Mississippi Supercells


On March 18, 1996, shear and buoyancy profiles were favorable for supercell development in Mississippi and Alabama. A series of isolated supercells passed through south-central Mississippi. These storms produced considerable damage due to straight-line winds, giant hail and flash-flood-producing rainfall. One large storm produced a tornado northeast of Hattiesburg. This series of images shows the storms at 2141 UTC.

Reflectivity Pattern at 8000 Feet
This image shows the base reflectivity pattern monitored by the Mobile, Alabama WSR-88D, located in the lower right hand corner of the image, at 1/2 degree tilt. The reflectivity pattern shows a large hook echo, or mesocyclone, associated with a High Precipitation (HP) -type supercell that had just passed through Hattiesburg. There is the suggestion of another pendant forming on a second storm just southwest of Hattiesburg. The first storm produced at least one tornado. The Mobile radar was intersecting both storms at roughly the 8000 foot level at this distance.

 

Reflectivity Pattern at 20000 Feet

This image shows the base reflectivity pattern monitored by the Mobile radar at 2 1/2 degree tilt. The radar was intersecting both storms at roughly the 20000 foot level at this distance. The reflectivity pattern shows considerable symmetry for both storms. The superposition of a symmetric echo on the notched lower level echo indicates the occurrence of a Bounded Weak Echo Region (BWER).

BWERs occur when significant rotation has developed at mid and lower mid-levels of a supercell. This area of rotation is surmounted by strong divergence in the upper portions of the storm. BWERs often precede the development of mesocyclone-induced tornadoes.

 

Bounded Weak Echo Region

The schematic at above right shows the radar echo pattern (contours are in Dbz) at the 10 km (19000 foot) level in a supercell. The figure at above left is a close up of the reflectivity echo at the 20000 foot of the Hattiesburg supercell.

The schematic at bottom right shows the radar echo pattern (contours are in Dbz) at the 3 km (10000 foot) level in a supercell. The figure at bottm left is a close up of the reflectivity echo at the 8000 foot of the Hattiesburg supercell.

Stacking the paired images in the "mind's eye" helps visualization of the BWER.

Velocity Field

The image above shows the velocity pattern monitored by the Mobile radar at 1/2 degree tilt. Colors correspond to whether the radar is detecting inbound (negative--blues and greens) or outbound (positive--reds and purples) velocities on radii extending outward from the radar site.

The radar has detected inbound velocities of 123 knots and outbound velocities of 122 knots in association with the color "couplet" evident on the south side of the storm just northeast of Hattiesburg. There is no rotation evident on the storm southwest of Hattiesburg, but weak rotation is evident on the much smaller storm just south of that city.

The strong color shift and speed shear across a small area with the largest storm is consistent with a very stromg mesocyclone. The storm was tornadic at this time.

The schematic at above right shows the couplet pattern expected in the velocity field of a mesocyclone associated with a supercell as observed by a Doppler radar. Compare with a closeup of the velocity field of the Hattiesburg HP supercell.

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