Tornadoes 2011 – Superoutbreak 2
Dr. Greg Forbes
Severe Weather Expert
The Weather Channel
2011 NWA Teacher’s Weather Workshop
Birmingham, AL
October 17, 2011
Radar echo of the
SUPERCELL
(rotating) thunderstorm,
spawning the EF4 tornado
at Tuscaloosa AL
Lavender-colored ball is due to debris
tossed aloft in the tornado
Strong radar return used to
simulate the tornado in 3D 
Personal Damage Observations from April 27
• I flew in a helicopter over part of the
Tuscaloosa tornado path and crossed part of
another tornado path farther east
• I thought some of the structural damage in
Tuscaloosa would have been F5 in the past
(original Fujita Scale)
• The trees northeast of Tuscaloosa often fell
inward TOWARD the tornado from the
northeast; inflow winds before the strongest
southwest winds arrived!
Tuscaloosa, AL
 EF-0  EF-1
Before April 27
EF-4
EF-3
 EF-1
to 2
Tornadoes are rated
based upon worst damage
After April 27
Now using the
Enhanced Fujita (EF) Scale
^
^
Original
F-Scale
F
^
Enhanced
Fujita
Scale
EF
Engineers said F3 and higher wind
estimates were usually too high. Homes
not built that strongly.
Enhanced Fujita Scale for Houses
Houses have “Achilles heels” that cause them
to fail in winds weaker than 200 mph
EF0
EF1
EF2
EF2
EF3
EF3
EF4
Wind speeds are ESTIMATED
afterwards based upon
surveying damage using
Enhanced Fujita Scale
Better initial home construction could
keep EF1 from meaning “totally destroyed”
 Need more in-home shelters to safe lives
• 87% of tornadoes are Weak (EF0
and EF1)
• Strong (EF2 and EF3) and Violent
(EF4 and EF5) tornadoes
are less common but cause
92% of deaths
Death rate is 50 to 100% greater
at night
EF0
-1
apartment debris
toward northeast
EF4+
home debris
down embankment
toward northwest
(inflow) EF3
trees converge
EF2
-3
trees down from northeast
(left-side inflow)
EF0
Tuscaloosa AL Apartment Complex – southeast corner two-story apartments leveled
(EF4, could have been F5)
Railroad bridge destroyed outside Tuscaloosa – no EF-Scale guidance on bridges
NWS-BMX
Hackleburg AL EF5 – brick home disintegrated
I never thought I’d see so many
deaths from tornadoes as in 2011
Deadliest Known Tornado Years in USA
1. 1925 – 794
2. 1936 – 552
3. 1917 – 551
4. 2011 – 546 (+ ?)
5. 1927 – 540
6. 1896 – 537
2011 had the deadliest tornadoes since 1957:
Joplin MO (159)
Hackleburg AL (72)
Tuscaloosa AL (63)
April 2011 Tornado Outbreaks:
April 2011
Tornado Tally:
- 748 tornadoes
- shattered old April
record (267 in
1974)
-broke record for any
month (542 in
May 2003)

2011 Tornado Tally:
-about 1371 tornadoes through June 30 (record 1304 in 2008)
- 546 killed from 57 killer tornadoes (most since 1936)
- deadliest tornado (Joplin, MO on May 22 killed 159)
since 1947 (Woodard, OK, 181 killed)
- Largest tornado outbreak on record (about 293 tornadoes, 4/26-28)
Factors in Tornado Formation
• Instability – needed to get intense thunderstorms to form; with low cloud bases
• Favorable winds and wind shear – winds turn
clockwise and increase speed in lowest 3000
feet  source of damaging winds, rotation
• A meteorological “trigger”: upper-air trough
or jet streak, low-level convergence or front,
etc.  to help storms break through initial
inhibition
PRIME TORNADO AREA – Large-scale factors
Supercell Thunderstorms:
Strong Instability and Shear
(component)
(Component)
Tail
Wall cloud
Tail Cloud, Independence, KS, 7/8/2008 – Dale Reynolds
Classic
supercell
thunderstorm
The worst tornadoes come
from “supercells”
Wall cloud
Thunderstorms that have large,
long-lived rotating updrafts
Overshooting top – where the updraft reaches storm top
anvil
Very anomalous, persistent
weather pattern in April 2011
Very warm from Ohio Valley to South
Strong and persistent
jet stream and trough
2011 Superoutbreak (AL portion)
Rainsville
AL EF5
Smithville
MS EF5
HackleburgPhil Campbell
AL EF5
 Ringgold, GA
EF-4
 Rainsville EF-5
MS
EF5
Philadelphia EF-5
April 26-28, 2011
Tornado Outbreak
