Abstract:
Cyclones from the central United States and south-central Canada were examined from 1982 and 1989 to determine how often they contained more than one baroclinic zone. A baroclinic zone was defined if a gradient of 8°F (4.4°C) per 220 km was found and a length of 440 km was achieved. Forty-three percent of cyclones were found to have multiple baroclinic zones. The greatest frequency of cyclones with multiple baroclinic zones occurred during the transition months of April, May, August, and September. In addition, the baroclinic zones appeared to follow a seasonal progression. Ninety-four percent of all baroclinic zones were coincident with a moisture gradient that was apparent through isodrosotherm analysis every 4°F (2.2°C), and 73% contained a veering wind shift across them of at least 20°. Of cyclones with multiple baroclinic zones, severe weather was found to occur along 57% of southern baroclinic zones, significant severe weather along 41%, tornadoes along 35%, and significant tornadoes along 24%. During the spring and summer, severe weather occurred along 83% of southern baroclinic zones, significant severe weather along 65%, tornadoes along 57%, and significant tornadoes along 39%. The occurrence of severe weather, significant severe weather, tornadoes, and significant tornadoes was relatively consistent along the southern baroclinic zones between 1982 and 1989. Finally, the formation of multiple baroclinic zones was examined and two main forms were found. A second baroclinic zone can be the result of an interaction with a historical cold/stationary front, or can result through the attachment of a baroclinic zone from the north.