Extratropical Cyclones with Multiple Warm-Front-Like Baroclinic Zones and Their Relationship to Severe Convective Storms

Abstract

Extratropical cyclones over the central United States and southern Canada from the years 1982 and 1989 were examined for the presence of two or more (multiple) warm-front-like baroclinic zones, hereafter called MWFL baroclinic zones. Of the 108 cyclones identified during this period, 42% were found to have MWFL baroclinic zones, where a baroclinic zone was defined as a magnitude of the surface temperature gradient of 8°F (4.4°C) 220 km?1 over a length of at least 440 km. The largest frequency of cyclones with MWFL baroclinic zones occurred during April, May, August, and September. Ninety-four percent of all baroclinic zones were coincident with a magnitude of the dewpoint temperature gradient of at least 4°F (2.2°C) 220 km?1, and 81% of all baroclinic zones possessed a wind shift of at least 20°, suggesting that these baroclinic zones were significant airmass and airstream boundaries. Although cyclones with MWFL baroclinic zones formed in a variety of ways, two synoptic patterns dominated. Thirty-eight percent of cyclones with MWFL baroclinic zones formed as a cold or stationary front from a previous cyclonic system was drawn into the circulation of a cyclone center, forming the southern baroclinic zone. Twenty-two percent of cyclones with MWFL baroclinic zones formed as a cold front to the north of the cyclone center was drawn into the circulation of the cyclone, forming the northern baroclinic zone. Other synoptic patterns included outflow boundaries (9%), chinook fronts (4%), return flow from the Gulf of Mexico (4%), and unclassified (22%). Although the frequency of severe weather in cyclones was roughly the same for cyclones with and without MWFL baroclinic zones, the presence of the southern baroclinic zone provided a mechanism to focus the location of severe weather, showing their utility for severe weather forecasting. Despite the potential for severe convective storms along these southern baroclinic zones, 51% were not identified on the National Meteorological Center (now known as the National Centers for Environmental Prediction) surface analyses, indicating the importance of performing real-time surface isotherm analysis.