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By Daniel Noah
Warning Coordination Meteorologist
Tampa National Weather Service
 Thunderstorms
As many Floridians know, the most awesome phenomena in the atmosphere develop
within and around severe thunderstorms. In these storms, the damaging effects
of high winds, flood-producing rains, lightning, hail, tornadoes and waterspouts
threaten Floridians. Thunderstorms are most common in the interior sections
of central Florida, with more than 100 days of storms each year. Coastal areas
of the state average more than 80 storm days each year.
Thunderstorms affect relatively small areas when compared
to hurricanes. Despite their small size, all thunderstorms are dangerous! The
typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes.
Nearly 1,800 thunderstorms occur at any moment around the world, an average
of 16 million per year. Of the estimated 100,000 thunderstorms that occur each
year in the United States, only about 10 percent are classified as severe. Your
National Weather Service classifies a storm as severe if it produces hail at
least three-fourths inch in diameter, winds of 58 mph (50 knots) or higher,
or tornadoes.
It is your responsibility to protect yourself and family
during times of severe weather. Keep in mind the following weather terms when
storms approach:
Severe Thunderstorm Watch: Conditions
are favorable for thunderstorms to produce wind gusts to 58 mph or stronger
or hail to three-fourths inch or larger in or close to the watch area. These
watches are issued for four to six hours at a time and for a number of counties.
Stay informed, watch the sky and take cover if a severe thunderstorm approaches.
Severe Thunderstorm Warning: A severe
thunderstorm has been detected by radar, or by a trained SKYWARN (editor’s
note: use insert box or footnote to explain) spotter. Take cover if you are
near the severe thunderstorm.
Thunderstorm Formation: Three ingredients
are needed for a thunderstorm to form, and Florida has an abundance of all three
during the summer rainy season. The first ingredient is moisture, which provides
fuel for the storm. When water vapor condenses into visible cloud material,
it releases heat that can strengthen a storm’s updraft. The second ingredient
is a triggering mechanism, or an atmospheric feature to lift the moisture vertically
so clouds form and updrafts can develop. Triggering mechanisms provide the initial
lift to the storm and include such features as cold fronts, sea breeze fronts,
and outflow boundaries from previous thunderstorms. The last ingredient is called
instability. Instability in the atmosphere allows updrafts to accelerate up
and down at an unusually high rate of speed. Think of instability as a hot air
balloon. If the air inside the balloon is warmer than the surrounding air, the
balloon will rise. This increase in upward motion causes the cloud tops to build,
thereby increasing the intensity of the storm.
The addition of another ingredient creates a special storm.
This ingredient is called wind shear. Wind shear is the changing of wind speed
and/or direction with height. Wind shear has the ability to keep updrafts and
downdrafts within a thunderstorm separate, allowing the storm to become stronger
than a storm that develops in the absence of wind shear.
Detection and Forecasting
 Many
sophisticated tools are used to forecast the occurrence of severe thunderstorms.
Satellite images allow forecasters to see the development of clouds that grow
into thunderstorms and to monitor the changes in weather patterns that make
the atmosphere more conducive to severe thunderstorm development.
Doppler radar allows forecasters to “see inside”
a thunderstorm and detect conditions that precede the development of a severe
thunderstorm, tornado or indicate a high probability of damaging “straight-line”
winds. Doppler radar also provides real-time rainfall estimates that give meteorologists
valuable information concerning flood potential.
Another type of technology includes automated surface observations,
lightning detection networks, and weather balloons. Super computers located
near Washington, D.C., are fed these data from around the world. The data are
entered into weather forecasting models that mathematically simulate the behavior
of the atmosphere. The computer takes current weather conditions and step them
forward through time. Meteorologists at the local National Weather Service office
integrate the computer output with past experience in order to arrive at the
forecast. Some computer forecast models provide information through 48 hours,
while other models provide extended forecast information through 10 days.
While the technological tools are extremely valuable to
forecasters when predicting severe weather potential, one of the most reliable
tools used for severe storm detection remains the eyes of trained SKYWARN storm
spotters. While weather radar provides information on what is likely occurring
within a storm, spotters can provide the critical ground-truth information that
can be incorporated into subsequent warnings and weather statements. To become
a spotter, contact your local National Weather Service office for details.
Conclusion
Many people are involved in efforts to provide advanced warning before a severe
thunderstorm strikes. Sophisticated technological tools, National Weather Service
forecasters, the media, SKYWARN spotters and public safety groups have developed
a system to provide information to alert you to dangerous weather conditions.
In order for the system to work, however, YOU must take the time to understand
severe weather safety rules and plan in advance what actions you will take if
a warning is issued. The decision to do this may be the most important one that
you will ever make for yourself and your family.
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