Microbursts not micro inside the burst

Microbursts not micro inside the burst

By George Barnes TELEGRAM & GAZETTE STAFF
gbarnes@telegram.com

For those living in the path of a microburst, the name may seem understated.

Several communities in the past week saw some mega damage from microbursts that developed in thunderstorms on July 3 and July 7.

Kim Buttrick, forecaster for the National Weather Service, said storm damage in Phillipston and Templeton in Worcester County and Whatley and Sunderland in Western Massachusetts was consistent with a microburst.

"There were microbursts on the 3rd that the Skywarn folks investigated," she said.

On that day, the National Weather Service did not send its own team to investigate because staff was busy plotting Hurricane Arthur, which was also heading toward Massachusetts. Ms. Buttrick said the weather service was not able to measure the severity of the winds in those towns, but it appeared to be from a downburst, a larger-scale microburst.

As the hurricane made its way up from the south, the thunderstorms that brought with them microbursts came from the west and landed especially heavily in Phillipston on Highland Avenue, Ward Hill and Blake Corner Roads, uprooting and snapping trees. No tornadoes were reported, but winds of 80-plus miles per hour were estimated.

On July 7, a microburst was confirmed in Bedford, but significant damage also occurred in Ashburnham, Athol and parts of Fitchburg and Leominster.

Ms. Buttrick said microbursts build up in thunderstorms when warm, dry air at the surface is cooled by upper level air.

"Then it just drops all the way to the ground, spreading out," she said. "It's like pouring milk on the floor."

Predictability of microbursts and other major wind events is an evolving science. Ms. Buttrick said the advent of cellphones and increased numbers of weather spotters helps with predicting and assessment, but predictions are still a challenge.

Greg Carbin, warning coordination meterologist for the NOAA Storm Prediction Center, said lead time for predicting a tornado path when a tornado has been verified is about 15 minutes, but for microburst events that come from unorganized storm systems the lead time is close to zero.

"There's not a lot to go on there," Mr. Carbin said.

The storms may appear in and be on the ground for as little as 20 minutes. Microburst can develop in almost no time at all.

A June 1989 microburst hit downtown Fitchburg, killing one person and causing extensive damage. It took down many trees and two steeples, destroyed the third floor of a factory building and caused extensive damage to a senior citizen apartment building. Winds were estimated well over 100 mph. There was little or no warning.

The storm hit at just after 3 p.m. and turned a perfectly clear day as dark as night. Wind and wind pressure were so strong, windshields shattered on cars

Fitchburg City Councilor Jeffrey A. Bean was mayor at the time and vividly recalls the storm.

"It's some of those things you don't forget," he said.

Mr. Bean was at home when it hit. His first inkling of severe weather was when wind picked up so strong that a pop-up camper set up in his yard was blown like a tumbleweed into another yard. Realizing there was a major problem and at the urging of his wife, he said, he then headed into his office in downtown Fitchburg and began gathering sporadic reports.

"You didn't know what was happening," he said.

Mr. Bean and city emergency workers struggled with lack of communications. He said eventually the National Guard was called in to help. Two days later, Gov. Michael S. Dukakis landed on Crocker Field to assess the damage. The field was in the middle of the worst of the storm and many large trees that lined the field had been knocked down.

There are several types of damaging wind storms, including tornadoes, downbursts, microbursts, gust fronts, derechos and haboobs.

A downburst is a strong downdraft resulting in a outward burst of damaging winds on or near the ground. They are generally more than 2.5 miles across. A microburst is a smaller version of the downburst that is less than 2.5 miles across. They generally last only five to 10 minutes. There are wet and dry microbursts. Generally in New England, microbursts are associated with heavy precipitation at the surface. In the west, especially in the high plains and mountains, the winds develop, but the precipitation never hits the surface.

Gust fronts occur when the leading edge of rain-cooled air clashes with thunderstorms and creates gusty wind ahead of the storm. Derechos are widespread, long-lived wind storms associated with a rapidly moving band of thunderstorms. The damage swath extends more than 240 miles and includes wind gusts of 58 mph or more along most of its length. Typically a derecho contains multiple microbursts, downbursts and downburst clusters.

A haboob is a wall of dust pushed along the ground at high speeds by a thunderstorm downdraft. On Monday, when severe weather was moving across Massachusetts, a haboob struck Phoenix, Ariz.

Microbursts have been responsible for sinking ships, causing wind shear that has brought down airliners, and wreaking significant damage to houses and other property.

Contact George Barnes at george.barnes@telegram.com. Follow him on Twitter @georgebarnesTG