Blizzard of 1978
Greenwood Floodinging – June 08
Special Weather Events
There are many types of hazardous weather
• Tornadoes
• Blizzards
• Thunderstorms
• Flooding
• Hurricanes
• Drought
Thunderstorms
• The 3 Key Ingredients of a Thunderstorm
▫ MOISTURE
 Large bodies of water (warmer temperatures = good)
▫ INSTABILITY
 Warm moist air underneath colder dense air
 As the warm air rises it will cool and form the
CUMULONIMBUS clouds
▫ A LIFTING MECHANISM
 Differential heating (Water vs. land), Fronts, Terrain
Thunderstorms
• The 3 Stages of a Thunderstorm
▫ TOWERING STAGE
 The warm air is forced upwards making the tall fluffy
cumulonimbus clouds.
▫ MATURE STAGE
 The air that has raised higher cools and the precipitation
begins to fall.
▫ DISSIPATION STAGE
 The storm runs out of the warm moist air that fuels it.
Towering Phase
Mature Phase
• “Anvil” Shaped Top
Mature Phase
• “Anvil” Shaped Top
Dissipation Stage
Types of Thunderstorms
• Ordinary Cell
▫ One cycle through the previous phases
• Multi-cell
▫ The storm builds and cycles through several of the
phases
• Squall Line
▫ The thunderstorm forms a horizontal line that
spreads several miles.
• Supercell
▫ Last for several hours traveling over long distances.
Produce almost all of the significant tornadoes
Supercell Thunderstorm
Thunderstorm Hazards
• Hail
• Damaging Winds
▫ Indiana State Fair
• Tornadoes
• Flash-Floods (Greatest cause of death in
Thunderstorms)
Tornado
• A violently rotating column of air descending
from a thunderstorm and touching the ground.
Formation
• Most tornadoes are spawned from supercell
thunderstorms. Supercell thunderstorms are
characterized by a persistent rotating updraft
and form in environments of strong vertical
wind shear.
• The exact processes for the
formation of a funnel are not
known yet.
Enhance Fujita Scale
EF
scale
Class
EF0
EF1
EF2
EF3
EF4
EF5
weak
weak
strong
strong
violent
violent
Wind speed
mph
km/h
65-85
105-137
86-110
138-177
111-135
178-217
136-165
218-266
166-200
267-322
> 200
> 322
Description
Gale
Moderate
Significant
Severe
Devastating
Incredible
What’s more dangerous than a tornado?
• The Fire Whirl
Flooding
• Basics – Too much water in, not enough water
out.
• Water “Out” sources – rivers, lakes,
INFILTRATION, CSO’s.
• Dangers
 People underestimate how powerful moving water
can be.
 Even at low levels moving water is strong.
Dangers of moving Water
• Of the three deaths which occurred as a result of
the Fort Worth tornado, March 28, 2000, one death
was due to flooding. The man who drowned was a
passenger in a car with his girlfriend, the driver.
They approached a low spot with water flowing
over the road due to very heavy rain. Flooding was
a common occurrence at this location with heavy
rains and the danger was well marked.
• As the driver drove her car into the water she
became frightened as the water rose higher and
higher around her vehicle. She backed out to
higher ground. The passenger said the water
was NOT too deep and he would prove it by
walking across to the other side. He never made it.
Lake Effect Snow
• is produced during cooler atmospheric conditions when
COLD winds move across long expanses of
WARMER LAKE WATER, providing energy and picking
up water vapor which freezes and is deposited on
the opposite shores.
Lake Effect Snow