tuscaloosa tornado - april 27th, 2011
TRANSCRIPT
TuscaloosaTornado-April27th, 2011Jonathan Christophersen
Department of Earth, Ocean and Atmospheric Sciences, Florida State University
IntroductionThe afternoon of 27 April 2011 saw one of the deadliest tor-
nado outbreaks in U.S. history. Among the numerous reports
of tornadoes during this day, Tuscaloosa, AL saw one of the
strongest. With a peak wind speed of 165 kts, this EF4 tor-
nado had a lifespan of nearly 2 hours and left a track of nearly
90 miles. The result of this particular cell would end up cost-
ing over 100 million dollars in repairs, approximately 1500
people injured and nearly 70 fatalities.
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 27APR 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 supercellpasses.
• ECMWF Interim Reanalysis full resolution data isused to show the synoptic conditions leading up to thetornado.
Upper Air Observations
Fig. 1: Soundings for BMX at 12Z-00Z on 27 and 28 April
Figure 1 shows the upper air observations for 12Z on 27 April
at station BMX. At this point in time minimal CAPE values
of approximately 500J/kg are present. Fig. 2 shows large
values of mid-level CAPE of nearly 4700J/kg, 12 hours after
the first figure. This shows that through the heating of the
day, potential energy built up in the mid to upper levels of the
atmosphere. Due to convection parcels were able to reach the
LFC and tap into the reservoir of energy. That entails strong
vertical acceleration of air yielding possible rapid updrafts.
Synoptic Overview• 25 April showed a cold front moving over the southern plains. The atmosphere began to further destabilize due to warm
moist air being funneled in from the Gulf of Mexico.
• An associated mid-level jet (near 500 mb) positioned itself at the base of the low pressure center on the morning of 27April 2011. Simultaneously an upper-level jet, near 200 mb, was oriented above the mid-level jet.
• This led to enhanced shearing of the already unstable airmass.
• As the system moved eastward it encountered areas of large surface convergence, vertical shear and CAPE, thus increasingthe likelihood of supercell activity.
Radar Imaging
Fig. 2: Radar imagery of the Tuscaloosa tornado at around 2215 UTC.
Radar imagery picks up rotation southwest of Tuscaloosa at around 2201. Already it is noticeable thatthere is an area of strong updraft and a prominent forward-flank downdraft (FFD) and rear-flank downdraft(RFD). The second panel at top shows the structure of the hook-echo after 5 minutes has elapsed. As wecan see, further wrapping of the hook-echo has taken place as the RFD pushes the rotating column to thesurface. Finally, by 2215 high dBZ returned is a result of debris located in the center of the rotation. Theimages below show a good representation of the tornado the time of the plot just described.
ReferencesNational Centers for Environmental Prediction, 2012: SPC Severe Weather Events Archive. [http://spc.noaa.gov/exper/archive/events/.]
National Climatic Data Center, 2012: Storm Events Database. [http://www.ncdc.noaa.gov/stormevents/eventdetails.jsp?id=314662.]
Markowski, P. and Richardson, Y., 2010: Mesoscale Meteorology in Midlatitudes. Wiley-Blackwell, 407 pp.
DynamicsThe above figure shows the change in surface convergencefrom the morning hours into the afternoon on 27 April 2011.Note the movement of the convergence as the airmass moveseastward. This dynamic effect provided aid to vertical motionof air parcels due to the continuity of mass.
Figure 4 shows the change in the vertical wind shear nearthe time the tornado touched down. The second hodographshows strong vertical curvature of the wind as a function ofheight, especially in the lower levels of the atmosphere. Thisis a result of the the synoptic effect of the upper-level jetinteracting with the mid-level jet creating high levels of shear.
Fig. 3: Surface convergence/divergence at 12Z-00Z.
Fig. 4: Hodograph at BMX - 12Z 27 APR and 00Z 28 APR
Summary & Conclusion• This case study has provided both synoptic
background as well as mesoscale analysis todetermine what factors played a role in theTuscaloosa tornado.
• Along with these analyses, both upper air andsurface observations were used to capture im-portant dynamic and thermodynamic ingredi-ents that contributed to the strengthening ofthe convection and updrafts, which eventuallyled into strong supercells.
• In conclusion, the conditions were ripe dueto create a violent tornado that demolished alarge portion of Alabama.