An Analysis of Model Track Forecast Errors in Hurricane Joaquin (2015)

Hurricane Joaquin, which affected the central Bahamas as a major hurricane on 1-3 October 2015, was a major forecast challenge that resulted in above-average track forecast errors from much of the dynamical track model guidance and official NHC forecasts. In particular, much of the model guidance, with the exception of the ECMWF global model, failed to bring Joaquin far enough south into the Bahamas in the short range and later showed Joaquin making landfall along the U.S. East Coast from the Carolinas into the mid-Atlantic states for several forecast cycles on 29-30 September.

Possible sources of track forecast error in the models will be presented, with a particular focus on the NCEP GFS model. There appear to be three critical factors that determined the ultimate track of Joaquin: 1) how far south the tropical cyclone moved over the Bahamas during the period from genesis on 28 September through 2 October; 2) the evolution of a closed mid/upper-tropospheric low that developed over the southeastern U.S. on 2 October and subsequently moved slowly eastward; 3) the track and amplitude of a mid/upper-level trough that moved westward from the central Atlantic and passed north of Joaquin on 3-4 October.

A preliminary investigation of model forecasts, analyses, and observations suggest that this southward motion of Joaquin over the Bahamas was sensitive to the depth of the tropical cyclone vortex in the models and the strength of a mid-tropospheric ridge to the north. The ECMWF global model had a stronger and deeper representation of Joaquin, and a track farther south relative to the GFS, which had a weaker, shallower vortex. Even using a very smoothed representation of the GFS large-scale flow, the deep-layer Beta Advection Model, which uses a deep-tropospheric steering flow, had a track farther south than the GFS for several model cycles.

Properly capturing the southward progression of Joaquin into the Bahamas appeared to be important to how the tropical cyclone subsequently interacted with both the closed upper-level low over the southeastern U.S. and the mid/upper-level trough moving westward from the central Atlantic. The interplay of all these factors in the model forecasts determines whether Joaquin made landfall along the U.S. East Coast or moved northeastward offshore.