#1276 Postby chaser1 » Mon Oct 05, 2015 4:08 pm
It's interesting that here in the Talking Tropics thread, we're discussing potential (GFS) model support for a system that may well already exist (91L). The purpose of my mentioning this is not to suggest that we move or limit such conversation to the 91L Model thread, but rather to bring to light a couple of interesting points for consideration and a different perspective regarding present GFS long range development of a W. Caribbean tropical system.
One point being that in spite of all of the models seemingly performing unsatisfactorily over the past couple of years, here we are "potentially" acknowledging a disturbance that might form in the W. Caribbean at some time a good 10-14 days from now and as already stated, one which would well fit Climo. The fact that such a disturbance may never form at all (especially in light of the Strong El Nino) is obvious, however what the GFS suggests is that at least for now, several downstream conditions such as generally lower surface pressures, warm SST's, and possibly conducive mid to upper level conditions appear to be setting up. Greater confidence will build or wane of course, based on whether there's continued continuity of future GFS model runs and of course any future EURO and other model runs that show supporting (or divergent) solutions. One thing I have given some thought to is that prior to 2-3 years ago, there seemed to be a good deal more confidence in model output. Then, for a couple of years SAL seemed to be unusually strong causing an unexpected impact mitigating Atlantic tropical development. This year, atypical conditions were forecast and have related to a range of marginal to volatile conditions across parts of the Atlantic, caused by El Nino. All i'm saying here is that just maybe... the models have not actually gotten that much worse, but perhaps a greater level of recent year climate variability has been part of what has seemingly been plaguing all of the models. Okay... enough model inconsistency discussion for this thread, which wasn't really the prime reason for my post LOL.
The other point which I find interesting is that not only "might" the GFS be trending toward downstream tropical development, but may be forecasting such from the evolution of a vigorous tropical wave (91L?) that appears to be both convective and clearly moving westward from the MDR... in October. What's interesting about this to me is that in so many years past, we might see a decent October tropical wave to our east that just seems to languish out there...plodding westward at a speed only slightly faster than the time it takes glue to dry. That's basically normal and simply a sign that the (N. Hemisphere) Easterlies throughout the tropics are weakening and losing their influence to the change of seasons. Quite often the storms that we do see form in the far S.W. Caribbean later in the year, may well come from westward moving tropical waves but from a further south latitude and where much of the wave axis has pretty much dragged through N. South America. In other cases, large gyres seemed to develop over Central America or even propagate north from the Pacific side. Only reason I'm even bringing this up though is because one would typically think that a strong El Nino would only further inhibit a strong tropical wave to continue westward in October, and especially at its present latitude. I think the jury is still out on the topic why such a strong El Nino hasn't had greater impact on this year's development, disrupt Joaquin from deepening into the major hurricane that it was, or hasn't already ended the 2015 Atlantic season with 40+ knot easterly winds consistently blowing through the Caribbean.
I personally doubt that we'll see anything stronger than a sloppy tropical storm form out of the W. Caribbean in the 10-14 day range, but that's simply given my own somewhat tainted expectations of harsh upper level conditions that we all expect El Nino to bring (especially to the Caribbean). That said, I think its fair to say that this Atlantic hurricane season has proved capable to producing a number of AND an intensity of hurricanes, greater than what most of us and most professionals really anticipated.
So, here we are in the 4th quarter of the game with the expectations of many that range anywhere from "season over" to strong chances of at least one more potentially damaging hurricane. Meanwhile, Climatology teaches us what might be expected when conditions are favorable. El Nino has taught us why tropical development is unlikely when disruptive conditions appear to be present. So all in all, one almost needs to still defer to the plausibility of the past-proven models because when it comes right down to it, their forecasts are purely objective. Global models do not care what the date is, nor forecast tropical genesis based on Climatology. At the same time, the Global models don't factor strong westerlies or other conditions "which past" strong El Nino's have presented. What we do know however is that conditions through much of the Atlantic this year have proved conducive for tropical storms or hurricanes to form during particular periods of time when favorable conditions have existed.
At this time 200mb - 500mb winds in the W. Caribbean presently range from Easterly to variable, but certainly not roaring out of the west. Therefore, until El Nino rears it's ugly head to the extent I would have thought upper level conditions would have looked 3 months ago, I've got to at least consider the model output, persistence of present conditions, and the reality that yes.... big bad hurricanes have and might still develop. For those so sure that this simply can't happen, try telling that to the families who's loved ones perished just days ago from Hurricane Joaquin. Had steering conditions been just slightly different during this event, the result could just as easily meant the devastation of Freeport and Nassau, parts of Florida, or any one of many points along the U.S. Eastern Seaboard.
So, season over? Um, maybe... or maybe not quite yet, and perhaps it'll end up going out with yet one more bang instead.
I have to think that until conditions in the tropics significantly change, its more than reasonable to expect some additional development to still occur. The biggest question is, will development occur where climatology would suggest (and as currently depicted by the GFS), or be limited to those W. Atlantic regions that most of us anticipated earlier in the season?
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Andy D
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I'd call this about as strong support as the GFS is going to get from the GEFS: 17 of ~20 members with W. Carib SLP between 985 and 1005 mb hour 312! But still only fwiw as we all know.
