Below is an animation I put together showing TS Lee undergoing extratropical transition as it makes landfall in Louisiana, then becomes captured by the trough digging into the Southeast U.S. Note the strong gradient from green to red located along the cold front near the end of the loop. This delineates warm, moist inflow from the Gulf with very dry continental air west of the boundary. This boundary separated dewpoints in the low to mid-70s from dewpoints in the mid-50s. Water vapor imagery (not shown) at this time showed very dry air charging south and east towards the Gulf at ~400 mb.