Clark
Meteorologist
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The long, strange ride continues
Mon Oct 10 2005 11:19 PM
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Well, as Stacy Stewart said in the 11p Vince discussion, the long, strange ride continues. Vince remains as a classified tropical cyclone with winds at 45mph based upon ship observations and enhanced convection on the east side of the storm. Located at 9.5W, it is the first system to be classified as a tropical cyclone in this basin to the east of 10W (not including the unclassified storm in the S. Atlantic near Congo in 1991). It is also the first storm to be making landfall in Portgual. If it maintains a more easterly heading in the short-term, it may even make it to the Mediterranean Sea; however, with the strong front/trough approaching from the west, it's more likely to end up just inland across Spain over the coming day.
Vince is yet another reminder that tropical cyclones do not always form from tropical waves over >26C waters. It's not something that redefines the tropical cyclone paradigm; in fact, it's formation is similar to both of those South Atlantic tropical cyclones (1991 and 2004). All three storms can be explained using our theories for tropical cyclone development -- the idea of the efficiency of a Carnot cycle as depicted by a hurricane is based upon sea surface temperatures and temperatures at the outflow layer/tropopause. This efficiency goes into the maximum potential intensity calculation (http://www.wxmaps.org/pix/hurpot.html) which, believe it or not, states that a significant tropical cyclone could form and maintain itself across that region toward Portgual/Morocco. While it is rare to see a storm form in such cold SSTs (~25C or less throughout), it is not impossible...nor is it impossible for such a system to acquire at least a weak low-level warm core.
Vince formed from an upper-level area of low pressure that slid to the southeast out of the mid-latitude pattern and became cut-off. This upper-level low induced (action at a distance) a surface circulation just west of the Madeira Islands; through time, the upper-level low weakened, leaving behind the surface circulation in a favorable upper-level environment for development. The initially cold-core surface low acquired a warm-core structure -- the microwave imager data from UWisconsin shows this pretty well and the operational model data show this to a lesser degree -- through the so-called WISHE tropical cyclone development mechanism.
This WISHE -- wind-induced surface heat exchange -- mechanism states that as you get winds impacting the ocean's surface, heat and moisture are extracted from the upper ocean. The convection associated with the system and the associated rising motion serve to transport this energy upward (note that it is not the convection itself that drives the heating within the storm!) where the accumulation of energy at upper levels maintains the storm, builds the warm core, and, through meteorological principles, helps to intensify the system. We saw the system acquire a warm-core, later develop an eye, and then weaken -- all like we see with tropical cyclones! The only difference is the unique location and underlying circumstances.
Such events are so rare, leading them to be nearly impossible to predict. While we have records of a few, there are likely hundreds more we've missed because we couldn't or were not looking for such systems. Whenever the model guidance suggests something might cut-off in a region marginally favorable -- even if it is out of our normal paradigms for tropical cyclone development -- it might just be worth keeping an eye on, just in case the unexpected does happen. At the very least, it can help you better understand how normal mid-latitude cyclones evolve. I should note that these systems are the types of systems Jack Beven at the NHC absolutely loves...he must've been pinching himself when this one popped up (and I mean that in a good way)!
Nevertheless, it's always a good idea to think outside the box -- but it's also reassuring to know that our tropical cyclone development theories explain such a development & transition to a warm-core system quite well. Otherwise, we'd have to think about redeveloping our TC development theories!
Enjoy Vince while it lasts -- you may not see another storm quite like it for a long time to come. 10W in the Atlantic basin has so little ocean -- and even less that is favorable for development even under the outside-the-box principle (in terms of MPI) -- that it's very tough to see something like this even happen.
In other parts of the basin much, much closer to home, ex-STD 22 is about to get caught up in the convergence zone/trough hanging out just off of the US east coast and is not likely to redevelop. I'm watching that area very near Puerto Rico, however, on the periphery of that big upper low. There's some model guidance supporting something getting going there and as it moves into a more favorable position with respect to the upper low, we could see something spin up later this week. I'd put it at 50/50 we finish the Atlantic list sometime this week...and overall better than 50/50 that we see Alpha sometime this season. This is the wacky season, where storms don't necessarily have the typical tropical origins, but with very few exceptions -- Lee is the one I can think of -- everything has been a legit tropical cyclone. Accumulated TC energy is going to be skewed with Rita, Katrina, Emily, and Dennis, but the total number of TC days is going to be relatively low for what one might expect for 20+ storms.
More later...having some trouble with the site lately, so updates on my end might be slow in coming.
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