Subtropical cyclones: Operational practices and analysis methods applied at the Joint Typhoon Warning Center

Subtropical cyclones present unique challenges for the Joint Typhoon Warning Center (JTWC). JTWC analysis and forecasting procedures for subtropical and tropical cyclones differ significantly. The Center, in most cases, does not issue detailed track, intensity, and surface wind forecasts for subtropical cyclones. However, JTWC forecasters are required to monitor these systems for potential transition into tropical cyclones, promulgate detailed subtropical cyclone analysis data in tropical area bulletins, and ensure that collaborating forecast agencies have the requisite information to account for localized impacts accordingly. Because US Government partners rely upon JTWC analyses and forecasts to tailor meteorological products for impacted customers, accurately distinguishing subtropical cyclones from tropical cyclones is essential. Cyclones located in the subtropics often exhibit well-documented physical characteristics common to both subtropical and tropical cyclones. However, a universal method to systematically differentiate such cyclones from tropical cyclones at operational forecast centers in ambiguous situations has not yet been established. To address this deficiency, JTWC has developed an “in-house” cyclone phase classification method that exploits available remote sensing datasets to systematically guide the classification process. This adaptable method reduces the uncertainty and inconsistency that may result from a purely subjective approach, and provides customers a clear representation of how these classifications are determined.