Event Summary
     National Weather Service, Raleigh NC

October 1, 2008 Severe Weather Event
Updated 2009/09/21





Event Headlines -
...An impressive supercell thunderstorm moved across the Sandhills and Southern Coastal Plain on October 1, 2008 producing up to baseball size hail...
...Despite a strong mesocyclone and a relatively deep layer of rotation, no tornado was observed. It is believed that insufficient 0-1 km storm-relative helicity and the rapid advancement of the rear flank downdraft inhibited tornadogenesis...
...A large and impressive Three Body Scatter Spike (TBSS) or hail spike was noted for several minutes during the event...



Event Overview -
A highly amplified upper level trough was centered across the Great Lakes with several embedded shortwaves on October 1, 2008. A stalled cold front across the eastern Carolinas was enhanced by one of the disturbances rotating around the upper trough. The air mass ahead of the front across southeastern North Carolina was warm and moist. The unstable air mass combined with sufficient deep layer shear resulted in the development of severe thunderstorms across the eastern Piedmont into the Coastal Plain with numerous reports of large hail.


Event Details -
On the morning of the October 1, 2008, the upper level pattern across the United States featured a highly amplified upper level trough situated over the Great Lakes and a western U.S. ridge. Water vapor satellite imagery from 1215 UTC showed the upper level trough with an embedded shortwave across the Ohio Valley. At 250 hPa, a 90 kt jet was dropping south across the far western Ohio Valley into the base of the eastern U.S. trough. At lower levels, the 850 hPa trough was located west of the Appalachians but the flow near and just east of the Appalachians was becoming westerly and northwesterly as shown at KGSO and KIAD with a west-southwesterly flow further east. At the surface, a weakening and slow moving cold front stretched from northeast to southwest across central North Carolina with a weak surface low along the front near Charlotte at 12 UTC.

A few showers moved across the northern North Carolina Piedmont during the mid morning hours. An area of persistent low clouds across the Sandhills and Coastal Plain gave way to a fair amount of sunshine by midday.

By midday, a vigorous short wave trough with winds in excess of 50 kts was rotating around the base of the long wave trough in the eastern Tennessee Valley. The cold front had weakened but the approach of the upper level shortwave and the development of a lee trough reinforced this surface boundary. The cold front/boundary had moved slightly eastward with a weak wave of low pressure located near Fayetteville. The southerly flow ahead of the front was transporting moisture into the Sandhills and Coastal Plain with surface dew points rising into the mid to upper 60s by 16 UTC. Precipitable water values were in excess of 1.1 inches across eastern North Carolina which was greater then 150 percent of normal and supportive of deep convection.

By 18 UTC, the atmosphere had destabilized with mixed layer CAPE values now in excess of 1000 J/kg across the Sandhills and Coastal Plain. Surface temperatures were approaching 80 degrees and dew points still ranged in the mid to upper 60s. Forcing for ascent was provided by an approaching upper level short wave and the strengthening mid level flow as well as the low level boundary and associated frontogenesis. An area of enhanced cumulus cloudiness developed ahead of the surface boundary and scattered convection was detected on radar just before 18 UTC. The convection quickly intensified and grew in coverage between 1758 UTC, 1828 UTC and 1858 UTC

The convective environment at 19 UTC featured moderate instability with mixed layer CAPE values of 1000-1500 J/Kg and surface based CAPE values approaching 2000 J/Kg. Low level lapse rates range between 7.0 to 8.0 degrees C/Km while mid level lapse rates were more anemic around 5.5 to 6.0 degrees C/Km. The mid level flow featured a 50 to 55 kt west-southwest jet across central North Carolina. The bulk shear values around 50 kts were supportive of supercells and multicell lines. The 0-3 km storm relative helicity values ranged between 125-150 m2/s2 and the 0-1km storm relative helicity values ranged around 50 m2/s2 were less then the values typically observed with tornadoes. The low level flow was rather weak as noted in the MSL pressure analysis. LCL heights ranged just under a 1000 meters.

The convection quickly intensified from a few discrete cells into a larger multicell line with a few embedded supercells. The first severe weather report was received at 245 PM EDT in Cumberland County. Between 300 and 400 PM, numerous large hail reports were observed across Sampson and Wayne Counties with hail up to the size of baseballs observed in Sampson County.


