The formula for destruction: Warm, moist air plus cold, dry air plus wind equals tornadoes.
The view along Upper Hampden Road in Monson the day after a June 1 tornado ravaged sections of forest in this hilly, heavily wooded Springfield exurb.They were the kind of ominous weather conditions that meteorologists usually detect in the Plains states, not in New England.
But on June 1 at around 1 p.m., at the National Weather Service’s Storm Prediction Center in Norman, Okla., meteorologist Greg W. Carbin and his colleagues were concerned enough at what they were seeing on their computer screens that they did something rare: They issued a “tornado watch” for the Pioneer Valley and surrounding counties in Western Massachusetts.
And within hours of issuing that alert, three tornadoes -- the most destructive outbreak in more than half a century in the Bay State -- touched down, wreaking havoc throughout the region.
“That part of New England gets less than one tornado watch a year. I grew up in Vermont and never saw a tornado,” Carbin said.
“But, when I looked at the conditions that morning, they looked like a pretty classic case. The pattern suggested it was going to be a big day for New England,” he said.
A big day, indeed.
The principal tornado to hit that day would have been a monster wherever it landed. It had a path 39 miles long.
According to the Storm Prediction Center, from 1981 to 2010, the median path length of tornadoes -- half were longer, half were shorter -- in the United States was just a half-mile.
More than 1,200 tornadoes touch down in the U.S. annually, more than in any other country. Perhaps two or three materialize in Massachusetts each year, but they are nearly always weak, short-lived phenomena. By contrast, the state of Texas gets an average of 139 tornadoes a year.
Tornadoes need certain elements to form: a mass of cold air, a mass of warm, moist air and winds.
In the Plains states, warm, moist air from the Gulf of Mexico can move north in the spring, meeting cold, dry air coming down from Canada. Along the boundary of the two air masses, warm air, which is less dense than cold air, rises and the moisture in it condenses into rain, forming thunderstorms that are being acted on by the strong winds.
Like a whirlpool forming where bath water goes down the drain, the currents of rising warm air can begin to spin if there are winds aloft -- especially winds going in various directions, called “wind shear.”
Winds going in opposite directions can spin a column of air, just as your palms moving in opposite directions can spin a pencil between them. In the atmosphere, the vortex that’s created can become a tornado.
Michael Rawlins, a climatologist at the University of Massachusetts Climate Research Center in Amherst, said the conditions that spawn large tornadoes are more common in the Plains states than in the Northeast.
“In New England, the contrast between cold, dry air from the north and warm, moist air from the south is not as extreme compared to the Great Plains region,” he said.
Added Carbin, “The Atlantic is a great source of moisture for snowstorms and nor’easters because it’s cool. But, it is not a great source of warm, moist air.”
On June 1, however, all of the ingredients for a tornado made a rare appearance in the region. There was a flow into New England of especially warm, moist air from the Southwest. There was also a flow of cold, dry air into New England from the Northwest. And strong winds, to get the storm and rising air currents spinning, were available.
“You have to get a series of events come together all at once to create the situation we had. It was rare,” said abc40 meteorologist Ed Carroll, who was on duty the day the tornadoes ripped through Western Massachusetts.
“The warm, moist air at the surface was the biggest factor,” Carroll said. “But, there was also a lot of wind speed at the surface -- at 5,000 feet, 10,000 feet and 20,000 feet -- and the wind was screaming. It was very high velocity and going in different directions at every level.”
The volatile winds were also a major factor, according to Rawlins.
“Tornadoes are more likely to form if a thunderstorm is rotating. Strong wind shear -- winds of differing direction and speed with increasing height -- together with strong updrafts, tend to produce thunderstorms which rotate,” he said.
The final event that day that created conditions ripe for a tornado was the appearance of the sun early in the day, Carroll said.
“It started to cook everything at the surface, raising the temperature and causing the air to lift,” he said. “I knew we were in trouble.”
However, regardless of the severity of the conditions as they developed, it was almost impossible to say with any certainty that a tornado would form -- almost until it did form, according to Carbin.
“There needs to be a really fine balance of ingredients. If you are off just a little bit, you have a rain storm instead of a tornado,” he said.
Carroll agreed.
“I’ve seen tornado watches get issued and sometimes thunderstorms don’t even develop. The atmosphere is so fluid that almost anything can happen.”
“But on that day,” Carroll said, “the situation only got worse.”