Event Summary
     National Weather Service, Raleigh NC

September 24, 2006 Severe Thunderstorm Event
Updated 2006/10/03


Event Headlines

...A supercell thunderstorm developed over northern Person county and tracked east across northern Granville, Vance and Warren counties...
...The thunderstorm had an elevated core with 72 dBZ reflectivity up to nearly 24,000 feet...
...Warning polygons were effectively used to pinpoint the threatened locations...

Event Overview

On the afternoon of Sunday, September 24, 2006, a thunderstorm with supercell characteristics developed over northern Person county and subsequently tracked 35 miles east through extreme northern Granville, Vance and Warren counties (loop of the storm's progression across northern N.C.). Fortunately, the storm was located in sparsely populated areas when it was most intense. The Kerr Lake Park Office reported the storm snapped 20 trees in half in campgrounds along Kerr Lake in eastern Vance and western Warren counties. The storm almost certainly produced large hail and high winds elsewhere in Granville, Vance and Warren counties during its lifetime. However, due to the fact that rural areas were most affected, no other severe weather reports were received.

09/24/06 4:13 PM EDT (2013Z) Composite Reflectivity 
image from KRAX showing supercell thunderstorm over northern Vance County.
 - Click to enlarge
(Click on the image to enlarge.)

09/24/06 4:13 PM EDT (2013Z) Composite Reflectivity image from KRAX showing supercell thunderstorm over northern Vance County.



Surface / Upper Air Overview

On the afternoon of the 24th a cold front was located along a line stretching from NY south southwest through central PA, TN, KY, MS and LA. Weak surface convergence associated with a wind shift was located along the mountains and foothills of NC. No significant surface forcing was present during Sunday afternoon across central NC. Possible mesoscale surface boundaries include a differential heating boundary induced by a northeast-southwest oriented line of pre-frontal cloud cover across the foothills and far western piedmont and horizontal convective rolls or HCR's (image courtesy of Peter S. Dailey and Robert G. Fovell) across most of the state induced by surface heating and strong low-level wind shear.

The 12Z GSO upper-air sounding on Sunday, September 24 showed strong low-level speed shear, with 10kt southwesterly surface winds, 30kt winds at 1000 ft and 40kt winds at 3000 ft. Modified 12Z soundings indicated MUCAPE values in the 1500 to 2500 J/kg range on Sunday afternoon assuming temperatures in the mid to upper 80s and dewpoints in the upper 60s to lower 70s. Minimal dry air aloft was noted from the surface up to 600 mb. The freezing level was located at approximately 14,000 ft and the wet-bulb zero was near 12,000 ft. SPC analyzed precipitable water values at 20Z were fairly high at around 1.8. A 75 to 90kt 500mb jet and a 100 to 120kt 300mb jet max were located over the Ohio Valley, entering the northern Mid-Atlantic during the afternoon and evening hours. 12Z upper-air analyses and short term model forecasts showed that the best upper level divergence and strongest cross height ageostrophic flow associated with the jet would be located north of NC in the DELMARVA area in vicinity of 60 to 100 meter height falls ahead of the approaching upper level trough.

With the best surface lift associated with the front still west of the area and the best upper level lift north of the area during peak diurnal heating on Sunday afternoon, there was a temporal and spatial disconnect between the positive factors for significant convective development. Therefore, it was thought that the severe weather threat would be marginal during the afternoon and evening with the possibility of isolated damaging wind gusts in convective cells with updrafts strong enough to survive the low-level speed shear.


09/24/06 12Z GSO Upper-Air Sounding
 - Click to enlarge
(Click on the image to enlarge.)

09/24/06 12Z GSO Upper-Air Sounding (Above)

09/24/06 3:57 PM EDT (1957Z) VAD Winds from KRAX
 - Click to enlarge
(Click on the image to enlarge.)

09/24/06 3:57 pm EDT (1957Z) VAD Winds from KRAX (Above)

09/24/06 1:01 PM EDT (1701Z) Visible Satellite Imagery showing pre-frontal band of clouds and horizontal convective rolls (HCR's) over NC. - Click to enlarge
(Click on the image to enlarge.)

09/24/06 1:01 PM EDT (1701Z) Visible Satellite Image showing pre-frontal band of clouds and horizontal convective rolls (HCR's) over NC. (Above)



Radar Analysis / Warning Decision Process

Convection developed during the early afternoon hours along horizontal convective rolls stretching from the southwest piedmont northeast towards Raleigh and Rocky Mount. Radar imagery showed convective cells having a tough time growing in the strongly sheared environment. Updrafts were unable to loft 35-40 dBZ echoes any higher than about 20,000 ft. Indeed, an analysis of echo tops and composite reflectivity showed that the echo tops were displaced down-shear (northeast) of the convective cores. As a result, only sporadic lightning strikes were observed.

At 152 PM a discrete cell developed in Forsyth county along horizontal convective rolls and a differential heating boundary. The cell proceeded to move northeast into Rockingham and Caswell Counties. The first lightning strike with this cell was observed in Caswell County. While in Caswell county, the cell slowly began intensifying with 35-40 dBZ echoes ascending to 25-30 kft. At this time, the cell began to turn right on a due easterly course into Person County.

At 341 pm, lightning strikes began to increase and a special weather statement for a strong thunderstorm was issued for Person and Granville Counties. At 355 pm, the combination of deviant storm motion, increasing lightning strikes, broad mid-level rotation and ascending reflectivity cores prompted the issuance of a severe thunderstorm warning for Granville County. By 410 PM the storm significantly strengthened, with a deep 65-75 dBZ core extending up to 24,000 ft. A severe thunderstorm warning for Vance County was issued as the storm progressed east. Enhanced wording was used for the Vance County warning as the storm had clearly become a supercell. The storm continued moving due east at a high velocity (45-50 mph), and another severe thunderstorm warning was issued for Warren County. After entering Warren County, at approximately 430 pm, the storm began to significantly weaken, with descending reflectivity cores of 55-60 dBZ. At this time, lightning strikes decreased and the storm began to turn more towards the ENE as deviant motion associated with perturbation pressure effects decreased.

A loop of 0.5 degree krax reflectivity imagery and warnings from 1748Z (148 PM EDT) Sunday, September 24, to 2102Z (502 PM EDT) Sunday, September 24, 2006 is available.


09/24/06 4:09 PM EDT (2009Z) KRAX 4.0 Degree Elevation Angle showing 72 dBZ at almost 24,000 ft aloft!
 - Click to enlarge
(Click on the image to enlarge.)

09/24/06 4:09 PM EDT (2009Z) KRAX 4.0 degree elevation angle showing 72 dBZ at almost 24,000 ft aloft! (Above)

09/24/06 4:13 PM EDT (2013Z) KRAX (from left to right) Composite Reflectivity, VIL, Echo Tops and Digital VIL.
 - Click to enlarge
(Click on the image to enlarge.)

09/24/06 4:13 PM EDT (2013Z) KRAX Composite Reflectivity, VIL, Echo Tops and Digital VIL. (Above)



Case Study Team -

Brandon Vincent
Jonathan Blaes

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


  • NWS Disclaimer.