Severe Weather Reports -
Text of severe weather reports across central North Carolina





Three Body Scatter Spike (TBSS)

Three Body Scatter Spike diagram The Three Body Scatter Spike (TBSS) radar signature is generally a 10-30 km long region of reflectivities aligned radially downrange from a highly reflective (>60 dBZ) echo core. The signature is a relatively reliable indicator of large hail in the thunderstorm and is often referred to as a "hail spike."

The TBSS is created when radar energy strikes a 60+ dBZ core and radar energy is scattered back toward the radar and toward the ground. The radar energy that is directed downward is reflected by the ground and then sent upward toward the 60+ dBZ core. Finally, the radar energy is reflected by the 60+ dBZ core and reflected back to the radar. The radar algorithm is confused by the delayed return of the echo and believes it is coming from further away and it is then displayed downrange on the radial as a 5 to 20 dBZ spike. Additional information on the Three Body Scatter Spike (TBSS) is available in this article from Weather and Forecasting: The Radar “Three-Body Scatter Spike”: An Operational Large-Hail Signature.

A large Three Body Scatter Spike (TBSS) or hail spike in the 8.0 deg reflectivity product from 1900 UTC - click to enlarge Characteristics of a TBSS:
  • Low Reflectivity values of 5 to 20 dBZ
  • Low radial Velocities
  • High Spectrum Width
  • More elevated hail events produce longer spikes
  • Strength of the TBSS is proportional to hail size, concentration, wetness, and vertical extent of core
  • Position of the TBSS along the radial is proportional to core altitude above the surface (why it appears at higher altitudes in the storm)
In this event, a large and impressive Three Body Scatter Spike (TBSS) or hail spike was noted at several elevation angles for multiple volume scans (look in the upper right pane and both of the lower panes to see the TBSS in the loop). The TBSS was particularly impressive in the 8.0 deg reflectivity product from 1900 UTC. The TBSS can also contaminate velocity data as shown in the 8.0 deg storm relative velocity product from 1900 UTC. The corruption of velocity data can also produce irregularities with derived products or output from algorithms. This hail spike was unusually long, extending more than 55 miles or 90 km.



NSSL's Rotational Track Product


NSSL Rotational Track product from 16 to 23 UTC on October 1, 2008 - click on the image to enlarge NOAA's National Severe Storms Laboratory (NSSL) has developed a gridded dataset that contains rotational shear from single and multiple radars that is accumulated over time providing tracks of radar detected rotation. The basic process for creating these products is initiated when velocity data from each radar is run through a Linear Least Squares Derivative (LLSD) filter creating an azimuthal shear field. The azimuthal shear fields in a 0-3 km layer from each radar across the CONUS are then combined and the maximum value at each 250 m² grid point is plotted over the time period providing the graphic.

The process was further improved when the WDSS-II (Warning Decision Support System - Integrated Information) group at NSSL made the "Rotational Tracks" data available for display in Google Earth. Using Google Earth with an overlay of near real-time rotational tracks allows forecasters to estimate where a storm’s low-altitude circulation was most intense and to determine locations of possible damage. The satellite images and high density maps in Google Earth often make it possible to determine the location down to a neighborhood or the street. This simplifies the verification process by reducing the amount of time that is spent searching for reports.

The image (click on it to enlarge) above, includes the NSSL rotational track product from 16 to 23 UTC on October 1, 2008. A long corridor of rotation associated with the supercell that moved across northern Cumberland, northern Sampson, and southern Wayne Counties is easily seen. The NSSL rotational track product shows a clear bend to the right that the supercell made as it moved northeast of Fayetteville across Sampson and Wayne Counties. This bend is indicative of deviant motion from other storm tracks in eastern NC.

The strongest rotation was located over a small area in northern Sampson County a few miles west of Newton Grove where rotational shear exceeded 0.014 s-1. The rotational shear associated with the same storm as it moved across southern Wayne County near Walnut Creek exceeded 0.014 s-1 as well. These values were similar to the November 15, 2008 North Carolina Tornado Outbreak and they exceeded the rotational shear values observed in the March 27, 2009 Tornado Event. The NSSL rotational track product can not provide definitive information as to whether a tornado touched down but the track and location of the greatest rotational shear is clear and this information can be used to focus and target follow up calls about damage.



