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© Hamilton County Florida Emergency Management
NATURAL HAZARDS
Severe weather events are a likely occurrence in Hamilton County. The
semi-tropical like climate offers sporadic tropical storms, tornadoes and
flooding events. While the county rarely deals with snow, sleet or ice,
winter storms and freezes can significantly impact the agricultural industry.
Many of the identified hazards are related (e.g., flooding can occur and
tornadoes can develop during tropical storms) in the sense that other
hazards may result from a disaster event, such as sinkholes stemming
from flooding; in such instances, these hazards are not listed separately
but concurrently.
Hazards that most frequently affect Hamilton County:
A. Hurricanes and Tropical Storms
B. Thunderstorms and Hail
C. Tornadoes
D. Flooding
E. Sinkholes
F. Wildland Fires
Hurricanes and Tropical Storms
Hurricanes and Tropical Storms are a fact of life in Florida and the effect
from these storms are felt equally in all of Hamilton County and the City of
Jasper, Town of Jennings and the Town of White Springs.
All hurricanes start off as tropical storms and the only difference between
the two is wind speed. Hurricanes are tropical cyclones with winds that
exceed 74 mph while tropical storms have wind speeds less than that.
They are essentially heat pumping mechanisms that transfer the sun’s heat
energy from the tropical to the temperate and polar regions. This helps to
maintain the global heat budget and sustain life. Hurricanes are formed
from thunderstorms that form over tropical oceans with warm surface
temperatures. The ambient heat in the sea’s surface and moisture in the
rising air columns set up a low pressure center and convective conditions
that allow formation of self-sustaining circular wind patterns. Under the
right conditions, these winds may continue to intensify until they reach
hurricane strength. This heat and moisture from the warm ocean water is
the energy source of a hurricane. Hurricanes weaken rapidly when
deprived of their energy source by traveling over land or entering cooler
waters.
Hurricanes and Tropical Storms are characterized by torrential rainfalls,
tidal flooding and damaging winds can spawn severe thunderstorms,
lightening and tornadoes.
Thunderstorms
Thunderstorms are a storm of heavy rain accompanied by lightning,
thunder, wind and sometimes hail. They occur when moist air near the
ground becomes heated, especially in the summer here in Hamilton
County. The air then rises forming clouds that produce precipitation.
Ordinarily, thunderstorms cause little damage unless it grows in strength
and reaches the severe level. The National Weather Service defines this is
as having large hail of at least 1 inch (2.5 cm), surface wind speeds of 58
miles per hour (93 km/h) or greater.
Hazardous conditions associated with thunderstorms include tornadoes,
lightening, hailstorms, flooding, downburst and microburst winds.
Strong “downbursts” (winds) are concentrated, straight-line winds created
by falling rain and sinking air that exceed 125 mph.
A separate wind phenomenon is the microburst, which are narrowly-
concentrated downdrafts that can exceed speeds of 150 mph.
Lightning occurs in all thunderstorms and can strike anywhere, both air
and ground.
Every thunderstorm produces lightning, which kills more people each year
than tornadoes or hurricanes. Heavy rain from thunderstorms can cause
flash flooding, and high winds can damage homes and blow down trees
and utility poles, causing widespread power outages.
Tornadoes
NOAA identifies damaging winds as occurring in two categories: rotational
(tornadic) or straight-line (downburst). Both forms of wind are bi-products
of a thunderstorm and can cause a similar degree of damage. Tornado
wind damage is often highly concentrated over narrow width in relation to
their damage length. Downburst wind damage is normally spread out over
a broader width but shorter damage length, in either case, both can cause
substantial damage to structures.
Tornadoes occur usually during daytime, from mid-afternoon till about early
evening. Their movement is usually from the southwest to the northeast.
Sometimes they move in any direction, and in the general path of the
thunderstorm. The spinning winds cover an area of about 300 — 400
yards, and can travel on a path for about 5 miles (some tornadoes travel
for over 80 miles), at a speed of about 5 — 60mph.
Sometimes tornadoes develop in a very short time frame, leaving very little
lead time for warning and preparation.
