Midlevel dry intrusions as a factor in Tornado outbreaks associated with landfalling tropical cyclones from the Atlantic and Gulf of Mexico

Abstract

Midlevel dry intrusions have been mentioned as a factor in tornado outbreaks associated with landfalling tropical cyclones for more than three decades, but a systematic analysis of historical outbreak cases with respect to this pattern has been missing. Herein, following an examination of all named tropical cyclone landfalls from the Atlantic and Gulf of Mexico for the period 1960-99, 13 "outbreak" cases (defined as storms producing 20 or more tornadoes) have been identified with 11 of the cases offering clear evidence of a dry intrusion at midlevels over the outbreak area. The outbreaks all occurred when the favored area for tornadogenesis (the right-front or northeast quadrant of the storm) coincided with a pronounced gradient of relative humidity, with the gradient best reflected at the 700- and/or 500-hPa level(s). Two distinct patterns were identified with respect to the origin of the midlevel dry air involved in these cases: in one, a mass of dry air that impinged on much of the northern or northwestern semicircle of the storm's outer circulation gradually divided into two separate areas (one to the northwest and the other to the northeast of the storm center) as the storm advanced; the other involved ingestion of a lobe of dry air from a reservoir most often located in the eastern semicircle of the storm. The outbreak cases were determined to have a strong diurnal signal detected in the temporal distribution of the tornadoes, as almost two-thirds (65%) of the tornadoes in the 11 "positive" cases occurred between 1500 and 2400 UTC (daylight hours in the region examined). A review of all tropical cyclone landfall events since 1960 revealed that there are only three cases with clear evidence of a midlevel dry intrusion that were not accompanied by an outbreak as defined herein. © 2004 American Meteorological Society.