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The April 1, 2001 Severe Event

 

Bryan P. McAvoy and Wayne A. Jones
NOAA/National Weather Service
Greer, SC

During the late morning of April 1st, 2001, a line of low-topped thunderstorms moved rapidly across the southern North Carolina Piedmont and foothills, and the northern Upstate of South Carolina.

This was an unusual event as most of the severe weather occurred in an area with dewpoints in the mid to upper 40s, and temperatures in the middle 50s. This was not an elevated convective event, with parcels lifted from the surface being the most unstable on local LAPS soundings. A javascript loop of satellite, lightning and RUCII 500 mb heights and vorticity data from the event can be found here (Note: This loop is quite large, around 1 MB, and can take a while to load). The loop reveals that the CWA was on the north side of the polar jet, in an area of strong upper divergence ahead of a strong shortwave trough. Even ahead of the wave, 500 mb temperatures were around -25 deg Celsius. The lightning data reveals that convection had developed over eastern Tennessee. However, when the MCS moved into the mountains it dissipated. Upon emerging into the foothills, it rapidly redeveloped and became severe.

While a large area of thunderstorms did develop ahead of the line as moved into central and eastern North Carolina, most of the convection in the Upstate and southern North Carolina Piedmont developed right along the surface front. The 13Z surface meso-analysis revealed a surface front extending form just east of Asheville, south into the western Upstate. Note that surface dewpoints are only in the mid 40s at this time ahead of the front. There was a somewhat more significant pool of surface dewpoints further east. This was along a weak "back-door" cold front which had moved in the day before. The 14Z meso-analysis showed that the front had made significant eastward progress. The KGSP Composite Reflectivity image valid at this time showed that a narrow MCS had developed along the front and was moving east between 40 and 50 mph. Meso-analysis done at 15Z had the front extending from Shelby to Newton to a little east of Spartanburg. At this time the attendant MCS had accelerated a little ahead of the front and had moved nearly into the western suburbs of Charlotte. The narrow MCS had storm tops of between 15 and 20 kft. A small bow-echo can be seen developing in the MCS over the northern part of the Upstate. By 16Z the front had moved into Charlotte. The MCS continued to exhibit a bow-echo signature on radar across the Chester and York counties. However there were several reports of wind damage further north, into North Carolina, along the MCS. The 18Z meso-analysis had the front nearly out of the CWA. It appears that a significant dry intrusion associated with the short wave trough was causing the airmass to mix out well ahead of the frontal boundary. Perhaps this boundary was more a lee trough. A considerable amount of convection had developed even ahead of the front, in the area of pooling moisture. A meso-low had also developed ahead of the front, in association with the MCS. Surface pressure with the meso-low fell to around 999 mb.

Several severe thunderstorm warnings were issued during the late morning. A list of all severe thunderstorm warnings for the event can be found here, and a list of local storm reports is here. This line was accompanied both by winds of 60 to 70 mph, and up to golf ball sized hail. The larger hail reports occurred in the extreme eastern part of the CWA with cells that formed just ahead of the main MCS.

Even though the atmosphere was unstable, the amount of instability was not that great. This LAPS sounding taken at 15Z at a point over Charlotte reveals MUCAPE of around 500 j/kg. Moderate shear was in evidence with winds increasing from 2.5 m/s to around 20 m/s in the lowest 3 km. However, there was tremendous upper forcing and a strong mid level dry intrusion as seen from the 700 mb upper air analysis from that morning.