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SEVERE WEATHER. Lecture 9. Definition of a Severe Thunderstorm. The National Weather Service defines a severe thunderstorm as a thunderstorm that produces Hail of 1 inch diameter or larger And/or Wind gusts 58 mph or greater A tornado Note: this does not include lightning or heavy rain!.

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definition of a severe thunderstorm
Definition of a Severe Thunderstorm
  • The National Weather Service defines a severe thunderstorm as a thunderstorm that produces
    • Hail of 1 inch diameter or larger


    • Wind gusts 58 mph or greater
    • A tornado
    • Note: this does not include lightning or heavy rain!
when do thunderstorms occur
When do thunderstorms occur?
  • During the spring and summer, certain atmospheric conditions can be set up which drive severe convective storms:
  • Conditionally unstable atmosphere
  • Moisture
  • Upward vertical motion (“Lifting”)
  • Wind shear
conditional instability
Conditional Instability
  • Recall from a last week…the atmosphere is conditionally unstable if the environmental lapse rateislessthan the dry adiabatic lapse rateandgreaterthan the moist adiabatic lapse rate
  • A “dry” (unsaturated) air parcel will cool at theDALRwhen lifted. It will always be colder than its environment, and sink back to its original level – STABLE
  • A “moist” (saturated) air parcel will cool at the MALRwhen lifted. It will always be warmer than its environment, and continue to rise - UNSTABLE
  • Since a conditionally unstableatmosphere is only unstable with respect to an saturatedair parcel, some moisture sourceis required to create severe weather

Cyclones can advect warm, moist air from the Gulf of Mexico into the central plains

Other sources of moisture include the Pacific and Atlantic oceans

vertical lifting
Vertical Lifting
  • An unstable atmosphere will only generate severe weather when it is given a “push”
  • Unstable air parcels can be lifted by the following mechanisms:
    • Convection
    • Convergence
    • Frontal forcing
    • Topography
wind shear
Wind Shear
  • Wind shear is a term which describes how the speed and directionof the wind change with height
  • It is critically importantfor the formation of severe weather, because wind shear will keep upward motion and downward motion in the stormseparated,allowing the storm to survive
directional wind shear
Directional Wind Shear
  • Wind changes direction with height
  • Wind is backing if the wind direction rotates counterclockwise with height
  • Wind is veering if the wind direction rotates clockwise with height
  • Generally, severe weather will only be found if the wind is veering with height
life cycle of a thunderstorm
Life Cycle of a Thunderstorm
  • Building block for any thunderstorm is a thunderstorm cell
  • Typical thunderstorm lasts approx. 30 minutes
  • Three stages
    • Developing stage
    • Mature Cumulus stage
    • Dissipating Stage
developing stage
Developing Stage
  • Warm, humid air rises and develops an updraft
  • Air parcels saturate and form a towering cumulus cloud
  • Little or no rainfall
  • Lasts near 10 minutes
  • No severe weather yet
mature stage
Mature Stage
  • Precipitation begins to fall, creating downward motion (downdraft)
  • When downdraft hits the ground, it spreads out and creates a “gust front”
  • Storm develops overshooting (“anvil”) top
  • Lasts an average of 10-20 minutes.
  • Most likely time for severe weather (large hail, gusty winds, tornadoes)

The change in the wind with height forces the downdraft to be separate from the updraft, which allows the storm to live longer and become severe

mature thunderstorm
Mature Thunderstorm

Roll cloud in Dodge County-not attached to parent thunderstorm cloud-associated with thunderstorm gust front. Sign of possible microburst activity


Shelf Cloud

  •  Associated with a thunderstorm gust front 
  • Attached to parent thunderstorm cloud 

- Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

dissipating stage
Dissipating Stage
  • Downdraft dominates and shuts off the updraft
  • Gust front moves out ahead of the storm and cuts off inflow of warm, moist air
  • Severe weather threat diminished
  • Lightning still a threat
  • Other storms may develop along outflow boundary
types of thunderstorms
Types of Thunderstorms
  • Ordinary Cell
  • Multi-cell Cluster
  • Multi-cell Line (aka Squall line)
  • Supercell thunderstorm
  • A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACT with the ground.” -NWS
  • Can sometimes last for more than an hour and travel several miles
  • Most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more
what causes tornadoes
What causes tornadoes?
  • Tornadoes in the winter and early spring are often associated with strong frontal systems that form in the Central US and move east
  • During the spring in the Central Plains, thunderstorms develop along a dryline – separates very warm, moist area to the east from hot, dry air to the west.
  • Tornadoes may form as thunderstorms fire along the dryline during the afternoon hours
how do tornadoes form
How Do Tornadoes Form?
  • Before thunderstorms develop, a change in wind shear creates an invisible, horizontal spinning effect near the surface.
how do tornadoes form25
How Do Tornadoes Form?
  • Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical

how do tornadoes form26
How Do Tornadoes Form?
  • An area of rotation, 2-6 miles wide, now extends through much of the storm.
  • Most strong and violent tornadoes form within this area of strong rotation

tornado facts
Tornado Facts
  • Some tornadoes may form during the early stages of rapidly developing thunderstorms.
    • Most common along range of Rocky Mountains, the Plains, and the western US
  • Tornadoes may appear nearly transparent until dust and debris are picked up
  • Two or more tornadoes may occur at the same time from the same storm.
tornado facts28
Tornado Facts
  • The average tornado moves SW to NE but tornadoes have been known to move in every direction.
  • The average forward speed of a tornado is 30 mph
  • Tornadoes can accompany tropical storms and hurricanes as they move over land
  • Tornadoes are most likely to occur between 3 pm and 9 pm
wedge tornado
Wedge Tornado

rope tornado
Rope Tornado

tornadoes in wisconsin
Tornadoes in Wisconsin

Stoughton, 8/2005 Tornado - Storm Chaser Scott A. Kampas

cool tornado video
Cool Tornado Video
tropical cyclones
Tropical Cyclones
  • A tropical cyclone is a low pressure system that develops over tropical or subtropical waters (between 30º N and 30º S)
  • Develop in areas without a horizontal temperature gradient
  • Does not have fronts
  • Has a warm core center
  • Has an organized circulation
tropical cyclone names
Tropical Cyclone Names
  • Depending upon location, tropical cyclones have different names around the world
  • In the
    • Atlantic/Eastern Pacific Oceans: hurricane
    • Western Pacific: typhoon
    • Indian Ocean: tropical cyclone

Do not typically form within 5° latitude of the equator due to the lack of sufficient Coriolis Force




tropical cyclone names40
Tropical Cyclone Names
  • Since 1953, Atlantic tropical storms have been named from lists made by the National Hurricane Center (NHC)
  • In 1979, men’s names were introduced and they alternate with the women’s names
  • Six lists are used in rotation
tropical cyclone names41
Tropical Cyclone Names
  • The only time that there is a change in the list is if a storm is so deadly or costly that future use of its name would be inappropriate
    • Essentially the storm name is “retired”
    • For example, Andrew 1992, Katrina 2005, Gustav and Ike 2008
  • In the event that more than 21 tropical cyclones occur in a season, additional storms will take names from the Greek alphabet
    • Alpha, Beta, Gamma, Delta, etc
how do tropical cyclones develop
How Do Tropical Cyclones Develop?
  • Several favorable environmental conditions must be in place before a tropical cyclone can form:
    • Some initial disturbance such as a thunderstorm complex, which may slowly develop
    • Warm ocean waters (at least 80°F)
    • Potentially unstable atmosphere favorable to convection
    • Moist air near the middle of the troposphere
    • Low values of vertical wind shear between the surface and upper troposphere
  • If these conditions persist for several days, a tropical cyclone may form
how do tropical cyclones strengthen
How Do Tropical Cyclones Strengthen?
  • Tropical cyclones strengthen and maintain themselves through latent heat release
  • As water vapor rises, it cools and condenses
  • Large quantities of latent heat release warm the air, causing it to be less dense than its surroundings
  • The ideal gas law tells us that the pressure should then decrease
  • The storm must be over warm ocean waters to supply enough moisture
stages of hurricane development
Stages of Hurricane Development
  • When these disturbances first appear, they are called tropical depressions
    • Not named yet
  • Once the disturbance has developed with surface wind speeds stronger than 39 mph, the storm is classified as a tropical storm
  • After further strengthening and surface wind speeds greater than 74 mph, the system is upgraded to a hurricane
  • Atlantic Hurricane season is June 1 – November 30