Records
(any outbreak)
 293 tornadoes
 316 deaths (second to
1925)
 2900+ miles of path
 67 tornadoes 400+ yds
wide
 21 states hit
(numbers preliminary)
Joplin MO tornado – May 22, EF5, 159+ killed
- Deadliest tornado in USA since 1947
Preliminary Comparison of Superoutbreaks – 1974 vs 2011
(24 hours each)
*
*
*
*
Forbes’ Impact Index (100 max)
(next in line, 37.46)
68.86
68.53
Until 2011, the benchmark
tornado outbreak was the
“Superoutbreak” of April 3-4, 1974
I studied the 1974 Tornado
Superoutbreak with Dr. Fujita
148
tornadoes
24 hours
Multiple
“suction”
vortices
I developed TORCON in 2009
to help inform people of tornado risk
Yellow – Thunderstorms expected
Red – Severe thunderstorms possible
day of Yazoo City, MS
EF4 Tornado
Apr 24, 2010
TORCON –
TORNADO CONDITIONS INDEX
• Scale of 0 to 10 (First 10 – north AL, 4/27/2011)
• Multiply by 10 to get approximate probability of
a tornado within 50 miles (e.g., 5  50%)
• Based upon forecasts of weather conditions
needed for tornado formation and how well
they may come together
• Combines numerical model guidance, human
expertise

Rear-flank downdraft
Supercell Structure
Tornado families
A supercell
thunderstorm
often spawns a
sequence of
tornadoes
called a
“tornado family”
The gaps between tornadoes are usually downdraft-induced
So the storm-scale downdraft can create (and
destroy) a tornado
Fujita and Forbes – tornado turns and
hook echo evolution during
Superoutbreak
“Classic” Mesocyclone Cycling  tornado family
Relates to Rear-Flank Downdraft
There Are Lots of
Unknowns, Issues
• As few as 10% of supercells produce tornadoes
• NWS Tornado Warning False Alarm Rate (FAR) is about
75% (2008)
• NWS Tornado Warning Probability of Detection (POD)
about 72% (2008)
• NWS Tornado Warning Lead Time 13+ min (2008)
• Radar beam wider than tornado, overshoots subcloud-base tornado at distances from radar
 Improvements if more radars?
Need more efficient warning system for people
to receive warnings
3 km
coverage
Good meso detection
1 km
coverage
Poor NEXRAD coverage
below 1 km AGL
degraded tornado detection
Denser network of low-power
low-cost radars on cell
towers?
McLaughlin et al 2009: BAMS 90(12),
1797-1817
Recent Extreme Events – Global Warming?
Rare/Unprecedented Northern and
Cold Season Events
• Feb 10, 2009 – Strongest and deadliest Feb OK tornado on record
• Jan 7, 2008 – 2 WI tornadoes (Jan - only one previously)
• Jan 7, 2008 – northern IL tornado (Jan – only one other
since 1950)
• Jan 10, 2008 – EF1 tornado near Vancouver, WA (Jan –
only 2 previously since 1950)
• Feb 5-6, 2008 Largest Feb outbreak on record; mid-South
• Oct 18, 2007 – unprecedented killer tornado, northern MI
• Dec 1, 2006 – 1st PA December tornadoes
• Nov 1, 2000 – tornadoes near Bismarck, ND; farthest north
so late in year
• Mar 11-13, 2006 – largest March outbreak
• Sept 22, 2006 – largest non-tropical outbreak
• Dec 29, 2006 – unprecedented TX Dec outbreak
Severe Weather and Global Warming
MAM
JJA
Trapp et al, 2007: Proc. Nat’l. Acad. Sci.
104(50), 19719-19723
CAPE increases – warmer, moister, more unstable
Difference in climate model parameters,
2072-2099 vs 1962-1989, RegCM3
Surface specific humidity increases
Different results for different
climate models
0-6 km shear mostly decreases
NDSEV = # Severe T-storm days, when
CAPEx(0-6 km shear) at least 10,000
2 to 3 more days with severe thunderstorms
NSDEV increases, up to 100% in South and East
Strong upper trough and
upper-level jet streak
Weather Maps, morning April 27, 2011
Approaching frontal system;
Warm moist air off Gulf of Mexico
Tornado Winds are Estimated from Damage
(Original)
Replaced by Enhanced Fujita EF Scale, 2/1/2007
Fujita Scale (F0 to F5)
F0 Damage, EF0 damage
Milestone Tap, Utica IL April 20, 2004
F3 to F4
EF3
EF3, Wheatland, WI, 1/7/2008 – Elizabeth Moore
Saved by a safe room, Wheatland, WI EF3, 1/7/2008 –
Elizabeth Moore
Fujita Scale
F5 Damage in past, now often EF4