Some examples of the damage tornadoes can cause include leveling
homes, flipping mobile homes upside down, toppling large trees, picking
up cars and dropping them miles away from their original location, and
pushing metal shards into tree trunks. The worst damage a tornado can
cause, however, is to the lives of the people involved, since injuries and
fatalities are common when a tornado hits.
Tornadoes materialize at the trailing edge of large frontal cyclones that
result from the clash of high pressure and low pressure weather systems
moving at continental scales across North America. Because of climatic
differences, southern states like Florida experience their most violent
tornadoes in winter. However a tornado event is not limited to winter.
They can also be generated during the summer in association with
afternoon thunderstorms. Most tornadoes are of short duration and do not
touch down as the Category 4 or 5 that make national headlines.
When a tornado threatens, only a short amount of time is available for life
or death decisions. The National Weather Service (NWS) issues two types
of alerts:
A Tornado Watch means that conditions are favorable for tornadoes to
develop, and
A Tornado Warning means that a tornado has actually been sighted.
Flooding
A flood is an overflow of water onto normally dry land. The inundation of a
normally dry area caused by rising water in an existing waterway, such as
a river, stream, or drainage ditch. Ponding of water at or near the point
where the rain fell. Flooding is a longer term event than flash flooding:
it may last days or weeks.
A flash flood is caused by heavy or excessive rainfall in a short period of
time, generally less than 6 hours. Flash floods are usually characterized
by raging torrents after heavy rains that rip through river beds or urban
streets. They can occur within minutes or a few hours of excessive rainfall.
Historically, floods have been a factor in over 80 percent of all Presidential
declared disasters.
Sinkholes
For the residents of Florida, the sudden appearance of sinkholes is not an
unusual occurrence. Sinkholes occur because the entire State of Florida is
underlain by limestone, a type of rock that is slowly dissolved by weak
natural acids found in rain and in the pore spaces in soil.
As rainfall is filtered through the ground to the limestone, it erodes and
dissolves the soluble rock and creating cavities in the subsurface.
The abrupt formation of sinkholes may follow extreme rain producing
events such as tropical storms or hurricanes. This is because the weight
of a large amount of rain water at the earth’s surface may bring about the
collapse of an underground cavity if its limestone “ceiling” has become
thin. This tendency for sinkholes to form following events that produce
large amounts of rainfall is made worse in times of drought. During periods
of drought, underground cavities that might normally be filled with water
may be only partially filled. These cavities are less likely to bear the weight
of flood waters without collapsing.
Ground water pumping in specific areas when water levels are already low
and are forced lower can trigger a more sudden collapse of overburdened
sediments and create sinkholes that might not have otherwise happened.
Increases in ground water pumping, loading at land surface, retention pond
building, and altering a landscape where you’re changing the
overburdened thickness are all activities that can induce sinkholes.
Wildfires
As people search for a place to live they often desire two conflicting
conditions: country living with urban services. This desire for urban/rural
living has been given the name “wildland/urban interface.” As described
by the Florida Forest Services, the wildland/urban interface “refers to that
geographical area where two diverse systems - wildland and urban – meet
and affect each other and give rise to conflicts between societal values and
expectations concerning the management of natural resources.” The
major problem resulting from the wildland/urban interface is wildfire.
A wildfire is any fire occurring in wildlands (i.e., grasslands, forest, brush
land, etc.) Prescription burning is the process of igniting fires under
selected conditions, in accordance with strict parameters. Wildfires have
burned across the woodlands of Florida for centuries and are part of the
natural management of much of Florida’s ecosystems. Forest fires from
natural causes such as lightning account for only a very small percentage
of Florida’s wildfires, whereas, man is by far the leading cause of wildfires.
Forest land is continuously susceptible to destruction by wildfires.
There are four types of forest fires:
• Surface - A surface fire is the most common type and burns along the
floor of the forest, moving slowly while killing or damaging trees.
• Ground - Ground fires (muck fires), which are usually started by
carelessness, burn on or below the forest floor. These fires are hard to
detect, and even harder to extinguish.
• Crown - Crown fires are spread rapidly by the wind and move fastest of
all types of fires by jumping along the tops of trees.
• Wildland-Urban Interface – WUI Fire is a wildfire in a geographical area
where structures and other human development meet or intermingle with
wild lands or vegetative fuels.