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Tuscaloosa Tornado - April 27 th , 2011 Jonathan Christophersen Department of Earth, Ocean and Atmospheric Sciences, Florida State University Introduction The 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 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. • ECMWF Interim Reanalysis full resolution data is used to show the synoptic conditions leading up to the tornado. 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 27 April 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 increasing the 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 that there 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 we can see, further wrapping of the hook-echo has taken place as the RFD pushes the rotating column to the surface. Finally, by 2215 high dBZ returned is a result of debris located in the center of the rotation. The images below show a good representation of the tornado the time of the plot just described. References National 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. Dynamics The above figure shows the change in surface convergence from the morning hours into the afternoon on 27 April 2011. Note the movement of the convergence as the airmass moves eastward. This dynamic effect provided aid to vertical motion of air parcels due to the continuity of mass. Figure 4 shows the change in the vertical wind shear near the time the tornado touched down. The second hodograph shows strong vertical curvature of the wind as a function of height, especially in the lower levels of the atmosphere. This is a result of the the synoptic effect of the upper-level jet interacting 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 to determine what factors played a role in the Tuscaloosa tornado. • Along with these analyses, both upper air and surface observations were used to capture im- portant dynamic and thermodynamic ingredi- ents that contributed to the strengthening of the convection and updrafts, which eventually led into strong supercells. • In conclusion, the conditions were ripe due to create a violent tornado that demolished a large portion of Alabama.
