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By: TropicalAnalystwx13 , 3:03 AM GMT on October 21, 2012
Note: The Tropical Cyclone Report (TCR) below contains comprehensive, yet easily understandable, information on each tropical cyclone, including synoptic history, casualities and damages, provided by a multitude of different, official resources, and the post-season analysis best track (six-hourly position fixes and intensities). A tropical cyclone is defined as a warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center. These include depressions—cyclones that did not attain 34-knot sustained winds—storms, and hurricanes. It should be noted that, while I strive to produce the most accurate information for the particular cyclone listed below, these reports...including the storms' position and intensity...are not official and are no way associated with the National Hurricane Center or any other branch of the National Oceanic and Atmospheric Administration. Please visit the Atlantic TCR page and the East Pacific TCR page for official reports on any desired cyclone within a particular season.
Tropical Cyclone Report
Tropical Storm Aletta (EP012012)
Duration: 14 May – 19 May 2012
Produced on: 20 October 2012
Tropical Storm Aletta was an early-forming tropical storm that remained in the open Pacific during May 2012. It is only the third tropical storm in recorded history to form before the official date of the Pacific hurricane season, which begins on May 15 and ends on November 30 every year.
a. Synoptic history
Aletta's origins are from a broad area of low pressure that developed within the Intertropical Convergence Zone (ITCZ), roughly 545 miles south-southwest of Acapulco, Mexico, on May 11. Over the next 24 hours, the low and associated area of shower and thunderstorm activity moved very slowly to the west in response to a potent ridge of high pressure centered over northern Mexico. With a favorable environment, characterized by wind shear of 5 to 10 knots, the disturbance slowly organized. By the afternoon hours of May 13, an Advanced Scatterometer (ASCAT) pass revealed a closed circulation in association with the system, but convection was not yet organized enough to be declared a tropical cyclone. However, a burst of convection the following morning led to the formation of a tropical depression at 0600z 14 May centered about 575 miles south of Manzanillo, Mexico. The best track positions and intensities are listed in Table 1 at the bottom of this entry (to be added).
Continuing to the south of the mid-level ridge over northern Mexico, the depression continued to organize atop sea surface temperatures of nearly 29°C and within a region of low vertical wind shear. Satellite intensity estimates from UW-CIMSS ADT, SAB, and TAFB all indicate that Tropical Depression One-E intensified into a tropical storm by 1800z 14 May, earning the name Aletta. Intensification continued, and Aletta attained its peak intensity with maximum sustained winds of 45 knots 18 hours after becoming a tropical storm. The following day, an upper-level trough passing to the north of the cyclone imparted strong southwesterly wind shear on the cyclone, subsequently leading to steady weakening. At 0600z 17 May Aletta weakened to a tropical depression and turned towards the north as the aforementioned trough eroded the western periphery of the ridge of high pressure. Despite periodic bursts of deep convection the following day, the cyclone no longer met the criteria to be considered a tropical cyclone, (persistent, deep convection) and it is estimated that Aletta degenerated into a non-convective remnant area of low pressure at 0000z 19 May about 530 miles south-southwest of the southern tip of Baja California. Now embedded within the quick east to west flow across the Pacific basin, the remnant circulation of Aletta turned westward and dissipated early on 20 May.
b. Meteorological statistics
Observations used to determine the peak intensity of Tropical Storm Aletta include many Advanced Scatterometer (ASCAT) passes, satellite intensity estimates from the University of Wisconsin's-Cooperative Institute for Meteorological Satellite Studies Advanced Dvorak Technique (UW-CIMSS), Tropical Analysis and Forecasting Branch (TAFB), and the Satellite Analysis Branch (SAB). Data from the Advanced Microwave Sounding Unit (AMSU) was also useful in constructing the best track of Tropical Storm Aletta.
Data from a 1642 UTC 16 May ASCAT pass depicted several 40-knot wind barbs in association with Aletta. Given the low-bias of the instrument, in conjunction with a satellite intensity estimates of T3.0/45 kts and T3.3/51 kts from SAB and UW-CIMSS, respectively, it is estimated that Aletta reached its peak intensity of 45 knots at 1800z 15 May.
The development of Aletta at 1800z 14 May marked only the third time in the satellite era (1966-current) that a tropical cyclone has formed in the East Pacific prior to the official start of the Pacific hurricane season at 0000z 15 May. Only Hurricane Alma of the 1990 Pacific hurricane season and an unnamed tropical storm during the 1996 Pacific hurricane season have accompanied Aletta in developing before 15 May.
Due to the short-lived nature of Aletta and its relatively compact wind-field, no reports of tropical storm-force winds were received by ships traversing the region.
c. Casualty and Damage statistics
There were no reports of damage nor fatalities in association with Tropical Storm Aletta as it remained far from any landmasses.
d. Forecast and Warning critique
Personal Tropical Weather Outlooks (TWOs) were not produced for any tropical cyclone until 1 August.
Figure 1. Satellite imagery of Tropical Storm Aletta at peak intensity.
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