Mesoscale simulations of dynamical factors discriminating between a tornado outbreak and non-event over the southeast US - Part II: 48-6 hour precursors

Using observational analysis and mesoscale numerical simulations we investi gate the subtropical jet (STJ) and its effects on the lower environment (as sociated mass and momentum adjustments, development of a low-level jet (LLJ ), and low-level PV) 48 to 6 hours before the Raleight tornado outbreak (19 88). We also compare the environment to a synoptically similar event in whi ch severe weather forecasted but did not develop over central North Carolin a. In the severe weather case a self-maintaining. low-level circulation ori ginated over Mexico, propagated across the Gulf Coast and moved over the Pi edmont at the time of the tornado. It is characterized by a surface trough, low-level PV maximum, mid-level jet, a warm Mexican airmass and STJ exit r egion that was co-located and moved across the Gulf Coast States as a coupl ed system. Initially, a STJ exit region (with thermally indirect ageostroph ic circulation) approached the Gulf Coast creating upper-level divergence a nd ascent, which helped to maintain a low-level trough. A warm Mexican airm ass was located over the Gulf Coast (southeast of the surface trough) creat ing a northwestward-directed PGF, which created a mid-level jet. The right entrance region of the mid-level jet and its associated thermally direct ci rculation (ascent) was over the low-level trough. These features created an environment favorable to deep convection and the release of latent heat th at generated low-level PV. In the non-event case, these features (low-level warm Mexican airmass, mid-level jet, deep convection, low-level PV and low -level trough) were absent over the Gulf Coast States.