Event Summary
     National Weather Service, Raleigh NC


May 9, 2008 Severe Weather Event
Updated 2008/08/20







Event Headlines

...Isolated supercells developed after sunset during the evening hours of May 9th and the early morning hours of May 10th, 2008...
...Multiple reports of quarter to golf ball size hail and strong damaging winds were received...
...The first severe weather event occurred at 945 PM with the last severe weather event occurring at 0144 AM...
...The Chapel Hill (KIGX) ASOS recorded a wind gust of 60 MPH at 1211 AM...


Event Overview

Isolated supercell thunderstorms developed over and moved across central North Carolina during the late evening hours of May 9 and the early morning hours of May 10. These severe thunderstorms came as somewhat of a surprise since atmospheric conditions did not initially look conducive to the development of severe weather. Numerous reports of quarter to golf ball sized hail were received in association with these storms, in addition to several reports of wind damage and a wind gust of 60 MPH recorded at the Chapel Hill (KIGX) ASOS site.

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





Surface Analysis

A manual surface analysis valid at 02Z 05/10/08, or 1000 PM EDT on the evening of 05/09/08 shows a weak area of low pressure over northern West Virginia, with a cold front stretching southwest through Tennessee and Kentucky. A weak stationary front was analyzed over central Virginia and far northeastern North Carolina. Additionally, a trough axis was located roughly west/east across the center of North Carolina.

The east/west surface trough axis across the state provided a focus (in the form of low level convergence) that helped thunderstorms develop during the overnight hours. The trough axis was a boundary between a more stable air mass across northern North Carolina and some low level instability across the southern portions of the state where MLCAPE values where greater than 500 J/Kg were analyzed. Dew points in the lower 60s across southern North Carolina provided additional moisture to the isolated convection that developed.




Upper Air Analysis

The upper air analysis on 00Z May 10, 2008 revealed several features that pointed to an increased potential for thunderstorms and the possibility for severe weather. The 00Z GSO RAOB sounding showed steep mid-level lapse rates around 7.5 deg C/km with MLCAPE values around 1000 J/Kg. LI values were generally below -5 and with a wet bulb zero value of around 9000 feet. The wind field was very impressive with a wind max of around 55 knots at around 5 km with a strong unidirectional flow providing impressive deep layer shear (surface to 6 km shear values of around 50 knots). However, there was a capping inversion at the 700 mb level with a temperature of 5 degrees Celsius.

A 00Z upper air analysis of the 700 mb level showed a shortwave trough upstream of the Carolinas over the Ohio/Tennessee Valleys. Notice that the 700 mb temperatures upstream were colder by several degrees (Nashville was 3 deg C at 700 mb while Greensboro was 5 deg C). The cooler mid-level temperatures were associated with the 700 mb shortwave trough that moved east into North Carolina a few hours after 00Z (via a 50 MPH westerly flow), helping erode the capping inversion present on the 00Z GSO RAOB.

At the 500 mb level, 00Z RAOB analysis showed a weak shortwave upstream of the Carolinas over the Ohio Valley, with 20 meter height falls at Wilmington, OH. Another shortwave was located in vicinity of the DELMARVA peninsula. An area of colder tempertaures at 500 mb can be seen over the southern Appalachians and the western Piedmont of North Carolina and Virginia with both Greensboro and Roanoke reorting 500 mb temperatures of -17 deg C. The 500 mb temperature at Greensboro had cooled nearly 5 degrees in the 12 hour period ending at 00Z. This cooler pocket of air aloft was responsible for the fairly steep mid level lapse rates analyzed at 01Z.

The 250 mb analysis showed a trough across portions of North Carolina and Virginia. A 100 knot WNW Jet was nosing into eastern Tennessee and western North Carolina placing central North Carolina near the left front portion of the southeastward digging jet and enhancing some modest upper level divergence analyzed at 01Z.



NSSL Hail Swath Product

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).

The NSSL Hail Swath product valid from the evening of May 9th and the early morning hours of May 10th, 2008 is shown to the right. You can clearly see the track of several long-lived supercells that moved across central North Carolina. If you look closely at the left-center of the image, you can see where a supercell in Randolph County split, with the left-mover continuing due east toward southern Wake County and the right-mover moving southeast toward Fayetteville. The track separation that looks like a split in central Chatham County was actually caused by a separate storm that developed ahead of the storm that split in Randolph County, heading southeast through Lee and Harnett Counties and eventually toward the NC coast.

