Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 12:35 AM GMT on September 10, 2013
Tropical Storm Humberto continues to gradually intensify in the eastern Atlantic. As of the 2100Z NHC advisory bulletin, the following information was available on the storm:
Wind: 50 mph, with higher gusts
Location: 13.7°N 25.3°W
Movement: W at 12 mph
Pressure: 1002 mb
While the satellite presentation looks impressive, with an abundance of deep convection, recent microwave and scatterometer data suggest that the low-level center remains along the northeastern edge of the convection, a direct symptom of northeasterly shear. Satellite estimates have not changed appreciably, with a recent UW-CIMSS raw ADT number coming in at 3.0. The shear is customary of systems in this portion of the Atlantic, and as Humberto becomes more distant to the axis of the deep-layer ridge centered over the Azores, the shear should lessen, with the GFS amplifying a large 200 mb anticyclone over the surface center. Such a wind pattern normally portends significant intensification, and as long as Humberto remains over sufficiently warm water, it should respond to that pattern by strengthening.
Figure 1. Latest infrared satellite image of Tropical Storm Humberto. Image credit: NOAA's Satellite Services Division (SSD).
My forecast still calls for a peak of 80 kt between 36 and 48 hours; thereafter, the cyclone is forecast to cross the 26C isotherm, encounter more stable air, and increasing southwesterly shear associated with a large upper trough. Weakening is thus shown beyond 48 hours, although the large size of the circulation will probably not allow for a rapid decay. Although sea surface temperatures warm slightly along the projected track at the end of the period, I don't see how Humberto can reintensify at those ranges in an environment characterized by southwesterly to westerly shear. It is possible that Humberto could experience a brief episode of rapid intensification tomorrow if an inner core becomes established, and there is still a chance of the cyclone reaching major hurricane status.
Synoptic data suggest that Humberto is south of a well-established subtropical ridge near or over the Azores. Water vapor imagery shows an upper low over the central Atlantic between Bermuda and the Azores moving southeastward, and another such low and attendant trough a few hundred miles west of the coast of Portugal moving southward. These two features are forecast to interact and create a large weakness in the western extent of the subtropical ridge, allowing Humberto to turn sharply northwestward to north-northwestward in the next 24-36 hours. At days four and five of the forecast, I anticipate the cyclone sharply turning back toward the west-northwest as mid-level ridging rebuilds to the north. The models continue to be in agreement with this, increasing confidence in the forecast.
Intensity forecast and positions
INITIAL 09/09 2100Z 13.7°N 25.3°W 45 KT 50 MPH
12 hour 09/09 0600Z 13.8°N 27.1°W 50 KT 60 MPH
24 hour 09/10 1800Z 14.4°N 28.6°W 65 KT 75 MPH
36 hour 09/11 0600Z 17.1°N 29.3°W 75 KT 85 MPH
48 hour 09/11 1800Z 18.5°N 29.6°W 80 KT 90 MPH
72 hour 09/12 1800Z 22.1°N 30.2°W 70 KT 80 MPH
96 hour 09/13 1800Z 24.9°N 31.4°W 60 KT 70 MPH
120 hour 09/14 1800Z 25.9°N 35.3°W 50 KT 60 MPH
Figure 2. My forecast track for Humberto.
An area of low pressure, associated with the remnants of Gabrielle, is located about 350 miles south of Bermuda. Convection has increased significantly during the diurnal minimum, which could be a sign that the system is attempting to regenerate. Based on microwave and earlier visible satellite data, the low-level circulation, albeit small, appears well-defined, and only a slight increase in the organization of the convection would result in the reformation of a tropical cyclone. For now though, the data suggests that the center is clearly to the west of the convection due to continued southwesterly to westerly shear. The GFS shows the upper flow remaining only marginally favorable, but the diffluence associated with such a flow might allow for enough convective sustenance to see Gabrielle regenerate despite the shear.
This system is expected to begin moving northeast ahead of a frontal zone over the western Atlantic.
Probability of development in 48 hours: 50%
Bay of Campeche development possible late in the week
The GFS and ECMWF show a tropical cyclone developing in the southwestern Bay of Campeche in about four or five days; the GFS is about a day faster with development. In either case, the upper flow pattern on the GFS is very favorable for the development and intensification of a tropical cyclone, with the possibility existing that the system could become a hurricane.
There is a window of opportunity for the system to move northward into Texas or Louisiana as a trough amplifies over the southern United States this weekend, but it will depend on the depth of the trough, the latitude of formation, and the vertical depth of the modeled tropical cyclone.
I assume this system comes from the tropical wave currently over the Yucatan Peninsula. This system is already generating deep convection in the western Caribbean, indicating some potency to the wave.
Regardless of development, heavy rains and possible flooding is expected over portions of eastern Mexico later this week.
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