As tropical storm Lee made landfall on the central Louisiana coastline, the synoptic pattern was already exerting influence on the cyclone. An upper-low to the northwest of Lee was forecasted to back away and allow for modest strengthening up until landfall. The upper-low did back away some, but then more or less got absorbed by the tropical cyclone. This is a rather unique interaction and the RGB airmass product from the GOES Sounder caught the action.
As you can see in the image above, the cyclone looks more extratropical at this point than the textbook tropical system. The red circle is highlighting some dry air that is being entrained into the circulation through the southwest inflow channel. This is approximately where the 500 mb low was located at this time. The small area of red shows dry air at mid-levels, although the tropical (green – warm, moist) air is still in close proximity. The low-level circulation does not show well on this image, but it is located on the central Louisiana coastline at this time.
In this image, the cyclone looks like an occluding mid-latitude system with a dry punch evident rounding the eastern quadrant. The red circle is again highlighting the drier air being entrained into and around the tropical storm.
At the same time, GOES water vapor imagery shows a significant dry punch being advected into the eastern quadrant and rotating around the northern portion of the mid to upper level circulation. The difference between this plot and the RGB Airmass plot is that you cannot see the warmer, moist air at lower levels also rotating around the cyclone. Looking at this image, one would question whether this is actually a tropical storm or a winter-time occlusion.
This final image is valid at 12z on 09/04/11, four hours later from the previous images and shows deep convection erupting to the southeast and east of the tropical cyclone near or just after landfall. The dry air is still evident on the northern portion of the circulation, slowly moving into the western quadrant. The deep convection is most likely due to the interaction of the moist, tropical airmass at lower levels becoming strongly unstable as dry air mixes in aloft. This creates higher convective available potential energy (CAPE) than one might expect with a tropical cyclone, thus leading to the assumption that TS Lee was becoming fully extratropical at this point.