Derecho: a unique little windstorm

Statement as of 3:55 PM EDT on August 4, 2006

... Derecho hit southern New England on Wednesday August 2 2006...

......snip......

A derecho is a long-lived... widespread... and rapidly moving convective windstorm that essentially is comprised of a family of downburst clusters. Downbursts can be either microbursts or macrobursts... depending on whether the damage swath is less than or greater than 2.5 miles. The result of this family of downburst clusters is widespread straight-line wind damage over a large area. The winds associated with derechos are not constant and may vary considerably along its path... sometimes being below severe limits (57 mph or less) and sometimes being very strong (greater than 100 mph in extreme derechos).

The word derecho was coined by Dr. Gustavus Hinrichs... a physics professor at the University of Iowa... in a paper published in the American meteorological journal in 1888. Dr. Hinrichs chose this terminology for thunderstorm-induced straight-line winds as opposed to the word tornado. Derecho is a Spanish word which can be defined as "direct" or "straight ahead" while tornado is thought by some... including Dr. Hinrichs... to have been derived from the Spanish word "tornar" which means "to turn."

The modern-day strict definition of a derecho requires that its length be greater than 240 miles... however the Storm Prediction Center acknowledges that true derechos can be somewhat shorter in length. The Wednesday storm originated in northwest Massachusetts in the slopes of the Berkshires and traveled southeastward... impacting much of Massachusetts south of the Mass Pike... northeast Connecticut... Rhode Island... and it finally exited the cape and islands. This was approximately 175 miles.

Trees and wires were reported down in Conway... Ashfield... and Charlemont in Franklin County Massachusetts shortly after 4 PM. Similar damage occurred in Hampshire and Hampden counties after that. There were reports of trees down on houses and some cars. Shortly after 5 PM... a very old roof was blown off of an apartment complex in Springfield... which made the 11 units on the fifth floor uninhabitable. It is estimated that straight line wind gusts of 70-80 mph had occurred there.

At approximately 515 PM... an am radio station tower was knocked over in Dudley Massachusetts in southern Worcester County. The tower was 240 feet tall. Based on conversations with the station engineer and the fact that 70 to 80 mph winds had reached ground level elsewhere... it is conceivable that wind speeds at the top of the tower approached 100 mph.

Damaging winds continued to cause trees and wires to be blown down in Granby... Enfield... Thompson... and Killingly Connecticut. Trees fell onto houses and cars in Medway and Foxboro Massachusetts. A very significant downburst occurred in Sharon... Stoughton... and Brockton Massachusetts. Large trees were snapped near their bases or uprooted entirely. Utility poles consisting of southern Yellow Pine were snapped 6 feet above ground level near routes 24 and 123 in Brockton.

Shortly after 7 PM... large tree limbs were downed in Portsmouth and Tiverton Rhode Island and wires were downed in Newport. In Warren Rhode Island... 12 students had to be rescued when their boats capsized. There were no injuries. The damage continued all the way to the southeast Massachusetts coast where trees and wires were downed in Dartmouth... New Bedford... and Fairhaven.

Cloud to ground lightning was extremely frequent and started several building fires as the storms crossed the region. In addition...
penny sized hail was reported with a few of the storms... in Holden
Massachusetts... Sharon Massachusetts... and Tiverton Rhode Island.

The aforementioned locations were chosen to illustrate the widespread nature of this event. There were several other towns not listed that reported trees... tree limbs... or wires downed.

The last derecho to affect southern New England was on July 15 1995... when a band of severe thunderstorms raced from the Upper Peninsula of Michigan at midnight to the coast of Massachusetts by 8 am. A wind gust to 93 mph was recorded at Otis Massachusetts in Berkshire County and severe weather was reported as far east as Worcester County before it finally began dissipating below severe thresholds.