The document summarizes a deadly EF4 tornado that struck Tuscaloosa, Alabama on April 27, 2011. It analyzed the synoptic conditions leading up to the tornado, including a cold front moving over the southern plains that destabilized the atmosphere. Upper-level jets interacted with a mid-level jet to enhance wind shear over areas of large surface convergence, vertical wind shear, and high CAPE, increasing the likelihood of supercell storms. Radar imagery showed the developing tornado southwest of Tuscaloosa with a prominent hook echo and debris ball. Soundings showed increasing CAPE and wind shear from the morning through afternoon, contributing to strengthened convection and updrafts that led to the violent tornado.

1. Tuscaloosa Tornado - April 27 th, 2011
Jonathan Christophersen
Department of Earth, Ocean and Atmospheric Sciences, Florida State University
Introduction Synoptic Overview Dynamics
The afternoon of 27 April 2011 saw one of the deadliest tor- • 25 April showed a cold front moving over the southern plains. The atmosphere began to further destabilize due to warm The above figure shows the change in surface convergence
nado outbreaks in U.S. history. Among the numerous reports moist air being funneled in from the Gulf of Mexico. from the morning hours into the afternoon on 27 April 2011.
• An associated mid-level jet (near 500 mb) positioned itself at the base of the low pressure center on the morning of 27 Note the movement of the convergence as the airmass moves
of tornadoes during this day, Tuscaloosa, AL saw one of the
April 2011. Simultaneously an upper-level jet, near 200 mb, was oriented above the mid-level jet. eastward. This dynamic effect provided aid to vertical motion
strongest. With a peak wind speed of 165 kts, this EF4 tor- of air parcels due to the continuity of mass.
nado had a lifespan of nearly 2 hours and left a track of nearly • This led to enhanced shearing of the already unstable airmass.
Figure 4 shows the change in the vertical wind shear near
90 miles. The result of this particular cell would end up cost- • As the system moved eastward it encountered areas of large surface convergence, vertical shear and CAPE, thus increasing the time the tornado touched down. The second hodograph
ing over 100 million dollars in repairs, approximately 1500 the likelihood of supercell activity. shows strong vertical curvature of the wind as a function of
people injured and nearly 70 fatalities. height, especially in the lower levels of the atmosphere. This
is a result of the the synoptic effect of the upper-level jet
Radar Imaging interacting with the mid-level jet creating high levels of shear.
Data Used
• NCDC’s Storm Events Database which contains a com-
prehensive assessment of the events that took place.
• SPC sounding analysis for Skew-T/Log-P charts for 27
APR and 28 APR.
• WSR-88D base reflectivity data from BMX (Birming-
ham, AL) is used to elucidate the structure of the su-
percell at peak intensity. The provided imagery at-
tempts to show track and intensity as the supercell
passes.
Fig. 3: Surface convergence/divergence at 12Z-00Z.
• ECMWF Interim Reanalysis full resolution data is
used to show the synoptic conditions leading up to the
tornado.
Upper Air Observations
Fig. 4: Hodograph at BMX - 12Z 27 APR and 00Z 28 APR
Summary & Conclusion
Fig. 2: Radar imagery of the Tuscaloosa tornado at around 2215 UTC.
• This case study has provided both synoptic
Radar imagery picks up rotation southwest of Tuscaloosa at around 2201. Already it is noticeable that background as well as mesoscale analysis to
there is an area of strong updraft and a prominent forward-flank downdraft (FFD) and rear-flank downdraft determine what factors played a role in the
Fig. 1: Soundings for BMX at 12Z-00Z on 27 and 28 April
(RFD). The second panel at top shows the structure of the hook-echo after 5 minutes has elapsed. As we Tuscaloosa tornado.
Figure 1 shows the upper air observations for 12Z on 27 April can see, further wrapping of the hook-echo has taken place as the RFD pushes the rotating column to the
• Along with these analyses, both upper air and
at station BMX. At this point in time minimal CAPE values surface. Finally, by 2215 high dBZ returned is a result of debris located in the center of the rotation. The
surface observations were used to capture im-
of approximately 500J/kg are present. Fig. 2 shows large images below show a good representation of the tornado the time of the plot just described.
values of mid-level CAPE of nearly 4700J/kg, 12 hours after
portant dynamic and thermodynamic ingredi-
the first figure. This shows that through the heating of the
ents that contributed to the strengthening of
day, potential energy built up in the mid to upper levels of the
References the convection and updrafts, which eventually
atmosphere. Due to convection parcels were able to reach the
National Centers for Environmental Prediction, 2012: SPC Severe Weather Events Archive. [http://spc.noaa.gov/exper/archive/events/.] led into strong supercells.
National Climatic Data Center, 2012: Storm Events Database. [http://www.ncdc.noaa.gov/stormevents/eventdetails.jsp?id=314662.]
LFC and tap into the reservoir of energy. That entails strong Markowski, P. and Richardson, Y., 2010: Mesoscale Meteorology in Midlatitudes. Wiley-Blackwell, 407 pp. • In conclusion, the conditions were ripe due
vertical acceleration of air yielding possible rapid updrafts. to create a violent tornado that demolished a
large portion of Alabama.