National Weather Service United States Department of Commerce
Severe Weather Preparedness Week in Southern New England: May 1 - May 5, 2017
For more information, please visit the Weather-Ready Nation Page and NWS Tornado Safety Tips Page
The National Weather Service in Taunton will feature a different Severe Weather related topic each day during the Awareness Week.
Definition of a Severe Thunderstorm
 

A 'Severe Thunderstorm' is defined as a thunderstorm that produces wind gusts of at least 58 mph and/or hail 1.00 inches in diameter or larger, the size of a quarter. Severe thunderstorms can and occasionally do spawn tornadoes.

A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma for large portions of the region when the potential exists for severe thunderstorms. A Severe Thunderstorm Warning is issued by the local National Weather Service Forecast Office, such as in Taunton, when severe thunderstorms are imminent based on radar or already occurring based on spotter observations.

 

Note that torrential downpours of rain that cause flooding are not part of the definition of 'severe weather.' They would prompt the issuance of Flood or Flash Flood Warnings, but not Severe Thunderstorm Warnings. It is important to note that frequent lightning also is NOT a criterion for what is termed 'severe weather.' Of course lightning can be extremely dangerous, but every thunderstorm has lightning. that is what causes the thunder. It is not practical to issue a warning for every thunderstorm, thus we issue Severe Thunderstorm Warnings for those storms that could produce large hail and damaging winds.

NOAA Weather Radios, with warning alarm tones, will alert you when a Severe Thunderstorm Warning is issued. However, they will not sound an alarm for non-severe thunderstorms, which still can produce deadly lightning. We recommend that lifeguards at beaches and pools have hand-held lightning detectors. The same is true for athletic coaches, camp directors, and parks and recreation workers. Even without equipment, you can protect yourself by moving indoors to a place of safety at the first rumble of thunder. If you can hear the thunder, the storm is usually close enough for you to have the potential to be struck by lightning.

Thunderstorms and Lightning
 

Summertime is a good time for outdoor recreational activities in New England. It is also the time of the year when thunderstorms are most likely. Thunderstorms can be beautiful, but they also can be deadly. While many people think they are aware of the dangers of thunderstorms and lightning, the vast majority are not.

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 Thunderstorm 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 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 and base of the cloud cause a positive charge to build up on the ground under and near the thunderstorm. Finally, when the difference between these charges becomes to great, a giant atmospheric spark that 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 and the storm is moving away. Most victims also report that at least a portion of the sky was blue when they were struck.

While inside a home or building

  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.
Tips while outdoors
  1. Plan outside activities so that you minimize the risk of being caught outside in a thunderstorm.
  2. 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.
  3. If the sky looks threatening, move inside immediately. Don't wait for the first stroke of lightning. It could occur anywhere under or near the storm.
  4. 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.
  5. If you are caught outside in a thunderstorm and can't reach a safe shelter, you can only minimize your risk of being struck by lightning. If lightning strikes near you, it will most likely strike the tallest object in your immediate vicinity. First, don't be the tallest object in the immediate vicinity and don't be near the tallest object. Second, get as low as possible to the ground, but minimize your contact with the ground. Do not lie on the ground.

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 of a thunderstorm.

 

 
Being Aware and Being Prepared

It is important to be aware and prepared when there is severe weather in the forecast. Make sure you know the 6 ways to receive a weather warning so that you can ensure that you and your family are always prepared. The first way to receive warnings is through the NOAA Weather Radio...this will alert you whenever a warning is put out for your area and keep you updated on our recent forecasts. A more common way to receive warnings and updates is through Local TV stations or from your friends, family, and coworkers. If you own a smartphone, make sure that your Wireless Emergency Alerts are enabled so that when a Tornado Warning is issued for your area, your phone alerts you! Outdoor sirens are another useful way of receiving warnings, although they are not common here in the Northeast. And finally, keep up to date with our forecast and warnings via our webpage at weather.gov/boston.

Another way to stay up to date on severe weather forecasts is through the The Storm Prediction Center /SPC/ in Norman, Oklahoma. SPC issues what are called Convective Outlooks for the nation, which can alert you to any heightened areas of possible severe thunderstorms. These can be found on our website at weather.gov/boston

Marginal risk implies that isolated severe thunderstorms are possible. Slight risk means that scattered severe thunderstorms are possible. Enhanced risk means that numerous severe storms are possible, and they are more persistent or widespread with a few potentially producing a tornado. A Moderate risk is issued when a widespread severe thunderstorm outbreak is likely. It often means that some of the storms could be long-lived and possibly produce a strong tornado. A High risk is rare but it means that severe storms are expected and will be very widespread, long-lived, and particularly intense. This will often result in a tornado outbreak or a derecho.

The categories described above are defined based on the statistical probabilities of a severe storm occurring within 25 miles of a given point for a given meteorological situation. For more details about this, please see http://www.spc.noaa.gov/misc/about.html

The Concord Tornado and Tornado Safety
 

At approximately 322 AM on the morning of August 22 2016 an EF1 tornado touched down in Concord, Massachusetts. Fortunately there were no fatalities or serious injuries. The tornado had a path length of a half mile and a path width of 400 yards. Maximum wind gusts were estimated at 100 mph. It was the fifth tornado on record to occur in Massachusetts between midnight and 6 AM.

On average, a few tornadoes occur each year in southern New England. Most are brief and weak with a rating of EF0 or EF1. They also are more common across the interior than along the coast because the proximity of the ocean often acts as a limiting factor. It is quite unusual for a tornado to occur during the morning hours although it is not completely unprecedented.

We know that tornadoes do occur here, such as the EF3 in Monson and Springfield, Massachusetts in 2011, the EF2 in Revere, Massachusetts in 2014, and several weaker tornadoes in southern New England. The question is, are people prepared? Take these steps to ensure that you and your family are safe during a Tornado Warning.

Make sure that you know the difference between a Tornado Watch and a Tornado Warning. A Tornado Watch means that you should be prepared because conditions are such that a tornado could form, somewhere within the large Watch area. However, a Tornado Warning means that you need to take action! A tornado is either occurring, or is imminent, based on radar or spotter observations! During a Tornado Watch, check for forecast updates, keep an eye to the sky, and know where to take shelter. During a Tornado Warning, take shelter immediately! Seek further forecast information on NOAA Weather Radio, the NWS website, or local media outlets for the latest updates.


DAMAGE IN REVERE

DAMAGE IN REVERE photo credit: NWS Taunton, MA
Downburst Winds From Severe Thunderstorms Can Be Powerful
 

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 and relatively small area of rapidly descending air beneath a thunderstorm. It can result from stronger jet stream winds 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 a common thunderstorm downdraft because the winds it produces have the potential to cause damage on or near the ground. Surface damage patterns have shown that whether the winds are straight or 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.