An Analysis of Multiple Steering Influences on the Track of Tropical Cyclone Joaquin (2015)

The track of Tropical Cyclone (TC) Joaquin in September-October 2015 was exceptionally difficult to forecast due to the large uncertainty in forecast tracks. The forecast uncertainty was attributed to competing synoptic-scale features that were expected to impact the steering motion of TC Joaquin. These steering features would ultimately guide Joaquin out to sea, east of the forecast model guidance majority, and result in large left of track errors operationally by the National Hurricane Center. This presentation is motivated in part by these large errors in the initial operational track forecast, and also by the need to clarify which synoptic-scale features were responsible for steering TC Joaquin out to sea.

This study will employ piecewise vorticity inversion and apply it to reanalysis and ensemble based forecast datasets of TC Joaquin to analyze the synoptic features responsible for track errors and steering of TC Joaquin. A preliminary investigation using the Climate Forecast System Reanalysis suggests that the cumulative time-dependent steering flow over TC Joaquin was comprised of 1) an upstream deep-layer trough in the eastern US, 2) a poleward deep-layer ridge across the western Atlantic basin, 3) a cutoff upper-tropospheric cyclone to the south of the deep-layer ridge, and 4) a building upper-tropospheric ridge over the Caribbean sea. All four synoptic features are needed to accurately capture the steering flow that guided Joaquin eastward out to sea. The role of errors in these synoptic features on Joaquin's track forecasts are further investigated using the European Center for Medium-Range Weather Forecasts (ECMWF) ensemble, available through the THORPEX Interactive Grand Global Ensemble (TIGGE) dataset. Composite differences will be constructed between the subset of members that inaccurately took Joaquin inland (left tracking members) versus a subset of members that accurately tracked Joaquin out to sea (right tracking members), with particular attention paid to the representation of the aforementioned synoptic features.