Hurricane Hunter aircraft found that the tropical wave passing through the Bahamas had acquired a closed low-level circulation and was sufficiently organized to warrant upgrading the system to TD 12. Located near 23.5N/76W as of the 11p advisory, the depression is generally headed in a west-northwesterly to northwesterly direction at about 7-8mph.
Currently, TD 12 is half of a storm. I say that as all of the convection is currently located on the eastern side of the system. An upper-low to the north and west of the system is imparting shear across the western half of the storm, impeding it's development. Nevertheless, a solid curved band of convection exists on the eastern half of the storm and will likely provide the focus for the depression to maintain itself over the next day or so. That upper-low (plus a trough through the Carolinas on the northern end) is also currently the weakness in the subtropical ridge, which is solidly in place centered near Bermuda between 50-70W and again centered between Mobile and Biloxi and extending southward into the central Gulf. The evolution of this weakness through time is going to be critical as to where TD 12 heads from here on out, while the evolution of the upper-low itself will be critical to the intensification of the depression until it reaches the Florida coastline.
Before going into my forecast reasoning, a brief synopsis of the current track and intensity forecast is in order. The 11pm track has been shifted ever-so-slightly to the right and trended slower, with a 5 day position about 60-75 miles north and east of the previous forecast. It now also brings the depression into southern Florida as a slightly weaker storm, with winds of 65mph as opposed to 70mph, though that is really picking at hairs. It still brings the storm up to hurricane intensity in the north-central Gulf of Mexico in 5 days. Their track forecast largely follows the model ensemble mean and their intensity forecast follows a weighted mean of the SHIPS and guidance. I should note that the models should be better initialized beginning tomorrow evening, once the upper-atmosphere data from the G-IV aircraft is collected and input into the models. At that time, we should have a better handle on where this storm might go.
The factors at play in forecasting the ultimately track of this storm are: 1) how slow does the system go initially as well as over the Florida peninsula? and 2) how strong is the trough projected to move across the United States late in the week going to be, and how far south will it extend? All of the models -- even the , which has been discounted by the (and by myself for an unrealistic depiction of the storm and its environment over Florida) -- show the same basic evolution of the environment, with the upper-low to the northwest of the storm gradually weakening, the system moving towards Florida, a weakness in the ridge developing as the deep-layered trough approaches from the west, and the storm ultimately being captured by this trough. They differ in track forecast ultimately because of the timing of the storm -- the slower models are more towards the right of the suite with their forecasts, while the faster models are more towards the left of the suite. They all agree (reasonably so) on the strength and positioning of the trough, however; I should note that the trough the models predict on capturing the storm is the one currently located over the NW US.
As the trough in the NW US becomes largely cut-off from the main steering flow, it will traverse slowly across the northern tier of states. The pattern and its strength suggest that as it approaches the eastern US in a few days, it should extend down into the SE US, but just so. This should be enough to ultimately capture the storm, and in that I don't disagree with the models. Thus, as noted before, the track forecast largely becomes one of how fast the storm will move in the next three days and, to a minor extent, what the weakness in the ridge will look like as it gets towards Florida.
The current trend with the forecast in the short-term has been to slow the system down; the current track has it making landfall around midnight-2am Friday morning. This is slower than the , which has it near the coast about 24hr prior to that, the UKMET, which is about 12hr prior to that, and the (6hr prior), but faster than the , which delays landfall by about 6-12hr. Not surprisingly, the has the system headed for Mississippi, the UKMET is awkward in taking it WSW towards where Emily made landfall, the suggests the central Florida Panhandle, and the is drastic in taking it to Florida but then offshore. Generally, the slower the progression in the short-term to Florida, the further east it will make landfall in the northern Gulf. I see no reason to disagree with this at this time.
Given the model projections, however, I believe the forecast track is currently too far to the left/west at 4-5 days. I don't disagree with the forward speed of the storm at all; given that forward speed, however, I believe the storm is more likely to move a bit further east than predicted once in the Gulf of Mexico. The synoptic pattern is relatively straightforward once it gets there (Florida); what isn't is the timing of it getting there and where it will be when it does get there -- the former of which is important and the latter of which is importantly only insofar as to determine where onshore it will make landfall. I see no reason at this time to disagree with the projection of a landfall near Ft. Lauderdale, perhaps slightly north of there (to account for the slight potential of interaction on the western side of the upper-low early in the period), late on Thursday or, more likely, early on Friday. The storm should turn towards the west-northwest as it crosses the Florida peninsula, only to turn to the NW, NNW, and finally north in the 4-5 day time span. My current target area is that from Mobile to Apalachicola, with the Ft. Walton-Panama City area that in the center of the path. Nevertheless, those from New Orleans eastward need to watch this storm, as any change in its forward speed will ultimately impact the final track of the storm.
In the short-term, I believe slow intensification is likely. The upper-low to the NW is currently limiting intensification at this point in time by imparting shear, dry air, and weak sinking motion aloft on the western side of the storm. As this feature gradually weakens, the environment should become marginally more favorable for development. Negative factors that will remain, however, are the many islands of the Bahama chain in its path. Waters are very, very warm in the area, but are also very shallow. While a weak storm will not churn up waters very deep, water depths in this area are shallow enough to potentially limit the amount of intensification before making landfall in Florida. I believe the system will likely remain a weak tropical storm as it passes through the Bahamas but intensify at a faster clip to the east coast of Florida. Thus, my initial intensity forecast is just slightly below that of the , while the intensity at first landfall is near that of the , perhaps slightly above. A hurricane at landfall cannot be ruled out at this time, but that is not my thinking. Given the slow motion of the storm across Florida and despite the relatively moist conditions in the northern Everglades, the storm should weaken back down to a weak-to-moderate TS as it crosses the state, only to reintensify over the Gulf waters. My intensity forecast in the Gulf is similar to the as it enters the Gulf, but I feel like the potential for the storm-relative shear to weaken as the storm begins to move northward plus the very warm waters will result in more intensification than forecast, to a strong category 1 hurricane (~90mph) by landfall.
This forecast is similar to that of Hurricane Erin of 1995, albeit weaker as it reaches Florida due to greater interaction with the Bahama Islands. Many references will be drawn to that storm over the coming days and, unlike a lot of those we've seen with other recent storms, probably a fairly apt one. We'll watch TD 12 over the coming days and provide more updates and information as events warrant.
Not much has changed with 97L since the last update, so in the interest of brevity, I will refer you back to the previous blog entry for details on that system.
I apologize for the length of this update, but feel it necessary to convey all of the basic information. Next update: tomorrow night if conditions warrant; otherwise, Thursday. Thanks for reading and, as always, if you have any questions or comments, please feel free to send me a PM or ask them on the main board (or in the appropriate folder).
Edited by MikeC (Wed Aug 24 2005 12:11 AM)
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