Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 6:22 AM GMT on June 29, 2013
A small but well-defined area of low pressure located several hundred miles south of the southern coast of Mexico is showing obvious signs of organization, and could become a tropical depression on Saturday.
Convection has significantly weakened over the last 6-12 hours, but this is probably a diurnal fluctuation.
Figure 1. Latest infrared satellite image of Invest 96E. Image credit: NOAA's Satellite Services Division (SSD).
While there have been no recent scatterometer passes over the disturbance, an ASCAT pass around 17z suggested that the surface circulation was becoming better defined in the surface wind field. There have also been no reliable microwave passes over the last several hours either, and the ones we did get provided little coverage to the internal organization of the system, and were thus of little use. While it is difficult to determine precisely the existence of a possible closed surface low pressure center using only nighttime satellite imagery, low cloud vectors on the shortwave infrared satellite channel strongly suggest that the circulation has consolidated since the time of the ASCAT pass (which actually showed that the south side of the gyre was destitute of westerly winds), and based on the cloud lines and organization of the cloud pattern, I strongly believe that the circulation associated with this system is now closed off.
Environmental conditions are seemingly favorable for intensification, possibly significant; the SHIPS and all of the global models forecast an environment of weak upper tropospheric vertical shear over the budding disturbance throughout the forecast period, and they even suggest a large and highly amplified upper-level anticyclone will accompany the system during this time. The only immediate impediment to development I see is that the system is still embedded in the monsoon trough, but this will become irrelevant if the circulation is truly closed off, or in the process of doing so. Also, I see evidence of a small upper low/cyclonic shear axis on water vapor imagery about 5 degrees west and north of Invest 96E. The GFS correctly recognized the presence of this low, but suggests that it should move slowly westward away from the disturbance. Understanding that there actually is evidence of this low and its associated cyclonic vorticity moving westward on a gradual basis, I believe the model forecasts of low shear; however, it is possible that this low will impart some modest southerly shear to the disturbance during the next 24 hours given its orientation and position relative to the surface vortex associated with 96E. Any shear should not be enough to prohibit development, and a southerly shear vector is not all that detrimental for a northward-moving system. In about four days, the system is expected to cross the 26C isotherm and should begin to weaken at that time, with more rapid weakening anticipated at day five.
The large-scale steering environment surrounding this low consists of a broad trough associated with the aforementioned upper low. The global models suggest the consequent weakness in the ridge will stick around for about 48 hours, at which time the model forecast fields show a low- to mid-level ridge building to the north of the tropical cyclone over southern Mexico. This is expected to cause the system to gradually bend to the west-northwest and west with time. Based on the pattern, I anticipate a track similar to Hurricane Beatriz in 2011, although probably a little farther offshore; there is enough margin for error, however, that interests along the southern coast of Mexico should carefully monitor the progress of what will very likely be a small but decent hurricane by the time it reaches its closest approach to the coast.
Regardless of whether or not the system makes landfall, periodic episodes of heavy rain and gusty winds will lash portions of southern Mexico over the next several days. In addition, dangerous rip currents and high waves will accompany the system.
Probability of development in 48 hours: 90%
Elsewhere, a vigorous tropical wave is moving through the Windward Islands and southeastern Caribbean Sea. While this wave is very well-defined in the surface wind field and is rather unusual for June, upper-level winds are not expected to permit development; given the well-marked wave signature, however, I would watch for the possibility of development in the western Caribbean or the eastern Pacific in the next 7 days.
By the way, the presence of this wave further validates the numerous forecasts of a hyperactive season from the many forecasting agencies.
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