Tropical Cyclone Report (TCR); Tropical Storm Aletta
Tropical Cyclone Report
Tropical Storm Aletta
14 May-19 May 2012
Aletta was a rare out of season tropical storm that formed about a day before the official start of the eastern North Pacific hurricane season. Since reliable satellite measurements were taken in 1966, there have only been two other documented tropical storms that formed before the official start of the season.
a. Synoptic History
As is typical for most early season tropical cyclones in the eastern North Pacific, the exact system from which Aletta originated is difficult to determine. Satellite, scatterometer, and lower- to middle- tropospheric vorticity analysis from University of Wisconsin CIMSS suggest that a tropical wave may have crossed the coast of west Africa on 1 May. This wave was very poorly-defined on satellite pictures and did not generate deep convection across its journey through the Atlantic. There were some indications in surface observations from Trinidad and Tobago of a poorly-defined cyclonic wind shift on 8 May that could have been related to this wave; however, the available data in the area is inconclusive since no marked pressure drop occurred accompanying the passage of the purported wave. The Intertropical Convergence Zone (ITCZ) was very active over the eastern North Pacific during the second week of May, and three separate disturbances developed within the gyre. The central disturbance, which was located about 1000 miles southeast of the southern tip of Baja California, became the dominant system and began to acquire central banding features on 11 May. The first Dvorak classifications were initiated at this time. Meanwhile, the tropical wave entered the basin early the next day, and may have contributed to the formation of a broad surface low within the central gyre later that day. The disturbance moved slowly to the west-northwest, then turned west as a weak mid-level trough over northern Mexico moved off to the east. Convection began increasing around the center beginning near 0600 UTC 14 May, and the low became a tropical depression near 1800 UTC that day while located about 500 miles south-southwest of Acapulco, Mexico. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added). The best track positions and intensities are listed in Table 1 (to be added). The cyclone became a tropical storm about 6 h later.
Aletta strengthened slightly to reach a peak intensity of 40 kt near 1200 UTC 15 May, when the center appeared to be embedded in a small area of deep convection that was surrounded by a well-defined curved band to the east. Easterly shear increased as Aletta began to execute a sharp clockwise loop, and it weakened to a tropical depression around 0000 UTC 17 May. Aletta generated several bursts of convection over the next couple days, at times garnering satellite classifications that were of tropical storm strength. The bursts were transient enough that the cyclone was unable to regain that strength. Overcome by southerly to southwesterly shear, Aletta's circulation continued to wind down, and it degenerated into a non-convective remnant low pressure area around 0000 UTC 19 May, centered about 650 miles south of the southern tip of Baja California. Aletta's remnants continued southeastward into an area of very strong southeasterly upper-level winds and quickly lost their identity by 20 May.
b. Meteorological Statistics
Observations in Aletta (Figs 2 and 3, to be added) include the satellite-based Dvorak intensity technique from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the University of Wisconsin CIMSS' Advanced Dvorak Technique (ADT). Various microwave units, along with scatterometer data, were also useful in tracking the center of Aletta.
The peak intensity of Aletta is estimated to be 40 kt and occurred at 1200 UTC 15 May. This was based on Dvorak estimates which supported an intensity from 35 to 45 kt during that time.
c. Casualty and Damage Statistics
No fatalities or damages were reported with Aletta.
d. Forecast and Warning Critique
The formation of Aletta was well anticipated in off-season Special Tropical Weather Outlooks (STWOs). The precursor disturbance was mentioned on 0000 UTC 12 May and given a "low" chance of development. About 30 hours later, genesis probabilities reached the "high" category, with explicit mention of tropical cyclone formation during that time.
Average track and intensity forecasts associated with Aletta were generally small, with intensification into a low to moderate tropical storm expected, which is concurrent with what occurred. Aletta's small clockwise loop prior to dissipation was also well predicted, largely due to the uniformity of the global models in recognizing it.
Infrared satellite image of Aletta at peak intensity.