Documentation of high based thunderstorms developing on a boundary which became tornadic

This paper will document two separate days (29 June 1999 and 02 Sept 1996) in which high based supercell thunderstorms developed over southeast Colorado and became tornadic. What is common between these two tornadic cases is:

1.) A well defined low level boundary was in place prior to storm interaction, and;

2.) The synoptic environment was favorable for rotating storms.

Although high based supercell thunderstorms which develop on the dryline are common over the eastern Plains of Colorado, most of these are of the "LP" variety. Studies have documented and personal observations from storm chasers indicate these storms do not typically become tornadic. Recent results from VORTEX indicate supercells which interact with boundaries are more likely to become tornadic. In addition, recent studies in the literature suggest storms which typically have lower cloud basis (low LCLs) are more likely to become tornadic then storms that have high LCLs. This poster will suggest high based supercells which become tornadic need enhanced low level vorticity to assist in tornadogenesis. This "vorticity enhancement" is due to the preexisting boundary which is in place. Although it is unknown if this boundary adds vertical vorticity (which forms the classic "landspout" tornado), or horizontal vorticity (which is necessary for traditional tornadogenesis), the fact that the boundary is collocated with the high based rotating updraft is believed to increase the likelihood for tornadogenesis. It is possible that a "hybrid" type of tornadogenesis occurs with these storms, in that the boundary adds vertical vorticity to the storm which is ingested into the high based mesocyclone to produce a tornado.

It is suggested that warning meteorologist should be more cognizant of tornadogenesis when a rotating high based storm develops over a boundary which is already in place.