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An Overview of the 27 April 2011 Tornado Outbreak

An Overview of the 27 April 2011 Tornado Outbreak. Dr. Scott M. Rochette Department of the Earth Sciences The College at Brockport. Overview. What are tornadoes and how do they form? How can you tell how strong (or weak) tornadoes are? What happened? Why did it happen? How bad was it?

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An Overview of the 27 April 2011 Tornado Outbreak

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  1. An Overview of the 27 April 2011 Tornado Outbreak Dr. Scott M. Rochette Department of the Earth Sciences The College at Brockport

  2. Overview • What are tornadoes and how do they form? • How can you tell how strong (or weak) tornadoes are? • What happened? • Why did it happen? • How bad was it? • Can tornadoes happen here? • Summary

  3. What are tornadoes? • Tornadoes are violently rotating columns of air that are: • in contact with the ground • connected to a thunderstorm cloud (Photos courtesy of NSSL)

  4. Are these tornadoes? No No Yes! (Photos courtesy of NSSL, NWS, and NASA)

  5. How do tornadoes form? • Tornadoes form from severe thunderstorms • Four ingredients for severe thunderstorms: • moisture • lift • instability • vertical wind shear (speed and directional) • ALL FOUR OF THESE INGREDIENTS NEED TO BE PRESENT AT THE SAME TIME IN THE SAME PLACE! • Large tornado outbreaks are quite rare because of this

  6. Supercells • The strongest tornadoes come from supercells (Diagram courtesy of quedoc.org)

  7. How do you tell how strong (or weak) tornadoes are? • Enhanced Fujita (EF) Scale • tornado strength determined by damage surveys • CANNOT BE FORECASTED • potential for strong/violent tornadoes can be predicted

  8. EF-0: weak tornado (65-85 mph) – minor damage • EF-1: weak tornado (86-110 mph) – moderate damage (All photos courtesy of NWS)

  9. EF-2: strong tornado (111-135 mph) – considerable damage • EF-3: strong tornado (136-165 mph) – severe damage (All photos courtesy of NWS)

  10. EF-4: violent tornado (166-200 mph) – extreme damage • EF-5: violent tornado (>200 mph) –massive/incredible damage (All photos courtesy of NWS)

  11. What happened? • 327 confirmed tornadoes occurred in 21 states and Canada from 25-28 April 2011 (record 875 in April 2011) • 321 dead (out of 361 dead for entire month) • Estimated $6,000,000,000 in damages

  12. 27 April 2011 • 292 tornadoes (almost twice as many as 3-4 April 1974) • Fourth deadliest tornado day (worst since 1932)

  13. Surface conditions

  14. Lower troposphere

  15. Middle troposphere

  16. Upper troposphere

  17. We knew it was coming!

  18. How bad was it?

  19. How bad was it? Tuscaloosa, AL (photo courtesy of Dusty Thompson, Tuscaloosa News

  20. How bad was it? EF-4 damage in Ringgold, GA (photo courtesy of NWS)

  21. How bad was it? EF-5 damage in Phil Campbell, AL (photo courtesy of NWS)

  22. How bad was it? Satellite view of Tuscaloosa tornado path (photo courtesy of NASA)

  23. How bad was it? …landed here (50 miles away!) Package from Tuscaloosa… (photo courtesy of hamwx.com)

  24. Can tornadoes happen here? • Yes, but… • They are somewhat rare around here (about two per year) • Generally very weak (EF-0 or EF-1), like most tornadoes Hilton, NY, 25 July 2009 (photo courtesy of author)

  25. Summary • 27 April 2011 outbreak due to a ‘rare’ combination of ingredients: • copious low-level moisture ahead of strong cold front • strong lift provided by combination of surface front, mid-tropospheric trough, and upper-tropospheric jet • significant instability from warm, moist air in lower troposphere and cool, dry air aloft • considerable vertical wind shear (fast southerly low-level flow vs. strong WSW flow aloft) • Accurate forecasts likely led to fewer deaths • Such outbreaks are rare in general, even more so here

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