Download Severe Weather

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Severe Weather
A thunderstorm is a weather event where there is heavy rain, lightning, thunder roars, and
possible hail.
o They occur in warm moist air masses and along fronts.
o The air is forced upward where it is cooled and
condensation occurs forming cumulonimbus clouds.
o Then precipitation happens in the form of rain or
hail.
o The rain cooled air sinks and strong updrafts of warmer
air causes strong winds
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How do tornadoes form?
o There MUST be thunderstorm conditions and cumulonimbus clouds.
When thunderstorms develop, winds can start to blow at high speeds in different
directions at different heights. This creates an invisible, horizontal spinning effect of
the air in the lower atmosphere; we call this wind shear.
o Rising air from the updraft of a thunderstorm tilts the rotating air from horizontal to
vertical.
o Once this updraft pushes the rotating air into the cumulonimbus cloud, it continues
to spin at high speeds, and it tries to touch the ground.
o ONLY, when this touches the ground is it a tornado.
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Where do most tornadoes form?
o The central part of the United States, known as the Great Plains, is perfect for
tornadoes to form because it is flat, and doesn’t block wind or air from mixing there.
o This includes Texas, Kansas, and Oklahoma. More than 500 tornadoes typically occur
in this area every year and is why it is commonly known as "Tornado Alley".
o What is happening is the cold air from the north such as Canada meets the warm
moist air from the Gulf of Mexico. This causes a front to form with two very
different air masses. This starts the process of thunderstorms, and sets the stage
for tornado formation
o The formation of cumulonimbus clouds takes place, and from there the conditions
associated with Thunderstorms (hail , lightning, strong winds, and heavy rain) start to
build up, and can lead to supercells, and tornadoes.
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TORNADO WATCH - Tornadoes are possible in your area. Stay tuned to the radio or
television news.
TORNADO WARNING - A tornado is either on the ground or has been detected by Doppler
radar. Seek shelter immediately!
Measuring Tornadoes
The Enhanced Fujita Scale
EF0
Some damage to chimneys; breaks
gale tornado
branches off trees; pushes over shallow40-72 mph
rooted trees; damages sign boards.
EF1
Lower limit is the beginning of hurricanemoderate
force winds. Peels surface off roofs; mobile
tornado
homes pushed over; moving autos pushed
73-112 mph
off roads.
EF2
Roofs torn off frame houses; mobile homes
significant
demolished; boxcars pushed over, large
tornado
trees snapped or uprooted; light-object
113-157 mph
missiles generated.
Severe damage. Roofs and some walls torn
EF3
off well-constructed homes; trains
severe tornado overturned; most trees in forest uprooted;
158-206 mph
heavy cars lifted off the ground and
thrown.
EF4
Well-constructed homes leveled; structures
devastating
with weak foundations blown off some
tornado
distance; cars thrown and large missiles
207-260 mph
generated.
EF5
Phenomenal damage. Strong frame homes
incredible
disintegrate or lifted off foundations and
tornado
carried considerable distance; trees
261-318 mph
debarked.
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HURRICANES
o What is a hurricane?
A huge storm that can be up to 600 miles across and have strong winds spiraling inward
and upward at speeds of 75 to 200 mph.
o Hurricanes gather heat and energy from contact with warm ocean waters.
o Hurricanes form over warm ocean water of 80°F or warmer.
o Hurricanes usually lasts for over a week, moving 10-20 mph over the open ocean.
o Evaporation from the seawater increases their power.
o Hurricanes rotate in a counter-clockwise direction around an "eye."
o The center of the storm or "eye" is the calmest part. It has only light winds and fair
weather.
o When they come onto land, the heavy rain, strong winds and large waves can damage
buildings, trees and cars. Similar storms are called typhoons in the Pacific Ocean and
cyclones in the Indian Ocean.
o Hurricanes typically form between 5 to 15 degrees latitude north and south of the equator.
The Coriolis effect is needed to create the spin in the hurricane.
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Saffir-Simpson Hurricane Scale
Category
Winds
(MPH)
Storm Surge
(Feet)
Damage
1
74-95
4'-5'
Minimal
2
96-110
6'-8'
Moderate
3
111-130
9'-12'
Extensive
4
131-155
13'-18'
Extreme
5
>155
>18'
Catastrophic
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Can Hurricanes cause damage? YES!
Storm surges usually cause the most damage and death
Winds can destroy homes and uproot trees
Extensive rain can also cause major flooding
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TROPICAL STORM WATCH - Tropical Storm conditions with sustained winds from 39 -74
mph are possible in your area within the next 36 hours.
TROPICAL STORM WARNING - Tropical Storm conditions are expected in your area within
the next 24 hours.
HURRICANE WATCH - Hurricane conditions with sustained winds of 74 mph or greater are
possible in your area within the next 36 hours. This WATCH should trigger your family's
disaster plan, and protective measures should be initiated. Especially, those actions that
require extra time such as securing a boat and leaving a barrier island.
HURRICANE WARNING - Hurricane conditions are expected in your area within 24 hours.
Once this WARNING has been issued, your family should be in the process of completing
protective actions and deciding the safest location to be during the storm
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BLIZZARDS
o How do blizzards form?
A blizzard is a long-lasting snowstorm with very strong winds and intense
snowfall.
o You need three things to have a blizzard; cold air at the surface, lots of moisture,
and lift.
o Warm air must rise over cold air. Blizzards have winds of at least 56 km/h,
temperature is below 20 degrees Fahrenheit, and visibility is less than 400
meters
Regional snowfall index (RSI) categories
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Category
RSI value
Description
1
1–3
Notable
2
3–6
Significant
3
6–10
Major
4
10–18
Crippling
5
18.0+
Extreme
Forecasting Weather
o What information do you need to know to forecast weather??
o Temperature
o Winds
o Air pressure
o Humidity
o Precipitation
The people who determine, predict, and forecast the weather are called Meteorologists.