Overview of U.S. Tornadoes

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Includes data illustrations and comparison of two major tornado outbreaks

Transcript of Overview of U.S. Tornadoes

  • Overview of Tornadoes in the United States Including comparison of Super Outbreak, April 3 4, 1974 with Tornado Outbreak, April 25 -28, 2011 Jeffrey Permuy | May 26, 2011
  • Section I What you need to know about tornadoes
  • Tornado: Definition (specific to the U.S.) Violently rotating column of air aided by: 1. Warm, moist air advancing northward from the Gulf of Mexico at low levels of the atmosphere. Near the surface (about 1000mb of atmospheric pressure). 2. Dry air advecting east of the Rocky Mountains at mid levels of the atmosphere. Near 10,000 feet (about 700mb of atmospheric pressure). 3. Colder air channeling southward from the Arctic at higher levels of the atmosphere. Near 18,000 feet (about 500mb of atmospheric pressure). Location of the Jet Stream: ribbon of strong moving air dictating steering currents for weather at the surface.
  • Tornadoes: Ingredients and Recipe How it comes together: Geography and topography of North America provides an ideal breeding ground for the development of severe and supercell thunderstorms and tornadoes, particularly in the flat basin of the Midwest, and the Southeast where the confluence and clash of different air masses adds to atmospheric instability and vertical wind shear. Rocky Mountains along the western terrain of the continent and the Appalachian chain in the east, forms an upside triangle (like a wedge) allowing cold air to funnel southward, mixing with dry air from the leeward side of the Rockies and with warm, moist air from the Gulf of Mexico.
  • Tornadoes: Seasonal shifts Frequency of tornadoes by season and region: Early spring: Tornadoes tend to occur in the Southeast, and Gulf states from February to April. Late spring: Tornadoes spread farther north into Kansas, Nebraska and the Tennessee Valley region. Mid-summer: Most of Tornado Alley is active and tornadoes may occur throughout the U.S. Late summer: Tends to bring some of the stronger tornadoes into the upper Midwest and Ohio valleys. Autumn: Pattern shifts southward. Winter: Fewest tornadoes occur.
  • Tornadoes: Other characteristics Did you know Vast majority of tornadoes develop within a supercell thunderstorm: a thunderstorm with a deep rotating updraft known as a mesocyclone. Supercells are the most powerful thunderstorms; hence, they spawn proportionally more dangerous tornadoes. The highest incidence of tornadoes are in late spring and early summer. Tornadoes typically move in a northeast trajectory with an average forward speed of about 30 mph. Twisters have been tracked at slower and much faster speeds. 95% of twisters rotate counterclockwise (cyclonic) because air moves west to east as it ascends the mesocyclone. The rare anticyclonic tornadoes, clockwise rotation, are the norm in the Southern Hemisphere.
  • Tornado from a supercell thunderstorm (illustration)
  • Tornado Detection: Aided by Observation Inflow bands Ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south. The presence of inflow bands suggests that the storm is gathering low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.
  • Tornado Detection: Aided by Observation Beavers Tail Smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. Beavers Tail usually skirts around the southern edge of the precipitation area, suggesting presence of rotation.
  • Tornado Detection: Aided by Observation Wall Cloud Isolated cloud lowering attached to the rain-free base of the thunderstorm, usually to the rear of the visible precipitation area. A wall cloud is about two miles in diameter and marks the area of strongest updraft in the storm. A wall cloud does not always produce a tornado. When it does, it exists for 10 - 20 minutes before a tornado appears. A wall cloud may rotate with strong surface winds flowing into it, and have rapid vertical motion indicated by small cloud elements rising into the rain-free base.
  • Tornado Detection: Aided by Observation Rear Flank Downdraft (RFD) Downward rush of air on the back side of the storm that descends along with the tornado. RFD looks like a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. It can also look like curtains of rain wrapping around the cloud base circulation. Eventually, the tornado and RFD will reach the ground within a few minutes of each other. RFD causes gusty surface winds that occasionally have embedded downbursts. It is the motion in the storm that causes the hook echo feature on radar.
  • Tornado Detection: Aided by Observation Condensation Cloud Water droplets that extend downward from the base of the thunderstorm. If it is in contact with the ground it is a tornado; otherwise, it is a funnel cloud. Dust and debris beneath the condensation funnel confirm a tornado's presence. Condensation cloud may or may not be connected with a wall cloud.
  • Tornado Detection: Aided by Radar Tornadic Vortex Signature (TVS) Mescocyclone Hook Echo These distinguishing features are picked up by Doppler radar data and analyzed by computer programs, making it easier to identify severe weather. Forecasters are trained to recognize precise radar signatures produced by sophisticated algorithms. Today's weather radars typically provide on average 11 minutes lead-time, pinpointing locations in harm's way with a high degree of accuracy.
  • Tornado Prediction: Aided by Integration Advanced Weather Interactive Processing System (AWIPS) Interactive computer system that integrates all meteorological, hydrological, satellite and radar data, enabling forecasters to prepare and issue more accurate and timely forecasts and warnings. This telecommunications system, with its advanced information processing, is the cornerstone of the National Weather Service (NWS) modernization and restructure.
  • Tornado from a supercell thunderstorm (illustration with descriptors)
  • Tornado from a violent supercell thunderstorm (radar signature with descriptors) Radar reflectivity image from May 3, 1999 indicating powerful (F5*) tornado that devastated Moore, Oklahoma (NE of twister at time of image). Mescocyclone and tornado make up the classic "hook" shape of the echo, a common feature with strong and violent tornadoes. *Tornado rating by wind speed and strength (F- scale; EF-scale) are covered in the next three slides.
  • Tornadoes: Rating by wind speed Fujita (F) Scale Developed by Dr. Tetsuya Theodore Fujita to estimate tornado wind speeds based on residual damage. Its ratings have been used since 1971 and was instrumental in the analysis of the Super Outbreak, April 3 4, 1974. Enhanced Fujita (EF) Scale Developed by a forum of meteorologists and wind engineers as an enhancement (more accurate measure) to the F scale. It has been approved by the National Weather Service (NWS) and replaced the original F scale in February 2007.
  • Tornadoes: Fujita Scale *Based on survey of 18,545 tornadoes occurring from 1950 to 1977, for which intensity and path could be determined.
  • Tornadoes: Enhanced Fujita Scale On average, 77% of tornadoes are considered weak (EF0 or EF1). The other 23% of tornadoes are significant (EF2 to EF5). About 5% of tornadoes are intense (EF3 or greater). Only 0.1% of tornadoes are the most violent (EF5).
  • Section II Data illustrations and trends
  • Tornadoes: From 1950 2010 2004 was the most prodigious year in twisters (1817) since the NWS began documenting and archiving tornadoes in 1950. The 10 most prolific years in tornadoes (1200+) have occurred since 1992. The bar graph illustrates an undulating pattern with an upward slope.
  • Tornadoes: From 1950 2011* Additional information: Much has changed since NWS began forecasting tornadoes in 1950: Population increase: More tornadoes are observed and reported. Better technology: More tornadoes are detected by meteorologists. Greater knowledge: Fewer tornadoes are mistaken for straight line wind damage; downbursts; gustnadoes, short-lived whirling gust front. Confirmation: Fewer tornadoes are double counted by separate eye witnesses, reporting the same twister. From 1950 to 1989, only two of the yearly totals exceeded 1,000 tornadoes (1973 and 1982). From 1990 to 2010 all but one year yielded more than 1,000 tornadoes (2002); annual average during this trend is 1,255. 1,228* tornadoes have already been documented in 2011, pending confirmation from the NWS (as of May 26th). No country or continen