Defining a Criteria for Transitioning an Extratropical Cyclone to a Tropical Cyclone

Numerous peer-reviewed papers discuss the evolution of tropical cyclones to extratropical cyclones and the hazards they produce (Evans et al. 2017, Jones et al. 2003, Anwender et al. 2008, etc.). However, there is limited research published on the inverse, extratropical to tropical transition (Davis and Bosart 2004, Bentley et al. 2017, etc.). The last decade of Atlantic Hurricane Seasons has seen an increase of weaker, subtropical or tropical cyclones that originate from extratropical source regions (e.g. upper-level lows, frontal boundaries, complex interactions among multiple features including African easterly waves, etc.) and this has led to the inconsistent naming assigned to some of these systems (Subtropical Storm #1 (2023), Danielle (2023), Rebekah (2019), etc.).

The National Weather Service (NWS) Ocean Prediction Center (OPC) and National Hurricane Center (NHC) are taking a closer look at the increased occurrence of these tropical transitions and whether a more concrete operational criteria is needed to ensure consistency and preserving the scientific integrity of the naming convention. At the same time, it is critical that OPC and NHC serve the marine community by identifying these systems for the unique hazards (greater than 35 kt in any quadrant, convective alignment with the center, etc.) that differentiate these storms from the larger extratropical cyclones that mariners experience more often. This presentation will document some of the recent cases of extratropical to tropical transition and focus on the new tools offered by GOES-R, Cyclone Phase Space (Hart 2003), and scatterometer winds, among others. One operational goal of this work is to build more confidence in the decision to transition an extratropical cyclone to a named subtropical or tropical cyclone.