Tropical weather analysis - May 16, 2012
Tropical Storm Aletta is holding its intensity over the open Pacific. As of the latest NHC advisory, the following was posted on the storm:
Wind: 40 mph, with higher gusts
Location: 11.5°N 114.3°W
Movement: W at 9 mph
Pressure: 1004 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
Some cold-topped convection has formed just north of the low-level center, and Aletta hangs on as a tropical storm for a little longer. One possible reason that Aletta has not weakened as quickly as forecast could be due to a lack of southwesterly shear, despite being consistently forecast by the GFS model since the cyclone's inception.
Figure 1. Latest infrared satellite image of Tropical Storm Aletta, courtesy of NOAA's Satellite Services Division (SSD).
Nonetheless, the cloud pattern is gradually deteriorating, brief convective upswings like the ongoing one notwithstanding. Water vapor imagery near the storm shows that it is enveloped by a large area of dry air on the subsident side of a mid-level ridge.
Water vapor imagery indicates that Aletta is nearing the western periphery of the ridge, and should soon turn poleward. Ordinarily, I would predict weakening systems like Aletta to move westward instead, but CIMSS analysis shows that this weakness extends as far down as 850 mb. However, it will probably still be another 12 hours before Aletta makes the turn, and about another 24 before she begins to move northeast. By Saturday, the cyclone is expected to turn eastward, then east-southeast, trapped amidst large-scale westerly flow associated with Invest 91E, which is forecast by the global models to become a tropical cyclone. This seems supported by current organization trends.
The GFS wants to keep the shear over Aletta a little bit less over the next 24-48 hours, and this is certainly supported by water vapor imagery. Since there are still no real signs of shear yet, I feel Aletta will weaken a bit slower than the global models are indicating, at least for the first 24 hours. Thereafter, the cyclone should encounter a sharp sea surface temperature gradient west of 115W, and begin to weaken more rapidly. This process will be further hastened by the very dry airmass lying west of the 26C isotherm. Given that Aletta will only be skirting said gradient, rapid dissipation is not expected, especially since weak storms like this one tend to survive better in cooler waters than stronger ones. Instead, the cyclone will likely hang on as a lingering tropical depression while gradually being entrained into the westerly monsoonal flow associated with 91E.
5-day intensity forecast
Initial 05/17 0300Z 35 KT 40 MPH
12 hour 05/17 1200Z 35 KT 40 MPH
24 hour 05/18 0000Z 30 KT 35 MPH
36 hour 05/18 1200Z 30 KT 35 MPH
48 hour 05/19 0000Z 25 KT 30 MPH
72 hour 05/20 0000Z 25 KT 30 MPH...post-tropical/remnant low
96 hour 05/21 0000Z 25 KT 30 MPH...post-tropical/remnant low
120 hour 05/22 0000Z...absorbed by developing tropical cyclone
A large tropical disturbance is located about 500 miles south of Acapulco. A small area of convection lies just north of the center, but the overall cloud pattern is poorly-organized. A poleward outflow channel has developed to the north, possibly in response to a mid- to upper-level low over central Louisiana, and its associated jet maximum over the Gulf of Mexico.
Figure 2. Latest infrared satellite image of Invest 91E, courtesy of NOAA's Satellite Services Division (SSD).
Upper-level winds appear favorable for additional development, but I must emphasize that the large size of this disturbance may limit the potential for rapid intensification in the short-term. Long term, as the system picks up speed and establishes a solid inner core, a greater degree of intensification is possible. This system has the potential to become the Eastern Pacific's first hurricane.
The future course of this disturbance is still up for speculation. The models agree on a trough moving across the central United States on Saturday. This trough is then forecast to deamplify and be replaced by a more zonal regime. This could deflect the storm to the west. Another, more potent trough, is forecast to enter the western United States on Monday. It is this secondary trough that the models suggest could recurve the system toward mainland Mexico. Recurvature depends entirely on how far west the system gets before the arrival of the second trough, a scenario impossible to verify. The GFS and ECMWF, both typically reliable, show a rather weak ridge in the wake of the first trough, as southwesterly flow lingers over the region. An upper low moving into western Colorado could help drag the trough currently over Wyoming southward, assisting in keeping a prevailing weakness over the central United States.
Given this, and the usual reliability of the aforementioned group, there is a decent chance of 91E eventually impacting some portion of the Mexican coast. However, since the other models, reliable in their own right, insist that 91E will veer away from land, I can't simply ignore that, either. As a course of least regret, I will forecast a slow northwestward motion for the next three days, followed by west-northwest until the next trough builds back in, at which point the system is expected to turn back to the northwest. Needless to say, this is a low-confidence forecast.
Interests along the entire coast of Mexico should follow the progress of this disturbance.
Probability of genesis within the next 48 hours: 40%
Atlantic development is looking less likely, especially in the Caribbean. However, there is still weak support for a brief subtropical storm to form off the southeast United States coast. Such a system would move northeastward away from the mainland, embedded within the prevailing mid-latitude westerly flow.