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Severe Thunderstorms Severe Thunderstorms AOSS 102: Extreme Weather 20 November 2015 Multiple Choice Thunderstorms All thunderstorms need three basic ingredients: 1) Moist surface air 2) A conditionally unstable or absolutely unstable atmosphere 3) A mechanism “trigger” that forces air to rise The thunderstorm classifications are: • Ordinary cell (Air mass) thunderstorm • Multicell thunderstorm • Squall-line thunderstorm • Mesoscale Convective Complex (MCC) • Supercell Thunderstorm Characteristics of a Multicell Thunderstorm: Cells at all stages Updrafts and downdrafts are tilted, supplying additional energy. Image Quiz Image Quiz Multicell Thunderstorms Gust Front leading edge of the cold outflowing air around 1000 feet deep with average speeds of 10-30 mph, wind gusts (called straight line winds) around 60 mph May form shelf cloud (if atmosphere is very stable at the ground) and roll cloud (spinning around horizontal axis) Many gust fronts can merge into an ‘outflow boundary' Microbursts localized downburst (less than 2.5 miles wide) that hits the ground and spreads horizontally in a radial burst of wind Winds as high as 150 mph, severe damage possible Downdrafts are mainly created by evaporating rain drops that cool the air. Downdrafts reach the surface, cool air spreads out, forming a gust front. Fig. 11.6, p. 303 Multiple Choice Outflow Boundary from Multicell Storm System Radar image of a outflow boundary, often triggering new thunderstorms Outflow Dust and sand can be picked up by the wind at the outflow boundary. New thunderstorms might form along the outflow boundary. Outburst Winds of a Microburst Forming a Dust Cloud Fig. 11.11, p. 305 Downburst Characteristics Microbursts are small downbursts: Winds do not extend more than 2.5 miles Precipitation evaporates before Virga reaching the ground, cools air Downburst Characteristics: Stationary & Traveling Microbursts Stationary Microburst: Damage Pattern Traveling Microburst: Damage Pattern Microbursts and Aviation Downburst Characteristics Microbursts: An Aviation Hazard Fig. 11.12, p. 306 Thunderstorm Categories: Squall-Line Thunderstorms Fig. 11.13, p. 306 Line of multicell thunderstorms with strong gusty winds directly along or ahead of a cold front Radar image of a pre-frontal squall Causes severe line from Indiana to weather over much Arkansas (red: of the length of the most intense) squall line Most common: Pre-frontal squall line thunderstorms ahead of cold front Triggered by upper-air gravity waves that originate at the cold front Squall-Line Thunderstorms Squall-line thunderstorm with a trailing stratiform cloud layer and tilted updrafts: • tilt promotes development of new cells as storm system moves • rear inflow jet may exceed 100 mph Fig. 11.15, p. 307 Squall Line Thunderstorms Side view of the lower half of the squall-line thunderstorm: If the strong straight-line winds extend horizontally for a considerable distance, the wind storm is called “Derecho” Fig. 11.16, p. 308 Bow-Shaped Squall Line: Bow Echos Preferred area where tornadoes develop: northern side of bow Fig. 11.17,p. 308 Damaging straight-line winds along the squall line: Derecho Bow-Shaped Squall Line: Bow Echos Small bows (mini bows) embedded in squall line Fig. 11.18, p. 309 Downbursts: Derecho Wind Storms • Thunderstorm-induced (non- rotating) straight-line winds • wind damage extends for more than 240 miles • includes wind gusts > 57 mph along most of its length • embedded several 75 mph or Derecho may be associated with a greater (hurricane-strength, single bow echo or with multiple bows. up to 130 mph) wind gusts Damaging Effect of a “Derecho” Straight-Line Windstorm (80 mph) Hurricane-strength wind speeds are often observed Fig. 11.19, p. 309 Derecho Climatology in the U.S. Predominantly from April to August, within minutes, extremely high windsFig. can 11.19, arisep. 309 June 2012 North American Derecho Fig. 11.19, p. 309 June 2012 North American Derecho Red: tornado warnings Yellow: severe thunderstorm warnings, Green: flash flood warnings Tornado / Hurricane-like damage https://en.wikipedia.org/wiki/June_2012_North_American_derecho Fig. 11.19, p. 309 Thunderstorm Categories: Mesoscale Convective Complexes (MCCs) During the summer a number of individual multicell thunderstorms may occasionally grow in size and organize into a large circular convective weather system Can be 1000 times larger than an air mass thunderstorm Can cover entire states, an area bigger than 100,000 km2 Move slowly (< 20 mph) and last for over 12 hours Provide significant portions of the growing season rainfall in the Central U.S., reach their max intensity at night Fueled by moist low-level jet from the south (brings moisture) Severe weather threat: hail, high winds, flash floods & tornadoes Fig. 11.20, p. 310 Satellite image showing the cold cloud tops (dark red) of an MCC in Kansas Radar image of an MCC moving SSEward into Kentucky. Red: strong thunderstorm activity Fig. 11.21, p. 311 Radar Image Short Answer What Makes a Thunderstorm Severe ? A thunderstorm is considered “severe” if it contains one of the following: • Hail with a diameter of three-quarter inch or greater • Wind damage or gusts of 50 knots (58 mph) or greater • A tornado Supercell Thunderstorm: A large, long-lasting thunderstorm with a single rotating updraft Fig. 11.23, p. 313.
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