Public Information Statement
Issued by NWS Portland, ME

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NOUS41 KGYX 022112

512 PM EDT WED MAY 2 2018

The National Weather Service has declared the week of April 30
through May 4th, SEVERE WEATHER AWARENESS WEEK in New England.  This
is the third in a series of five Public Information Statements on
various topics related to severe weather awareness.


By definition, a severe thunderstorm is one which produces
wind gusts of 58 mph or more, or hail 1 inch in diameter or
greater. Severe thunderstorms can also produce tornadoes.

In New England, severe thunderstorms are not uncommon
during the summer.  Every year, the National Weather
Service gets numerous reports of wind and hail damage
throughout Maine and New Hampshire.  Severe thunderstorm
winds down trees and branches onto homes, buildings,
vehicles, and power lines.  Scattered power outages are
often the result of lightning or wind-fallen trees and
branches.  Also, wind-driven hail from thunderstorms
flattens and/or damages crops in the states.  On rare
occasions, large hailstones damage homes, buildings, and
vehicles.  In addition to the lightning, falling trees and
large hail also pose a significant threat to people, as


During the development of a thunderstorm, warm air rises
upward in the atmosphere (an UPDRAFT) causing the formation
of clouds and precipitation.  As a thunderstorm matures,
cool, precipitation-laden air sinks downward through the
atmosphere (a DOWNDRAFT).  When a downdraft reaches the
ground it spreads out causing the cool, gusty wind that
often accompanies a thunderstorm.

In some thunderstorms, intense downdrafts develop.  When
these downdrafts reach the ground, they spread out very
quickly causing strong and often damaging winds at the
ground.  These intense downdrafts are called DOWNBURSTS and
can cause significant wind damage over large areas.  In the
case of downbursts, the damage is generally referred to as
straight-line wind damage since fallen trees generally line
up in the same direction.  In Maine and New Hampshire, most
thunderstorm wind damage is caused by downbursts.

A special type of downburst is the MICROBURST.  Microbursts
get their name because they generally affect a much smaller
geographical area, but the winds in a microburst can be
very intense.  Like the general downburst, most of the
damage with microbursts lines up in one direction,
although, there may be a tendency for the damage to radiate
outward.  Microbursts are usually accompanied by heavy rain
and/or hail and can have winds as strong as those in a
small tornado.

Under certain atmospheric conditions, thunderstorms can
begin to develop a circulation within the thunderstorm
cloud.  These storms are often called MESO-CYCLONES because
of the counter-clockwise circulation that develops within
the storm.  The updrafts and downdrafts in these storms can
persist for hours as the storm moves along its path.
Severe winds and hail are also more likely with meso-
cyclones, and if the rotation within the storm becomes more
intense, there is an increasing possibility that the storm
might produce a tornado.  National Weather Service Doppler
RADAR allows meteorologists to monitor air movement within
these storms and to see the development and strength of any
circulation within the storm.


The circulation that accompanies a meso-cyclone is also a
factor in HAIL formation.  Hail initially forms when liquid
water droplets are carried upward by the updraft to a level
where the droplets freeze.  Eventually, the small hail
stone may begin to fall downward, only to be caught once
again by the persistent updraft of a meso-cyclone.  Each
time the hailstone goes through this process, it gets
larger and heavier.  Eventually, the hailstone will be
blown away from the updraft or will become too heavy for
the updraft to support and the hailstone will fall to the

In Maine and New Hampshire, hail is fairly common during
well-developed summertime thunderstorms.  Although most
hail that reaches the ground in northern New England is an
inch or less in diameter, occasionally hailstones over 2
inches in diameter will fall.  Large hailstones can fall at
speeds faster than 100 mph and can do considerable damage
to cars, homes, and buildings, and can be a significant
threat to people, as well.

During 2017, the National Weather Service received 11
reports of severe hail (1 inch or greater in diameter) in
Maine and 15 reports in New Hampshire. In Maine, the most
significant hail reports were 1.50 inches in Guerette on
July 21st and near Augustine Pond on August 3rd.  In New
Hampshire, the most significant reports were 1.75 inches
near Silver Lake (Carroll County) on May 31st, in Center
Conway on July 1st, and in Effingham Falls on August 2nd.

Here are some of the larger hailstones reported in Maine
and New Hampshire since 1995.

3.00 Inches
     Jun 19 1995  Springfield, Maine
     Jun 22 2008  Mt. Cardigan, New Hampshire

2.75 Inches
     Aug 24 1998  Tamworth/Freedom, New Hampshire
     Jun  1 2011  Kingfield, Maine
     Jul 19 2015  North Berwick

2.50 Inches
     Aug  4 2007  Ft. Kent, Maine
     May 27 2011  Bingham, Maine
     Jul 19 2015  Rochester, New Hampshire
     Aug  4 2015  Waterboro, Maine

2.00 Inches
     Aug 18 1996  Lincolnville, Maine
     Jul 11 2006  Exeter, New Hampshire
     Jul 11 2006  Hampton Falls, New Hampshire
     Jul  9 2007  Concord, New Hampshire
     Aug  4 2007  Grand Isle, Maine
     Aug  4 2007  Caribou, Maine
     Aug 30 2007  Rome, Maine
     Jul 18 2008  Strafford, New Hampshire
     Jun  5 2010  South Paris, Maine
     May 29 2012  Westmoreland, New Hampshire
     Jun 29 2013  Andover, Maine
     Sep 11 2013  Windham, Maine
     Sep 12 2013  Upton, Maine
     Jul  3 2014  Statham, New Hampshire
     Aug  4 2015  York Harbor, Maine

For both severe winds (58 mph or greater) and large hail (1
inch or greater in diameter), the National Weather Service
indicates that the atmospheric conditions are favorable for
severe weather to develop.  A WARNING indicates that severe
weather is imminent or is already occurring.  If you hear a
WARNING for your area, be prepared to seek a safe shelter
if you are in the path of the storm.

Here is a list of the other topics that will be covered in
Public Information Statements issued by the National
Weather Service this week.

Friday......Flash Floods


National Weather Service
Gray, Maine

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