Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 2:41 AM GMT on May 27, 2013
The persistent area of low pressure we have been tracking well to the southwest of Manzanillo, Mexico for the last several days has become better defined today. Last light visible satellite images of the day suggested a small but well-defined low-level circulation, which is in direct contrast to the state of affairs just 24 hours ago. The system is accompanied by an area of deep convection about 50 miles west of the exposed low-level center due to strong easterly shear associated with a mid-tropospheric ridge over northern Mexico.
Figure 1. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).
There are mixed signals amongst the global model forecast fields as to how favorable the large-scale environment will be. There is some easterly shear over the system, and the GFS doesn't recognize it. The 12z CMC seemed to have a better depiction of the reality of the upper air pattern over the area, so that forecast was relied more heavily upon than that of the GFS. The former model shows the shear lessening in about 24 hours, which would allow for some slow development beyond that time. Despite this, the odds of 91E eventually becoming a tropical cyclone appear low, as the SHIPS/GFS/CMC forecast an abrupt increase in easterly shear in about 72 hours, presumably in response to a developing vortex to the east -- modeled tropical cyclone in the global model forecast fields. In addition, the 850 mb vorticity fields within the models show 91E gradually getting absorbed into the secondary low during that time while the 500 mb center -- the mid-level center -- meanders aimlessly in weak steering between a mid-level ridge to the north and broad-scale cyclonic flow to the east.
This low is expected to move slowly westward under the influence of a weak low-level ridge before moving eastward beyond day three as the secondary system begins to dominate the cyclonic flow in the region.
Probability of development in 48 hours: 20%
A second area of low pressure is located about 250 miles southeast of Salina Cruz, Mexico, is showing some signs of organization, with a small burst of cold-topped convection ongoing over the center.
Figure 2. Latest infrared satellite image of Invest 92E. Image credit: NOAA's Satellite Services Division (SSD).
Analysis of the model fields and water vapor imagery this evening shows a very favorable environment for development of this low; the aforementioned high over northern Mexico is gradually retograding eastward as weak upscale troughing develops over the four corners region. This is causing the flow over the eastern Gulf to back and become more diffluent, a pattern which would theoretically enhance the anticyclonic outflow in the eastern semicircle; this is supported by the GFS as well, which shows the subtropical jet in a very favorable position to furnish a poleward outflow channel with this disturbance. Interestingly, the CMC shows an environment that is only marginally conducive for development, and accordingly barely acknowledges the existence of a future tropical cyclone in the 12z run. Since this model does not appear to have a good handle on the upper pattern in this area of the basin, its solution has been discounted. Underlying sea surface temperatures are very warm, and the system will be passing over progressively higher heat content over the next couple of days.
With low to nonexistent shear, a very moist atmospheric column, a well-established outflow pattern, and warm sea surface temperatures, there appears to be little reason why this low will not gradually strengthen. The only inhibitor I can detect right now is the fact that the low is still embedded in the monsoon trough. Since the system currently lacks much in the way of a reflection in the surface wind field (as denoted by satellite, microwave, and scatterometer data), I do not anticipate rapid development of this system, despite the SHIPS giving a whopping 78% chance of an increase of 25 kt or more during the next 24 hours. Once the system detaches from the monsoon trough, however, then rapid development is a distinct possibility given the favorable environment and the small size of the system.
This system is expected to move west-northwest to northwest and make landfall along the coast of Mexico in about four or five days. Interests from Acapulco to Puerto Angel should monitor its progress carefully through midweek.
Probability of development in 48 hours: 50%
Western Caribbean development remains viable during the first week of June
While the operational runs of the global models have remained inconsistent, the GFS/CMC/ECMWF ensembles continue to indicate lowering pressures over the western Caribbean region during the first week of June. This would coincide with the passage of the upward MJO in this area of the world, a pattern which can be seen enhancing convection over the tropical Pacific (91 and 92E).
It is still too early to definitively state whether or not a tropical cyclone will develop from this pattern, but regardless, very wet weather appears to be on tap for this region over the next 7 - 10 days as the monsoon trough lifts northward.
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