National Weather Service Forecast Office - Mobile/Pensacola - 12/20/2012 Mobile and Avalon Point Tornadoes

20 Dec 2012 Mobile, AL and Avalon Point, FL Tornadoes

Figure 1
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During the morning hours of 20 December 2012, an EF-1 tornado developed over Mobile, AL in association with a thunderstorm moved northeastward of the Gulf of Mexico into coastal Mississippi and then further northeastward into central Mobile County Alabama. Figure 1 yields a high resolution path of the tornado that formed around 449 AM CST very near Davidson High School and ended near Telegraph Road in Prichard, AL. The path length was 7 miles and was 50 to 75 yards wide.

A powerful December storm system centered over Missouri was dumping snow on its northwest side over the U.S. Upper Midwest while simultaneously producing severe thunderstorms over portions of the U.S. Deep South (see Figure 2). Along the Central Gulf Coast, the atmosphere had begun to destabilize late on the 19th, and this trend continued into the earlier morning hours as unseasonably mild conditions prevailed during the very early morning of the 20th as minimum temperatures only fell into the 60s in the interior to lower 70s along the coast. Around 205 AM CST, radar first began to detect a very weak developing rainshower located just off the southeast coast of Louisiana (Figure 3a). It was moving to the northeast. By 321 AM CST, significant rotation developed throughout a great depth of the thunderstorm’s updraft and the parent mesocyclone formed. This would become the parent circulation of the EF-1 Mobile, AL tornado (Figure 3b). The first tornado warning was issued by the NWS office in Mobile, AL at 411 AM CST (Figure 4).

Official Damage Survey Results

Figure 2
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Developing Cell becomes radar detectable

Figure 3a
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Figure 3b
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Figure 4
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Based on a near immediate ground survey, the tornado formed on Michael Boulevard near the YMCA and the Davidson High School baseball field around 449 AM CST. The series of Figures 5a-f show the tornado’s parent circulation’s evolution from beginning to end. Each image depicts radar base velocity on the left-handside and radar storm-relative velocity on the right-handside. Figures 5a-b shows radar data approximately two minutes prior to and after tornado formation (see white dot for initial tornado formation location). Figures 5c-f show the remainder of the storm’s evolution between 456 AM and 505 AM CST. The highest winds estimated along the path were from 86 to 109 mph (EF-1). Many homes along the path had significant roof damage.

Figure 5a
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Figure 5b
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Figure 5c
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Tornado formed and Moving along Path at 501 AM CST

Figure 5d
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Figure 5e
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It is interesting to note that on October 25th of 2010, a weak tornado also initially formed just west of the intersection of Michael Boulevard and Azalea Road and moved east while creating sporadic damage to the Festival Center (click the following link to see the path of that tornado-> http://www.srh.noaa.gov/images/mob/tornado/2010-2/path.jpg). Visit the NWS Mobile, AL Tornado Museum for more in-depth information on this storm (http://www.srh.noaa.gov/mob/?n=tornado_museum). Finally, a tornado formed over Pensacola Bay very near Avalon Point, FL. Figure 6a shows the vantage point where the picture was of the tornado was taken at 1009 AM CST (lefthand-side) and a picture of the tornado apparently just before it dissipated because it never moved onto land (at least as of this writing). It should be emphasized, for preparedness and educational purposes, this is a tornado over water – NOT A TYPICAL SUMMERTIME WATERSPOUT which is usually characterized by having much weaker wind speeds. Figure 6b shows the tornado’s parent circulation on radar at the same time (1009 AM CST). A tornado warning was in effect for this thunderstorm at the time the picture was taken.

Figure 6a
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Figure 6b
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It should be emphasized that most of the parent circulations (what meteorologists term mesocyclones) that developed during this event were not very tall in height (i.e., less than 25 thousand feet above ground level for the most part). The overwhelming majority of the mesocyclones were not even associated with lightning despite the production of tornadoes.