Event Summary
     National Weather Service, Raleigh NC

April 3, 2006 Severe Weather Event
Updated 2006/04/12

(photos courtesy of Mike Shumate)

Event Headlines -
...The National Weather Service in Raleigh issued 26 Severe Thunderstorm Warnings on Monday, April 3, 2006...
...The severe weather came in three waves with the first wave arriving around daybreak and the second wave arriving around mid morning to midday...
...The most significant wave of severe weather arrived during the late afternoon and early evening...

Event Overview -
Several rounds of severe thunderstorms marched across central North Carolina on Monday, April 3rd ( loop of KRAX radar imagery from 629 AM to 1158 PM EDT Tuesday, April 3, 2006 ). The first round occurred during the pre-dawn hours associated with a northward moving warm front. These storms were the first significant convection to occur across central North Carolina since the middle of January. These storms produce several reports of small hail. Another round of storms crossed the region during the morning hours after daybreak. This second round of storms produced intense lightning, torrential downpours and hail that varied in size from pea to penny size.

The third outbreak of convective activity developed during the late afternoon and evening hours on April 3rd. Following the morning thunderstorms, skies cleared by early afternoon allowing the airmass over central North Carolina time to destabilize. The afternoon and evening round of severe weather would prove to be the worse of the three. The initial afternoon storms developed along an old outflow boundary from the morning convection as well as a pre-frontal trough over the eastern Sandhills. The storms intensified quickly, evolving from showers to severe storms in a span of 20 minutes. These storms produce large hail with reports of ping pong ball hail south of Fayetteville and numerous reports of penny to nickel size hail across eastern Cumberland into northern Sampson County.

While the storms in the southern coastal plain were developing, storms ahead of a mid level trough and associated surface cold front were developing in the foothills of western North Carolina. These storms intensified as they moved into the northwest piedmont of central North Carolina. As storm approached the Triad from the west they quickly evolved into a broken line of storms with weak bow features noted in reflectivity pattern. These storms had more upper level support from the mid level jet crossing the deep South/Tennessee Valley. As the line organized, a severe gust front with winds of 50 to 60 mph developed ahead of the line. This line began to producing numerous wind damage reports just east of the Triad in Alamance County, continuing east into the far northeast Piedmont and the northern Coastal Plain.

Severe Weather Reports -

Text of severe weather reports across central North Carolina

Surface / Upper Air Perspective Before the Event

The potential for significant severe weather was well anticipated several days before the event. On the preceding day (Sunday, April 2nd), a weak high pressure area was moving off the mid-Atlantic coast. A front which stretched across central Tennessee and along the Georgia/South Carolina border in the afternoon was expected to shift northeastward as a warm front overnight and bring scattered convection to the area. The best convective coverage was expected across the northern half of NC beneath a weak lobe of vorticity and upper divergence maximum, coincident with isentropic upglide on the 300K surface.

After passage of the warm front just prior to daybreak, a brief lull in precipitation was expected to be followed by more strong to severe thunderstorms during the afternoon. A deepening 500 mb low pressure center tracked northeast across the Great Lakes, with a trough extending southward which took rapidly took on a negative tilt as it approached North Carolina. Both the NAM and GFS predicted strong height falls aloft and vigorous large scale lift within the right entrance region of a southerly 300 mb jet extending from eastern Kentucky to southern Ontario. Another upper jet, diving down the southwest side of the trough, further enhanced lift over the area later in the day.

In addition to these broad lift mechanisms, thermodynamic parameters on the NAM model indicated a strong likelihood for organized strong to severe storms. Convective available potential energy (CAPE), a measure of instability, was forecast to exceed 2500 J/kg over much of central and eastern North Carolina by the afternoon. The 850-500 mb lapse rates were expected to reach 7.0 to 7.5 C/km (observed at 7.3 C/km at KGSO on 00z 2006/04/04), and forecast lapse rates within the prime hail growth region of -10°C to -30°C were close to dry adiabatic. Other parameters were favorable for supercells, including the energy helicity index (EHI) and the bulk Richardson number (BRN), which both factor in buoyancy and shear (KGSO 00z 2006/04/04 RAOB). Wind profiles also pointed to a severe threat with a 35-40 kt southwesterly jetlet at 850 mb and surface-to-6 km wind shear of 40-45 kt.

The only potential limiting factor was expected to be the moisture. The initial return of moisture appeared brief and would have to overcome quite a bit of dry air, noted on the Sunday soundings at GSO. In particular, the models predicted much drier air moving in aloft by the afternoon. While instabilities were expected to be high, it was thought that the lack of good low level moisture convergence noted on the models going into the event would keep thunderstorms more scattered. As it happened, the lift across northern sections of central North Carolina was strong enough to make coverage quite widespread, while far southern sections saw very little or no rainfall.


Visible satellite imagery at 2115Z on Monday, April 3, 2006 (515 PM EDT)

The visible satellite image below shows the line of convection moving east of the mountains of North Carolina and Virginia on Monday, April 3, 2006. A few isolated thunderstorms can be seen across the Sandhills and Piedmont regions of North Carolina and South Carolina.

