Public Information Statement
Issued by NWS Boston, MA

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NOUS41 KBOX 051259

Public Information Statement
National Weather Service Taunton MA
859 AM EDT Fri May 5 2017


The National Weather Service has declared the week of May 1 to
May 5 as Severe Weather Preparedness Week in southern New
England. This is the final in a series of five public information
statements on various topics related to severe weather awareness.

While not as notorious, or perhaps as spectacular to witness as
a tornado, straight-line winds are responsible for most
thunderstorm wind damage, especially across southern New

A downburst is a strong, relatively small, area of rapidly
descending air beneath a thunderstorm. It can result from
stronger winds aloft being transported downward to the surface,
or it can result as air within the downburst is cooled
significantly as rain evaporates into initially drier air. This
cool, thus dense, air sinks rapidly to the surface. A downburst
is differentiated from common thunderstorm winds because the
downburst winds have the potential to cause damage near the
ground. Surface damage patterns have shown that whether the
winds are straight or even a little bit curved, they tend to
spread out, or diverge, considerably as they reach the surface.
Conversely, damage patterns resulting from a tornado generally
converge toward a narrow central track.

Intense downbursts can be phenomenal. Speeds have been clocked as
high as 175 mph near Morehead City North Carolina and at 158 mph
at Andrews Air Force Base in Maryland. Closer to home, 104 mph
downburst winds were measured at both Worcester Massachusetts on
May 31 1998 and Whitman Massachusetts on May 21 1996. Strong
downbursts will definitely cause roaring sounds and people may
often refer to a sound like a freight train, terms typically
associated with tornadoes. Although downbursts are not
tornadoes, they can cause damage equivalent to that of a small
to medium tornado. After all, wind is wind.

Downbursts are classified as either macrobursts or microbursts,
depending on the areal extent of the damaging wind swath. A
macroburst`s damage extends horizontally for more than 2.5 miles.
A microburst is a small downburst with its damaging winds
extending 2.5 miles or less. The small horizontal scale and
short time span of a microburst makes it particularly hazardous
to aviation.

The National Weather Service issues Severe Thunderstorm Warnings
for thunderstorms that are expected to produce damaging wind
gusts of 58 mph or greater, or hail that is one inch or greater
in diameter.


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