Happy New Year -- from Zeta!?!

"...via baroclinic enhancement leading to an enhancement of the processes that lead to tropical development ..." Thanks much, Clark that is the key to what I was wondering. I knew it had to be baroclinic in nature yet result in convection of a tropical nature, but I had no idea how that could occur. It sounds very interesting and yes I want to hear more about that when you get an opportunity.

Beven's early morning discussion was thoughtfully detailed. The steering environment is clearly complex and changing from hour to hour, and there is a lack of data on the specifics of all the atmospheric features surrounding Zeta. Ironically as soon as he pegged the SW movement Zeta took off like a shot for the west. We know this is because of changes in the environment, but it is so much more fun to blame it on Zeta being deliberately perverse.

Zeta stayed in the area of high upper level shear all night (30 to 40 kts), but right on the border of a low shear environment at the mid level, so the convection shows little sign of shear. However, unexpectectly, once again the LLC is mostly exposed with just a small amount of convection to the NE, but we know Zeta can exist just fine with that small amount of convection. The LLC is beautifully defined, especially the center (look at the 1515Z vis sat image).

Convection has not built up again, so in the absence of strong shear at the mid level there are some other factors inhibiting the increase of intensity, perhaps water temperatures or the combination of winds at different levels around Zeta. But perhaps it is simply that in this kind of marginal environment major intensification is not possible unless there is a baroclinic element, and otherwise only a low level but nevertheless steady state of intensity can be maintained, until (inevitably) encountering strong shear which will completely remove the convection and finish the TC.

The 10am discussion mentions oncoming unfavorable winds which will weaken Zeta today. With such a small amount of convection left, and no obvious improvement in the environment forthcoming, it does look like the repeated forecast of gradual weakening is now becoming the case, unless we see a repeat of intensification for a short time as already occured from interaction with a trough.

There is always so much to learn from the TC that do not make it to hurricane status. This winter the ATL has provided an interesting laboratory to study (or to simply be puzzled) by one hybrid TC after another. The automatic adaptability of the hybrid TC to ensure its survival in the marginal changing environment, by taking on various combinations of subtropical and tropical presentations, is facinating.

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