The northwest-southeast oriented track that originates in southwestern Virginia and ends in Wayne County, NC was associated with a long lived supercell that developed across the southwest VA mountains, propagating over a North Carolina. Reports of large hail and/or severe winds were received along the entire track of this storm before it dissipated.



Radar Imagery

Overview of the entire event with images from every volume scan between 0057Z (857 PM EDT) through 0604Z (204 AM EDT) May 10, 2008.
Java Loop of KRAX reflectivity imagery every volume scan between 0057Z through 0604Z (204 AM EDT) May 10, 2008.
Note - this loop includes 68 frames

The reflectivity loop below contains imagery from 0210Z through 0400Z showing a splitting supercell over southern Randolph County and another long lived super cell that moves southeast across Guilford, Alamance, and Durham Counties.




The KRAX 0.5 degree base velocity at 0410Z (1210 AM EDT) shows an area of 45-50 knot inbound velocities in Chapel Hill. The Chapel Hill ASOS (KIGX) reported a 60 mph wind gust at approximately 0411Z (1211 AM EDT).



The KRAX 0.5 degree base velocity at 0509Z (109 AM EDT) shows an area of 50 knot (60 MPH) outbound velocities at roughly 400 feet AGL just northwest of Smithfield in Johnston County. A half dozen trees were knocked down by severe winds at the location marked in the image.



The KRAX 3.1 degree reflectivity at 0516Z (116 AM EDT - shortly after the previous velocity image) shows 65-70 dBZ reflecitivities at 3500 ft AGL above the town of Smithfield in Johnston County. Ping pong ball sized hail was reported at this time at the 911 center in Smithfield.




The Capping Inversion and AMDAR Aircraft Soundings

AMDAR is an acronym for Aircraft Meteorological DAat and Reporting (AMDAR) which is an international effort within the World Meteorological Organization to coordinate the collection of environmental observations from commercial aircraft. In the United States, we often refer to the Meteorological Data Collection and Reporting System (MDCRS) which is a private/public partnership facilitating the collection of atmospheric measurements from commercial aircraft to improve aviation safety.

AMDAR data is very useful for short term forecasting situations where conditions are changing rapidly. Past experience has shown that the data is useful for aviation forecasting, precipitation type forecasting in winter events, and convection. Many of the AMDAR aircraft soundings that are available in central North Carolina do not include dew point data which would be very helpful in forecasting convection. Even without the dew point data, the AMDAR aircraft soundings on May 9th and 10th still provided important details useful to forecasters.

Several factors were working to limit convection during the event including a westerly flow with a fair amount of dry air and a significant capping inversion that was present at 700 mb. Despite a widespread field of cumulus clouds during the afternoon, convection was inhibited at least to some degree by the capping inversion. As the mid level jet and a series of disturbances in the westerly flow approached, the cap eroded during the evening hours as seen in the composite sounding image below.

The red line in the composite image below is the AMDAR aircraft sounding at KGSO from 2147 UTC (547 PM EDT) on 2008/05/09 which shows a significant capping inversion at around 700 mb. The blue line is the KGSO RAOB sounding released at 2315 UTC (715 PM EDT) on 2008/05/09 still shows the capping inversion but it is slightly weaker. The purple line is the AMDAR aircraft sounding at KGSO from 0229 UTC on 2008/05/10 (1029 PM EDT 2008/05/09) which shows the capping inversion had eroded away.





Mesoscale Data

Analyzed mean sea level pressure (black) and surface wind barbs from SPC at 03Z on Saturday, May 10, 2008
An east-west surface boundary can be seen in the pressure and wind analysis below.

SPC Analysis at 03Z on Saturday, May 10, 2008



Analyzed surface temperatures (red/purple), dew points (brown/green), and wind barbs from SPC at 03Z on Saturday, May 10, 2008
A region of convergence can be seen along a surface boundary in the analysis below with dewpoints in the lower 60s south of the boundary and in the mid to upper 50s north of the boundary.

SPC Analysis at 03Z on Saturday, May 10, 2008



700 MB heights, temperatures (red/blue), dew points (green), and wind barbs (black) from SPC at 03Z on Saturday, May 10, 2008
A relatively fast westerly flow was analyzed with the flow at around 35 knots across central North Carolina. The temperatures had cooled to around -3 degrees C at Greensboro.

SPC Analysis at 03Z on Saturday, May 10, 2008



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 general flow pattern from the west and northwest with the exit region of a 60 knot jet approaching the Carolinas from the northwest. A cool pocket of temperatures can be seen across central North Carolina and Virginia where temperatures were -16 degrees C.

