Event Summary
     National Weather Service, Raleigh NC

April 17, 2006 Severe Weather Event
Preliminary - Updated 2006/04/21





Event Headlines -
...The National Weather Service in Raleigh issued 14 Severe Thunderstorm Warnings on Monday, April 17, 2006...
...The majority of severe weather reports were from damaging straight line winds with a few reports of hail...
...Using the EL + 20% provided a threshold for the issuance of severe thunderstorm warnings that resulted in a high Probability of Detection (POD) across the far western counties...


Event Overview -
During the early morning hours of Monday, April 17, 2006 a potent upper level wave and surface low tracked southeast across Illinois, Indiana and Kentucky. A complex of thunderstorms at the leading edge of these systems tracked across the Ohio Valley overnight. At the same time, a stationary front was sitting across the state of North Carolina.

The thunderstorms at the leading edge of these systems weakened as they moved across West Virginia and Virginia around daybreak. Other thunderstorms near the core of the upper level system located over eastern Kentucky intensified.

By mid morning the surface low had advanced to southern West Virginia and the thunderstorms continued to intensify and organize. The convection weakened a bit as it crossed the North Carolina mountains but conditions were favorable for the thunderstorms to intensify as the system moved into central and southern North Carolina.

The thunderstorms were often severe as they swept across central and southeastern portions of North Carolina. The threat ended during the late afternoon and early evening hours as the storms moved east off the coast.



Severe Weather Reports -

Text of severe weather reports across central North Carolina





Satellite

Visible satellite imagery from 1845Z (245 PM EDT) on Monday, April 17, 2006

The visible satellite image below from 1845Z (245 PM EDT) shows the line of convection developing and moving east into the Piedmont and Sandhills regions of North Carolina. Severe thunderstorms were reported around this time in Davidson and Montgomery counties.

A Java Loop of infrared satellite imagery from 1515Z (1115 AM EDT) through 2145Z (545 PM EDT) Monday, April 17, 2006 is available.





Surface Analysis

Analyzed surface map from 1800Z (200 PM EDT) Monday, April 17, 2006

The surface analysis from 18Z (200 PM EDT) Monday, April 17, 2006 depicts a cold front stretching across central and southern North Carolina. A surface low pressure system located in the Foothills at 1800Z is moving east and southeast along the front.

A Java Loop of surface analysis imagery from 18Z Sunday, April 16, 2006 through 06Z Tuesday, April 18, 2006 shows the approach of the cold front, the passage of the surface low and then the cold front moving offshore.

Surface analysis from 1800Z (200 PM EDT) Monday, April 17, 2006



Mesoscale Data

Analyzed mean sea level pressure (black) and surface wind barbs from SPC at 19Z on Monday, April 17, 2006 (3 PM EDT).
A cold front can be seen stretching across southern North Carolina with a weak low pressure center located just east of Charlotte.

SPC Analysis at 19Z on Monday, April 17, 2006.



Analyzed surface temperatures (red/purple), dewpoints (brown/green), and wind barbs from SPC at 19Z on Monday, April 17, 2006 (3 PM EDT).
The frontal zone is clearly seen across North Carolina. Note the more unstable airmass with dewpoints in the lower 50s and surface temperatures in the mid to upper 80s across far southern North Carolina.

SPC Analysis at 19Z on Monday, April 17, 2006.



Analyzed low level lapse rates in the 0-3 km layer (blue, green, orange, and red) from SPC at 19Z on Monday, April 17, 2006 (3 PM EDT).
Note the low level lapse rates in red across southern North Carolina and South Carolina which represent a layer that is close to becoming "absolutely unstable."

SPC Analysis at 19Z on Monday, April 17, 2006.



300 MB wind barbs (brown), 300 MB isotachs (red) and analyzed 300 MB divergence (purple) from SPC at 19Z on Monday, April 17, 2006 (3 PM EDT).
Note the 70-80 knot jet max located over southeastern North Carolina and eastern South Carolina with a large area of divergence and implied upward vertical motion over central North Carolina and central South Carolina.

SPC Analysis at 19Z on Monday, April 17, 2006.



NWS Composite Reflectivity Imagery from 1942Z on Monday, April 17. 2006 (342 PM EDT).
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 1942Z on Monday, April 17. 2006.




Radar Loops

Java Loop of KRAX radar imagery from 1706Z (106 PM EDT) through 2201Z (601 PM EDT) Monday, April 17, 2006.
This loop includes imagery from every radar scan.

