Chapter 14 THUNDERSTORMS Thunderstorms A storm containing lightening

Chapter 14 THUNDERSTORMS

Thunderstorms A storm containing lightening and thunder; convective storms may have heavy rain hail Ordinary Cell Thunderstorms Air-mass thunderstorms: limited wind sheer Stages: cumulus, mature, dissipating Entrainment, downdraft, gust front

A dissipating thunderstorm

Thunderstorms Multi-cell Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top

Multicell storm

Thunderstorms Multi-cell Thunderstorms Micro-bursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind; wind shear, virga

Dust clouds from microbursts

Thunderstorms Multi-cell Thunderstorms Gust Front: leading edge of the cold air out- flowing air; shelf cloud, roll cloud, outflow boundary

Thunderstorms Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50 kts, or tornado, tilted updraft/downdraft

Shelf cloud

Thunderstorms Multi-cell Thunderstorms Squall-line thunderstorms; line of multi-cell thunderstorms, pre-frontal squall-line, derecho

Pre frontal Squall line

Pre frontal Squall line

The thunderstorms are producing strong straight-line winds called a derecho

Thunderstorms Multi-cell Thunderstorms 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, 000 km 2

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 Rain free base

A supercell thunderstormwith a tornado sweeps over Texas

Thunderstorms Supercell thunderstorms Strong vertical wind shear Surface, 850 mb, 700 mb, 500 mb, 300 mb conditions low-level jet

Thunderstorms Supercell thunderstorms Cap and convective instability

Thunderstorms and the Dryline Sharp, horizontal change in moisture Thunderstorms form just east of dryline c. P, m. T, c. T

Fig. 14 -23, p. 384

Thunderstorms Floods and Flash Floods Large floods can be created by training of storm systems, Great Flood of 1993 Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow moving thunderstorm

downtown Des Moines, Iowa, during July, 1993 Fig. 14 -25, p. 387

Thunderstorms Topic: Big Thompson Canyon July 31, 1976, 12 inches of rain in 4 hours created a flood associated with $35. 5 million in damage and 135 deaths Slow moving Thunderstorm

Fig. 1, p. 386

Flash Floods Slow moving or Stalled thunder Storm, especially in canyon areas 135 deaths in 1976 flood 12 inches of rain in 4 hours (normal ~16 inches /year)

Thunderstorms Distribution of Thunderstorms Most frequent Florida, Gulf Coast, Central Plains Fewest Pacific coast and Interior valleys Most frequent hail Central Plains