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.' 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 criteria for what is termed 'severe.' 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.
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).
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
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.
The Storm Prediction Center /SPC/ in Norman, Oklahoma issues what are called Convective Outlooks for the nation. These can be found on our website at weather.gov/boston in the Current Hazards tab and the Thunderstorms selection. You may have noticed that there is now more detail in the Day 1, 2, and 3 Convective Outlooks. The prior system had 3 thresholds, Slight, Moderate, and High and the new system gives more definition with 5 thresholds. A Marginal category has replaced the See Text section in the previous system and an Enhanced category has now been added to the higher-end Slight and lower-end Moderate situations.So, rather than the entire area from southern New England to the Mid-Atlantic states being covered in a Slight risk of severe storms, there might be an Enhanced risk portion just from western MA to Philadelphia, as an example.
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
At approximately 932 AM on the morning of July 28, 2014 an EF2 tornado touched down in Revere, Massachusetts. It was the first tornado ever confirmed in Suffolk County since records began back in 1950. Fortunately there were no fatalities or serious injuries. The tornado had a path length of 2 miles and a path width of 3/8 of a mile. Maximum wind gusts were estimated at 100 to 120 mph.The tornado resulted in damage throughout Revere. Numerous houses on Revere Beach Parkway were severely impacted, with one roof completely blown off. Numerous trees were downed, a few falling onto and crushing cars. A car was overturned at the intersection of Malden Street and Carlson Avenue. More than 100 homes had damage that ranged from siding torn off to portions of roofs lifted or blown off. Several store signs were destroyed. The tornado moved north-northeast and was on the ground for only 4 minutes. This led to a sharp cutoff of damage east of American Legion Highway/Route 60 as it dissipated.
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 in 2011, the EF2 in Revere in 2014, and weaker ones in Connecticut and Massachusetts in 2014. The question is, are people prepared? We would now like to focus on tornado preparedness for schools.
Every school should be inspected and tornado shelter areas designated by a registered engineer or architect. Rooms with exterior walls should never be used as tornado shelters. Basements offer the best protection. Schools without basements should use interior rooms and hallways on the lowest floor away from windows.
Schools should delay assemblies or lunch in large rooms with wide roof spans, such as gymnasiums, auditoriums, and cafeterias. These rooms offer little or no protection for tornado-strength winds and the wide span roofs can collapse.
Students and staff should know the protective position, sitting and facing an interior wall, elbows to knees, and with hands over the back of their heads.
Each school should have a NOAA Weather Radio with battery backup. Remember that the National Weather Service issues a Tornado Watch when conditions are favorable for tornado development and a Tornado Warning when a tornado has been spotted or indicated by radar. If the alarm system of a school relies on electricity, there should be an alternative method to notify teachers and students in case of power failure.
Please remember to make special provisions for faculty and students with disabilities, those with portable classrooms, and those outdoors. Keep children at school beyond regular hours during a tornado warning. For that matter, it would be good practice to delay departure of school buses with severe thunderstorm warnings too, since they can produce large hail and damaging winds without a tornado.
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.