Interesting discussion simmering here. And what a very interesting storm we have!
Classic case of (attempted) tropical cyclogenesis on the trailing end of a decaying frontal boundry. I've watched this storm sprout up, like many of us, from it's inception, as its parent front moved off the atlantic coastline a few days ago.
And while it certainly has been looking rather impressive the past day or so, it's also true that "appearances can be deceiving".
First, here's a few bits & snips from *Yesterdays* posts on this interesting system from Raymond, Rodney, Hugh and others:
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Part of Raleigh NWC forecast discussion:
MEANWHILE GFS SUGGEST THAT THE SYSTEM MAY GAIN SOME SUB-TROPICAL CHARACTERISTICS AS IT DEPICTS A WARM CORE AT 850MB BY THIS EVENING INTO THU.
This was the NWS thinking, based on the GFS, as of yesterday morning. But I don't think these exact figures were attained. Instead of a strong warm core developing at 850 mb by last evening, as the GFS had forecast, I believe we have only a shallow 'surface' warm core thus far, but steadily improving in that department, which I will get to in more detail shortly.
The the casual eye, the low now spinning offshore certainly *looks* like a hurricane, or at least like some kind of tropical storm.
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NHC has just issued STDS and I think that 'Kyle" has already formed, off of NC coast, ... either sub-tropical or tropical. In fact IMHO I think NHC will start issuing advisories before recon can get there.
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Yes, 94L could become Kyle ... I see an "eye-like feature" forming in the center of 94L!
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Looking at the long-range Wilmington, NC, radar... I'd have to say 94L looks like it is closer to tropical than the NHC reports. ... there is convection building near the LLC, and the radar presentation is improving. To me, if it looks like a hurricane, why not call it one, particularly when hurricane conditions are onshore?
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NHC stated the following...
THE LOW IS PARTLY RESPONSIBLE FOR AN AREA OF WINDS TO HURRICANE FORCE WELL TO THE NORTH AND NORTHWEST OF THE CENTER.
So why not start advisories? It would appear this meets sub-tropical requirements at least ...
Interesting observations and very good questions. It's a combination of the old "appearances can be deceiving" on the one hand, and strict criteria for cyclone classification on the other.
But the real key in 94L's attempted tropical transition is the requisite development of the all-important warm core.
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... LIGHTNING ACTIVITY HAS GRAVITATED MORE TO THE CENTRAL CORE OF THE DEVELOPING SYSTEM ... THERE ARE INDICATIONS THAT THE SYSTEM COULD BECOME WARM CORE ...
...
94L is a bit more complex, it seems to be warm core right now, at least according to phase analysis. ... and the frontal zone (now) appears to be breaking down, which would give the system a much stronger case for being considered tropical / subtropical.
...
As NHC states, 94L looks to be essentially a well-developed and tightly-wound coastal "LOW" that is in the process of acquiring some (warm core) tropical characteristics.
And from today's posts, just recently ...
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Have a vort that came in for 94L a little more supportive of naming it. 94L has developed a shallow warm core, and appears to be bringing in its radius of maximum sustained winds, while also exhibiting a little bit of banding-like structures around the center of circulation.
Excellent recap of the data collected from the recent recon mission into the storm. So, there are now definite trends, unlike yesterday, that tropical transition may now be well underway.
The visual sat. loops are showing gradually deepening convenction that is now tightly wrapping *all* the way around a relatively clear 'eye-like' formation, which presumably has led to the development of the 'shallow warm core' feature mentioned in the recon vortex message. And ...
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Long range Wilmington radar shows an almost large "eye" like feature present, that I don't think was there before. It also ... is moving or soon will be moving over the relatively warmer waters of the gulf stream.
Which brings me to the crux of the matter. The development of the warm core. It's this innermost convection that will 'make or break the case' for tropical or sub-tropical transition. Because it's precisely these thunderstorms with will, under the proper conditions, lead to the development (formation) of the tropical warm core.
And this ties in well with the question ... "Could somebody please point me to a good (basic) definition of what a system needs to do to be considered extratropical, subtropical or tropical?"
Above all, to be classed as a *true* tropical system, you need for the storm to have a 'warm core', relative to it's surroundings, through a deep layer of the atmosphere.
And so far, this just hasen't happened. But it now appears to be (finally) taking place as the LLC center is just beginning to move over the warm gulf stream waters.
These warmer waters will result in stronger thunderstorms, and more of them which, when acting in an organized fashion as they are now, and over time, will will release such a tremendous amount of latent heat of condensation, that it will actually *warm the entire atmosphere* over hundreds of square miles, and a bona-fide tropical cyclone is formed.
Although the wrapping we've seen previously certainly *looks* impressive, especially on the visible loops, the IR and Water Vapor loops tell a different story:
Until quite recently, this inner convection hasn't been partucullary impressive and certainly not very deep, as shown by the relatively warm cloud tops (compared with 93L), and the water vapor imagery suggests that not very much vertical transport of moisture (deep convection, with it's attendant latent heat) has occured yet, until quite recently.
Also, until recently, the doppler returns out of Wilmington, although suggesting the formation of a ring-like feature, have not been especially strong, either. And most of the deeper storms seem confined to the NW semi-circle.
But now, that all seems to changing, and quite rapidly, too. The warmer waters of the Gulf Stream now appear to be imparting better 'fuel' (has to do with higher vapor pressure) for the increasingly organizing thunderstorms, and as more and deeper convection continues to develop, the result will surely be the formation of the infamous warm core and an 'upgrade' to at least subtropical status, if not 'true' tropical transition sometime before landfall.
Keep one eye on the doppler radar for stronger and 'thicker' echo returns and the other on IR cloud top temps for dramatic cloud top cooling (especially after sunset, with the onset of the diurnal convective max cycle) and I think we will be witnessing tropical cyclogenesis right before our very eyes! Will we be having a landfalling 'Laura' ? Down to the Wire!
Edited by CoconutCandy (Thu Sep 25 2008 04:47 PM)
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