NSSL SWAT Case Study - 08 April 1998 Alabama Violent Tornadoes

Repeatedly during the spring of 1998 the Southeast U.S. experienced very dynamical severe weather and tornado events.  On April 8th one particularly dangerous supercell was responsible for producing three very strong to violent tornadoes, making a profound human impact in North Central Alabama.  The second in the series of tornadoes killed 32 people along its 30.6 mile track.  According to the American Red Cross, there were 1164 families whose homes were either destroyed or made unlivable (Storm Data).  Ironically, the situation could have been made considerably worse had the tornado not lifted when it did.  Had the tornado remained on the ground for only four more miles its trajectory would have taken it into the high rise buildings of downtown Birmingham and eventually into that city's airport.

As one might expect, the Birmingham supercell displayed many of the radar signatures that are more frequently associated with tornadic thunderstorms.

Presented here are various WSR-88D radar images from the Birminham, Alabama (KBMX) radar. Included in some of the images is output from NSSL's Mesocyclone Detection Algorithm (MDA) and Tornado Detection Algorithm (TDA).  A yellow circle represents a mesocyclone as detected by the MDA.  The red-in-yellow circle represents a mesocyclone whose base is at the lowest radar scan (where it is a more likely tornado threat).  A red triangle depicts a Tornado Vortex Signature (TVS) as detected by the NSSL TDA.

Description:  The first tornado touched down in far eastern Pickens county.  Many surrounding storm cells exhibited organized rotation as well, but the tornado producing cell became dominant as it moved quickly eastward into Tuscaloosa county.  Note the broad areal extent of the reflectivity with this storm as well as the tight rotational signature in the storm relative velocities. Algorithm output concurs; the meso of interest is #703 in the algorithm output table.

By 0058 UTC, the Birmingham supercell was unfurling its peak intensity as it approached the city's suburbs.  It was around this time that the storm echoed back its most impressive radar signatures.  Warnings were issued well in advance of the storm's arrival, but, unfortunately, the most violent of tornadoes are difficult to survive without the aid of underground shelter. It is important to remember, though, that such violent tornadoes are very rare and affect isolated locations.

Description:  Tornado vertical structure varies greatly both by region and across the spectrum of atmospheric environments in which they form.  The Birmingham Cross Section is an example of a strong, deep circulation in a highly sheared environment.  The circulation (where red outbound velocities meet green inbound velocities) is strongest (brighter colors corresponding to greater velocities) in the 0-5 km height range.  However, rotation within this storm appears to extend up to 10 km.  The approximate height of the radar beam for each sweep is indicated at the bottom of the 6-panel image.

The Birmingham circulation leans downwind toward the east.  The lean may be partly attributed to having the storm translate eastward inbetween each elevation scan of the radar.  The main contributing factor, though, is strong vertical wind shear in the large scale environment.

For comparison, the Jarell Cross Section represents an environment with very little vertical wind shear.  Yet, this storm, too, produced a devastating tornado.  How this can occur may depend on several factors, and remains a key research topic.  This storm was not as close to the nearest radar as the Birmingham storm, so nothing can be inferred about the strength of the circulation in the lowest levels.  The depth of the two storms' circulations, however is comparable, about 10 km.  The Jarrell storm is standing straight up in its low wind shear environment.

Furthermore, compare these storms with a Dozier Cross Section.  The tornadic circulation near Dozier, TX (Case Study - June 11, 1997), has a depth of only 2.5 km.  This, in spite of having formed in the Texas Panhandle which is much higher above sea level than the previous two cases.  The Dozier tornado's east-west lean is not a product of wind shear in the large scale environment; it is a product of rain-cooled outflow undercutting the circulation.  The tornado was relatively much weaker than those at Birmingham and Jarrell, but it was able to sustain itself for about six minutes.

These are only a few of the countless different types of tornadic circulations being sampled by WSR-88Ds.  The Birmingham tornado represents a storm in a highly sheared, unstable environment.  This storm displayed many of the radar signatures more frequently associated with tornadic thunderstorms.  Examples of other types of tornadic storms can be found by exploring the many cases presented on SWAT's case study page.

Birmingham Case Summary (from the Birmingham NWSFO)
SWAT Case Study - Dozier, TX, June 11, 1997
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