2. TORNADOES
GENERAL
CHARACTERISTICS
Tornado:
Tornadoes
are the most erratic, unpredictable, and violent of natures storms.
The
destructive power of a tornado is due to the combined action of its strong
rotary winds and the partial vacuum in the center of its vortex.
Tornadoes are violently whirling wind, characteristically accompanied by
a funnel-shaped cloud extending from a cumulonimbus cloud.
A tornado can be from a few yards to about a mile wide where it touches
the ground, with an average width of a few hundred yards. It can move slowly or
speed along at more than 60 mph and may travel for distances ranging from short
hops to several miles, causing extensive damage. The funnel is made visible by
the dust that it sucks up and by condensation of water droplets in its center.
Most tornadoes spin counterclockwise in the Northern Hemisphere and clockwise in
the Southern Hemisphere. They develop within low-pressure areas of high winds;
wind speeds often exceed over 300 miles per hour.
Tornadoes
are directly associated with dark, heavy cumulonimbus (thunderstorm, squall
line, supercell) clouds with intense lightning. Movement is usually from southwest to northeast.
However, they have moved in other directions and tornadoes associated
with hurricanes may move in a westerly direction. A tornado is formed when cold
air overriding a warmer air mass breaks through the upper boundary formed by the
two air masses and in a violent rotating column descends because of its heavier
weight. This creates a hole in the air mass boundary allowing the warm, lighter
air to rush upward into the cold air. This
cold-warm exchange of air is most violent and causes a funnel-shaped rotating
extrusion, which is nearly always from a thunderstorm cloud filled with intense,
continuous lightning.
While
tornadoes in Texas can occur in any month and at all hours of the day or
night, they occur with greatest frequency during the late spring and early
summer months and during late afternoon and early evening hours.
This is particularly true in Polk County. The county has a history of
tornadoes, direct line winds and a term used to describe a storm in March 1997,
as a gust-tornado. These storms
have cost property owners millions of dollars in property damage.
Frequency of Occurrence
Ø
March 4, 1994: A storm described as a gust-tornado
by the National Weather Service moved from Lake Livingston cutting a path about
two hundred meters wide and about 6 miles long that damaged several hundred
homes and destroyed approximately eight. This
storm downed trees and damaged roofs. Trees
falling on structures were the major cause of severe damage to homes.
Ø
May 9, 1994: A severe thunderstorm that produced a
tornado moved across northern Polk County producing large hail and heavy rains.
Damage was primarily to out buildings and numerous downed trees.
Ø
March 13, 1995:
A tornado traveling East from Lake Livingston
cut a path about a hundred yards wide, skipping for about 7 miles
damaging numerous homes and commercial buildings. Several homes in the Scenic Loop area were severely damaged
from high winds and falling trees. The
Ogletree Lumber Mill suffered approximately 1.2 million dollars in damages, the
Polk County Courthouse was also damaged, and the Campbell/Foreman building roof
was blown off. Heavy hail fell in a
two-block radius in Eastern Livingston depositing hail in banks that were better
than two feet deep.
Ø
May 28, 1997:
A tornado with multiple vortexes touched down about a mile north of
Livingston and was stationary for approximately twenty-five (25) minutes.
This storm caused very little damage, however it did cause us to
re-evaluate tornado warning and conduct tornado drills at all county school
districts.
Ø
February 10, 1998:
Several tornadoes cut paths of destruction all across Polk County. Within
an area of several blocks in West Corrigan, numerous homes were damaged and five
were destroyed. An apartment
complex located about a mile East of Corrigan was seriously damaged.
The Champions International Mill suffered about 2.5 million in damages to
the roof. Wonder Chemical suffered
about 1.6 million in damages to their plant.
Several hundred homes in the Scenic Loop area had roof damage, several
trailer houses were completely destroyed. At
Texas 146 and FM 943 tornadoes caused heavy losses to trees and homes, one
business was completely destroyed. It
was estimated that as many as four (4) tornadoes were the cause of all this
damage, however that number was not confirmed.
Polk County has a history of violent thunderstorms, high winds and tornadoes, however there has not been any recorded injuries or loss of life. This is due primarily to good fortune, not because of our warning system. Polk County currently does not have an effective tornado warning system. Local radio, radio scanners and local television crawls are the extent of our warning system. Tornadoes provide very little or no warning, therefore our present system is ineffective and needs to be improved. Plans to install a NOAA weather radio transmitter facility is currently in the planning stage. The cost of a NOAA tower is about $90,000. It is our hopes that by the end of March, 2001, a NOAA weather radio tower will be operational in the Onalaska Area.
This Homepage was prepared by Kenneth F. Hambrick, Polk County Coordinator. Contact at E-Mail address:
webmaster@pcoem.org for additional information about our program.
The
information contained in this Homepage is considered public
domain and the Office of Emergency Management encourages
interested persons to utilize any portions of it that might be of educational
benefit or enhance their local programs.
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