Event Summary
     National Weather Service, Raleigh NC

March 28, 2005 Severe Weather Event
Updated 2005/04/01



(photos courtesy of Tom Paylor)


Event Headlines -
...Numerous reports of large hail were received across Wake County including a report of 4 inch hail...
...A large hail spike or three-body scatter spike (TBSS) was noted shortly before very large hail was reported over North Raleigh...


Event Overview -
A strong, closed upper level low pressure system was observed at 300 MB over the Tennessee valley at 12Z (700 AM) on Monday, March 28, 2005. At the same time, an intense mid and upper level jet was observed at 500 MB with a 90 knot southwesterly flow and at 300 MB with a 140 knot southwesterly flow. A warm front advanced northward from near the North Carolina/South Carolina border at 09Z (400 AM) to near the Virginia/North Carolina border at 12Z (700 AM). A warmer and more unstable air mass moved into central and eastern North Carolina after the passage of the warm front. A composite image depicting the location of the lightning associated with the thunderstorms in relation to the upper low, dry slot, NCEP analyzed surface fronts, the mean relative humidity and the mid level theta-e provides a more complete overview of the meteorology associated with this event. The combination of the strong vertical motion associated with the jet stream energy, the shear associated with the advancing warm front, and the increasingly unstable air mass behind the warm front produced an environment favorable for the development of severe thunderstorms.

Forecasters were closely monitoring the threat of severe weather on Monday morning. The large hail that was observed across central North Carolina was produced by a left moving supercell thunderstorm that raced northward across the state. A supercell is a thunderstorm that possesses a deep, persistent rotation that maintains it's identity over several hours. It has a high probability of producing severe weather. This supercell originated near Charleston, SC earlier in the morning and then traveled northward at over 80 MPH taking less than 2 hours to travel 150 miles from the NC/SC border southwest of Lumberton to the VA/NC border north of Oxford. The storm was unusual in that it was a left-moving supercell. Most supercells move to the right of the mean wind flow, this storm favored a track that was left of mean wind flow. The track and rapid motion of the supercell can be seen in a loop of KRAX Radar imagery from 1100Z (600 AM EST) to 1258Z (758 AM EST).


Severe Weather Reports -

Text of severe weather reports across central North Carolina





Mesoscale Data

Analyzed mean sea level pressure (black) and surface wind barbs from SPC at 12Z on Monday, March 28, 2005 (7 AM EDT).
A low pressure center was located across northwestern North Carolina with a warm front extending south and east toward the coast.

SPC Analysis at 12Z on Monday, March 28, 2005.



Analyzed surface temperatures (red), dewpoints (green), and wind barbs from SPC at 12Z on Monday, March 28, 2005 (7 AM EDT).
Note the more unstable airmass with dewpoints in the lower to mid 60s across southern and eastern North Carolina.

SPC Analysis at 12Z on Monday, March 28, 2005.



Analyzed precipitable water (green) and wind barbs from SPC at 12Z on Monday, March 28, 2005 (7 AM EDT).
Note the axis of higher precipitable water values extending from the southern coast of North Carolina northward to south central Virginia.

SPC Analysis at 12Z on Monday, March 28, 2005.



300 MB wind barbs (brown), 300 MB isotachs (red) and analyzed 300 MB divergence (purple) from SPC at 12Z on Monday, March 28, 2005 (7 AM EDT).
Note the 130-140 knot jet axis located over Georgia and South Carolina. An area of enhanced divergence is noted over northern and northwestern North Carolina.

SPC Analysis at 12Z on Monday, March 28, 2005.



NWS Composite Reflectivity Imagery from 1944Z on Thursday, March 28, 2005 (344 PM EDT).
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

SPC Analysis at 12Z on Monday, March 28, 2005.




Satellite

Infrared satellite imagery at 1214Z on Monday, March 28, 2005 (714 AM EDT)

Infrared satellite imagery at 1214Z on Monday, March 28, 2005 (714 AM EDT) with positive and negative cloud to ground lightning strikes overlaid. A line of enhanced cloudiness associated with deep convection can be seen stretching across central North Carolina. The cluster of cloud to ground lightning strikes across southern Wake and northern Harnett Counties is associated with the thunderstorm that produced many of hail reports.

A Java Loop of Satellite imagery from 1045Z (545 AM EDT) through 1415Z (915 AM EDT) Monday, March 28, 2005 is available.





Radar Loops

Java Loop of KRAX Radar Imagery from 1100Z (600 AM EST) to 1258Z (758 AM EST).





Java Loop of Regional radar imagery from 10Z (500 AM EST) to 14Z (900 AM EST) Monday, March 28, 2005





Radar Analysis

KRAX 4-Panel Reflectivity Imagery from 1208Z Monday, March 28, 2005.
Four panel reflectivity imagery from 1208Z (708 AM EDT) is shown below. The imagery shows the thunderstorm shortly before producing very large hail across Wake County. Note the large hail spike or three-body scatter spike (TBSS) shown in the 19.5 degree elevation angle (shown in the image in the lower right).

A somewhat scientific explanation of the three-body scatter spike (TBSS) can be found at: ftp://ftp.werh.noaa.gov/share/ILN/88D-Techniques/6.TBSS/tbssweb.htm.

A more simplistic description from another web page ( http://www.srh.noaa.gov/bmx/special/3body.html) explains: "In brief, simple terms, this is caused by the radar beam hitting the wet hail, scattering to the ground below, then scattering back upward, and finally being scattered once again by the hail aloft. The 3 scatterings illustrate the triple reflection, thus the term 'three-body scatter spike'. "

Basically, the energy that has 'bounced around' as described above, is received back at the radar later than the energy reflected directly back from the storm. The increased time causes the radar to think this energy is coming from a target that is farther away from the radar, thus the reflectivity extending farther beyond the actual storm.

Click on the image to enlarge.

 - Click to enlarge





Case study team -
Michael Brennan
Rod Gonski
Gail Hartfield
Brandon Locklear
Douglas Schneider
Jeff Orrock
Kermit Keeter
Jonathan Blaes

For questions regarding the web site, please contact Jonathan Blaes.


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