NSSL MESH Hail Swath Product


NSSL MESH hail swath product from 16 to 23 UTC on October 1, 2008 - click to enlarge NOAA's National Severe Storms Laboratory (NSSL) has been developing techniques for getting popular WSR-88D cell-based hail information from the Hail Detection Algorithm (HDA) into formats that users can more effectively use. Some of the cell-based hail information has been incorporated into high-resolution gridded products that allow users to diagnose which portions of storms contain large hail. One such product is the "Hail Swath" product which accumulates hail size data over a period of time to provide hail swath maps, showing both maximum hail size by location, and hail damage potential (a combination of hail size and how long the hail has been falling).

More specifically, the "Hail Swath" product is a derivative of the MESH (Maximum Expected Size of Hail) output from the HDA. Reflectivity data from all of the CONUS NEXRAD radars are merged into a three dimensional (latitude/longitude/height) grid. A modified version of the NSSL HDA is then run on this grid producing a MESH grid at 60 second intervals. The maximum MESH value at each 1 square kilometer grid point is plotted over a chosen time period in order to create the areal “swaths” of MESH.

The NSSL Hail Swath product valid from 16 to 23 UTC on October 1, 2008 is shown along with the storm reports of large hail (H) and wind damage (W) in the image above and/or to the right (click on the image for a larger view). You can clearly see the track of this hail producing supercell that moved across Cumberland, Sampson, and Wayne Counties. The color scale on the image indicates the maximum expected hail size during the period with the light blue indicating hail estimated at around a half inch in diameter, the darker blue indicating hail greater then an inch in diameter, and the lighter green indicating hail potentially larger then an inch and a half.

More information on the MESH is available from the Verification of multi-sensor, multi-radar hail diagnosis techniques by Kiel L. Ortega, Travis M. Smith, and Gregory J. Stumpf.


Convective Evolution and Discussion

krax 4 panel image of composite reflectivity, VIL, LRM3 and echo tops from 1900 UTC - click to enlarge The upper air pattern featured a vigorous short wave trough that was rotating across the Ohio Valley and southern Appalachians. SPC mesoanalysis at 19 UTC indicated 0-6km shear values of around 45-55 kts with most of the shear located above 3km. The air mass behind the front in the western Piedmont featured surface temperatures in the 70s and dew points falling through the 50s. East of the front, the temperatures warmed into the upper 70s to lower 80s.

To further complicate the environment, a couple of boundaries were located to the east of the front. The first boundary resulted from a differential heating boundary produced by a persistent area of morning fog and stratus that stretched west to east across the Sandhills and Southern Coastal Plain. Another boundary can be seen extending southwestward from an area of early morning convection that moved across the northern Piedmont and the northern Coastal Plain.

The convection initially developed in the Sandhills just before 18 UTC. The thunderstorms developed into a supercell that appeared to travel along and then just north of a surface boundary located in the Southern Coastal Plain. The boundary can be seen in a loop of 0.5 degree reflectivity imagery as well. While the best deep layer shear was present a few kilometers above the surface, the presence of the surface boundary likely contributed additional vorticity that was ingested by the storm and likely enhanced the rotation within the storm.

Forecasters had a good handle on the mesoscale environment with this event and expected organized convection with supercells possible. The NSSL 24 hour radar precipitation product ending at 17Z on October 2, 2009 shows the track of the supercell which featured a turn to the right and a more easterly motion across far northern Sampson and far southern Wayne Counties. The NSSL rotational tracks and hail swath products also show the turn to the right. Based on the anticipated storm motion, warning polygons were adjusted to include a larger area to the right of the storm motion. Warning polygons were also trimmed on the upstream side as the storm moved out of a given area. Since the event occurred during school time, trimming the polygons provided an “all-clear” message to various schools and school districts to alert them of the diminished threat.

