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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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  1. SEVERE WEATHER Lecture 9

  2. 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!

  3. 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

  4. 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

  5. Moisture • 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 http://www.srh.noaa.gov/jetstream//tstorms/images/moisture.jpg

  6. Warm, moist air is being advected from the Gulf into the Plains

  7. 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

  8. 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

  9. 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

  10. Directional Shear

  11. Speed shear often causes severe weather with strong straight-line winds

  12. 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

  13. 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

  14. 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)

  15. 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

  16. Mature Thunderstorm • http://www.geography.hunter.cuny.edu/~tbw/wc.notes/10.thunderstorms.tornadoes/thunderstorm.jpg

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

  18. 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.

  19. 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

  20. Gust Front / Outflow Boundary on Radar

  21. Types of Thunderstorms • Ordinary Cell • Multi-cell Cluster • Multi-cell Line (aka Squall line) • Supercell thunderstorm

  22. Tornadoes • 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

  23. 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

  24. How Do Tornadoes Form? • Before thunderstorms develop, a change in wind shear creates an invisible, horizontal spinning effect near the surface.

  25. How Do Tornadoes Form? • Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical http://www.nssl.noaa.gov/edu/safety/tornadoguide.html

  26. 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 http://www.nssl.noaa.gov/edu/safety/tornadoguide.html

  27. 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.

  28. 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

  29. Tornado on Radar

  30. Wedge Tornado http://greek.wunderground.com/data/wximagenew/m/MikeTheiss/453.jpg

  31. Rope Tornado http://www.outdoor.com/wp-content/uploads/2008/01/ropetornado.jpg

  32. Tornadoes in Wisconsin Stoughton, 8/2005 Tornado - Storm Chaser Scott A. Kampas

  33. Cool Tornado Video • http://www.weather.com/multimedia/videoplayer.html?clip=14517&from=tv_program_vortex

  34. 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

  35. Hurricane Katrina- 2005

  36. 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

  37. Do not typically form within 5° latitude of the equator due to the lack of sufficient Coriolis Force Hurricane Typhoon Cyclone

  38. 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

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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 http://www.srh.noaa.gov/jetstream//tropics/tc_structure.htm

  44. 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

  45. 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

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