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Chapter 15: Thunderstorms and Tornadoes 40,000 thunderstorms occur everyday somewhere in the world. Thunderstorms produce Heavy rain High winds Lightning & Thunder Hail Tornadoes Thunderstorms occur as cumulonimbus clouds. Two kinds of thunderstorms 1. Air Mass 2. Severe 1. Air Mass Thunderstorm most common least destructive “self-extinguishing” fairly upright downdrafts cut off warm moist air to updraft that initially created thunderstorm Characteristics Usually live less than one hour Form within air mass Form away from frontal boundaries Some definitions… Cell – a single, individual thunderstorm Updraft – a column of warm, moist, rising air in a thunderstorm Downdraft – a column of cold, descending air in a thunderstorm Air Mass Thunderstorm Life Cycle 3 stages I. cumulus stage II. mature stage III. dissipating stage Cumulus Stage Unstable, warm parcels rise Cu clouds form Cu 7-9 km deep Updraft speeds up to 20ms-1 (45 mph) Cu stage ends when precipitation begins to fall. I. Mature Stage Begins when precipitation starts to fall Falling precipitation drags on air around it to initiate downdraft Updrafts and downdrafts, both distinct, dominate cloud Lightning, thunder, precipitation most intense Anvil forms, giving the thunderstorm its characteristic appearance. Dissipating Stage An increasing amount of the cloud ends up precipitating So…an increasing amount of the cloud is represented by downdrafts Dominant downdraft cuts off inflow to updraft Bottom half precipitates out of evaporates Top may persist a while Severe Thunderstorm A thunderstorm that produces Winds of at least 58 mph (50 knots, 93 km/h) Hail of at least 3/4" (1.9 cm) Tornadoes This is the official definition of a severe thunderstorm… ***No accounting for rainfall*** Extreme flash flooding can eliminate a town or city - but it's not "severe." Any thunderstorm can become severe Criteria may be met for only a few minutes---hail, winds, even small tornadoes (“gustinadoes”) BUT… Severe thunderstorms usually have more organization that the average air mass thunderstorm. Updraft and downdraft typically work together in a severe thunderstorm Supercell Thunderstorm - a thunderstorm with a single, rotating updraft. Distinguished from air mass thunderstorm by organized updraft Diameters: 20-50 km (12-30 miles) (Air mass thunderstorms typically smaller) Smaller than organized thunderstorm systems (squall lines, mesoscale convection systems) Lifetime: 2 to 4 hours (Air mass thunderstorms last < 1 hour) Organized Thunderstorm Systems Far larger than single air mass or supercell thunderstorm Mesoscale Convective Systems (MCS) groups of individual thunderstorms clustered together Individual thunderstorms come from a common origin (warm, moist, unstable air) But consider when… These favorable condition to form thunderstorms often occurs over large areas (10-1000 km) In these regions, thunderstorms often lead directly to the formation of others Two primary MCS types: 1. Squall line (line-type MCS) 2. Mesoscale Convective Complex (circular MCS) 1. Squall Line - Large number of individual thunderstorm cells arranged in a band. 500 km (300 mi) long band (average) 10 hour life span (average) usually parallel to and 300-500 km (180-300 mi) ahead of cold front. Gust front - a cold, dense wedge of air that surges ahead of the squall line. Formed by spreading out of downdraft air Can initiate new or maintain current updrafts 2. The Mesoscale Convective Complex (MCC) - An MCS having certain satellite-based size & shape criteria Shape - not a line---criteria require round or oval shape Structure - more complex than squall Line Downburst - a strong downdraft which induces an outburst of damaging winds on or near the ground may be straight or curved, but always highly divergent Macroburst - large downburst with outburst winds extending in excess of 4 km (>2.5 miles) in horizontal dimension. Intense macrobursts can cause widespread, tornado-like damage Damaging winds last 5 to 30 minutes Can be as high as 60 m/sec (134 mph). Microburst - small downburst with its outburst, damaging winds extending only 4 km (≤ 2.5 miles) or less. intense microburst winds as high as 75 m/sec (168 mph) From UIUC Microbursts pose a distinct danger to aircraft (esp. on landing) From UIUC Tornadoes rotating Def - a rapidly column of air with extreme horizontal winds o most often beneath a Cb cloud Average Characteristics Diameter Path Length Lifespan Wind Speeds Forward Speed 100 yards 2.5 miles A few minutes 40 – 280 mph 30 mph Waterspouts Similar to a tornado, but over a water body - Usually in tropics, over warm water body - Usually smaller than tornadoes Diameter - 17-330 feet Wind speeds ~ 90 mph - Develop from Towering Cu - Tropical waterspouts usually develop up from water surface Funnel Cloud A tornado that has not yet reached the surface Tornadoes: When and Where Everywhere in the world, all months, all hours, BUT… - More in the U.S. than anywhere **Due largely to topography** - > 1,000 tornadoes in U.S. annually Florida and Oklahoma are the tornado capitals of the U.S. Florida Hurricane induced May largest number of tornadoes Late afternoon generally preferred Missouri Tornadoes Late May – June Western MO more prone Average 7 per year La Nina year, positive phase of Pacific Decadal Oscillation up to 12 per year Tornado Formation Gustinadoes – outflow driven; bottom up Supercells – well-developed from wind shear changes; top down Remember wind shear??? 55 15 5000 feet Rotation from shear becomes tilted into the vertical Mesocyclone – large vortex up to 12 miles in diameter deep inside cloud several km above surface can precede tornado development up to 30 minutes Tornado Damage “Equalizing pressure” by opening windows is a MYTH Fujita Scale - a damage scale used to rank tornado intensity - most applicable to damage to residential homes “F-4 tornado” no such thing “A tornado that does F-4 damage” that I’ll buy… Severe Weather Safety Watch – a statement from the National Weather Service (NWS) that says weather conditions are right for the formation of severe weather - covers large areas - issued by the Storm Prediction Center Q: What’s your job? A: Keep an eye on the sky--keep a radio or TV on. Severe Weather Safety Warning – a statement from the National Weather Service that says severe weather has developed or is imminent. - Usually covers one or several counties - Issued by your local NWS Warning and Forecast Office (WFO) ***FIND SHELTER*** Severe Weather Safety - Don’t open windows - Go to an internal room on the lowest floor Basement Bathroom – pipes form a “cage” around you - In a car, don’t try to outrun the storm A ditch is probably ideal Underpasses are NOT necessarily safe