KRAX 4 panel storm relative velocity image from 1946 UTC on October 1, 2008 - click to load 4 panel image The best storm rotation was located a few elevation angles above the lowest elevation angle at 1.3 degrees which intersected the storm at approximately 6,300 feet. The storm relative velocity data indicated an outbound velocity of 58 knots and an inbound velocity of 45 knots resulting in a rotational velocity of 50 knots and a V/R shear value of 0.0959 s-1. The 50 knots of rotational shear at 37 nm from the radar with a mesocyclone diameter of around 1 nm corresponds to a strong mesocyclone on the 1.0 nm mesocyclone nomogram. At lower elevations angles and especially the 0.5 degree slice, the rotation was much weaker and more of a convergent pattern. An outflow boundary emerged south of the storm after it moved into Sampson County suggesting that the cooler rear flank downdraft overtook or undercut the updraft and inhibited tornadogenesis.

A large and impressive Three Body Scatter Spike (TBSS) or hail spike was noted at several elevation angles for multiple volume scans with one of the thunderstorms. The TBSS was particularly impressive in the 8.0 deg reflectivity product from 1900 UTC. The TBSS can also contaminate velocity data as shown in the 8.0 deg storm relative velocity product from 1900 UTC. The corruption of velocity data can also produce irregularities with derived products or output from algorithms. This hail spike was unusually long, extending more than 55 miles or 90 km. Note that the strength of the hail spike is proportional to hail size, concentration, wetness, and the vertical extent of core. In addition, more elevated hail events produce longer spikes. Additional information on Three Body Scatter Spikes (TBSS) or hail spikes is available in a previous section of this event summary.

Numerous follow up calls were made by several staff members to county officials, schools, and the Wayne County Agricultural extension office, to find potential wind damage or a tornado touchdown. No tornado touchdown was reported and no wind damage was observed. In addition, staffing arrangements were made and a storm survey team investigated the area the following day. No tornado could be confirmed and little if any wind damage was identified.

Given that there was a long lived supercell, with an impressive mesocyclone, and a storm that turned right increasing the storm relative helicity, the lack of a tornado seems surprising. Funk (2002) summarized previous research activities and noted that three ingredients were needed for significant (EF-2 or greater) tornadoes produced from supercells.
  • Persistent, rotating updraft - high values of CAPE (especially in the 0-3 km layer) and SRH is favorable for the development of a significant mesocyclone. The presence of high values of ambient CAPE and SRH is important to the rapid development and persistence of a rotating updraft.
  • Special Rear Flank Downdraft (RFD) - a warm RFD with more CAPE, less CIN, and higher theta-e allows more RFD air to be ingested into the updraft and enhance low level rotation. This can be monitored by examining low level dew point depressions and LCL heights.
  • Enhanced Storm-Relative Helicity (SRH) - high values of SRH in the 0-3 km and especially the 0-1 km layer are needed for a rotating updraft and the SRH values can be augmented near boundaries
Both the persistent rotating updraft and the special rear flank downdraft criteria appear to have been met but the enhanced Storm-Relative Helicity (SRH) in the 0-3 km and especially the 0-1 km layer were likely insufficient for tornado production. An analysis of the fixed layer Significant Tornado Parameter (STP) included in the Mesoscale Data section below suggests that the 0-1 km storm-relative helicity was a limiting factor and that the surface parcel CAPE (sbCAPE) may have also contributed to the lack of a tornado.



KRAX Radar Loops

Overview of the entire event with 0.5 degree reflectivity images from every volume scan between 1732 UTC and 1959 UTC on October 1, 2008.
A Java loop contains KRAX 0.5 degree reflectivity imagery from 1732 UTC through 1959 UTC on October 1, 2008. This loop of reflectivity data uses the feature following zoom option which loops the data over a moving point, allowing better interpretation of radar data. Note - this loop includes 34 frames

The KRAX reflectivity image below is from 1908 UTC or 208 PM EDT on October 1, 2008 as a supercell thunderstorm moved across northern Sampson County. Hail up to 2.75 inches in diameter was observed around the time of this image.

KRAX reflectivity image from 1908 UTC on October 1, 2008 - click to load loop



Overview of the entire event with 4 panel reflectivity images from every volume scan between 1732 UTC and 1959 UTC on October 1, 2008.
A Java loop contains KRAX 4 panel reflectivity imagery from 1732 UTC through 1959 UTC on October 1, 2008. The 4 panel imagery includes reflectivity data from the 0.5 degree elevation in the upper left, the 1.5 degree elevation in the upper right, 2.4 degree elevation in the lower right, and the 3.4 degree elevation in the lower left. This loop uses the feature following zoom option which loops the data over a moving point, allowing better interpretation of radar data. Note - this loop includes 34 frames

The KRAX 4 panel reflectivity image below is from 1934 UTC or 334 PM EDT on October 1, 2008 as a supercell thunderstorm moved across southern Wayne County. At this time, there was a developing Bounded Weak Echo Region or BWER which is a nearly vertical area of weak radar echoes, surrounded on the sides and above by significantly stronger echoes. This feature is associated with a very strong updraft.

