Severe Weather Awareness Week - Wednesday Statement

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE GRAY ME
800 AM EDT WED MAY 1 2019

The National Weather Service has declared the week of April
29th through May 3rd...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.

...SEVERE THUNDERSTORMS - DOWNBURSTS, MICROBURSTS, MESO-
CYCLONES, AND HAIL...

By definition, a severe thunderstorm is one which produces
wind gusts of 58 mph or more, or hail 1 inch in diameter or
greater. However, 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. 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 well.

THUNDERSTORM WINDS, DOWNDRAFTS, DOWNBURSTS, AND MICROBURSTS

During the early stages of 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 stronger thunderstorms, intense downdrafts often
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. Those circulations are called MESOCYCLONES because
the circulation occurs within the thunderstorm and is
classified as being on the meteorological mesoscale. 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 mesocyclones, 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 the velocity within these storms
and to see the development and strength of any circulation
within the storm.  

HAIL

The circulation that accompanies a mesocyclone 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 mesocyclone. 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 it, and the hailstone will then fall to the ground.

In Maine and New Hampshire, hail is fairly common during
well-developed 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.

In 2018, the National Weather Service received 8 reports of
severe hail (1 inch or greater in diameter) in Maine and 6
reports in New Hampshire. In Maine, the most significant
hail report was 1.75 inches near Telos Lake on August 29th.
In New Hampshire, the most significant report was 1.75
inches near Canterbury on July 10th.
 
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
issues SEVERE THUNDERSTORM WATCHES and WARNINGS. A WATCH
indicates that atmospheric conditions are favorable for
severe weather to develop. A WARNING indicates that severe
weather is imminent or is already occurring. If a WARNING
is issued 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.

Thursday....Tornadoes
Friday......Flash Floods

$$

Watson
National Weather Service
Gray, Maine

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