Observational and Numerical Model among Tornadic Outbreaks over the Southeast US

Observations and mesoscale simulations show dramatic similarities among three severe weather episodes over the southeast US. Prior to the tornado outbreak, the Carolina tornado outbreak in March 1984, the Raleigh tornado of 1988, and the Palm Sunday tornado over the southeast US in 1994 were synoptically very similar.

All had a deep mid-level trough of low pressure over the Ohio valley and a strong cold front moving into the southeastern US. In the upper-levels, the polar jet entrance region was juxtapositioning with a subtropical jet exit region. In the low-levels, there was extremely warm air to the south of the frontal boundary. This airmass was warmed over the Mexican Plateau, moved over the Gulf, and advected over the southeastern US. Cold air was moving in from the northwest creating a strong low-level thermal gradient.

In the upper levels, the transverse ageostrophic circulations associated with the jet streaks create an area of strong divergence between these two jet streaks. The upperlevel divergence leads to the removal of mass from the air column. This integrated mass loss also leads to the development of a mesolow and the low-level winds accelerating toward the area of mass removal.

The low-level return branches associated with the jet streaks bring differing airmasses into proximity. The low-level return branch associated with the subtropical jet exit region advects very warm air from the southeast and the low-level return branch associated with the polar jet entrance region advects cold air from the northwest. Together these circulations produce substantial low- level frontogenesis in an area between the upper-level jet streaks. Also, the wind field accelerates dramatically (over Mississippi, Alabama, Georgia, and the Carolinas) as a response to this strengthening pressure and temperature gradients.

During the mutual mass-momentum adjustment process a thermally direct ageostrophic circulation develops between the jet streaks. This adjustment represents a subsynoptic jetogenesis process. The vertical wind shear caused by this process creates an environment above the inflowing tropical air, which is highly sheared, highly stretched and convectively unstable.