20th Conference on Weather Analysis and Forecasting/16th Conference on Numerical Weather Prediction

18.2

Extratropical Cyclones with Warm Sector Baroclinic Zones and their Relationship to Severe Weather

Nicholas D. Metz, Oklahoma Weather Center Research Experience for Undergraduates and Valparaiso University, Valparaiso, IN; and D. M. Schultz and R. Johns

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 8F (4.4C) 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 4F (2.2C) 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 (22%).

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.

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Session 18, Cyclogenesis and Winter Weather (ROOM 607)
Wednesday, 14 January 2004, 4:00 PM-5:30 PM, Room 607

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