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2013 Severe Weather Awareness Week in Maine

April 29 through May 3 is Severe Weather Awareness Week in New England. The week has been divided into specialized topics each day. These topics appear on this page, for your convenience.

Follow these links for more weather safety and educational material. A variety of brochures can be found here. For example, their color brochure, titled Thunderstorms, Tornadoes, Lightning—Nature's Most Violent Storms, includes the topic "Tornado Safety Information for Schools." There are many more valuable goodies on that page, including info tailored for kids and families. Please check it out!

Table of Contents

Severe Weather Terminology

With severe weather season rapidly approaching, it is a good idea to refresh your memory in regard to severe weather terminology. In Maine, severe weather season generally runs from May through August. In northern Maine, mid-June to around August 1 is the most active period for severe local storms.

The term watch, when used in conjunction with tornado or severe thunderstorms, means that tornadoes or severe thunderstorms are possible. Watches are usually in effect for several hours and indicate that atmospheric conditions are favorable for severe storms to develop. When a watch is in effect, you should keep your eye to the sky and make plans for what you need to do if severe weather occurs.

The term warning means that either a severe thunderstorm or tornado is imminent or is already occurring. Warnings are usually in effect for 1 hour or less. If a warning is issued for your area, take action immediately.

A severe thunderstorm is any thunderstorm that produces winds of 58 mph or more and/or hail of 1inch in diameter or greater. Severe thunderstorms can produce damaging straight line winds that affect a relatively large area. These are usually associated withy rapidly moving squall lines. Severe thunderstorms can also produce Microbursts, which are damaging straight line winds that affect a relatively small area. In both cases, winds can exceed 100 mph.

A tornado is a violently rotating column of air in contact with the ground. The funnel usually descends from the base of a severe thunderstorm. They are usually wedge shaped with the narrowest end at the ground. Tornado witnesses have heard a "roaring" sound similar to a freight train. Tornadoes vary in intensity. Weak tornadoes have winds less than 100 mph while violent tornadoes have winds in excess of 200 mph.

A funnel cloud is a violently rotating column of air that descends from the base of a thunderstorm but does not make contact with the ground. A tornado usually passes through the funnel cloud stage during its development and dissipation. Not all funnel clouds become tornadoes, but you should still take cover if one approaches.

Thunderstorms and Lightning


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. 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 falling trees and large hail, lightning also poses a significant threat to people, as well.


There are three basic ingredients needed for the formation of a thunderstorm. They include low-level moisture, an unstable atmosphere and a trigger (a source of lift).

Low-level Moisture:
This moisture is needed for cloud forma­tion, growth and the development of precipitation within the cloud.

Unstable Atmosphere:
An unstable atmosphere allows warm, moist air near the ground to rise rapidly to higher levels in the atmosphere where tempera­tures are below freezing. An unstable atmosphere also allows air at higher levels in the atmosphere to sink to the ground level rapidly, bringing stronger winds from the higher levels to the ground.

A Trigger:
Something to set the atmosphere in motion.

All three ingredients contribute to the formation of a thunder­storm. In fact, as the magnitudes of these ingredients increase, so do the chances that a thunderstorms could become severe. 

In the summertime, listen to the latest forecast and learn to recognize the signs which often precede thunderstorm development. Warm muggy air is a sign that ample low-level moisture is available for thunder­storm development. Towering cumulus clouds indicate an atmosphere that is, or is becoming, unstable. And, the trigger could be continued heating from the sun; an approaching front or sea breeze front; or a cooling of the upper atmosphere.

All thunderstorms go through various stages of growth and develop­ment. As a thunderstorm cloud continues to grow, snow and ice begin to form in the middle and higher levels of the cloud where temperatures are below freezing, and electrical charges start to build up within the cloud. Negative electrical charges near the middle of the thunderstorm cloud cause a positive charge to build up on the ground under and near the thunderstorm. Finally, when the difference between these charges becomes too great, a giant atmospheric spark we call lightning occurs.