KRAX 4 panel reflectivity image from 1934 UTC on October 1, 2008 - click to load loop



Overview of the entire event with 4 panel storm relative velocity images from every volume scan between 1732 UTC and 1959 UTC on October 1, 2008.
A Java loop contains KRAX 4 panel storm relative velocity imagery from 1732 UTC through 1959 UTC on October 1, 2008. The 4 panel imagery includes storm relative velocity data from the 0.5 degree elevation in the upper left, the 1.5 degree elevation in the upper right, 2.4 degree elevation in the lower right, and the 3.4 degree elevation in the lower left. This loop uses the feature following zoom option which loops the data over a moving point, allowing better interpretation of radar data. Note - this loop includes 34 frames

The KRAX 4 panel storm relative velocity image below is from 1946 UTC or 346 PM EDT on October 1, 2008 as a supercell thunderstorm moved across southern Wayne County. At this time, there was 53 knots of rotational shear in the storm in the 1.3 degree elevation slice around 6300 feet with 56 knots of wind toward the radar and 49 knots of wind blowing away from the radar.

KRAX 4 panel storm relative velocity image from 1946 UTC on October 1, 2008 - click to load loop



Regional Radar Loop

A Java loop of regional reflectivity imagery from 1658 UTC through 2228 UTC on October 1, 2008 is available here. Note - this loop includes 25 frames.

The reflectivity image below is from 1928 UTC on October 1, 2008. This was around the time in which golf ball to tennis ball size hail fell across northern Sampson County.


Regional reflectivity image - click to load loop



Mesoscale Data

Forecasters at RAH routinely use the SPC mesoanalysis products during severe weather operations. During this event, the SPC mesoanalysis products were consulted frequently to monitor the evolving environment. The images and discussion below highlight several of the SPC mesoanalysis products that provided insight into the evolution of the severe weather event. These images are not only used in real time but they are archived locally for use in post event analysis and training.


500 MB heights, temperatures (red), dew points (green), and wind barbs (black) from SPC at 03Z on Saturday, May 10, 2008
The analysis shows a vigorous mid level trough across the Appalachian Mountains with a 50 knot southwesterly flow across the Sandhills and Southern Coastal Plain. A temperature gradient from 12 to 14 degrees C is located across central North Carolina.

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



Analyzed surface temperatures (red), dew points (blue) and shaded, and wind barbs from SPC at 19 UTC on October 1, 2008
A surface boundary is evident across the Coastal Plain with a northwesterly wind behind the trough in the Piedmont and a southwesterly flow across the coastal plain. Dew points reached the mid 60s across the Coastal Plain and approached 70 degrees along the coast. An axis of warmer temperatures approaching 80 degrees extended northward into the Southern Coastal Plain.

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



Analyzed mixed layer convective available potential energy (MLCAPE) (red) and mixed layer based convective inhibition (MLCIN) (blue lines - shaded) from SPC at 19 UTC on October 1, 2008
MLCAPE values ranged between 1000-1500 J/Kg in the Sandhills and Southern Coastal Plain. At the same time, the most unstable CAPE and the surface based CAPE were larger ranging 1500-2000 J/Kg in the Sandhills and Southern Coastal Plain with even greater instability closer to the coast.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



3 hour analysis of mixed layer convective available potential energy (MLCAPE) change from SPC at 19 UTC on October 1, 2008 The instability across the Sandhills and Southern Coastal Plain increased slightly during the 3 hour period.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



0-6 km Bulk Shear (blue) and storm motion (brown) from SPC at 19 UTC on October 1, 2008
The 0-6 km bulk shear values range between 45-55 knots across the Sandhills and Southern Coastal Plain. Given sufficient instability, thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 30-40 knots and the analysis at 19 UTC supports the potential of supercells.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



