it lasts. The Washington, D.C., center predicts the jet stream will continue to plague the Northeast for the next month.
“It suggests temperatures will be near normal, and for precipitation it suggests above normal,” Taber said. “So that suggests what we’ve been in.”