In his 9:30pm discussion Mike mentions that Isabel has been stationary and that is what I also found to be the case between 1715Z and 2115Z but since then, the lady seems to be reversing, which I am sure is not the case but simply an eye reorganising.
Hope he does not mind but his discussion is below and that is followed by a discussion by Gary Gray ... so this post is loooooooooooooooong!
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NOT OFFICIAL...SEE NHC PRODUCTS FOR OFFICIAL INFORMATION.
TECHNICAL FORECAST #1
...Note...this forecast was made using the initial position from 00Z on 09/07/03 (8PM EDT this evening) and is/was available 1.5 hours before the TPC forecast is available to the public...
Satellite images from this evening reveal a well-organized tropical storm. Banding features in IR2 imagery...and visible imagery just before sunset...showed a good deal of wrapping low-cloud and convective bands and a CDO-like feature. In fact...at first glance Isabel looks like a hurricane in IR satellite imagery. Satellite estimates are up to 55 knots from both SAB and TAFB...and with a close to ideal environment and plenty of increasingly warm water out ahead...Isabel will likely become a hurricane in the next 12 hours or so.
Isabel appears to have been close to stationary over the last 4 hours or so based on fix-to-fix position estimates...but the overall cloud envelope has been progressing west or slightly south of west. It's possible that earlier fixes were a little too far to the west and or the center had been reorganizing...but with the strong deep layer ridge in place to the north...Isabel should continue to move westward for the next 3 days or so. During days 3 and 4...global models depict a broad and not very sharp 500MB trough passing through the central Atlantic. This should slow Isabel and turn the storm to the west-northwest for a time...but by day 5...Isabel is expected to turn back to the west as the trough passes by. Since this is forecast to happen at toward the end of this forecast period and beyond...it's probably a little too early to do more than mention these features...future forecasts will go into more detail as the downstream picture comes into place. It should be noted that the BAM models are starting to show a hook to the west and even to the west-southwest toward the end of the period...reflecting the relative strength of the western Atlantic high in the GFS model. In addition...the most recent run of the ECMRF model also latches on to the westward trend.
The intensity forecast below reflects the inherent uncertainty in forecasting intensity 5 days out...but it is entirely possible that Isabel will become another powerful hurricane in the coming days. There is plenty of time to determine which...if any of the islands of the eastern Caribbean will encounter Isabel.
Next forecast at 9:30PM EDT tomorrow.
Watkins - 2003090700-0115Z
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Tropical Weather Discussion
by Gary Gray
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...Attention Turns To Isabel [9/6 7PM EDT]...
With Henri flaily rather pathetically earlier today, albeit near the U.S. coastline and looking a little better this evening, and with Fabian running on out into the north Atlantic poised to become extratropical, we shift our focus now to Tropical Storm Isabel. Isabel's long-range future is very much underdeterminable at this point. There is model unanimity that she'll go out to sea, but there is also great variation as to when and how. With such variation we can't even be sure that she will at all! So, we've got a perplexing forecast in the long range. In the short range it is exactly the opposite; the forecast is rather "yawnful"... Isabel is almost guaranteed to move manily west and continue to develop over the next 2-3 days. So, this forecast will focus on the long-range, since that is the only real area of debate. But I'd emphasize that this is very much a questionable forecast right now. Isabel is still 4-6 days (depending on forward speed) away from the Lesser Antilles, if she even threatens the area. So, to consider any eventual threat to land, we're looking at at least a four day forecast... something which the models are not presently in very good agreement on.
At any rate, Isabel is looking quite healthy, and we've yet to even "introduce" her, except in a special update to subscribers. So, let's just get to it...
Current Conditions: Isabel became a tropical storm this (Saturday) morning. She was instantly put at tropical storm strength, with no period at depression status. But this was, frankly, more of an "accounting" matter than any swift development. That is, a depression is typically defined when a system has a closed circulation, but there are alternative systems regarded as "tropical lows" which, by definition as a low, must have a closed circulation as well. So, what's the difference? It's a matter of organization, so it's very subjective and there is a lot of gray area between a depression and a simple "low". Therefore, if you hang onto "low" status a bit too long, you can easily have the need to jump to tropical storm status immediately. That's all that happened.
But, let me be clear as well... that's not to say that Isabel was not developing at all. In fact, she was; and, she has continued to do so. She looks quite healthy this evening and the only surprising element is just how low the satellite estimates are. My subjective view of it would easily put Isabel at about 50kts... maybe even up to 55-60kts. She's definitely NOT a hurricane at this point, so the 60kts may be stretching it, as that's only 5kts from hurricane strength. But, on the flip side, as I write this her official intensity is only listed at 45kts. I suppose it the lower end of my estimate... 50kts... is correct, then I'm just splitting hairs. So, let's just put it this way... Isabel is at least as strong as her present official intensity.
For those unaware, Isabel is out in the Central Atlantic near 13N latitude, 36W longitude as of Sat evening. Some satellite position estimates are a bit south of the official position (my 13N approximation given is nearer to the satellite position). As such, the motion obtained from from satellite fixes is south of due west. TPC/NHC implied that this may, in part, be due to center reformation. Given her very well organized structure this morning, but with the actual low center fixed on the NE edge of the convection, this assertion seems reasonable. However, I do think there is likely SOME southward component truly occurring. Why? Well, predominantly because the satellite fix positions have shown southward components in two consecutive fixes, which makes a simple reformation seem very unlikely as the only cause. Also, Isabel has not passed the center of the subtropical ridge yet. So, the flow could certainly be slightly north of due easterly... though there is, obviously, no real-time data in the area to support that, the GOES Winds do indicate that this is possible. So, Isabel does appear to be moving slightly south of due west at the present time.