0-1 km Storm Relative Helicity (SRH) (shown in blue) and storm motion (brown) from SPC at 19 UTC on October 1, 2008
Note that the 0-1 km SRH values ranged between 50 and 100 m²/s² across the Southern Coastal Plain of North Carolina. The SRH is a measure of the potential for cyclonic updraft rotation in right-moving supercells. Studies have shown that larger values of 0-1 km SRH, greater than 100 m²2/s², suggests an increased threat of tornadoes and that very large values of 0-1 km SRH (perhaps greater than 200 to 300 m²/s²) are indicative of significant tornado potential.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



0-3 km Storm Relative Helicity (SRH) (shown in blue) and storm motion (brown) from SPC at 19 UTC on October 1, 2008
Note that the 0-3 Km SRH values ranged around 150 m²/s² across the Southern Coastal Plain of North Carolina. The SRH is a measure of the potential for cyclonic updraft rotation in right-moving supercells. Larger values of 0-3 km SRH (greater than 100 m²/s²) suggest an increased threat of supercells and tornadoes. Some studies suggest that the 0-3 km SRH is a better indicator of storm rotation, which is related to tornadoes, but not directly the potential for tornadoes themselves.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



Analyzed Lifting Condensation Level (red, blue, and green) from SPC at 19 UTC on October 1, 2008
The LCL height is the height at which a parcel becomes saturated when lifted dry adiabatically. The importance of LCL height is thought to relate to sub-cloud evaporation and the potential for outflow dominance. Low LCL heights imply less evaporational cooling from precipitation and less potential for a strong outflow that would likely inhibit low-level mesocyclone development. Thunderstorms that produce significant tornadoes generally have a lower LCL height with LCL heights less than 1,000 meters typically favorable for tornado development. The LCL values across the Southern Coastal Plain ranged between 750 to 1000 meters.
(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



Analyzed Significant Tornado Parameter (STP) (fixed layer) and the mixed layer convective inhibition (MLCIN) from SPC at 19 UTC on October 1, 2008
The STP is designed to highlight areas favoring right-moving tornadic supercells. The STP is a multiple ingredient, composite index that includes mixed layer CAPE (mlCAPE), mixed layer LCL height (mlLCL), 0-1 km storm-relative helicity (SRH1), 0-6 km bulk wind difference (6BWD), and surface parcel CIN (sbCIN).

The modified STP formulation is as follows:
STP = (mlCAPE/1500 J kg-1) * ((2000-mlLCL)/1500 m) * (SRH1/100 m2 s-2) * (6BWD/20 m s-1) * ((200+sbCIN)/150 J kg-1)

When the mlLCL is less than 1000 m AGL, the sbLCL term is set to one, and when the sbCIN is greater than -50 J kg-1, the sbCIN term is set to one. Lastly, the 6BWD term is capped at a value of 1.5, and set to zero when 6BWD is less than 12.5 m s-1. A majority of significant tornadoes (F2 or greater damage) have been associated with STP values greater than 1, while most non-tornadic supercells have been associated with values less than 1 in a large sample of RUC analysis proximity soundings. Additional information can be found here. Analyzed values across the Southern Coastal Plain were around 0.5. The STP value is a function of variables similar to:
mlCAPE which ranged between 1000 and 1500 J/Kg which would have set the mlCAPE term to just less then 1, around 0.8.
mlLCL ranged between 750 and 1000 m which would have set the mlLCL term to 1
SRH1 ranged between 50 and 100 m²/s² which would have set the SRH1 term to less then 1, around 0.8.
0-6 km bulk shear of 45-55 kts which would have capped the shear term to 1.5
sbCIN was near zero which would have set the sbCIN term to 1.

This suggest that the mixed layer CAPE (mlCAPE) and the 0-1 km storm-relative helicity (SRH1) terms were the limiting factor.


(Click on the image below to enlarge)

SPC Analysis at 19 UTC on October 1, 2008 - click to enlarge



NWS composite radar reflectivity imagery from 1858 UTC on October 1, 2008.
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 1858 UTC on October 1, 2008 - click to enlarge



Archived Text Data from the Severe Weather Event

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EDT time + 4 hours).

Product ID information for the most frequently used products...

