Is the Offshore Storm Subtropical?

Yikes, I've been slipping on posting for this one, especially since it's right up my alley!

From the outset, I gave this one about a 30% shot of getting to the subtropical level and 10% to the tropical level. My reasoning was mostly guided by the cyclone phase space analyses, as I have been following severe weather much more so than anything tropics-related lately, but was governed by the various model solutions for the storm in the phase space. Basically, they were (and still are) very similar to my mental set of "null case" tropical transition cases, where the cold core representation goes very close to a low-level warm core but sorta fades away. It showed a tightening of the wind field but ultimate decay of the storm, whereas the case of systems that completed tropical transition would more or less maintain themselves but go back cold-core in the phase space. It's tough to get across in words without having any pictures to help, but I don't have access to old phase space analyses of Epsilon (2005) -- probably the best case -- to really show this that well.

This system has and had many of the parameters necessary to complete transition but lacked the vertical temperature differential to get the tropical transition process started. Basically, part of the theory on how tropical cyclones develop is based upon a "heating efficiency" based on the difference in temperatures from the ocean's surface to the upper atmosphere, with a greater difference being better. There is a threshold for getting the engine going, to coin a term, and I don't think that it has been met here.

Satellite analyses show that the storm has never quite separated from its frontal structures nor has it shed the markings of an extratropical low. In particular, a cold front-type boundary is still evident to the east of the storm (http://www.ssd.noaa.gov/goes/east/nwatl/avn-l.jpg -- time sensitive) and a descending dry air jet is present south and west of the storm (http://www.ssd.noaa.gov/goes/east/nwatl/wv-l.jpg -- also time sensitive). These are both hallmarks of extratropical systems. Convection in the inner core has become better organized over the past day or two but is still not very well organized nor present near the center of the storm. The splotchy nature of it suggests that it is more of a slightly deeper form of the open cell (dubbed because of how it looks like numerous scattered cells) convection common with extratropical cyclones.

Essentially, while it has taken many of the steps toward becoming a subtropical or tropical system, it hasn't quite completed them all. The radius of maximum winds has tightened, perhaps in response to the slightly better convective organization -- particularly that ring evident about 200 miles from the center of the storm in the images above --and the system may have briefly developed a lower level warm core. But, it never developed that aloft, thanks to not getting organized enough convection-wise (as I believe HF alluded to) and now is more likely in the filling, or weakening, stages of its lifecycle. SSTs cool from here to the coast and land is just ahead.

It basically is an oddly-traveling occluded mid-latitude cyclone (say that three times fast!) that will go down as an interesting oddity and early precursor to what could be an active 2007 season.

Hope everyone's ready for the ride!

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