Clark
Meteorologist
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Posts: 1710
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Ivan, Ivan, Ivan
Sun Sep 12 2004 10:48 PM
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Had a big long post here, but the server and my foolishness with the back button ate it up. D'oh!
I'll quickly summarize...
Ivan is currently finishing up an eyewall replacement cycle, as recon has recently reported just one, larger eyewall of 28 n. mi. in diameter. Satellite photos confirm this, as the eye became less distinct an hour or two ago, but has shown signs of reappearing in recent images. From here, another 6-12hr of consolidation can be expected followed by one last shot at restrengthening before a combination of factors -- interactions with Cuba, eyewall/internal system dynamics, and a weakened outflow channel to the NE -- result in slow but steady weakening entering the Gulf.
Right now, outflow is impressive in all quadrants but to the NW, where it is only fair. However, this has been the case for a day or two now, and we can all see what effect this has had on the storm -- not much. Last pressure was 917mb, while the height of the 700mb surface remains below 2400m...or over 600m below "normal." A 20% difference shows you how strong this storm is on the whole. A little bit of dry air appears to be impinging upon the storm on the north & northwest sides, but this is mainly a result of dry air off of Cuba than anything else.
Essentially, the dry air you see surrounding Ivan is a result of the storm itself and nothing that should impact the strength of the storm. In the atmosphere, generally rising and sinking motion should approximately balance out over a large area. Inside Ivan, there is a large amount of rising air, creating the convection powering the storm. Only a small fraction of this rising motion is counterbalanced by sinking motion in the eye; the rest has to balanced out along the periphery of the storm, thus the resultant "moat" of dry air around the storm. This dry air is thus not a product of actual dry air that will be infused into the storm, but storm-"created" dry air that should have little impact on its intensity.
In the short term, Ivan should continue to move WNW - roughly 295 degrees - then gradually turn NW and NNW over the course of the next 24-36 hours. On this path, it will come very close to the extreme western tip of Cuba; if we thought Charley took the path of least resistance over the island, Ivan may well redefine that notion. Hurricane warnings should go up for the western part of the island in the very near future, as the storm should pass over or very near to the tip sometime late tomorrow.
Unfortunately, this will probably be when the storm is at its peak intensity again...but fortunately, this will probably be the last time it reaches that intensity. The outflow channel provided by the upper-low in the Atlantic is gradually weakening as Ivan moves westward and the low retreats; furthermore, interactions with Cuba and eyewall replacement cycles should put an end to further intensification sometime late tomorrow, followed by perhaps a *slight* increase in shear having an impact at later times.
From there, forward motion should gradually begin to increase. Right now, Ivan is currently rounding the base of the ridge and entering somewhat of a "col" region of very weak steering currents. The current forward motion -- speed-wise -- will likely continue for another day to day and a half before the system gets caught up in the midlatitude steering flow. The shortwave over the S. Central US has dug a bit further south than expected and has additional support on the way from the Pacific Northwest. A slight bit further push to the south and east with this system is likely; I don't anticipate it weakening to any large degree nor retreating before the storm gets there. With this in mind, I feel confident that the Biloxi area is as far west as the storm gets. It's not inconceivable that it goes further west, I just view it as extremely unlikely.
After passing near the tip of Cuba through the Yucutan channel, the storm should roughly parallel the 85°W line before turning NNE or even NE as it approaches the coast. On this path, the Panama City to Apalachicola area (and points from Apalachicola to St. Marks on the NE-SW oriented coast) are most likely to be affected. The NHC forecast path is a bit further west than my thinking here at landfall (though the landfall locations are similar, I think a more pronounced turn late in the Gulf is more likely), as are the majority of models. Landfall in about 3 to 3 and a half days -- late Wednesday most likely -- is what I anticipate somewhere in the Panama City-Apalachicola area. I'm trending towards the eastern side of that swath, with a forecast point early Thursday about 35 mi. west of Tallahassee, but that is subject to change. Remember, even 3-3.5 days out, track errors are subject to large biases.
Intensity is going to be the big question. We saw Bonnie not do much in this general region as it approached the coast, but the synoptic conditions were quite a bit different then - not to mention the intensity and size of the storm - than they are now. Shear should increase in the Gulf, yes, but not by much. Waters are still warm, as no storm has touched these waters to any large degree this season (note that Bonnie was moving 15-20mph throughout this area and took place almost a month ago). The storm will weaken before landfall, but by how much? As it approaches shore is the time when much of the weakening will take place, so it is not inconceiveable to see many worried folk in the Fl. Panhandle fretting over a 140-150mph hurricane approaching the coast, as with Opal. The storm should not make landfall at that intensity, however, thanks to increasing interaction with the trough as well as shallower waters (resulting in less overall energy for the storm); an intensity of 120-135mph, category 3/4, is more likely.
Again though, everyone from Biloxi east to Cedar Key should be paying close attention to this storm. I don't believe that this is one for the peninsula in any way, though don't be surprised to see tropical storm force conditions approach the shore as the storm moves northward. On the projected path, hurricane watches will likely be issued along the northern Gulf coast late tomorrow or early Tuesday, with hurricane warnings coming early-midday Tuesday. Tropical storm watches will likely be issued for areas to the east along the Fl. west coast, mostly as a precautionary device in case the wind field -- currently extending out ~175mi -- impacts the coast. But, in any case, people along the northern Gulf coast need to take heed of this one -- it's not pulling hard right between the Keys and Cuba and then hard north between the Bahamas and Florida and missing land, nor is it going to pull a Roxanne from 1995 and sit and spin and die in the Gulf.
Pressed for a landfall point into a mythical landfall pool, I would say a bit east of Mexico Beach (alternatively, a bit west of Cape San Blas) as a 125-130mph cat 3 storm around midnight Wednesday night/Thursday morning.
I'll try to add a bit more tomorrow and answer any questions I see, though it's going to be a rather busy day on my end.
So much for quickly summarizing things, though!!
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