I remember Nash when I was a kid back in the late 70s. To answer your question; it's similar to a trough or shortwave...it is the area of lowest pressure or lowest height; thus the least resistance. Many may wonder why the storm isn't going to move north despite what appears to be two anticyclones in the upper air; reason is there's an axis or you may think of it as a longwave ridge given it is a full latitude ridge. A ridge axis if strong enough; currently 5930 over Bermuda and 5920 near Daytona Bch. FL is a barrier to hurricanes. I wouldn't at all be surprised if you see Frances wobble west a bit over the next 24 hours and as I mentioned don't be surprised if she stalls a bit as the ridge has actually built over the last 12 hours by about 20 decameters. I've noted the cooling tops as I mentioned a moment ago. Everybody has to remember eyewall replacement weakens the storm; once done, it begins to reintensify. This is a normal process. As I said, there is some shear over the system too, but the huge area of subsidence from yesterday is almost gone.