Nexrad radar image (reflectivity, 0.5 deg tilt) at 12:40am on May 6, 2006 (Radar: GRK)
(For a smoothed version, click on the image)


Nexrad radar image (storm-relative velocity, 0.5 deg tilt) at 12:40am on May 6, 2006 (Radar: GRK)
(For a smoothed version, click on the image)



Nexrad radar image (reflectivity, 0.5 deg tilt) at 12:50am on May 6, 2006 (Radar: GRK)
Ten minutes after the previous radar data, the storm is battering the area from Robinson to downtown Waco to Bellmead. The tornado has likely dissipated at this time. Note the broad area of lighter reflectivity (green pixels) from near Hallsburg and Riesel westward toward downtown Waco. This is the area of strong inflow wind wrapping into the storm from the east and southeast. In the next three images, we'll examine the storm by looking east to west into the heart of the storm along this inflow track, using new software to visulaize various storm features.

Nexrad radar image in volume mode at 12:50am on May 6, 2006 (Radar: GRK)

["Volume Mode" explained: The nexrad (WSR88D) radars deployed across the U.S. by the Nat'l. Weather Service (some are actually maintained by the Dept. of Defense) operate in an automated volume-scan pattern. Volume scan means that the radar (for example) starts at 0.5 degrees elevation, makes a scan, tilts up to 1.5 degrees elevation, makes another scan, tilts up to 2.5 degrees, makes another scan, etc. When the radar has completed this process, which can take as little as 4 1/2 minutes, the digital data collected forms a 3D 'picture' of the storms detected by the radar, as well as other data. Until the new software used for this image became available, meteorologists could only look at one elevation at a time, or at multiple elevations but only in a thin slice through a storm. With the new software, we can now see storm features that previously we could only theorize were there.]


Nexrad radar image in volume mode at 12:50am on May 6, 2006 (Radar: GRK)
In this view we can see the mesocyclone inbound/outbound velocity couplet just left of and below the "W" in "Waco". The radar site is located off the left side of this view (the storm has been electronically rotated ~90 degrees clockwise), but the colors mean the same (bright green is toward the radar and bright red is away from the radar). Note that the couplet is more or less co-located with the vault area seen in the previous image.

Nexrad radar image in volume mode at 12:50am on May 6, 2006 (Radar: GRK)
This gets very technical, so hang in there. This is a view of spectrum width, one of the three basic moments, produced by Doppler radars. Notice that in this image there is an area of dark gray colors that, when compared to the previous two images, coincides with the inflow notch and the vaulted updraft region. This data confirms that this is indeed strong inflow feeding up into the storm. To understand why this image has that meaning, you need to know that spectrum width tends to be "low" (gray colors) in inflow/updraft regions and "high" (red and magenta colors) in outflow/downdraft regions.
[Spectrum width is a representation of the variability of of measured velocity within each data bin in the data. The data bins are computer-generated pockets, arranged along each radial down which the radar beam is firing, and into which the velocity data from each pulse along that radial is placed. The computer averages the data in each bin to arrive at an average velocity for that bin. The computer has to do this because it fires hundreds of times per second along a radial before the antenna has advanced very far. The radar alternately fires and listens for return data, and dumps the values of each returned pulse into the appropriate bin, then averages the data. Where the air flow is smooth and non-turbulent, there isn't much variation in the values (low spectrum width); where the air flow is uneven and very turbulent, there is a lot of variation (high spectrum width).] 

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE FORT WORTH TX
857 PM CDT SAT MAY 6 2006

..RESULTS OF THE SURVEY OF THE MCLENNAN COUNTY STORM OF MAY 6

ON SATURDAY AFTERNOON...STAFF FROM THE NATIONAL WEATHER SERVICE SURVEYED THE DAMAGE IN WACO AND MCLENNAN COUNTY. BASED ON THE RESULTS OF THIS SURVEY...IT WAS DETERMINED THAT THE DAMAGE IN AND NEAR WACO WAS CAUSED BY A COMBINATION OF DOWNBURST WINDS AND A TORNADO.

