Using observational analysis and mesoscale numerical simulations we investigate the subtropical jet (STJ) and the associated mass and momentum adjustments, the low-level jet (LLJ), and low-level potential vorticity (PV), 6 hours before the 1988 Raleigh (RDU) tornado outbreak. We also compare the environment to a synoptically similar event with severe weather forecasted but nothing developed over central North Carolina. In the event case there is a self-maintaining, low-level circulation, which was characterized by a surface trough, low-level PV maximum, mid-level jet and a warm Mexican airmass, that originated over Mexico, propagated across the Gulf Coast and moved over central North Carolina at the time of tornado. A meso-cyclone and low-level PV propagates over the Piedmont at the time of the RDU tornado outbreak. The low-level PV maximum is maintained by low-level forcing: specifically, tilting about a diabatic heat source (convection). In the non-event case, this feature is absent along the Gulf Coast states. In the event case, the polar jet right entrance region moves near the Sn left exit region which creates strong ascent and upper-level divergence over the Piedmont. The lifted index indicates the airmass over the Piedmont is unstable. We developed a Divergence Profile Buoyancy Index (DPBI) based on: upper-level divergence, airmass buoyancy and low-level tilting effects associated with shear and thermal gradients. We found that DBPI values over 15 correspond to tornadic activity.
