Brief Climatology Of Severe Weather Events and Thunderstorms In The NWS Taunton County
Warning Area by DAVE HENRY
statistics summarizes severe weather and thunderstorm data that occurred in Southern New
England during the warm months from May 1997 through May 2001. Severe weather days
are days that damage from wind, hail, or flooding was reported in an LSR (local storm
report). During the summer months, LSR reports are usually only issued as the
result of severe thunderstorms or tornadoes. Severe thunderstorms vary by month and
are most likely to occur in warm and humid air and are often triggered a synoptic feature
such as a cold front or other mechanism that provides the necessary lift to initiate a
||The graph at the
right shows the average number of convective events per month that required the issuance
of an LSR. The criteria for a severe thunderstorm and a report in an LSR include hail .75
inch or larger, thunderstorm wind gusts of 50 knots or more, trees blown down or uprooted,
power lines blown down, large limbs or large branches blown down, permanent signs blown
down, roof damage from the wind, damage to homes, buildings, or other structures from the
wind, injuries or deaths to people as a result of wind or hail. The graph shows the
largest number of damaging storms occur during summer when temperatures are highest.
|A severe weather event may include large hail,
damage from wind, or flooding from torrential rain that a thunderstorm may produce.
The graph at the left shows the number of days in each of these categories .
Usually, thunderstorms that produce hail also cause strong wind. But, this is not
always the case. When the airmass is very unstable with warm and moist air above a
cooler layer near the ground hail may occur but the downburst from the thunderstorm that
causes the wind can be somewhat reduced by the cool air below and hail occurs without
damaging wind. Strong wind may occur without hail when humidities are high aloft and
less favorable for hail formation.
||At the left is a graph that shows the average
dew point during severe weather events. High dewpoints near the ground indicate
significant latent heat which is released during thunderstorm development and intensifies
Temperature is also important. Using the highest temperature reported during a
severe event we see that almost all severe weather that has large hail and damaging wind
occurs as a result of above normal temperatures. Flooding can occurs from the
thunderstorms but also results from other convective processes that do not create strong
thunderstorm winds or hail and does not require the very warm air near the ground.
||The average temperature departure for severe
weather events is shown at the left. The data was derived using the highest reported
temperature in the county warning area for each event. As one might expect,
temperatures average 5 to 10 degrees above normal when severe weather occurs.
Temperature departures for hail and strong wind are similar. An 8 degree above
normal temperature over the interior forecast zones would be translate to a maximum
temperature of 92 degrees in July and 90 degrees in August.
|Not all thunderstorms create severe weather.
The average number of days with thunderstorms over the county warning area is shown
at the right. These numbers not only include severe events but also more isolated
thunderstorms such as those that sometimes occur in southern coastal sections from an
approaching warm front. The number of thunderstorms that occurs in late spring is
disproportionally higher than the severe event. One reason could be that dew points
near the ground average lower than during the summer so there is less latent heat to be
released and so the severity of the convection is less.
||The ratio of days with severe weather to days
with thunderstorms is show at the left. The graph indicates that thunderstorms that
develop in July and August when it is warmer and dew points are higher are more likely to
produce severe weather. 50 percent of the days with thunderstorms in these months
have at least some severe event which is about 1.7 times that which occurs in May and September.