ISOLATED AREAS OF WIND DAMAGE WERE NOTED IN WESTERN MCLENNAN  COUNTY...NEAR CRAWFORD...NEAR VALLEY MILLS...AND WESTERN WACO. MORE WIDESPREAD WIND DAMAGE WAS NOTED IN WOODWAY...HEWITT...ROBINSON... AND SOUTH WACO. THIS DAMAGE EXTENDED ALONG AND SOUTH OF THE HIGHWAY 6 CORRIDOR. MINOR ROOF DAMAGE AND DAMAGE TO TREES...SIGNS...AND AWNINGS WAS NOTED IN THIS AREA. DAMAGE PATTERNS AND CORRELATION WITH RADAR DATA SUGGEST DOWNBURST WINDS OF 50 TO 80 MPH WERE LIKELY RESPONSIBLE FOR THE DAMAGE.

NORTH OF THE DOWNBURST AREA...EVIDENCE OF A CIRCULATION WAS FIRST OBSERVED SOUTH OF NEW ROAD AND JUST WEST OF SPUR 298...OR JUST  NORTH OF RICHLAND MALL. THE TORNADO REACHED ITS PEAK INTENSITY...  AND CAUSED THE MOST SIGNIFICANT DAMAGE...JUST EAST OF SPUR 298. SEVERAL COMMERCIAL BUILDINGS HAD CONSIDERABLE SECTIONS OF ROOF REMOVED AND WALLS DAMAGED. NUMEROUS TREES WERE DOWNED AND POWER POLES WERE SNAPPED IN AND EAST OF THIS AREA. THE CIRCULATION WEAKENED AS IT APPROACHED INTERSTATE 35...AND THE LAST EVIDENCE OF CIRCULATION WAS NOTED IN BEVERLY HILLS...APPROXIMATELY 3/4 MILE EAST OF INTERSTATE 35. DOWNBURST-RELATED WIND DAMAGE CONTINUED TO THE EAST OF WACO.

WHILE DEBRIS IN THE AREA NEAR AND SOUTH OF HIGHWAY 6 WAS CARRIED MAINLY EAST AND SOUTHEAST...DEBRIS IN THE CIRCULATION AREA WAS CARRIED TO THE NORTH AND NORTHWEST IN SOME LOCATIONS. EVIDENCE WAS ALSO SEEN OF A CONVERGING WIND FIELD IN THIS AREA. THIS SUGGESTS THAT A TORNADO WAS RESPONSIBLE FOR THE NORTHERN PORTION OF THE DAMAGE SWATH.

BASED ON THE DAMAGE...THE TORNADO WILL BE GIVEN A PRELIMINARY RATING OF A LOWER F2 ON THE FUJITA SCALE. MAXIMUM WIND SPEEDS IN THE TORNADO WERE LIKELY IN THE 115 MPH RANGE. PATH LENGTH WAS AROUND 2.6 MILES...AND AVERAGE PATH WIDTH WAS APPROXIMATELY 150 YARDS.

THIS WAS A METEOROLOGICALLY COMPLEX EVENT. THE TYPE OF THUNDERSTORM WHICH MOVED ACROSS THE WACO AREA IS CALLED A SUPERCELL THUNDERSTORM. SUPERCELLS ARE CHARACTERIZED BY AN AREA OF STORM-SCALE ROTATION... KNOWN AS A MESOCYCLONE. A STRONG DOWNDRAFT ...CALLED A REAR FLANK DOWNDRAFT...LIKELY WRAPPED AROUND THE WEST AND SOUTH SIDE OF THE MESOCYCLONE...STRUCK THE GROUND...AND PRODUCED THE DOWNBURST DAMAGE.  JUST NORTH OF THE DOWNBURST AREA AND NEAR THE CENTER OF THE MESOCYCLONE...A SMALL VIGOROUS AREA OF ROTATION DEVELOPED AND EVOLVED INTO THE TORNADO.

GARY WOODALL, WARNING COORDINATION METEOROLOGIST
NATIONAL WEATHER SERVICE - FORT WORTH