Tropical weather analysis - May 17, 2012
Aletta weakened to a tropical depression in the wee hours of Thursday morning. As of the latest NHC advisory, here is the information on this system:
Wind: 35 mph, with higher gusts
Location: 13.6°N 114.3°W
Movement: NNE at 6 mph
Pressure: 1006 mb
Category: Tropical depression (Saffir-Simpson Hurricane Scale)
A bursting-type pattern of convection continues, but remains west of the low-level center. Although this convection when it erupts is fairly cold, tropical cyclones, as a general rule, do not strengthen amidst bursting patterns. It is a bit of a mystery to me why the convection is confined west of the center as opposed to east. One would think that in an environment characterized by southwesterly shear, an inverse relationship would apply. Careful analysis of water vapor imagery, corroborated by CIMSS mid- to upper-level wind data, shows no evidence of easterly shear beneath the outflow layer. Satellite imagery does reveal arc clouds emanating from the eastern quadrant of the depression, which is usually a sign of dry air, so maybe that's it. But in the end I suppose, it's best not to read too deeply into a triviality that will ultimately not alter Aletta's fate.
Figure 1. Latest infrared satellite image of Tropical Depression Aletta, courtesy of NOAA's Satellite Services Division (SSD).
A small weakness prevails north of the storm toward the Mexican coast in association with a weak frontal trough. Although this trough was apparently strong enough to recurve Aletta, its ultimate demise will be met not at the hands of cold water and wind shear, but at eventual absorption into a larger tropical disturbance to the east; large scale imagery suggests that this disturbance is beginning to dominate the low-level flow in the vicinity of Aletta, and this evolution should absorb the storm by Monday. The global models are in excellent agreement with this scenario.
Aletta should weaken in the interim, and degenerate into a remnant low in about 24 hours. An SSMI overpass near 0z was kind enough to catch the important half of the circulation -- i.e. the center -- and a comparative analysis of the mid- and upper-level circulations suggests a vertical decoupling, courtesy of the southwesterly shear impacting the tropical cyclone, which finally materialized as the GFS said it would. To add insult to injury, a large expanse of dry air lies to the west of the storm, associated with the stable stratocumulus cloud deck intrinsic to this portion of the Pacific west of 115W.
The models are discrepant on exactly where the storm will move in such a weak steering environment, but broadly agree on an eventual turn to the southeast or east-southeast as it becomes embedded within the environmental westerly flow in the lower troposphere being created by Invest 92E. I have done what seems appropriate, and have taken the GFS forecast, which seems supported in light of current synoptic trends. However, I'm sure Aletta will move in fits and wobbles until absorption.
5-day intensity forecast
Initial 05/17 0300Z 30 KT 35 MPH
12 hour 05/17 1200Z 25 KT 30 MPH
24 hour 05/18 0000Z 25 KT 30 MPH...post-tropical/remnant low
36 hour 05/19 1200Z 20 KT 25 MPH...post-tropical/remnant low
48 hour 05/20 0000Z 20 KT 25 MPH...post-tropical/remnant low
72 hour 05/21 0000Z 20 KT 25 MPH...post-tropical/remnant low
96 hour 05/22 0000Z...absorbed by developing tropical cyclone
Shower activity in association with a broad area of low pressure centered several hundred miles south of Acapulco has become a little better organized, with a recent burst of very cold cloud tops exceeding -80C in some locales. The center is difficult to locate using nighttime imagery, but earlier microwave data and careful analysis of recent satellite visuals place it just inside the eastern edge of the convective canopy.
Figure 2. Latest infrared satellite image of Invest 92E, courtesy of NOAA's Satellite Services Division (SSD).
Upper-level wind analysis from University of Wisconsin CIMSS says 92E is experiencing 20 kt of easterly shear, and current satellite trends authenticate that. However, upper-level winds are forecast to become more favorable in the path of the system, especially beyond 24 hours. Pretty much all the global models turn this into a hurricane, and it appears pretty likely that this low will become the next named storm. If so, it would be named 'Bud'.
This disturbance is expected to remain virtually stationary until Sunday, when a trough over the central United States if forecast to be replaced by a ridge. This should steer the system on a gentle westward track until the next trough arrives on Tuesday. The amplitude of this trough is fairly impressive in the global model forecast fields, and if true, would be sufficient to recurve it toward the Mexican coast by midweek. The models have come into better agreement on the evolution of the synoptic pattern in today's cycles, which increases confidence in an eventual threat to the Mexican coast, especially from Puerto Vallarta eastward to Manzanillo. It is not uncommon for this portion of the coast to receive a direct hit from a tropical cyclone, and interests across the entire coastline, as a course of least regret, should closely follow the progress of this disturbance.
Probability of genesis within the next 48 hours: 60%
Atlantic (sub?)tropical development still possible
The global models are continuing to sing their tune of enthusiasm in regards to developing a subtropical or tropical entity -- or cyclone -- off the southeast United States coast in about three days. Given the harsh environmental westerlies in this area, any development here would probably be subtropical. However, the GFS indicates that the system could find a small hole of lower shear off the coast of the Carolinas, which would allow it to develop tropical characteristics. Preexisting cloudiness is already there, so we will need to watch this area carefully over the next several days.
Some of the models now suggest the system could loop back around to the west as the trough fails to shoot it into the westerlies.