Weather service says clash of air masses produced Arlington microburst, but no tornado

By Melissa M. Werthmann, Globe Correspondent

A clash between two air masses Wednesday in Massachusetts generated the violent thunderstorms that pounded Eastern Massachusetts and produced a microburst in Arlington. But the storms did not spawn a tornado, a National Weather Service meteorologist said Thursday.

Charlie Foley said the service issued a tornado warning at 2:04 p.m. Wednesday for parts of Essex and Suffolk counties after radar equipment picked up rotation in the clouds. But no tornado touched down, and the warning was called off after 30 minutes.

The service also fielded reports of a funnel cloud over Peabody, but Foley could not confirm that one actually occurred.

“We didn’t have anything on radar to back that up,” he said.

Foley said ominous-looking, black cloud formations, known as “wall clouds” are often confused for funnel clouds.

Meanwhile, in Arlington, a weather service crew met with the fire chief and confirmed that a microburst hit the area, ripping down trees and knocking out power in dozens of homes.

A microburst is a strong rush of wind downward, which spreads out as it reaches the ground, said meteorologist Rebecca Gould.

“It’s like throwing a rock in a pond,” she said. Once the rock touches the pond, water ripples outward. When a microburst touches the ground, strong gusts of winds radiate outward, drastically changing wind direction and speed. Microbursts typically affect an area of 1 to 2½ square miles and last less than five minutes, she said.

The crew was able to confirm a microburst by using radar information, analyzing how trees fell, and checking the size of the damaged area.

The weather service estimated the microburst winds at 70 to 80 miles per hour and reported that the path length was 2 miles long. More than 100 trees were knocked down during the storm in the area.

“The wind was blowing so hard. All I saw was swirling masses of debris in the air, I thought it was a tornado. I told all my kids to hit the floor and it came out of nowhere, lasted about 30 seconds and it was gone and everything just dropped. It was violent, very violent,” said Arlington resident Julie Somlyody.

The dangerous weather that hit the region Wednesday was a result of a cold front that began moving into New England in the beginning of the week, Foley said.

By the time it reached Southern New England, the cold front became the boundary between two air masses: one, a warm, humid, tropical air mass; the other, a cool, dry, Canadian air mass.

The clash between the two caused the heavy rain, frequent lightning, and widespread hail, he said.

Foley said thunderstorms started moving through Massachusetts at midday Wednesday and lasted until 8 p.m. on the South Shore.

“It was a fairly slow-moving frontal system,” he said. “It didn’t zip right through.”

The slow speed caused heavy rain and most of the precipitation fell on the eastern part of the region, Foley said.

He said there was a very defined line, right along Route 128, that determined where rain fell.

The storm dumped 1.8 inches of rain on Boston, while sprinkling only .03 inches on Worcester.

Thunderstorms clustered “pretty much along the coastline,” he said, while in the western part of the state, “the activity wasn’t as intense.”

Melissa Werthmann can be reached at melissa.werthmann@globe.com.