TropicalAnalystwx13's WunderBlog

Tropical Depression Nine forms; watching the Bay of Campeche this week

By: TropicalAnalystwx13, 3:50 AM GMT on September 09, 2013

The ninth tropical cyclone of the unusually slow 2013 Atlantic hurricane season is here. The African wave we've been monitoring for the past 48 hours has finally gained enough organization for advisories to be initiated on it. And as of the most recent advisory from the National Hurricane Center, Tropical Depression Nine was located within 25 nautical miles of 13.2N 21.9W...about 160 miles southeast of Praia, Cape Verde. Maximum sustained winds were up to 35 mph, the minimum barometric pressure was down to 1006 millibars, and the system was tracking off towards the west at 12 mph. This trajectory tracks the system just south of the southernmost Cape Verde Islands over the next two days, and a Tropical Storm Warning has been issued for the islands of Maio, Santiago, Fogo, and Brava accordingly. Hourly infrared satellite imagery updates suggest a well-defined and organizing tropical depression, with convection - shower and thunderstorm - cooler than -80C firing in association with the system. The low-level center of Nine remains slightly east of these thunderstorms due to common northeasterly wind shear off Africa, but it does not appear to be completely exposed. The latest satellite intensity estimates from SAB, TAFB, and UW-CIMSS ADT were T2.0/30 kt, T2.0/30 kt, and T2.4/34 kt, respectively. A 23:07 UTC ASCAT-B pass caught a portion of the tropical depression, revealing a few 25 wind vectors in the northwestern quadrant; it is likely that stronger winds were occurring farther west in the aforementioned deep convection. For this reason, I believe Nine is nearing, or already at, tropical storm status.

Forecast for Nine
The initial motion for the tropical depression is towards the west at 10 knots. This trajectory is a result of a deep-layer area of high pressure to the cyclone's north-northeast and should continue over the next 24-36 hours. After this time, a mid- to upper-level low currently analyzed in the north-central Atlantic is forecast to dig southward, eroding the western periphery of the ridge. This should allow for a weakness to develop ahead of Tropical Depression Nine and induce a turn towards the northwest, if not north. By the end of the forecast period, this weakness should begin to close as high pressure begins to build in once again, a west-northwest track may ensue; this idea is supported by the most recent runs of the GFS and ECMWF.

Tropical Depression Nine is located within a very favorable environment. Maps courtesy of the University of Wisconsin-Madison reveal that the cyclone is embedded within a very deep and large pouch of atmospheric moisture, characterized by 700-500 relative humidity values of near 80%. Sea surface temperature analysis courtesy of the 0z SHIPS revealed the system was over waters of 28C. Northeasterly shear is currently impacting the system as evidenced by the displaced low-level center, but this is expected to decrease over the next 12 hours as the depression tracks westward and an upper-level high establishes itself atop the system. Furthermore, an analysis of the GFS upper-level wind forecast reveals that the upper-air pattern will become quite phenomenal, with both an equatorward and poleward outflow channel predicted to develop. What little Saharan Air Layer remains across the East Atlantic has been shunted far to the north of the system, so this is also not expected to be an issue. My forecast follows a blend of the SHIPS, LGEM, and GFS, all of which show the system steadily strengthening over the next 4 days. It should be noted that if an inner core becomes established quickly that rapid intensification is a distinct possibility. The current setup is reminiscent of the one that Fred 2009 and Julia 2010 were able to take advantage of. After 96 hours, the system is expected to cross the sub-26C isotherm; this should induce slow weakening.

My current forecast calls for the system to attain hurricane status late Tuesday night. If this does indeed occur before 12z on September 11, we will not break the record of having the latest first Atlantic hurricane. The current record is held by 2002's Hurricane Gustav.


INIT 09/0300Z 13.2N 22.0W 30 KT 35 MPH
12H 09/1200Z 13.4N 23.6W 40 KT 45 MPH
24H 10/0000Z 13.6N 25.7W 50 KT 60 MPH
36H 10/1200Z 14.2N 27.8W 55 KT 65 MPH
48H 11/0000Z 15.0N 28.7W 65 KT 75 MPH
72H 12/0000Z 18.3N 30.1W 75 KT 85 MPH
96H 13/0000Z 22.4N 31.7W 70 KT 80 MPH
120H 14/0000Z 23.8N 33.2W 60 KT 70 MPH

Watching the West Atlantic
Turning our attention away from Nine, it appears that the Bay of Campeche will need to be watched over the coming days as the northern portion of a tropical wave axis currently sitting in the Gulf of Honduras slides westward into central America and the southern Gulf. This axis is likely to interact with a surface trough left behind from short-lived Tropical Depression Eight, resulting in the formation of an area of low pressure. One inhibitor for this disturbance may be a competing low on the other side of Mexico, in the East Pacific. If this low becomes dominant, shear from the developing system will weaken the Atlantic-side low and eventually cause it to be absorbed into the former's circulation; and vice versa. Model support for this system remains on and off; a majority of the 12z GFS ensembles were in agreement for a moderate tropical storm moving inland near Veracruz in 6-7 days...many of the 18z GFS ensembles did not show a storm, just an elongated area of lower pressure. How far north in latitude this system gets in the Bay of Campeche will be crucial for development. If it is able to form far enough offshore, atmospheric conditions will favor steady development and the system is likely to spend several days over the tropical cyclone-enhancing Bay of Campeche. Whatever forms will likely be shunted westward as a result of high pressure over the United States.

Figure 2. 18z GEFS MSLP anomaly average for days 1 to 5.

I'll have a new blog tomorrow,


Tropical Cyclones of 2013 Tropical Weather

Updated: 3:36 AM GMT on March 10, 2014


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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Small town USA guy. Politics nerd. Soccer fan. Interested in eyewalls, deformation zones, and hook echos.

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