Model/Technical Discussion: As stated up front, the models are really widely varied with Isabel. Unfortunately, we're limited somewhat, to about a handful of models, since we only want those that go into the medium-range. But, it seems like we're used to that now, so it's not much of a change from the norm... it only drops a couple of models from our view. However, as one can see on the latest (12Z) GFS, even the west solutions, because they're drawn up by another shortwave, curl Isabel back out to sea. The problem with that is that we almost need the models to go out 8-10 days... at least for those with western tracks! Only two models do. But, here's a crack at the various models we can examine...
The tropical model suite presents the biggest challenge, because all four members are among the furthest SW solutions... all threatening some of the Caribbean Islands. But, they only go out to 120hrs and are not accompanied by other atmospheric field maps, so I cannot determine whether or not their southern tracks are far enough south to keep them from breaking the unanimity (i.e., not turning out to sea). One thing is certain, though, all four members threaten the Caribbean Islands... and three of the four place the threat towards the NE portion of the Caribbean. The GFDL is rather head-scratching as well. While it is north far enough to miss the Caribbean (and moves outside the 20/60 benchmark), the GFDL also turns Isabel to the westnorthwest right at the end of the forceast. But, telling whether this is a motion that will hold or not is darned near impossible, as the GFDL shows a terribly unstable motion (possibly a tribute to the complex field)... at about 290 degrees, then 305, then 295, then 320, then 290... each at six hour intervals towards the end of the period. The NOGAPS has enough turning such that, eventually, Isabel will kick out to sea. She has not done so by the end of the NOGAPS forecast, but she is well on her way. Isabel never seems to make in into the GEM domain throughout the forecast period. So, it's tough to see exactly what that model does, but clearly, Isabel heads out to sea. The Fri ECMWF (Sat run is not in as of this writing) whipped Isabel out to sea far, far to the east. The 12Z UKMET has shifted well west of its previous forecast, but still carries Isabel just outside the "benchmark" and APPEARS to have enough troughing to turn her out to sea... I emphasize "appears", because the trough is weakening as Isabel moves in on it. Finally, the GFS has shifted west every progressive run. The 12Z run now threatens the entire tier of extreme northern Caribbean islands except Cuba. This would seem to indicate an easy threat to the U.S.; but, instead, a weakness north of Hispaniola abruptly halts Isabel and eventually lifts her NE out to sea.
Synoptically, when one examines the models it looks as thuogh most are allowing the northward turn due to an upper low moving out ahead of Isabel and helping produce a weakness. Then, one she gets far enough west to pull around the east side of the ridge, there is another weakness that lifts her our to sea. But on the more western models, this weakness is a weak shortwave embedded under a new ridge. With such complex and, frankly, questionable features, I can't say that there can be any confidence in any of the model solutions. What about the more eastern solutions, since they are responding to a more severe trough weakness and, therefore, should be more reasonable? Well, they aren't much more convincing. Though the ridge weakness is more substantial where those models turn Isabel out, the shortwave is just about to lift out when it picks up Isabel, and Isabel is starting from a further south point (since she's further east and moving WNW in the models), making her harder to dig out and curl out to sea. But I'm also not saying these solutions are wrong. In fact, they are all quite reasonable for the scenarios presented. My problem is that the scenarios presented hinge on a number of close calls and speculative features which, in a 4+ day forecast, are bound to have at least minor errors in them.
My thinking: Much like when Fabian was in this area, I must emphasize that a four to six day forecast, and beyond, is fairly low confidence. That is especially true in the rather complex environment ahead of Isabel. However, I am REASONABLY confident that Isabel will not follow the path of the furthest east models, like the ECMWF (and possibly the NOGAPS and GEM... but they're not nearly as far east as the ECMWF). Why? The weakness in the ridge, though plainly evident even on the western models, is simply too narrow, and it does not represent a real trough down at Isabel's latitude. Meanwhile, she is moving on a path which is currently south of almost all of the guidance... further protecting her from the weakness. Incidentally, I do think this weakness could do enough to stall intensification, as shear increases, for 12-24 hours; this owuld occur in 2-3 days.
What this means in the end is that, in the long run I'm expecting a track something like the tropical model suite, the GFS, the UKMET, or, in an extreme case, the GFDL. The GFDL is extreme because it is the further northeast of these western solutions (unless one includes the NOGAPS in this group), and it is already off a bit to the north (i.e., 6+ hours into its most recent run and Isabel is already to the left of the GFDL track). This range of solutions does allow for the possibility of everyone getting off the hook (like the UKMET and GFDL show), but it also runs a significant risk to the Caribbean (as seen in the GFS and the tropical model suite members). As for any eventual threat to the U.S., I'm not even going to guess on that yet. There's enough room for error in the approach up to the Caribbean, combined with my uneasiness in the models' prediction of the weakness off the U.S. East Coast, such that I'd have absolutely zero confidence making such a guess. Check back over the next few days as we begin to address that issue.
With that, just a few words about Isabel's projected intensity before closing... Over the next 5 days I see only that one brief aforementioned period where conditions degrade enough to stall development or even cause a bit of weakening. Otherwise, the conditions look almost idea... SSTs are warming along Isabel's track, upper level ridging to the north should provide good conditions aloft, and Isabel is not moving too quickly (as has been a common problem this year). She should very easily reach hurricane strength and, depending on the severity of the interruption in intensification, she may well become the next major hurricane of the season.
-Gary
Mike Watkins confirms what I suspected
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