A Java Loop of Satellite imagery from Visible satellite imagery from 1603Z (1203 PM EDT) through 2233Z (633 PM EDT) Monday, April 3, 2006 is available.

Radar Analysis

KRAX 4-Panel Composite Reflectivity/Base Reflectivity/VIL/Enhanced Echo Top Imagery from 2100Z Monday, April 3, 2006.
A four panel Composite Reflectivity/Base Reflectivity/VIL/Enhanced Echo Top image from 2100Z (500 PM EDT) is shown below. The imagery shows the severe thunderstorm as it was producing large hail across Cumberland county. Several reports of hail were received around 500 PM EDT from locations 10 miles southeast of Fayetteville, near Cedar Creek and southwest of Stedman.

Note the VIL values in the lower left panel than range between 50 to 55 J/Kg (max value was near 51) and the Enhanced Echo Top values in the lower right panel around 39 Kft.

Click on the image to enlarge.

 - Click to enlarge

KRAX 4-Panel Velocity Imagery from 2306Z Monday, April 3, 2006.
A four panel velocity image from 2306Z (706 PM EDT) is shown below. The imagery shows severe thunderstorms across Guilford and Alamance counties with damaging winds. These thunderstorms produced a wind gust of 58 MPH about 12 minutes earlier at the Burlington Airport at 654 PM EDT. There were numerous other reports of wind damage across Guilford and Alamance counties between 630 and 700 PM EDT. Additional reports of wind damage produced by these storms were received across Orange, Person, Wake, Vance and Nash counties later in the evening.

Note small pixels of light blue in the yellow circle in the upper left panel of the image below. The upper left panel is the 0.5 degree velocity data. The light blue represents wind velocities greater than 60 knots. At this range the radar is sampling the data at 3970 ft above sea level.

Click on the image to enlarge.

 - Click to enlarge

Radar Loops

KRAX base reflectivity image from 2353Z on Monday, April 3, 2006 (753 PM EDT)

The radar image below with the severe thunderstorm warnings highlighted in yellow is from 2353Z or 753 PM EDT at the height of the storminess across central North Carolina. Note that there are 8 active severe thunderstorm warnings in effect across central North Carolina covering 9 counties with several other warnings in effect along the coast and near Charlotte.

Java Loop of KRAX Radar Imagery from 1029Z (629 AM EDT) on Monday, April 3, 2006 to 358Z (1158 PM EDT) Monday, April 3, 2006.

Warning Decision Process

Both the overnight and morning severe events were dominated by marginally severe hail. During these events, the 50dbz versus freezing level hail technique was applied with some success. The evening severe event began as a hail event over southern Cumberland and Sampson counties. It was noted in the afternoon that the freezing levels had lowered since morning and that lower values of VIL and elevated 50 dbz would likely produce severe hail. Base reflectivity products, radar All Tilts, Echo Tops and VIL’s all worked rather well though warnings were primarily issued from the base products. The EL +20% technique also proved useful.

As the severe thunderstorm producing ping-pong ball size hail moved across Sampson County, other storms were starting to blossom along the Yadkin River as they moved into Forsyth and Davidson counties. These storms were developing ad intensifying as they moved into an environment with higher CAPE combined with a little more CIN. This combined with steepening lapse rates in the mid levels resulted in thunderstorms with intensifying updrafts in the Triad region.

The first reports of severe weather in Forsyth County were of hail, but the event quickly began to transition into a wind event. The low level wind profile was unidirectional but with increasing speed shear. The speed shear allows the updraft to keep pace with the outflow that races out ahead of the storm. While forecasters were still watching for hail signatures in these cells, warnings from the Triad area east across Alamance and Orange were issued with a bit more urgency given the changing nature of the threat. By the time the storms passed east of Greensboro the storms were primarily wind threats.

As the line pushed east of Burlington the reflectivity cores weakened and a well defined gust front was evident on both base reflectivity and base velocity. The Burlington ASOS measured a peak wind around 60 mph and forecasters quickly realized the gust front was still producing severe wind despite the low reflectivity’s. Warnings were quickly issued for Durham, Wake, Franklin and Granville counties and at this time 3 forecasters were involved in issuing warnings and SVS’s. While the gust front was still near Chapel Hill a call was made to the RDU tower to notify them of the wind threat. Numerous warnings were also issued for the NC/VA border counties and east to Nash and Halifax counties. At the same time storms were redeveloping over Montgomery and Moore counties and that part of the county warning area was sectorized to handle the threat. By the time the gust front reached I-95 winds were down to 40-50 mph and warnings were ceased at the appropriate time.

Overall warning strategies were changed 3 times during the event and good communication and situational awareness allowed for quick adaptability.

Case study team -
Scott Sharp
Gail Hartfield
Michael Strickler
Jeff Orrock
Jonathan Blaes

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