Tropical weather analysis - May 18, 2012
Aletta
Aletta is finally showing signs of succumbing to the shear. The storm is poorly-organized on satellite imagery, with the low-level center completely exposed well to the west of a small area of convection. As of the latest NHC advisory, the following information was posted:
Wind: 30 mph, with higher gusts
Location: 14.5°N 113.2°W
Movement: NE at 3 mph
Pressure: 1006 mb
Category: Tropical depression (Saffir-Simpson Hurricane Scale)
There is no doubt that Aletta looks worse than 24 hours ago, and satellite estimates have plummeted accordingly. The cyclone is likely to decay to a remnant low very soon, probably in about six hours. It is possible that the cyclone is already a remnant low, and I will verify this in post-season analysis.
Figure 1. Latest infrared satellite image of Tropical Depression Aletta, courtesy of NOAA's Satellite Services Division (SSD).
Large scale imagery and model guidance indicates that the current shearing environment will persist. As Aletta turns southward, the upper flow will veer southeasterly, perhaps more detrimental than the ongoing southwesterly regime, since such a path would move the cyclone directly into the shear. Models are unanimous in absorbing Aletta into a larger cyclonic circulation to the east.
Satellite imagery suggests that Aletta is turning eastward. A sharp southward bend is expected in the track tomorrow, as the cyclone/remnant low slowly becomes entrained into the circulation associated with Invest 92E.
5-day intensity forecast
Initial 05/18 2100Z 30 KT 35 MPH
12 hour 05/19 0600Z 25 KT 30 MPH...post-tropical/remnant low
24 hour 05/20 1800Z 25 KT 30 MPH...post-tropical/remnant low
36 hour 05/21 0600Z 20 KT 25 MPH...post-tropical/remnant low
48 hour 05/22 1800Z 20 KT 25 MPH...post-tropical/remnant low
72 hour 05/23...absorbed by developing tropical cyclone
Invest 92E
In an interesting turn of events, Invest 92E has become considerably less organized today. The system consists of an elongated and shapeless band of convection extending westward for over 100 miles. I'm not going to pretend the center is easy to locate. Presumably, it is under the southern edge of the convection near 11.2N 100.3W, based on low-cloud motions gleaned from earlier visible imagery.
Figure 2. Latest infrared satellite image of Invest 92E, courtesy of NOAA's Satellite Services Division (SSD).
Courtesy of a large ridge over the western Caribbean, 92E is under about 20 kt of southeasterly shear, and this is strongly supported by satellite trends. The system has an impressive equatorial outflow channel due to this shear, and thus it may not die completely. The shear is forecast to abate in about 24 hours, although the environment will still not quite be anticyclonic. Most of the models respond by making this system a hurricane in about five days. I usually don't agree well with model intensity forecasts, since we know so very little about tropical cyclone intensification procedures, but in this case they could turn out to be correct.
Water vapor imagery shows a strong upper-level trough approaching the central United States. The upper low supporting this front is currently moving across western Utah. With a predominant northwesterly flow in place, said low should move southeastward for at least a day, dragging the cold front with it. Given the amplitude of this front, weak southwesterly flow is likely to linger as far south as the Gulf of Tehauntepec. This should prevent the storm from making too much westward progress. However, this trough is arriving sooner than the models were predicting yesterday, which may throw a wrench into my track predictions by allowing the storm to move more westerly by late Sunday into Monday as the high pressure area to the north of the low briefly rebuilds in the wake of the trough.
By Monday, another trough is forecast to enter the western United States. The models are unanimous that this trough will have a rather large amplitude, though they disagree on the exact placement of it. The GFS and ECMWF are farther north and slower, with the system still offshore next Friday, while the other models are a bit farther south and somewhat quicker. The 18z GFS looks a little suspect, even with the more northward placement of the trough, as a then hurricane is still well offshore. This is probably because that model initializes and develops a spurious vortex to the east of Invest 92E. Westerly flow associated with that feature (then tropical cyclone) attempts to grab the storm and scoot it east and ends up losing out -- but not without slowing the system's progress. Smoothing out the bogus vortex lends to a similar timing and position to the ECMWF.
It appears that 92E will be with us for awhile. One thing I'm confident of is that this system poses a long-range threat to the southwest coast of Mexico, and interests there should monitor it. I suppose it's worth noting that the NOGAPS shows a much more gentle recurvature, toward southern Baja. While none of the other models agree with this, depending on the evolution of the western US trough, it's a faint possibility.
Most of the models aren't developing this system until around Monday evening.
Probability of genesis within the next 48 hours: 20%
Atlantic subtropical development less likely
While the models are still hinting that the large baroclinic zone off the southeast United States coast could lead to the development of a weak surface low, they are less enthusiastic today than in previous runs. This area still needs to be watched, but I am not impressed. Perhaps some increased instability will overspread this region, however.
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