Air converges in the center in a cyclonic pattern and diverges at the top. In the center of the storm, called the “eye”, the storm is mostly cloud-free

tropical cyclone structure
Tropical Cyclone Structure
  • The main parts of a tropical cyclone are the eye, the eyewall, and the rainbands
  • The eye
    • Strong rotation of the cyclone causes a vacuum of sinking air at the center which suppresses cloud formation
    • This creates a pocket of generally clear, calm conditions in the center
    • Typically 20-40 miles across
    • Will usually develop when the winds exceed 74 mph but cause of eye formation still not fully understood
tropical cyclone structure50
Tropical Cyclone Structure
  • The eyewall
    • Consists of a ring of thunderstorms that produce heavy rains
    • Usually produces the strongest winds of the storm
    • Changes in the structure of the eyewall and eye can cause changes in wind speed
    • Double eyewalls can form if the eye changes size
tropical cyclone structure52
Tropical Cyclone Structure
  • The rainbands
    • Curved bands of clouds and thunderstorms that trail away from the eyewall in a sprial fashion
    • Capable of producing heavy bursts of rain and wind, as well as tornadoes
    • Sometimes gaps in the spiral bands where no rain or wind is found
    • if one were to travel between the outer edge of a hurricane to its center, one would progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again





what is storm surge
What is storm surge?
  • As the surface winds converge toward the center of the storm, they “pile up” ocean water
  • Also, the decreased surface pressure raises the sea level slightly
  • Strong waves also can increase the water height
what causes the most damage
What causes the most damage?
  • A hurricane can cause damage by many different aspects
    • Very strong winds can damage structures
    • Heavy rainfall can cause flooding
    • Storm surge can inundate low-lying areas
    • Occasionally, lightning strikes and/or tornadoes can cause damage
  • Storm surge generally causes the most damage and deaths, particularly in strong hurricanes
2005 atlantic hurricane season
2005 Atlantic Hurricane Season
  • Was the most active Atlantic hurricane season in recorded history
  • Forecasts fell far short of the actual activity
  • Most tropical storms, hurricanes, and Category 5 hurricanes recorded in one season
  • 27 named tropical storms, 17 became hurricanes
  • 7 major hurricanes
  • 4 Category 5 hurricanes – Emily, Katrina, Rita, Wilma
  • Widespread economical effects
hurricane katrina 2005
Hurricane Katrina - 2005
  • Katrina was the 6th strongest hurricane on record for the Atlantic Ocean
  • After entering the Gulf, rapidly intensified from Cat. 3 to Cat. 5 in 9 hours on August 28
  • Lowest pressure as Cat. 5 storm was 902 mb with max sustained winds of 175 mph
  • Weakened slightly before landfall to a Category 3
  • Hurricane force winds extended 120 miles from the center

Katrina’s Path

Made landfall August 29, 2005 as Cat. 3

katrina s impact
Katrina’s Impact
  • Largest natural disaster in the history of the U.S.
  • Attributed to over 1830 deaths and 700 missing people, mostly in Louisiana and Mississippi
  • Damages well in excess of $100 billion
  • Federal disaster declarations covered 90,000 sq miles (size of United Kingdom)
  • 8-16 inches of rain
  • Breaching of levees caused 80% of New Orleans to be flooded
time lapse of hurricane katrina
Time Lapse of Hurricane Katrina