Tropical weather analysis - September 4, 2013

By: KoritheMan , 6:51 AM GMT on September 04, 2013

Invest 97L

A tropical wave accompanied by a broad low pressure system is located in the eastern Caribbean about midway between Dominica and Puerto Rico. Satellite images show an increase in convection at this hour, but it is quite difficult to locate the actual low-level center, which appears to be rather poorly-defined. Upper-level outflow is restricted along the east side of the wave axis, implying some southeasterly shear; this means that although the large-scale cyclonic flow in this area is lessening (and thus the shear), the consequent upper-level anticyclone is not quite collocated with the tropical wave.

Figure 1. Latest infrared satellite image of Invest 97L. Image credit: NOAA's Satellite Services Division (SSD).

Although the shear is improving as water vapor imagery shows the TUTT weakening, the broad nature of the circulation could be an inhibiting factor, not to mention interaction with Hispaniola over the next day or two. One interesting meteorological factor that I didn't notice a few days ago, is the area of convection to the east of the Lesser Antilles, which appears to be connected with another tropical wave. The global models hinted the other day at a vorticity lobe heading westward and another one heading northward. I ignored it then, considering it an interesting triviality but with little bearing on reality; however, it seems they might have been onto something with those predictions after all. It will be most difficult to pinpoint the evolution of this wave over the next several days, and its possible influence on 97L in front of it. I believe the tropical wave is one of the reasons why the models have shown an abrupt recurve with this system after it passes Hispaniola; I surmise that the cyclonic flow associated with the tropical wave interacts with a trough over the western Atlantic to help pull 97L northward out to sea ahead of the trough. While I would expect some convective deepening of the secondary tropical wave over the next 12-24 hours, the global models show the upper low centered north of the system that has been responsible for providing the diffluence and lifting mechanism for thunderstorm development retreating northwestward and weakening, a conclusion that is already supported by water vapor analysis. With this in mind, there should be no real influence on 97L's track from the secondary tropical wave unless it unexpectedly becomes a tropical cyclone, which seems unlikely in the face of 20 to 30 kt of northwesterly shear north of the Greater Antilles as shown in the GFS fields.

This is one of the more complex situations I've had to forecast, and I could certainly be wrong. I don't generally defy a consensus, but I think the models are deepening 97L too fast, and are also overestimating the influence of the tropical wave to the east. My personal expectation is that 97L will move west-northwest into the Dominican Republic -- a bit to the west of where the currently clustered guidance is -- and then proceed slowly west-northwestward. At that point, either it recurves near or east of Bermuda, or it continues westward toward the Bahamas and south Florida under increased mid-level ridging behind the trough. Some of the GFS ensemble members have started to offer support for the more westward track once the system emerges into the Atlantic north of the Dominican Republic, and the 0z ECMWF shows the system moving westward into the central Bahamas before deepening and turning northward. The pattern in either direction (ridging or troughing) is rather fragile, and I cannot confidently say what will happen at this stage The trough seen carving out over the western Atlantic, I feel, does not possess enough amplitude to dig far enough south to recurve 97L nearly as sharp as the models are saying. Indeed, there has actually been a noticeable westward shift in the dynamical guidance envelope over the last 24 hours, particularly at longer ranges, which reflects my current thinking.

Regardless of development, heavy rains and possible flash flooding will impact portions of Puerto Rico and Hispaniola over the next couple of days, particularly along mountainous areas. Wind gusts to tropical storm force may also spread over the higher elevations of those regions.

Probability of development in 48 hours:

Invest 99E

An area of low pressure centered off the southwest coast of Mexico a few hundred miles south of Manzanillo is generating some showers and thunderstorms. While satellite data suggests that the circulation is gradually becoming better-defined, the associated convection has not been deep or persistent. If there is a significant blowup of organized thunderstorms over the center tonight due to the diurnal convective maximum period, we could see a tropical cyclone form tomorrow. However, the system is forecast to reach cooler waters in about three days, arresting development after then.

Figure 2. Latest infrared satellite image of Invest 99E. Image credit: NOAA's Satellite Services Division (SSD).

This disturbance is expected to move west-northwestward while staying offshore mainland Mexico or Baja.

Probability of development in 48 hours: 50%

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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2. galvestonhurricane
6:11 AM GMT on September 06, 2013
Good stuff, Kori! Thanks for the update!
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1. WunderAlertBot (Admin)
6:07 AM GMT on September 06, 2013
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