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Trouble in the Gulf, Aid for the Southeast
      #75183 - Thu May 31 2007 11:30 PM

This evening, a well-defined tropical disturbance is located in the extreme southern Gulf of Mexico, with the surface circulation and lowest pressures found on the northeastern tip of the Yucatan Peninsula per a QuikSCAT pass from earlier this evening. Maximum sustained winds associated with this feature are on the order of 25-30kt, higher in convection and localized gusts.

QuikSCAT pass (time-sensitive!)

Convection associated with this feature is highly disorganized at this time, with a band of deeper convection found southeast of the center in the NW Caribbean Sea. Convection over Honduras and Central America at this time is not associated with this disturbance; it is moreso associated with the diurnal convective cycle across the region. The system is quite lopsided to the downshear side of the storm, or toward the direction that the wind shear is pointed towards. Wind shear in the region is lowest in the NW Caribbean and highest in the central Gulf, increasing from 10-20kt in the former to 50-60kt in the latter. This is associated with a cut-off trough of low pressure in the upper levels of the atmosphere over the central Gulf; it is this feature that will provide clues on the ultimately intensity and path of whatever comes out of this disturbance.

On a broader scale, the flow over the continental US is quite stagnant thanks to a cutoff upper level low over the central United States and the aforementioned cutoff trough in the Gulf of Mexico. A Pacific jet is serving both to enhance severe weather potential across the central United States tonight but also to amplify the trough in the Gulf. There are no significant features in play over the northern Pacific to cause the cutoff low over the northern Plains to move significantly in the next couple of days; the same may hold true for the trough in the Gulf, but that's a tad trickier to diagnose just from water vapor satellite imagery. Of course, that's where the ultimate track forecast uncertainty lies -- can't ever have anything easy!

So, two questions -- track and intensity/structure. Let's cover the latter one first. Sea surface temperatures across the Gulf of Mexico are favorable until about 28N latitude -- about the latitude of Tampa, basically -- and then quickly drop off in both temperature and oceanic heat content toward the north. Any sort of warm core (re: tropical or subtropical) development is likely going to have to occur sooner rather than later as a result. Furthermore toward this point, upper level winds and vertical wind shear only increase to the north in association with an upper level jet streak aimed directly at the eastern Florida panhandle, suggesting an increasingly hostile environment for tropical or subtropical development as the disturbance goes north. However, as it accelerates north, storm-relative wind shear will tend to decrease somewhat, potentially allowing it to maintain itself somewhat better if it gets organized sooner. So, the key is, when does this thing get sufficiently well organized to draw further interest? If it's going to happen tropically, it needs to do so tonight. Watch the convective evolution over the southern Gulf tonight; if there is a flareup somewhere in there tonight, tropical development is more likely. If it can only develop a low-level warm core that recon can detect, the NHC is likely to classify it as tropical if for no other reason than for public notification purposes for an eventual landfall. No convective burst and tropical development potential starts to dwindle somewhat, particularly given the asymmetric structure it is already displaying on satellite imagery -- not really extratropical, but not purely tropical either. My best bet is a hybrid structure from here on out, but that says nothing about what it actually gets classified at; see the sentence a couple of lines back for my thoughts on that matter. Supporting evidence for the hybrid structure call comes from the various cyclone phase space forecast depictions for what low can be tracked by the models.

Cyclone phase space diagrams

Onto the ultimate track of this feature. Recall that tropical cyclones tend to be steered to first order by the mean wind in the atmosphere, with weaker storms going moreso with the lower-level wind flow and stronger storms moreso with the upper-level wind flow. To second order, the center of circulation can jump around in response to convective development and other things of that ilk; we saw this with Alberto last year, where the center jumped toward the convection a couple of times. In general, with the upper level flow pattern the way it is, this would suggest potential jumps north and/or slightly east. Right now, the lower-level wind field would tend to steer the disturbance more westerly, whereas the upper-level wind field would tend to steer it more northerly or northeasterly. This is well-captured by the Univ. of Wisconsin mean layer steering flow products linked below.

UWsic Steering Flow Products

Extratropical and hybrid storms, however, tend to move moreso with the strongest mid-level height falls and vorticity advection and/or toward the areas of greatest baroclinic energy. In general, this is going to be ahead of the upper-level feature and often embedded within an upper level jet streak, such as the one pointed at the eastern Florida panhandle right now. This isn't much different than what the tropical steering flow diagrams suggest, for what it's worth. However, is this going to change at all? Is the upper trough in the Gulf going to move, amplify, etc.? Tough to tell just looking at water vapor imagery, but it at least does give us an area to look at.

Thus far, this discussion has focused more on the observations than the models and their forecasts. So, what do the models say? The model tracks are across the place from the western Florida panhandle to the Tampa Bay area. The upper-level pattern is such that areas further west don't have a lot to worry about while areas further down the Florida peninsula should just watch this for the rain potential. In general, the models disagree on how much pivoting the upper level trough in the Gulf is going to do over the next two days; the models that pivot it a bit more east have the disturbance making landfall further east, as expected. Couple of things to note...

1) The upper ridge over the southeast and western Atlantic, while weakening, has been strong for quite an extended period of time and at least partially responsible for the drought conditions across the southeast -- possibly slightly enhancing the intensity of the ridge through surface heating feedbacks.
2) A strong Pacific jet across the southwest United States is heading toward the Gulf and the upper trough, but indications are that the flow is moreso being redirected into digging the feature rather than moving it along. Nothing in the upper air data suggests that the cutoff low over the northern Plains will help to kick this along either.

There has been some discussion elsewhere about using the NAM to look at this disturbance and its forecast evolution. While the NAM is generally not a good tropical model in the least, given the strong extratropical (re: midlatitude) influences at play and the overall poor performance of the model suite at large lately, I believe it should be given slightly more weight than would normally be warranted for a tropical-type of situation. It has generally been consistent with its forecasts but also west of most of the other models. However, the other models through the day Thursday were gradually trending toward the north and west again, for what it's worth (not necessarily a whole lot). What does that all mean?

Model guidance -- see Jonathan Vigh's page for many output plots -- is generally clustered in the Cedar Key to Tampa Bay area, with some outliers further west. The ECMWF, last season's best model particularly in the medium-long range, has been on the western end or slightly further west of that guidance envelope. This gives me confidence in an Apalachicola to Crystal River, FL impact area, aiming at the same general area that Alberto made landfall at last season. Timing? Looking at about 2-2.5 days out right now, toward the earlier end of that range if the center doesn't redevelop back in the NW Caribbean near the convection overnight.

What impacts can be expected? Mostly a rain event. Even if this develops tropically, wind speeds are not likely to be significant -- weak to moderate tropical storm at best given the environment -- and as a result waves and surge aren't likely to be all that significant either. Areas along and east of the track can expect to see several inches of much-needed rainfall. This includes coastal areas of the SE as well. Note that extratropical storms -- or storms that undergo extratropical transition from tropical storms -- sometimes have a tendency to have precipitation distributions that shift from predominantly NE of the center to NW of the center. I don't think that this will happen to any large extent for Florida or coastal Georgia right now, but as the system moves up the coast into the late weekend/early next week and topographic/orographic effects take hold, this might become a tad more significant. In short, except 1-2" of rain over a broad area from Panama City, FL north and east, with locally heavier amounts where the greatest moisture feed sets up -- 3-6" if not 8" in the most significant areas.

If anything, we're officially bringing in the 2007 with a bang tonight and we have a lot to look at into our weekend.

Current Tropical Model Output Plots
(or view them on the main page for any active Atlantic storms!)

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