Hurricane Earl strengthened to category 4 intensity overnight, as convection exploded and became more symmetric about the center.  The NHC currently estimates maximum sustained winds of 115 knots in the eyewall with a minimum central pressure of 939 mb, the latter of which has increased slightly given the new eyewall replacement cycle that initiated earlier this morning.  Eyewall replacement is very common with major hurricanes such as Earl, which occurs as the original eye shrinks in size with rapid intensification of the inner convection, and a second eye wall of wider diameter develops, eventually taking over as the primary eye wall of the hurricane as the inner eye becomes unstable and disappears.  Hurricanes typically weaken slightly during this "hand-off" between concentric eye walls, before intensifying once again as a single, dominant eyewall prevails.  Hurricane Katrina (a little over 5 years ago during another La Nina season) was undergoing a similar eyewall replacement cycle as it was making landfall in southeast Louisiana, and was just about to re-intensify rapidly as it was coming ashore on the Mississippi Gulf Coast, thus believe it or not could have been an even worse case scenario if this cycle had occurred earlier.  Different from Katrina, Hurricane Earl is forecast to recurve to the north and eventually northeast over the next 72+ hours around the western side of a strong North Atlantic subtropical ridge, and east of a midlatitude trough which will traverse the northern U.S. through the weekend.  Still though, recent model guidance has inched the major hurricane closer and closer to the North Carolina Outer Banks (around 66-72 hours), and even New England thereafter.  Since Earl will not encounter detrimental upper-level wind shear until beyond 48 hours, the NHC expects the 'cane to maintain category 4 status for the next few days as the center rides the warm waters of the Gulf Stream toward the Outer Banks of North Carolina.  The latest GFDL run and NHC forecast track for hurricane Earl are shown below.  As the storm turns to the north near the Outer Banks, however, dry continental air will likely be pulled into the circulation, and a slow transition to extratropical cyclone will begin as the forward speed accelerates toward New England.  The track of Hurricane Earl actually reminds me of the first hurricane I ever intercepted -- Hurricane Floyd near Cape Fear, NC, but I think Earl will pass a little further east, and likely stay off-shore (at least until Nova Scota).  We will continue to monitor the progress of Hurricane Earl, and if it looks like a landfall is imminent, TVN will deploy in the Dominator.  Stay tuned!