Numerical simulations and the analysis of observational data were employed to define the sequence of events which resulted in the development of an environment conducive for the tornadic convection that produced extensive damage and injury in Raleigh, North Carolina on 28 November 1988. This particular event was quite unusual in that a deadly F4 tornado occurred at ~1 AM local time during the late fall. Furthermore, the storm developed within an environment that was not considered dynamic enough to warrant a severe weather watch by the National Weather Service. Additionally, the environment occurred within the subgeostrophic entrance region of a polar jet streak. This opposes conventional synoptic wisdom in that severe weather is more frequently observed in the polar jet exit region. Positioned above the polar jet streak entrance region, a highly ageostrophic circulation associated with a subtropical jet exit region was present. The numerical model results as well as conventional asynoptic data analysis indicated that key features within the precursor environment could be traced back in time to at least 1200 UTC 27 November along the Gulf Coast. In-depth analysis of numerical model results and observational data show that the precursor processes which play a significant role in the development of the Raleigh tornado could be organized into four distinct "stages".
During the first stage, four different mesoscale pressure perturbations phase to produce a distinct cyclonic mesoscale circulation over southwestern Georgia. This mesoscale circulation will be referred to as the "Georgia mesocyclone". These pressure perturbations, which are all coupled to preexisting convection, include: 1) a strong cold frontal trough/squall line propagating through the Mississippi and Tennessee River Valleys, 2) a surface mesolow propagating along a weak quasi-stationary cold front/trough over the western Appalachian Piedmont within which convection is occurring, 3) an outflow boundary lying just north of the Florida Panhandle, and 4) a mesoscale high pressure region, which is wedged along the central and eastern Appalachian Piedmont.
The second stage involves the development of a mesoscale jet streak aloft over the Tennessee River Valley which rapidly propagates over the Appalachian Piedmont. The subsequent adjustment processes result in the destruction of the shallow hydrostatic surface ridge which prevents the northeastward propagation of the Georgia mesocyclone prior to 2000 UTC. This mesoscale "jetlet" is the result of diabatically-forced geostrophic adjustment processes wherein convective latent heating accompanying the strong cold front modifies the northwestward-directed pressure gradiant force within the subgeostrophic polar jet entrance region and subtropical jet exit region. The inertial-advective response to this pressure perturbation accelerates the mid-upper tropospheric winds at right angles away from the region of mass perturbation resulting from convective heating. Subsequently, this produces an independently propagating region of mass flux divergence which itself induces propagating gravity waves and significant pressure falls over western North Carolina and Virginia. Furthermore, the ageostrophic circulation associated with the new mesoscale jetlet results in the descent of dry air over the Western Piedmont. This mesoscale region of mass flux divergence propagates northeastward above the shallow dome of rain cooled air along the Carolina Piedmont.
The third stage represents an intensification of the Georgia mesocyclone through wave-CISK and its northeastward propagation along the Appalachian Piedmont. As the convectively-induced upper-level mass flux divergence amplifies the mesocyclone, the surface inflow increases, thus intensifying the low-level jet and low-level vorticity through vortex tube stretching. Part of the amplification also involves the development of upstream confluent flow aloft behind the CISK mode(s) resulting in downward momentum fluxes and the injection of dry air behind the propagating mesocyclone(s). Two well-developed gravity waves, organized by the intersecting pressure perturbations along the Gulf coast, sustain the Georgia mesocyclone as well as a precursor mesolow which propagate in tandem northeastward along the Piedmont frontal boundary. Each mesocyclone is accompanied by a mesoscale jetlet aloft contributing to mass flux divergence (convergence) ahead of (behind) the propagating mesoscale jetlet.
Finally, during stage four, the Georgia mesocyclone and its accompanying low-level southeasterly jet intensify due to diabatically-induced vortex tube stretching. Coincidentally, the descent of westerly momentum in the rear of the mesocyclone increases the vertical wind shear and theta-e gradients above the system. The enhanced vertical wind shear in proximity to vortex tube stretching intensifies the tilting of horizontal vorticity into the vertical. This results in a simulated region of increased buoyancy and vertical vorticity approaching Raleigh from the southwest very close to the time of the actual tornadic event.
