It really is too early to dwell on individual models, but the 18z GFS is scary. It takes Katrina west to west northwest to a point right on top of the high heat content pool south of Lousiana, then turns it right and accelerates the storm northward to near or just west of New Orleans... and the whole time it keeps an upper level high centered over the storm with little or no shear. The GFS really must think Katrina will get darn strong because it has the pressure below 980 mb for the last day and a half leading up to landfall and you rarely see the GFS go that low for tropical cyclones.