Good morning folks,
Hurricane Zeta has been making its trek across the Yucatan overnight. As of this writing, it is still over land near the northwest tip of the peninsula. The interaction with land has resulted in a significant decrease in the deep convection with this storm.
There are at least a couple of things that happen when a storm moves over land that causes it weaken. One is that the core of the storm loses access to the large amounts of heat energy contained in the ocean waters. That has resulted in Zeta losing some of its deep thunderstorms.
The other thing that happens is friction. Normally, as air moves toward the center of a hurricane due to the lower pressure in the center (pressure gradient force), Coriolis effect causes the air to start a spiral (counterclockwise in the northern hemisphere) journey toward the center. This spiraling rotation causes centrifugal force. Centrifugal force pulls the air away from the center. Of course, the air has to eventually get to the center so we have to have a perfect balance of these forces to allow the storm to maintain its intensity. A decrease in central pressure causes the air to move to the center faster. Faster winds cause more centrifugal force. If anything upsets the balance, a change in strength of the storm will happen. So when a storm moves over land, friction causes the surface winds to slow down. Slower winds means less centrifugal force. But if the central pressure hasn’t changed yet, the air will still get pulled toward the center of the storm. With less centrifugal force pulling the air away from the center, the air moves in faster, “filling” the center of the storm. The central pressure begins to rise and the storm weakens.
The situation Zeta is in right now is a bit different though. In this case, only the core of the storm has moved over land. Most of the rest of the storm is still out over the western Caribbean Sea, the southern Gulf of Mexico, and the Bay of Campeche. This means that the air at the surface out away from the core of the storm isn’t having to deal with friction so it isn’t slowing down. Yet as the central pressure starts to rise because of the core interaction with land, in the outer areas of the storm, centrifugal force overpowers pressure gradient force. So the air moves further out away from the center. This results in the storm spreading its energy out over a wider area of the surface. Mother Nature always maintains a balance. Energy is never lost, it’s merely translated into a different form.
So on satellite this morning we see Zeta losing a lot of its deep convection near the center but we see a broadening of the footprint of the storm as convection continues well to the north and northeast of the storm in the convective bands over the Caribbean and the Gulf.
One of the biggest issues Zeta will have to overcome now is all that dry air in the Gulf of Mexico. I think that will be challenging for this storm in its current state. I won’t be surprised if it is unable to recover but we’ll have to watch as the system gets out over the southern Gulf today.
The models have trended toward a weaker storm making landfall which makes sense. A strong trough over Mexico is expected to scoop it up over the next 24 hours and drive it quickly northeast tomorrow. This will result in pretty strong shear. Combined with the dry air and cooler waters of the northern Gulf, this storm doesn’t stand too much of a chance. But we still have to be weary of the storm surge that will come with this system,. On the plus side, it will be so fast moving that whatever we get, won’t last for long.
Everyone along the northern Gulf Coast should be prepared for this storm in whatever form it arrives Wednesday.
I’ll keep an eye on this storm and post more later today.