Public Information Statement
Issued by NWS Boston, MA

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Public Information Statement
National Weather Service Boston/Norton MA
805 AM EDT Wed May 2 2018


The National Weather Service (NWS) Boston, MA has declared April
30 through May 4 as Severe Weather Preparedness Week. Each day
this week we will highlight a different preparedness topic.

While hail and straight-line winds generally do not garner the
same attention or respect as tornadoes, they can be just as
deadly! Hail can exceed the size of softballs and fall at speeds
of over 100 mph, seriously injuring or killing anyone in its path.
Straight-line winds can topple trees onto cars, houses, and power
lines. Many deaths from straight-line winds are the result of
trees falling onto the person, whether they are outside, in their
house, or driving in their car. Strong straight-line wind events
can even destroy buildings, especially mobile homes and
manufactured homes.

When damaging straight-line thunderstorm winds or large hail is
expected, the National Weather Service will issue a Severe
Thunderstorm Warning. When a Severe Thunderstorm Warning is issued
for your area, or when threatening thunderstorms approach your
area, you should seek shelter immediately! To stay safe during
high winds, the same safety rules that are used for tornadoes also
apply during straight-line wind events, namely, you should seek
shelter in an interior room on the lowest floor of a sturdy
building or shelter, get away from windows, and get down low to
protect yourself from possible flying debris and falling trees.
During large hail situations, you should move indoors and stay
away from windows. Wind-blown hail can shatter windows. If you are
driving during a large hail episode, pull over into a parking lot
or gas station and use blankets or coats to cover yourself in
case the windshield shatters and hail enters the vehicle.

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 England.

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

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

Be sure to take some time this week to learn more about severe
weather safety. Learning and practicing severe weather safety
when the weather is good will allow you to react more quickly when
the weather turns bad.


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