Wednesday, December 1, 2010

Atlanta, LA EF-4 Tornado

Monday afternoon strong to severe storms developed over East Texas and West Louisiana as a cold front moved across the area. As we see many times this during cool season severe weather events, this situation involved a low instability high shear environment. The storm that produced the EF-4 tornado developed in the Toledo Bend area of East Texas early Monday afternoon south of Hemphill in an area of MLCAPE just under 1000 J/kg.
You combine this with 0-6km bulk shear around 40 knots, and the atmosphere was set for the possibility of supercells thunderstorms. As the storm moved to the northeast it entered an area where MLCAPE was approaching 750 J/kg, not overly impressive when one thinks of tornadic supercells. However, when you look at the surface based lifter index you will see the values are around -4°C,
which shows a decent amount of instability, more than likely produce what is called short fat CAPE. This would allow for strong updrafts to form in the lower level of the atmosphere where the overall shear was very high. 0-1km shear was on the order of 40-45 knots near Atlanta one hour before the tornado was produced.
At the same time the 0-1km Storm Relative Helicity was over 400 in southwestern Winn Parish.
At 2:46PM the circulation associated with the developing tornado was about 5 miles south of Natchitoches, LA moving into the area of 400+ 0-1km SRH. 20 Minutes later the circulation was very intense just to the west of Atlanta, 4 minutes before the tornado touched down. Shortly after touching down, this tornado destroyed a well built brick home causing EF-4 damage.

Another interesting observation is the decrease in the vertical intensity of the reflectivity values before the tornado was produced. The storm was at its most intense, at least by the depth of reflectivity, 10 minutes before the tightest velocity couplet occurred. At this time 60dbz reflectivity was nearing 30,000 feet in the atmosphere.
At the time of the strongest velocity couplet the height of the 60dbz reflectivity was nearly cut in half, just reaching above 15,000 feet in the atmosphere. Could this collapse help in the sudden intensification of the tornado to EF-4 strength where winds were estimated to have reached 170MPH for a short path?Studies have shown in the past that supercell mesocylones have strengthened after a weakening phase in overall vertical thunderstorm strength. It is interesting to see the short path of the EF3 to EF4 damage with this storm and that it occurred after this decrease in intensification.

The tornado finally dissipated just northwest of Winnfield for a path length of 14 miles. Later, this same thunderstorm would produce another tornado rated EF-1 near Bosco before moving into Mississippi.

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