NSSL SWAT Case Study - 4 May 1998:

Tornadic Meso-anticyclonic supercell in Sunnyvale California

On May 4 1998, a rare pair of tornadoes touched down in the San Francisco Bay area near the city of Sunnyvale.  Both tornadoes cuased F1 to weak F2 damage.  The tornadoes occurred from a left-moving anticyclonically-rotating low-topped supercell.  The tornadoes were each reported to be rotating anti-cyclonically (clockwise) as well, making for an extremely rare event.  To our knowledge, this is the only documented case of a tornadic anti-mesocyclone in the United States that has been captured with WSR-88D Level-II data.

The near-storm enviroment of the day was characterized by an anticyclonically curved hodograph, and moderate instability.  For more information on the storm's environment, consult this excellent summary Web page developed by Dr. John Monteverdi of the San Francisco State University.

Presented here are various WSR-88D radar images from the Monterey, California (KMUX) radar. Included in some of the images is output from NSSL's Mesocyclone Detection Algorithm (MDA).  A cyan circle represents a 2D vortex feature (from a constant elevation angle) as detected by the MDA.  Yellow, and red-in-yellow circles represent 3D vortex detections. However, that is unique about this case is that the overlays are of anti-mesocyclones!

Description:  The storm has reflectivity characteristics of a typical Northern Hemisphere supercell, except that the E-W axis is reversed.  The storm instead moves left (poleward) of the mean flow, rotates anti-cyclonically, and has a hook echo on the left (pole) side of the main core.  This can be seen in the following reflectivity loop.  We have created the same loop, but with "east and west flipped" to simulate the same storm as a classic right-turning cyclonic supercell.

We have done a similar thing (but this time flipped north and south) with this case of a left (equatorward) moving cyclonic supercell near Sydney Australia (the hailstorm of 14 April 1999).  Note that in this case, the supercell is a classic  supercell comon to the Southern Hemisphere, and not an anticyclonic storm moving poleward as in the Sunnyvale case.

Description:  The first tornado, in Sunnyvale, gained the most notariety, with many pictures and video shot of the tornado and its resulting damage.  The vortex is very small, and at 2334 UTC is too small to be accurately detected by the Mesocyclone Detection Algorithm.  The NSSL Tornado Detection Algorithm also misses the vortex, due to a combination of small size and weak rotational shear as seen by the WSR-88D.

Unfortunately, this is the first volume scan on the Level-II archive tape, and it is unknown from these data the origins of rotation in the tornado.  The fact that the vortex is more apparent a few elevation scans above the 0.5° tilt might be an indication that this tornado was from a descending vortex.

Description:  A second tornado, which apparently caused less damage and was less newsworthy, occurred in Los Altos near the high school.  It appears that the origins of rotation for the second tornado were different than for the first tornado.  A separate anti-cyclonic vortex develops to the north-northwest of the original Sunnyvale vortex, and produces yet another anti-cyclonic tornado.  There is some speculation that both tornadoes were non-supercell in origin, but it appears that this storm is an anticyclonically rotating supercell, and produces at least two tornadoes, either of the descending or non-descending type.  This eight-panel reflectivity display at 2355 UTC  and an eight-panel velocity display shows that the storms were also low-topped, about 30,000' high.

Finally:  Putting these storms in perspective, we can look at another case of a similar looking supercell, one which is left-turning, rotating clockwise, and has a hook on its left side.  However, this storm is a classic cyclonic supercell in the Southern Hemisphere, the Sydney hailstorm of 14 April 1999.

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