Mesoscale simulations of dynamical factors discriminating between a tornado outbreak and non-event over the southeast US Part I: 84-48 hour precursors

Observational analysis and mesoscale numerical simulations are in agreement concerning key dynamical processes which occurred over Mexico and the Gulf of Mexico 84 hours prior to the 1988 Raleigh (RDU), NC tornado outbreak. The subtropical jet (STJ) over northern Mexico and its associated transverse ageostrophic circulation forced air down the eastern side of the Sierra Madre Mountains creating adiabatic warming due to compressional heating. Along with this warm air, a low-level trough of low pressure formed and a low-level jet (LLJ) developed over the western Gulf of Mexico. This LLJ began the process that transported very warm and potential vorticity (PV) rich air from the Mexican plateau to the Carolina Piedmont. The low-level PV maximum over central NC at the time of the tornado was a coherent entity traceable back 84 hours to the Mexican plateau. Over the Mexican plateau, the STJ transported the PV rich air southward then down to the midlevels. There was substantial heating over the plateau producing a deep well-mixed layer and a mountain-plains solenoid. An area of strong vertical convergence developed in the 500-600 hPa layer which increased the thermal gradient and maintained the PV. This mid-level PV was transported to the low-levels by a hydrostatic mountain wave. As the PV maxima moved down the lee of the mountains it increased due to strong static stability, tilting and frictional effects. Finally, the PV maxima moved along the Gulf Coast and up the East Coast to central NC.