A lot of the question surrounding the track of the storm, and the cause of the GFDL bending westward, have been directly related to how Irene interacts with the trough over the northern US.
There are two thoughts on this: (1) the trough will "suck" Irene toward it and (2) the trough will not affect Irene much. GFDL is taking much more of the 1st thought while other models are running more with #2.
I don't know enough about how these sorts of upper air dynamics play out to tell you which is more likely, but from what I've read #1 is definitely capable of happening, but no one really knows if it will. From what I can tell, there is a lot of blind throwing of darts at dart boards about which scenario will actually happen.
Official forecasts are tracking more with #2, and as seen over the past day the models that have been trending toward #2 are more closely aligned with the actual track Irene has taken. However, there is no guarantee that that trend will continue as Irene draws closer to the trough.
A couple things about the GFDL and GFDN models: These models are based on global models. The GFDL uses the AVN while the GFDN uses the NOGAPS. Neither the AVN nor the NOGAPS is showing the westward jog, even though over the past two days the GFDL/N have both been showing westward jogs. Given that the GFDL/N are regional models with boundary conditions determined by the global model output, it is possible that they are developing the trough differently from the global models. Really hard to tell, though, with the output data available to the public.
Given the track record of the GFDL in past storms, I never dismiss that model. It is right a surprising amount of the time (though I have also seen it be very wrong). Therefore one must realistically consider that the jogs seen in the various Geophysical Fluid Dynamics Lab models are possible (this includes GFDL, GFDN, GFDT, GFDI, GFNI, and GFTI). BTW, anyone know what global model the T variant is based on? (FYI: the GF*I models are the runs that focus on intensity over position; while the GFD* runs focus on position over intensity.)
Hope this helps answer your question!
Ed Dunham or Clark Evans would be able to much better explain this than me!
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