RDUAFDRAH - Area Forecast Discussion
RDUZFPRAH - Zone Forecast Products
RDUAFMRAH - Area Forecast Matrices
RDUPFMRAH - Point Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDUSPSRAH - Special Weather Statement
RDULSRRAH - Local Storm Reports (reports of severe weather)
RDUSVRRAH - Severe Thunderstorm Warning
RDUSVSRAH - Severe Weather Statement
RDUTORRAH - Tornado Warning


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Lessons Learned

In potential severe thunderstorm and tornado events, the upper level dynamics and instability are critical in the initiation, maintenance and mode of vigorous convection. In this event, both the dynamics and instability supported severe thunderstorms with supercells, but the lack of significant storm relative helicity in the lowest kilometer or two limited the tornado potential.

The supercell thunderstorm that moved across the Sandhills and Southern Coastal Plain had a long lived rotating updraft. The right turning nature of the storm and its proximity to a surface boundary suggested an enhanced tornado threat. Analysis of the local environment suggests that the limiting factor in tornado development with this event was insufficient 0-1 km storm-relative helicity (SRH1). The surface parcel CAPE (sbCAPE) values of 1500-2000 J/kg were moderate, and somewhat greater instability at the lowest levels would have been more supportive of tornadogenesis.

SREF output available from the SPC Short Range Ensemble Forecast (SREF) Page can be used to evaluate the potential of a significant tornadoes via probability forecasts of the Significant Tornado Parameter (STP).

This case highlights the need for several ingredients to come together to produce a tornado from a supercell thunderstorm. When one or more of the ingredients are lacking, tornadogenesis is often inhibited. While the improved understanding of tornadogenesis has provided forecasters with the knowledge of some of the ingredients needed for the development of tornadoes, the objective measure of some these ingredients (such as enhanced storm-relative helicity or special rear flank downdraft) is lacking, making the tornado warning decision very difficult.

Issuing a tornado warning was the correct decision given that fact that there was a long lived supercell, with an impressive mesocyclone, and a storm that turned right increasing the storm relative helicity.

A large and impressive Three Body Scatter Spike (TBSS) or hail spike was noted for several minutes during the event. Hail spikes are a good indicator of large hail and are in general sufficient justification to issue a severe thunderstorm warning. The TBSS can contaminate velocity data and care must be used when interrogating radar data near a TBSS.

Forecasters used surface and mesoanalysis throughout the warning process. The SPC mesoanalysis page was invaluable and it was used frequently to find the locations that had the greatest tornado and severe thunderstorm threat. It should be noted that the SPC mesoanalysis products are produced from an objective analysis of surface data using the latest RUC forecast as a first guess. The surface data is then combined with the latest RUC forecasts of upper-air data to produce an analysis of surface and 3-dimensional fields which are subject to errors in the surface analysis and especially the RUC forecasts.

The feature following zoom option in AWIPS which allows users to loop radar or other data over a moving point, allowed forecasters to maintain continuity by viewing and interpreting radar data over a long period of time.

The event occurred during the afternoon shift change which can complicate severe weather operations. Despite this, continuous, uninterrupted radar coverage was ensured by identifying a radar operator who could work the entire event. The organized nature of the convection and its limited areal extent also made it manageable.



References

Funk, T., 2002: Tornadogenesis in Supercells: The Three Main Ingredients http://www.crh.noaa.gov/lmk/soo/presentations/tornadogenesis.pdf

Lemon, L.R., 1998: The Radar “Three-Body Scatter Spike”: An Operational Large-Hail Signature. Wea. Forecasting, 13, 327–340.

Thompson, R.L., R. Edwards, and C.M. Mead, 2004: An Update to the Supercell Composite and Significant Tornado Parameters. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Met. Soc. (CD ROM).


Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of the NWS Raleigh. The surface analysis graphics were obtained from the Hydrometeorological Prediction Center. GOES satellite data was obtained from National Environmental Satellite, Data, and Information Service. SPC mesoanalysis graphics provided by the Storm Prediction Center. Google Earth map imagery used under license. NSSL's Rotational Track and Hail Swath products provided by the WDSS-II (Warning Decision Support System - Integrated Information) group at NSSL. Local storm reports and warning polygons KMZ data provided by the National Climatic Data Center. The photo of Baseball size hail was anonymously reported by a spotter in Spivey's Corner, near the intersection of Timothy and Easy Street in Sampson County.


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Jonathan Blaes

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