SECTION 3 - THUNDERSTORMS,
SEVERE WEATHER SPOTTING & DETECTION
A. Basic facts about
Thunderstorms
Thunderstorms occur
in all 50 states. They can occur, at any time, day or night, throughout the
entire year. Thunderstorms are most common in the late afternoon and evening
during the warm months. It has been estimated that 1800 thunderstorms are in
progress at any given moment around the world and that lightning strikes the
earth 100 times every second. Thunderstorms are basically beneficial providing
necessary rainfall. In the United States, only about five percent of thunderstorms
become severe and only about one percent produce tornadoes.
A list of weather brochures and learning aids are available at http://www.nws.noaa.gov/om/brochures.shtml
B. Storm Spotting
Techniques
Your local National
Weather Service Office provides severe weather spotter training under a program
called SKYWARNTM. It is provided free-of-charge with the request that when you
do encounter severe weather, you report it to the National Weather Service.
The spotter training class includes computer and video presentations that help
you learn how to pick out visual clues from clouds to help determine the severity
of a storm and other tips on reporting hazardous weather. It is highly recommended
that the assigned district "Severe Weather Administrator and each school
assigned "Severe Weather Coordinator" take the training. Although,
any number of teachers and administrators are welcome to attend and receive
the training. The enclosed books provide information about storms and spotting,
however, they are not a substitute for official training.
C. NWS Methods of
Detecting and Tracking Severe Weather
The National Weather
Service uses a combination of radar, satellite, lightning detection, and surface
observations, including volunteer spotter reports for detecting and tracking
severe weather. The new sophisticated Doppler radars (WSR-88D) around the country
have greatly increased the National Weather Service's ability to pinpoint severe
thunderstorms and possible tornadoes and warn the public as to where the storm
is moving and what actions to take. Spotter reports tell forecasters the size
of hail, the depth of snow or flood waters, if wind damage is occurring, or
if a tornado is sighted. These reports provide ground truth to the images seen
on the Doppler radar.
Doppler Radar
The WSR-88D (Weather
Surveillance Radar - 1988 Doppler) is the new radar system for NWS, the Federal
Aviation Administration, and the Department of Defense (DOD). It is a very sensitive
radar designed specifically for the detection of weather phenomena. The computers
that compile the radar data can produce as many as 100 different radar products
every 5 minutes for forecaster interpretation.
Typically radar
has been used to tell meteorologists where precipitation is occurring, how intense
it is, and where it is moving. The ability of the new radars to detect radial
velocity (movement of radar targets, such as rain, toward or away from the radar
derived from the "Doppler Effect") allows meteorologists to see rotation
of thunderstorm updrafts and sometimes the development of tornadic circulation.
Supercell thunderstorms displaying strong radar signatures (storm rotation)
may allow forecasters to provide up to 20 minutes lead-time on warning for a
tornado before it touches down. Computer and mapping skills with the new radar
system also help meteorologists determine rainfall amounts and pinpoint areas
with potential flash flood problems.
Like all technology,
radars have their limitations. Radar beams can not see through mountains. This
means that weather at the valley floor between mountain ranges will not be detected.
Because of the curvature of the earth, as the radar beam moves away from its
source, it gets higher and higher in the atmosphere and is no longer sampling
the lower portion of the storm clouds. The NWS compensates by using trained
severe weather spotters which help forecasters to fill in the gaps and provide
ground truth information.
Satellites
Geostationary satellites
(stationary above a point over the equator) and polar orbiting satellites allow
meteorologists to watch the development of clouds and weather systems. Satellites
are extremely useful for tracking weather systems over the vast ocean areas
where no radar exist and only a few surface observations are taken. For example,
satellites greatly improved meteorologists' ability to detect the formation
and movement of hurricanes over the tropical waters. Satellites also help meteorologists
to track movement of air masses that are either very dry (such as off the mountains)
or very moist (such streams from the Eastern Pacific Ocean). This can greatly
influence a storm's development. Cloud patterns also tell forecasters about
the strength and movement of the jet stream which plays a large role in storm
development.
While satellites
provide meteorologists with much information, they also have limitations. The
satellite views a cloud from above, but does not tell what has formed below
it (such as a tornado). Severe thunderstorms and tornadoes can develop and dissipate
fairly rapidly. Even if a satellite picture indicated a severe storm, the time
delay for receiving the satellite picture may be 15 to 30 minutes which can
be too long to properly warn for an event such as this.
Lightning Detection
Lightning detection
systems map where cloud-to-ground lightning strokes are occurring. While this
allows meteorologists to know that thunderstorms are indeed in progress and
the frequency of lightning strokes, it tells little about the severity of the
thunderstorm. People often report "vivid lightning" or "severe
lightning," but lightning is not a criterion for whether a storm is severe,
according to the NWS definition of "severe." Lightning assists forecasts
in preparation for Fire Weather Forecasts and Products used by land management
agencies and local fire fighters.
Surface Observations
Surface observation
are the "ground truth" for tools such as radar and satellite. Because
thunderstorms are very localized (the severe weather potion of the storm may
only affect an area one mile wide), it would be impossible to have weather observers
everywhere.
It is the combination
of surface reports and remote sensing tools that brings the entire picture together
for forecasters and increases their ability to issue effective, informative,
and timely warnings. While the new technology has enhanced the meteorologist's
ability to issue a timely warning, it will be of little use if the people who
receive the warning do not know what safety actions to take.
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