Dr. Scott M. Rochette Department of the Earth Sciences The
College at Brockport
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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
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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)
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Are these tornadoes? No Yes! (Photos courtesy of NSSL, NWS, and
NASA)
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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
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Supercells The strongest tornadoes come from supercells
(Diagram courtesy of quedoc.org)
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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
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Enhanced Fujita Scale
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EF-0: weak tornado (65-85 mph) minor damage EF-1: weak tornado
(86-110 mph) moderate damage (All photos courtesy of NWS)
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EF-2: strong tornado (111-135 mph) considerable damage EF-3:
strong tornado (136-165 mph) severe damage (All photos courtesy of
NWS)
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
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27 April 2011 292 tornadoes (almost twice as many as 3-4 April
1974) Fourth deadliest tornado day (worst since 1932)
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Surface conditions
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Lower troposphere
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Middle troposphere
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Upper troposphere
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We knew it was coming!
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How bad was it?
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Tuscaloosa, AL (photo courtesy of Dusty Thompson, Tuscaloosa
News
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How bad was it? EF-4 damage in Ringgold, GA (photo courtesy of
NWS)
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How bad was it? EF-5 damage in Phil Campbell, AL (photo
courtesy of NWS)
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How bad was it? Satellite view of Tuscaloosa tornado path
(photo courtesy of NASA)
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How bad was it? Package from Tuscaloosa (photo courtesy of
hamwx.com) landed here (50 miles away!)
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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)
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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