Lightning is an underrated killer, usually claiming its victims one at a time. Lightning also leaves many victims with life-long serious injuries. Lightning can strike as far as 10 miles from the side of the thunder­storm cloud. In fact, many lightning victims are struck before the rain arrives or after the rain has ended. Many victims also report that at least a portion of the sky was blue when they were struck.

Although Maine has less lightning than most states east of the Rocky Mountains, Maine ranks 8th highest in the country in terms of lightning casualties (per capita).

This summer, the National Weather Service will conduct a nationwide awareness campaign to reduce the number of deaths and injuries from lightning. Although more information on lightning and lightning safety will be provided during Lightning Safety Awareness Week which will be during the week of June 23-29, here are some basic tips to help keep you and your family safe this summer.

  1.Avoid any contact with corded phones.
  2.Avoid any contact with electrical or electronic equipment or cords that are plugged into the electrical system.
  3.Avoid any contact with the plumbing system. Do not wash your hands, do not wash the dishes, do not take a shower, or do not do laundry.
  4.Do not stand next to a concrete wall and do not lie on a concrete floor.
  5.Stay away from windows, outside doorways, and porches.

1. There is no safe place outside in a thunderstorm. To be safe,
you must get inside a substantial building or hard-topped metal
2. Plan outside activities so that you minimize the risk of being caught outside in a thunderstorm.
3. If you hear thunder, move inside a safe shelter immedi­ately.  Generally, if you can hear the thunder, you're within striking distance of the storm.
4. If the sky looks threatening, move inside immediately. Don't wait for the first flash of lightning. It could occur anywhere under or near the storm.
5. Stay inside a safe shelter for at least 30 minutes after the last rumble of thunder was heard. Many lightning victims are struck after the worst part of the storm has passed.

Remember, when it comes to thunderstorm safety, it's your own actions that will determine your personal risk of being killed or seriously injured by the hazards that accompany thunderstorms.


Severe Thunderstorms - Wind 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. 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. 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 well.


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, 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. 

During the summer of 2011, Maine had numerous thunderstorms that produced damaging straight-line winds. Tragically, one person was killed when a tree fell on their car. Falling trees caused by thunderstorm winds were also responsible for two deaths in the area in 2006. The first death occurred in Fryeburg, Maine on June 19 when a tree fell on a tent in which people were camping. The second occurred in Waterboro, Maine on September 9 when the top of a tree fell on a vehicle killing the driver.


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 ground.

In Maine, 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.

Here are some of the larger hailstones reported in northern New England since 1995.

3.00 Inches
Jun 19 1995  Springfield, Maine

2.75 Inches
Jun 01 2011  Kingfield, Maine

2.50 Inches
Aug 04 2007  Ft. Kent, Maine
May 27 2011  Bingham, Maine

2.00 Inches
Aug 18 1996  Lincolnville, Maine
Aug  4 2007  Grand Isle, Maine
Aug  4 2007  Caribou, Maine
Aug 30 2007  Rome, Maine
Jun  5 2010  South Paris, 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 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.



Tornadoes are nature's most violent storm. By definition, a tornado is a violently rotating column of air that extends from the base of the thunderstorm cloud to the ground.

In addition to the three basic ingredients needed for the formation of thunderstorms and severe thunderstorms (low-level moisture, an unstable atmosphere, and a source of lift), winds at various levels in the atmosphere factor into the development of tornadoes.

Usually, prior to the development of a tornado, a pre-tornadic thunderstorm develops a circulation, that is, it starts rotating (a meso-cyclone). As this rotation becomes stronger, the chance that a tornado may develop also increases. Although the National Weather Service's Doppler RADAR generally can not see the actual tornado, the RADAR does detect rotation of the thunderstorm cloud, and thereby gives some indication of the possibility that a tornado may be forming or has formed.