Java Loop of KRAX radar imagery from 1706Z (106 PM EDT) through 2201Z (601 PM EDT) Monday, April 17, 2006.
This loop includes imagery at 15 minute intervals.


KRAX base reflectivity image from 1847Z on Monday, April 17, 2006 (247 PM EDT)





Warning Decision Process


Overview

At approximately 130 PM EDT on April 17, 2006 a cluster of thunderstorms associated with a vigorous surface low and upper level wave was approaching the Northwest Piedmont of North Carolina. A stationary front was situated across the southern half of the state. Abundant sunshine south of the stationary boundary allowed temperatures to soar into the upper 80s in many locations, significantly increasing the amount of energy available for thunderstorm development . At this time, thunderstorms also began to develop in northern Cabarrus and southern Rowan counties just west of NWS Raleigh's CWA (County Warning Area) in the Southwestern Piedmont.

The decision to sectorize warning efforts was made shortly after 130 PM EDT. One forecaster concentrated on the cluster of thunderstorms approaching the northwest piedmont and another forecaster concentrated on the developing thunderstorms in the southwest piedmont. Over the next few hours, the storms entering the northwest piedmont moved along or north of Highway 64 through Forsyth, Guilford, Davidson and Randolph counties producing a trail of wind damage along the way. The storms would produce more wind damage as they moved through Wake, Johnston and Wayne counties along Highway 70 later in the afternoon.

The storms developing in the southwest piedmont over northern Cabarrus and southern Rowan counties moved east into Stanly county at approximately 200 PM EDT. At this time, the developing storms did not show any reflectivity or velocity signatures indicative of severe weather. However, NLDN data showed lightning strikes becoming much more frequent and storm echo tops exceeded the EL plus 20% criteria. Additionally, the environment at the time was very favorable for the development of severe thunderstorms with strong upper level lift juxtaposed with a stationary boundary and a warm unstable atmosphere over the southwest piedmont and southern portions of central NC. As the storms moved through Stanly county, one of them split, with the left moving storm moving NNE into southern Davidson and Randolph counties and the right moving storm moving ESE into Montgomery county. At approximately 2:40 pm a Severe Thunderstorm Warning was issued for Montgomery and Moore counties as the right mover began to intensify. The primary threat at the time appeared to be large hail as the storm developed a strong reflectivity core between 65-70 dBZ. Severe Thunderstorm Warnings were issued for this storm thereafter until it moved east of the NWS Raleigh CWA. As the storm moved southeast from Moore into Hoke and Cumberland counties it became more multicellular, quickly evolving into a line of thunderstorms. At this time the severe weather threat transitioned from hail and high winds to primarily high winds as reflectivities decreased and 55-65 knot outbound radial velocities developed at the leading edge of the line.



MARC Signature

MARC signatures are useful because they are detectable in convective lines or bow segment at large distances 150-220km (90-135 miles) from the radar. Velocity data from the Raleigh (krax) radar which is a considerable distance from Forsyth County was essentially useless with range folding (shown in purple) obscuring distinguishable features. However radar data from Blacksburg (kfcx) revealed a persistent velocity couplet in the mid-levels or MARC Signature. The MARC signature is visible in the kfcx 0.5 degree base velocity imagery from 1746Z and the corresponding kfcx 0.5 degree base reflectivity imagery from 1746Z. A zoomed in view of the kfcx 0.5 degree base velocity from 1746Z clearly shows the MARC signature. Note the velocity couplet of 36kts of outbound and 26kts of inbound. Trees were reported down in Lewisville and Clemmons at 1805Z and 1820Z respectively. The MARC signature fluctuated up and down as the line segment continued to push southeast, with a strong or highly visible MARC signature at 1811Z. This portion of the line went on to produce wind damage all throughout Davidson and Randolph Counties. Two radar loops of kfcx reflectivity and velocity imagery are available.

KFCX 0.5 Reflectivity Loop

KFCX 0.5 Velocity Animation




Enhanced Echo Tops and EL + 20%

It has been noted that using the echo tops derived product worked well for the initial storms across the far western County Warning (CWA) Area. Using the sounding toolkit earlier that day, the day shift determined the EL + 20% would be around 38 kft. A loop of the echo tops of the storms as they entered the western CWA is shown below. Using 38kft echo top for a threshold of severe resulted in a high Probability of Detection (POD) across the far western counties, despite being such a long distance from the radar.

KRAX 0.5 Enhance Echo Tops Loop


Case study team -
Brandon Vincent
Brandon Locklear
Brandon Dunstan
Jeff Orrock
Jonathan Blaes

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


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