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Thunderstorms and Tornadoes

Chapter 14. Thunderstorms and Tornadoes. Thunderstorms. A storm containing lightening and thunder; convective storms Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado Ordinary Cell Thunderstorms Air-mass thunderstorms: limited wind sheer

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Thunderstorms and Tornadoes

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  1. Chapter 14 Thunderstorms and Tornadoes

  2. Thunderstorms • A storm containing lightening and thunder; convective storms • Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado • Ordinary Cell Thunderstorms • Air-mass thunderstorms: limited wind sheer • Stages: cumulus, mature, dissipating • Entrainment, downdraft, gust front

  3. Thunderstorms • Multi-cell Thunderstorms • Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top • Gust Front: leading edge of the cold air out-flowing air; shelf cloud, roll cloud, outflow boundary • Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; wind shear, virga

  4. Thunderstorms • Multi-cell Thunderstorms • Squall-line thunderstorms; line of multi-cell thunderstorms, pre-frontal squall-line, derecho • Meso-scale Convective Complex: a number of individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system; summer, 10,000km2

  5. Thunderstorms • Supercell thunderstorms • Large, long-lasting thunderstorm with a single rotating updraft • Strong vertical wind shear • Outflow never undercuts updraft • Classic, high precipitation and low precipitation supercells • Cap and convective instability • Rain free base, low-level jet • Surface, 850mb, 700mb, 500mb, 300mb conditions

  6. Thunderstorms • Thunderstorms and the Dryline • Sharp, horizontal change in moisture • Thunderstorms form just east of dryline • cP, mT, cT • Floods and Flash Floods • Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow thunderstorm • Large floods can be created by training of storm systems, Great Flood of 1993

  7. Thunderstorms • Topic: Big Thompson Canyon • July 31, 1976, 12 inches of rain in 4 hours created a flood associated with $35.5million in damage and 135 deaths • Distribution of Thunderstorms • Most frequent Florida, Gulf Coast, Central Plains • Fewest Pacific coast and Interior valleys • Most frequent hail Central Plains

  8. Thunderstorms • Lightening and Thunder • Lightening: discharge of electricity in mature storms (within cloud, cloud to cloud, cloud to ground) • Thunder: explosive expansion of air due to heat from lightening • Electrification of Clouds: graupel and hailstones fall through supercooled water, ice crystals become negatively charged • Upper cloud positive, bottom cloud negative

  9. Thunderstorms • Observations: Elves • Blue jets, red sprite, ELVES • The Lightening Stroke • Positive charge on ground, cloud to ground lightening • Stepped leader, ground stroke, forked lightening, ribbon lightening, bead lightening, corona discharge

  10. Thunderstorms • Observation: Apple tree • DO NOT seek shelter during a thunderstorm under an isolated tree. • Lightening Detection and Suppression • Lightening direction finder detects radiowaves produced by lightening, spherics • National Lightening Detection Network • Suppression: seed clouds with aluminum

  11. Tornadoes • Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. • Tornado life cycle • Organizing, mature, shrinking, decay stage • Tornado outbreaks • Families, super outbreak

  12. Tornadoes • Tornado Occurrence • US experiences most tornadoes • Tornado Alley (warm, humid surface; cold dry air aloft) • Highest spring, lowest winter • Tornado winds • Measurement based upon damage after storm or Doppler radar • For southwest approaching storms, winds strongest in the northeast of the storm, 220 kts maximum • Multi-vortex tornados

  13. Tornadoes • Seeking shelter • Basement or small, interior room on ground floor • Indoor vs. outdoor pressure • The Fujita Scale • Based upon the damage created by a storm • F0 weakest, F5 strongest • Enhanced Fujita Scale

  14. Tornadic Formation • Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear • Supercell Tornadoes • Wind sheer causes spinning vortex tube that is pulled into thunderstorm by the updraft • Mesocyclone, BWER, rear flank downdraft, vertical stretching, funnel cloud, rotating cloud, wall cloud

  15. Stepped Art Fig. 14-46, p. 402

  16. Tornadic Formation • Nonsupercell Tornadoes • Gustnadoes • Land spout • Cold-air funnels

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