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Tropical weather analysis - September 30, 2013

By: KoritheMan , 5:54 AM GMT on September 30, 2013

Tropical Depression Eleven

Tropical Depression Eleven has not strengthened yet, according to the 0300Z NHC advisory:

Wind: 35 mph, with higher gusts
Location: 27.0°N 47.0°W
Movement: ENE at 9 mph
Pressure: 1010 mb

The 6z satellite estimates from TAFB and SAB have not yet culminated, but a recent UW-CIMSS ADT estimate assumes the depression has become a tropical storm. Indeed, the shear does appear to have decreased from earlier, likely because the adjacent upper low that has been afflicting the system is quickly moving north of the cyclone, a process that is altering the upper flow from southwesterly to northwesterly. Though neither are particularly favorable for intensification, water vapor imagery suggests that -- for now -- the shear may actually be assisting in the development of outflow in the western semicircle. The low-level center is rather difficult to locate lacking any recent microwave or scatterometer passes, but with the help of shortwave infrared imagery and continuity, I surmise that it's more or less embedded within the southwestern edge of the convective cloud shield.

Figure 1. Latest infrared satellite image of Tropical Depression Eleven. Image credit: NOAA's Satellite Services Division (SSD).

Although the large-scale environment is hardly ideal, the shear has lessened, so the window for strengthening appears to be upon us. Satellite estimates are beginning to trend upward, and the depression may already be a tropical storm. A couple of scatterometer passes over the last 12 hours indicate that the low-level center associated with the depression is extremely small and rather fragile, so any abrupt increases in shear could quickly dishevel the system. Primarily due to this uncertainty, my forecast will not yet bring the intensity above 40 kt, but I would not be surprised to see the cyclone get a little stronger. Late in the period, there are some signals within the GFS that the 200 mb flow may transition to southwesterly again ahead of another upper-level trough now over the western Atlantic, which could act to shear the depression during that time. The timing and magnitude of this possibility, however, are highly uncertain.

As alluded to above, the center of the depression is not easy to locate this morning. It appears to be within the southwest edge of the convection, and more or less abiding by the latest NHC track, but I am not certain at all in either the location or the motion of the depression, and a few decent microwave passes would certainly be welcome right now. Regardless of the initial status of the cyclone, water vapor imagery shows the upper low that has been providing the primary steering mechanism quickly lifting out, perhaps even a bit faster than I had expected. This requires me to shift my track a little to the left of the National Hurricane Center track, under the assumption that the clockwise loop that I expect the depression to undertake will not be as sharp and theatrical as I was originally thinking. In a few days, another trough is forecast to approach the system and gradually recurve it around the western periphery of a deep-layer ridge over the eastern Atlantic. The model guidance remains in unwavering agreement on the general evolution of the synoptic pattern, and this forecast is thus of high confidence.

Intensity forecast and positions

INITIAL 09/30 0300Z 27.0°N 47.0°W 30 KT 35 MPH
12 hour 09/30 1200Z 27.1°N 46.7°W 35 KT 40 MPH
24 hour 10/01 0000Z 26.9°N 46.5°W 40 KT 45 MPH
36 hour 10/01 1200Z 26.7°N 46.7°W 40 KT 45 MPH
48 hour 10/02 0000Z 27.2°N 46.9°W 40 KT 45 MPH
72 hour 10/03 0000Z 27.8°N 48.1°W 40 KT 45 MPH
96 hour 10/04 0000Z 29.7°N 48.3°W 35 KT 40 MPH
120 hour 10/05 0000Z 31.3°N 44.8°W 30 KT 35 MPH

Track forecast

Figure 2. My forecast track for Tropical Depression Eleven.

Invest 97L

A weak surface trough in the south-central Caribbean about 350 miles south of Jamaica remains poorly-organized. Shower activity is quite limited, surface pressures are not falling, and earlier scatterometer and microwave data during the evening showed that the low-level center is very poorly-defined.

Figure 3. Latest infrared satellite image of Invest 97L. Image credit: NOAA's Satellite Services Division (SSD).

Westerly shear is impacting the system, diagnosed at 20 kt by UW-CIMSS, and this appears to be preventing significant development despite a relatively moist atmospheric column and amply warm sea surface temperatures. The GFS continues to suggest that upper-level winds will gradually improve over the next couple of days, which does actually look supported via a glance at water vapor imagery, which shows mostly northeasterly flow ahead of the system in the wake of a departing trough over the Gulf of Mexico. This seemingly favorable upper flow pattern could allow for some gradual development of this disturbance as it heads northwestward toward the northwestern Caribbean Sea. None of the models currently develop the disturbance, though.

Regardless of development, locally heavy rains and possible flooding will impact portions of Jamaica, the Cayman Islands, and Cuba over the next several days.

Probability of development in 48 hours: 10%

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. WunderAlertBot (Admin)
6:33 AM GMT on October 01, 2013
KoritheMan has created a new entry.
1. AtHomeInTX
6:08 AM GMT on September 30, 2013
Thanks Kori. What a season. Lol.
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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