It is not a given that Wilma will weaken in the Gulf, at least because of shear (lower SSTs may have an impact). Assuming it turns NE, it will moving in the same direction as the deep layer flow, so the only environmental shear will be due to changes in wind speed with height (speed shear). If it makes a further south landfall, the speed shear affecting the storm may not be very significant at all. Increasing southwesterly flow to the north of the system may temporarily enhance the outflow as well. Finally, an increase in forward speed may enhance the maximum winds in the right-front quadrant of the system. The best bet for weakening will be for the storm to pass near or over the Yucutan (like in the 00Z and 06Z GFS/GFDL) or near/over western Cuba (as in the official forecast). If the system becomes strong enough, eyewall-replacement cycles, which can't be forecast reliably, could also play a major role.

Generally speaking, storms coming under the influence of SW aloft from the mid-latitudes will tend to weaken, but there are factors that can cause the opposite to happen as well in some cases. I can recall both Opal and Charley rapidly intenisfying as they came under the influence of SW flow aloft. Opal weakened quite a bit before landfall, but Charley did not have time to cycle down before striking Florida. Wilma figures to be a larger system and probably less prone to rapid intensity changes if it enters the Gulf, but it is still hard to predict exactly how it would react to an environment where both the shear (inhibiting factor for intensity) and upper-level divergence (positive factor) will be at least slowly increasing.