The scale used to measure tornado damage is the Enhanced Fujita scale (named after Theodore Fujita, a famous tornado damage expert). This scale is commonly referred to as the E-F scale. Based on scientific studies of tornado damage, the original Fujita scale was modified and the new "Enhanced Fujita Scale" was officially implemented in 2007.

EF-0 - Light damage (winds 65 to 85 mph)
EF-1 - Moderate damage (winds 86 to 110 mph)
EF-2 - Considerable damage (winds 111 to 135 mph)
EF-3 - Severe damage (winds 136 to 165 mph)
EF-4 - Devastating damage (winds 166 to 200 mph)

Peak tornado activity in northern New England occurs between June and August, but tornadoes have occurred as early as May and as late as November (as was seen in 2005). Most tornadoes occur between 3 and 9 pm and have an average forward speed of about 30 mph. For the 40 year period between 1950 and 1990, 74 tornadoes occurred in Maine. Based on this, Maine averages about two tornadoes per year. During this period, the average path length of the tornadoes was 1.08 miles. The strongest tornado observed in Maine was an F2.

To alert the public to the threat of tornadoes, the National Weather Service issues TORNADO WATCHES and WARNINGS. A TORNADO WATCH indicates that atmospheric conditions are favorable for the development of tornadoes. A TORNADO WARNING indicates that a tornado is imminent or is already occurring. If you hear that a TORNADO WARNING has been issued for your area, seek safe shelter immediately if you are in the path of the storm.

Due to the usual short life-span of tornadoes in northern New England, there is often little, if any, advance warning. Tornadoes in New England generally touch down and then lift off the ground very quickly. Many of the tornadoes that have occurred in the past, have occurred while severe thunderstorm warnings have been in effect. If you hear that a severe thunderstorm warning is in effect for your area, be alert for the possibility of a tornado. A low rotating cloud, large hail,
and/or a load roar are all signs that may precede the touchdown of a tornado.

Here are some tornado facts and safety tips.

* Flying debris causes most deaths and injuries in tornadoes.
* The safest place in your home during a tornado is your basement.
* Stay away from windows.
* Get out of vehicles or mobile homes, they offer little protection.
* Seek shelter in a substantial building.
* Do NOT seek shelter under a bridge overpass. Bridge overpasses offer little, if any, protection from wind-driven debris.


Flood / Flash Flood Terminology

Flooding has overtaken lightning nationally in weather related fatalities. Heavy rain can turn a usually tranquil stream into a life threatening torrent.

General river flooding occurs most often in the winter months, as heavy rain combines with snow melt; however, flash flooding can also occur in these months due to ice jams on rivers and streams.

Flash floods are most common in the warm season of the year when thunderstorms drop large amounts of rain in a short period of time. Hilly and mountainous terrains are especially prone to flash floods.

A flood watch is issued when flooding and/or flash flooding is possible but not imminent. Watches are can be issued up to 36 hours in advance of a flood/flash flood event.

A flash flood warning is issued when imminent flooding requires immediate action to protect lives and property. The flooding may arise from events such as short-term excessive rainfall, the sudden release of water held by an ice jam, or a dam failure. Your immediate action is required if you are in the path of the waters.

A flood warning is issued for actual flood situations not covered by the flash flood warning. The urgency for protection of life and property is lessened only by the longer term of the warning. Your action is still required; flooding is imminent.

All three can be issued for counties, parts of counties, river basins, rivers or specific points on rivers, depending on how widespread the flooding is expected to be. The two warnings can also cover general small stream or urban area flooding.

Often times flooding develops quickly, so it is important to know where you need to go to find safety. Sometimes only minutes or seconds are available to get out of harm's way in a flash flood situation.


Home - NWS Forecast Office - Caribou, Maine

National Weather Service
Caribou Weather Forecast Office
810 Main St
Caribou ME 04736
(207) 492-0170
Page last modified: April 23, 2007
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