SPC Analysis at 03Z on Saturday, May 10, 2008



Analyzed mid level lapse rates in the 700-500 MB layer (blue, green, orange, and red) from SPC at 03Z on Saturday, May 10, 2008
A lapse rate is the rate of temperature change with height and the image below is for the layer from the surface to around 9,000 feet. Note the mid level lapse rates below range in the 7.0 to 8.0 deg C/km range with the greatest instability across southern portions of central North Carolina.

SPC Analysis at 03Z on Saturday, May 10, 2008



Analyzed mixed layer convective available potential energy (MLCAPE) (red) and mixed layer based convective inhibition (MLCIN) (blue lines - shaded) from SPC at 03Z on Saturday, May 10, 2008
The greatest MLCAPE values were around or slightly greater than 500 J/kg across the eastern Sandhills and southern Coastal Plain at 03Z with MLCAPE values less than 500 J/kg across much of the remainder of central NC. Increasing amounts of MLCIN were analyzed across central NC, especially just south of the area where severe weather was observed. This is to be expected with a stabilizing boundary layer. The 01Z MLCAPE analysis shows that the atmosphere was more unstable a few hours earlier.

SPC Analysis at 03Z on Saturday, May 10, 2008



Analyzed most unstable convective available potential energy (MUCAPE) (red) and lifted parcel level (dashed black lines and shaded in yellow and green) from SPC at 03Z on Saturday, May 10, 2008
MUCAPE values ranged between 500 to around 1000 J/kg across central North Carolina. The shading shows that the instability was not surface based but rather on the order of 1000 to 2000 meters above the surface.

SPC Analysis at 03Z on Saturday, May 10, 2008



Analyzed precipitable water (green) and wind barbs from SPC at 03Z on Saturday, May 10, 2008
Precipitable water values were around 1.1 inches or less across central North Carolina.

SPC Analysis at 03Z on Saturday, May 10, 2008



NWS Composite Reflectivity Imagery from 03Z on Saturday, May 10, 2008
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 03Z on Saturday, May 10, 2008



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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Summary

The isolated supercells that affected central NC late on the evening of May 9th and during the early morning hours of May 10th were able to develop and sustain themselves for several reasons. The 00Z GSO sounding showed an environment with strong westerly speed shear and steep low/mid level lapse rates. Although an inversion at 700 mb initially capped the atmosphere across central North Carolina, cooler mid-level air (associated with a 700 mb shortwave over the Tennessee/Ohio valleys) moved into central NC within a few hours after sunset, eroding the inversion and allowing more available instability.

The modest upper level divergence associatied with the jet exit region across the area likely provide some forcing for ascent and crontributed to the development of convection. The 500 mb shortwave upstream of the area over the Ohio Valley was likely too far north to provide much in the way of upper level lift but the flow was increasing at 500 mb as indicated by the 55 knot wind observed in the 00Z Nashville RAOB. The notable 700 mb shortwave over the Tennessee/Ohio valleys may have been a more significant player with the increasing westerly flow and cooler air to support destablizing the mid levels. The approach of the mid level trough and the subsequent convection can be seen in a water vapor loop from 12Z on May 9 through 12Z on May 10, 2008.

In terms of convective initiation, the 02Z 05/10/08 surface analysis showed an west-east trough axis across the entire state, from Cape Hatteras west to Asheville, with 60F dew points across most of central North Carolina. The first cells that developed across central North Carolina developed in an area of convergence associated with this boundary in Davidson and Randolph Counties at roughly 0145Z (945 PM EDT), becoming severe by 0215Z (1015 PM EDT).

The following factors are most likely to be responsible for the isolated severe thunderstorms that affected the area on the evening of May 9 and early morning hours of May 10:

1) The mid level ~ 700 mb shortwave approaching the state
2) Fast mid-level westerly flow
3) Significant mid level cooling
4) Steep lapse rates
5) 60F surface dew points
6) Surface trough axis across central NC
7) 100 knot upper level jet nosing into western NC


Acknowledgements

Some of the images and graphics used in this review were provided by parties outside of the National Weather Service in Raleigh. The Hail Swath product was obtained from the NSSL On-Demand website, and was displayed in Google Earth. Radar images were obtained by downloading archived level II radar data and displaying them in GRlevel2.


Case Study Team

Brandon Vincent
Jonathan Blaes


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