A Brief Climatology Of Severe Weather Events and Thunderstorms In The NWS Taunton County Warning Area by DAVE HENRY

 

The following 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 severe event.

 

Average Number of LSR Events Per Month (1997-2000) May=2, June=4, July=5, August=4, September=2 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. Number Of Days Of Severe Weather 1997-2000 based on LSR data.  Wind without hail=18, Wind and Hail=27, Hail without wind=12, Flooding=12
Average Dew Point for Severe Weather Events Wind=65, Hail=65, Flooding=64 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 the convection.

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.
Percent Of Severe Weather Event With Temperature Above Normal Flooding=42, Hail=90, Wind=94
Average Temperature Departure From Normal During Severe Weather Events: Flooding=-1, Hail=+7, Wind=+8 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. Average Number Of Days Per Month That Thunderstorms Occurred In The County Warning Area: May=8, June=10, July=10, August=8, September=6
Percent Of Thunderstorm Days That Produced LSR Events: May=29, June=37, July=49, August=50, September=30 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.