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.