Tropical weather analysis - May 11, 2012

By: KoritheMan , 04:19 AM GMT am 12. Mai 2012

Invest 90E

The Eastern Pacific hurricane season is right around the corner, but apparently, it wants to show signs of life early. A tropical disturbance has developed about 500 miles southwest of Acapulco, Mexico. The center of rotation of this disturbance is officially estimated at 8.9°N 104.7°W as of 21z. My fix is a bit farther north, closer to 10N 105W.

Figure 1. Latest infrared satellite image of Invest 90E, courtesy of RAMMB imagery from Colorado State University (CSU).

Whatever the case, this system definitely has that developing look to it. The biggest hurdle appears to be its slow movement, with little overall motion shown on satellite pictures over the last few hours. The result will be a binary interaction with the Intertropical Convergence Zone, which lies just to the south of the budding disturbance. This may try to interfere with development by limiting lower-level inflow. Although strong upper-level shear lies just to the west, as per CIMSS, this shear is clearly not reaching the system at this time. Although cloud tops are cold, there are no signs of organization yet.

Although strong upper-level winds lie in close proximity, water vapor imagery and objective steering analyses suggest that the environmental flow surrounding the system remains weak. Indeed, the synoptic pattern over this section of the Pacific currently resembles an Omega type blocking pattern, with 90E caught amidst that pattern. Since the system is pretty much quasi-stationary at this time, it remains effectively shielded from the dangerous southwesterlies which lie ahead. A very slow northwestward motion can be expected, in line with the models. Some slight acceleration will be possible over the ensuing 24-48 hours.

Water vapor imagery depicts a well-defined cold low centered far to the west of the system, near 30N 138W. Large scale models quickly evacuate this feature over the next 12-24 hours, leaving the weak cold front that currently trails the low to dissipate. The result will be an increase in ridging, which should turn the system westward with time. The models are split, so I have taken the middle road in calling for a slow west-northwest movement by Sunday with a gradual acceleration as the easterlies to the north of the system rebuild. This track is in good agreement with the 18z run of the BAMM. Eventually, the system should encounter cooler waters and dissipate. I do not currently anticipate a hurricane, but I have been wrong before, and the Eastern Pacific has been shown to produce strong hurricanes during this part of the year.

Should the system track north of where I think it will, its path would take it closer to strong upper-level westerlies, as well as cooler sea surface temperatures.

Probability of development into a tropical cyclone during the next 48 hours: 10%


Elsewhere in the tropics, the global models, including the typically reliable GFS and ECMWF, forecast the development of a secondary tropical cyclone to the east of Invest 90E in about a week. Satellite imagery does show the beginnings of a manifestation of disorganized cloudiness in that area. This may be reinforced by a large, convectionless swirl moving across Panama and the extreme southwestern Caribbean. There is not enough evidence to say if this is a tropical wave, but given the time of year it's not impossible. In addition, the ITCZ is rather active in this area of the Pacific, and with the upward MJO forecast to move eastward, the potential certainly exists for two tropical cyclones.

Also, I'd like to point out that the models, mainly the GFS, have been pretty consistent in the long-range in developing a low pressure area -- and possible tropical cyclone -- somewhere in the western Caribbean in about 10 days. This is undoubtedly due to a large flux of instability associated with the upward pulse of the MJO. If any development were to occur this early in the year, climatology certainly favors that area. However, there are simply too many uncertainties at this time to justify anything beyond a mention. It's also worth noting that with El Nino coming on, the Pacific is warmer than normal, which would tend to keep the MJO around for longer. This could be why the models are developing Bud behind possible Aletta. Given the depiction of the former system within the models, any tropical cyclone would have a tough time getting going.

Regardless, it appears that a multi-day period of heavy rain is possible across south Florida in about 7 - 10 days, as a large conglomeration of moisture overspreads the western Caribbean. In addition, the western Atlantic ridge is forecast to erode with the passage of a weak cold front late next week, and the southerly flow associated with this feature could generate some shower activity over the state.

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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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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