an overview of the 27 april 2011 tornado outbreak

An Overview of the 27 April 2011 Tornado Outbreak

Dr. Scott M. RochetteDepartment of the Earth Sciences

The College at Brockport

OverviewWhat 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

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)

Are these tornadoes?

NoNo

Yes!

(Photos courtesy of NSSL, NWS, and NASA)

How do tornadoes form?Tornadoes form from severe thunderstormsFour ingredients for severe thunderstorms:

moistureliftinstabilityvertical 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

SupercellsThe strongest tornadoes come from

supercells

(Diagram courtesy of quedoc.org)

How do you tell how strong (or weak) tornadoes are?Enhanced Fujita (EF) Scale

tornado strength determined by damage surveys

CANNOT BE FORECASTEDpotential for strong/violent tornadoes can be

predicted

EF-0: weak tornado (65-85 mph) – minor damage

EF-1: weak tornado (86-110 mph) – moderate damage

(All photos courtesy of NWS)

EF-2: strong tornado (111-135 mph) – considerable damage

EF-3: strong tornado (136-165 mph) – severe damage

(All photos courtesy of NWS)

EF-4: violent tornado (166-200 mph) – extreme damage

EF-5: violent tornado (>200 mph) – massive/incredible 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

EF-0 EF-1 EF-2 EF-3 EF-4 EF-50

40

80

120

160

105134

5122 12 3

Number of Tornadoes Per Category

27 April 2011292 tornadoes (almost twice as many as 3-4

April 1974)Fourth deadliest tornado day (worst since

1932)

Surface conditions

Lower troposphere

Middle troposphere

Upper troposphere

We knew it was coming!

How bad was it?

How bad was it?

Tuscaloosa, AL (photo courtesy of Dusty Thompson, Tuscaloosa News

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

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

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

How bad was it?

Package from Tuscaloosa…(photo courtesy of hamwx.com)

…landed here(50 miles away!)

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

tornadoesHilton, NY, 25 July 2009 (photo courtesy of author)

Summary27 April 2011 outbreak due to a ‘rare’ combination

of ingredients:copious low-level moisture ahead of strong cold frontstrong lift provided by combination of surface front,

mid-tropospheric trough, and upper-tropospheric jetsignificant instability from warm, moist air in lower

troposphere and cool, dry air aloftconsiderable vertical wind shear (fast southerly low-

level flow vs. strong WSW flow aloft)Accurate forecasts likely led to fewer deathsSuch outbreaks are rare in general, even more so

here