1. Chapter 14

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

5. 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

10. 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, 10,000km 2

17. 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

23. 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

25. 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

28. 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

32. 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

34. 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

37. 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

38. 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

43. 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

44. 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

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

49. Tornadic Formation  Nonsupercell Tornadoes Gustnadoes Land spout Cold-air funnels

52. Severe Weather and Doppler Radar  Doppler radar measures the speed of precipitation toward and away radar unit  Two Doppler radars can provide a 3D view  TVS, Doppler lidar  NEXRAD

53. Waterspouts  Rotating column of air that is connected to a cumuliform cloud over a large body of water  Tornadic waterspout