Wednesday, 14 January 2004: 4:15 PM
Extratropical Cyclones with Warm Sector Baroclinic Zones and their Relationship to Severe Weather
One hundred and eight midlatitude cyclones in the central United
States and southern Canada from 1982 and 1989 were manually analyzed.
A baroclinic zone was identified if a gradient of 8°F (4.4°C) per 220
km over a length of at least 440 km was found. Forty-three percent of
the cyclones were found to have a baroclinic zone within the warm
sector at 18 UTC. The greatest frequency of cyclones with warm-sector
baroclinic zones occurred in April, May, August, and September.
Eighty-eight percent of all baroclinic zones were coincident with a
moisture gradient of at least 4°F (2.2°C) per 220 km. Seventy-eight
percent of the baroclinic zones had a wind shift of at least 20 deg,
suggesting that many of these baroclinic zones may have been fronts.
Although several mechanisms for forming warm-sector baroclinic zones
were identified, the two most common were the interaction between the
cyclone and a preexisting cold/stationary front from a previous
cyclone (37%) and the attachment of an arctic front to the cyclone
Warm-sector baroclinic zones were associated with a high probability
of severe weather. Severe weather was reported within 220 km of the
18 UTC position of the warm-sector baroclinic zones 57% of the time.
These numbers were 43% for significant severe weather, 35% for
tornadoes, and 24% for significant tornadoes. For warm-sector
baroclinic zones during the spring and summer, these values increased
to 83% for severe weather, 65% for significant severe weather, 57% for
tornadoes, and 39% for significant tornadoes. Despite the high
probability of severe weather associated with these warm-sector
baroclinic zones, 54% were unanalyzed on the 18 UTC National
Meteorological Center surface analysis. This research argues that
forecasters need to revise their conceptual models for central United
States cyclones to include these features in order to improve
forecasting severe weather.