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.