Each year, billions of dollars of damage and countless deaths are attributed to tropical cyclones (TCs). The main purpose of this research was to provide a guideline for forecasters from the Joint Typhoon Warning Center (JTWC) to use in differentiating between the stages of extratropical transition (ET). This research compared the results of a conceptual model of ET using subjective satellite analysis along with the results of objective indicators (OIs) using Navy Operational Global Atmospheric Prediction System (NOGAPS) model analyses. The ultimate goal was to find a way to reduce the negative impact of incorrect TC forecasting by providing tools which are more objective in defining the stages of ET.

This work discusses the birth, growth, and death of TCs by describing what energy sources are necessary and what causes them to dissipate. Studies have been done in the recent past providing a conceptual model of ET with definitions of two stages and ways to use satellite analysis to identify them. While this conceptual model was being analyzed in the western North Pacific, objective indicators had been created and were being analyzed in the Atlantic Ocean. The research in the Pacific resulted in the creation of a simple three-by-three matrix illustrating the effects of mid-latitude circulation and TC remnant contributions. In contrast, the research in the Atlantic led to the exploitation of these OIs in a hodograph-like display.

This study utilized the results of the conceptual model on 13 TCs in the western North Pacific to verify the results of the OIs on the same 13 TCs. The OIs worked very well in the analysis of a “classic” TC that had undergone ET (Typhoon David in September 1997); however, the OIs used in the analysis of Typhoon Halong (July 2002 - a TC that was difficult to forecast) worked even better than the subjective conceptual model. The overall findings showed that the OIs gave results similar to, if not better than, those achieved through the use of the conceptual model; specifically, the start and stop times of ET were within 12 hours of each other and the duration times were also similar. This study strongly encourages the use of OIs by JTWC forecasters (and others) to differentiate between the stages of ET.