Opportunities for Probabilistic Forecasts of Tropical Cyclone Formations and Tracks on Intraseasonal Timescales

Abstract

ABSTRACT An opportunity that now exists for probabilistic forecasts of western North Pacific tropical cyclone formations and tracks on timescales of 7-30 days is demonstrated from the 51-member European Center for Medium-range Weather Forecasts 32-day ensemble predictions made once a week during the 2008 and 2009 seasons. A methodology has been developed to match the ensemble member vortices to form ensemble storms, and a weighted-mean vector motion technique is applied to generate the track. An objective validation relative to the Joint Typhoon Warning Center best-tracks is supplemented with a subjective evaluation of the quality of the agreement. Success of the approach was first demonstrated with three intense typhoons during the 2008 season. Although some successful predictions were also achieved with some moderate typhoons and tropical storms, the early and late season typhoons and the tropical depressions were not predictable on 7-30 day timescales. Evaluation of the performance during the more active and typical 2009 season again indicated success in predicting typhoons and most of the weak tropical storms and tropical depressions. The exceptions were some multiple storm events and track bifurcation scenarios due to midlatitude interactions. Among the challenges to produce an operational product are the requirements to make the entire procedure more automated, more objective, and with forecaster-friendly displays. A documentation of the reliability and of the limitations of the predictions will be helpful in gaining forecaster acceptance.