Subjective methods for assessing tropical cyclogenesis and intensity change at the Joint Typhoon Warning Center

Assessing tropical cyclogenesis potential and predicting intensity changes in developed tropical cyclones are two major forecast challenges facing the Joint Typhoon Warning Center (JTWC). Forecasters subjectively evaluate dynamical and statistical model output, tropical circulation structure, and key environmental parameters to predict both of these phenomena. Comprehensive training and forecasters' cumulative experience have typically guided these subjective forecasting processes. However, in an effort to standardize the genesis assessment process and increase forecast lead times, JTWC Techniques Development has designed a systematic analysis process to classify low, medium, and high tropical cyclogenesis potential from regularly observable and predicted parameters associated with genesis. Key genesis factors include symmetry of the low level circulation center, 850mb vorticity, subjective Dvorak final T-numbers, global model development, status of the Madden-Julian Oscillation, vertical wind shear, and upper-level outflow pattern. Typhoon Duty Officers and Satellite Analysts subjectively determine values of these parameters and assign them to corresponding value “bins” on a classification worksheet. A completed classification worksheet provides "trigger-based" recommendations for classifying genesis potential. That is, if certain factors or combinations of factors exist, the worksheet advises the forecaster to upgrade or downgrade development potential. This technique has been successfully integrated into the JTWC forecast process, and it is hypothesized that a similar approach to predicting intensity change should be considered.