Atlantic Subtropical Storms. Part I: Diagnostic Criteria and Composite Analysis

• DOI: 10.1175/2009mwr2468.1
• Publication Date: 2009-01-29T20:06:07Z
• Authors: Jenni L. Evans; Mark P. Guishard
• Keywords: Cyclogenesis; Westerlies; Anticyclone; Subtropical ridge; Trough (economics); Extratropical cyclone

Subtropical cyclones (ST) have only recently gained attention as damaging weather systems. A set of criteria for identifying and classifying these systems is introduced here employed to identify 18 ST cases forming in the 1999–2004 hurricane seasons. To be classified an ST, must near-surface gale-force winds show hybrid structure more than one diurnal cycle. The are partitioned into four classes based upon their genesis environments. Genesis over waters with SST excess 25°C observed almost 80% warm-season cases, contrast 55% climatology presented a companion study. low-shear magnitude constraint recognized tropical cyclogenesis less apparent formation 50% two partitions characterized by shear 10 m s −1 . This relatively high-shear environment corresponds equatorward intrusion upper troughs warm present mid–late season. Anomaly composites confirm that associated trough westerlies region weak static stability, corresponding reduction environmental near time genesis. These conditions correspond well transition identified Davis Bosart. Indeed, exhibit propensity continue development cyclone; eventually became named result consistent recent but had not been widely previously. raises possibility storms evolving from may overlooked or tracks truncated National Hurricane Center Database (HURDAT).