Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 12:06 AM GMT on February 19, 2013
Tropical Cyclone Report
28 August-2 September 2012
Kirk was a small category two hurricane (on the Saffir-Simpson Hurricane Scale) that remained over the central Atlantic without affecting land.
a. Synoptic History
Kirk formed from a large tropical wave that left the west coast of Africa on 22 August. The wave passed south of the Cape Verde Islands the next day before moving slowly west-northwestward toward a weakness in the Atlantic subtropical ridge partially left behind by Tropical Storm Joyce. The wave was poorly-defined on satellite images, and did not generate much deep convection due to a stable airmass. An area of low pressure developed along the wave axis on 24 August as the wave was passing a few hundred miles to the west of the western Cape Verde Islands and the cloud pattern exhibited some small banding features. While deep convection was sporadic and not well organized due to the dry environment, upper tropospheric shear over the area was fairly light, which allowed the wave to maintain a vigorous lower-tropospheric circulation as it moved into higher latitudes. The circulation began to become better defined in the surface wind field around midday on 25 August, but dry air continued to inhibit development despite progressively warmer sea surface temperatures. Zonal flow over the north Atlantic enhanced the outflow over the northern semicircle. The wave slowly became better organized as it moved into an increasingly moist environment, and it became a tropical depression near 1800 UTC 28 August, while centered about 1000 miles southwest of the Azores. 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 in the final rendition) The best track positions and intensities are listed in Table 1 (to be added in the final rendition). Some moderate southwesterly shear affected the tropical cyclone beneath the outflow layer due to a weak upper trough over the central Atlantic. The shear appeared to lessen a bit thereafter, and the cyclone became a tropical storm, albeit one with minimal deep convection, near 0600 UTC 29 August. Kirk intensified only slightly over the next day or so as the cyclone moved northwestward between an amplifying mid-level trough to the west and a mid-level ridge to the east.
The cyclone intensified rather quickly beginning around 0600 UTC 30 August, and a ragged eye began to become apparent on geostationary satellite images a few hours later. Kirk reached hurricane status near 1200 UTC that day, while centered about 900 miles east-southeast of Bermuda. Kirk's rapid intensification continued, and the hurricane assumed its peak intensity of 90 kt at approximately 0600 UTC the next day; by that point, the small hurricane had already turned northward ahead of a large mid-latitude trough. The satellite signature of the hurricane deteriorated only a short time later, as Kirk entered an environment of increasing westerly shear. Kirk turned north-northeastward late that day as it became fully captured by the aforementioned trough. The small cyclone weakened as quickly as it strengthened, becoming a tropical storm near 0000 UTC 1 September. Kirk gradually turned more toward the northeast and weakened as it moved into increasingly hostile conditions. The cyclone's acceleration over the cold north Atlantic waters soon caused it to encounter a highly baroclinic environment as a large cold front approached the storm from the west. Due to interaction with this front, Kirk became extratropical near 1800 UTC that same day while it was racing northeastward over the north Atlantic at about 30 kt. At the time of extratropical transition, post-tropical Kirk was located about 750 miles northwest of Flores in the southwestern Azores.
b. Meteorological Statistics
Observations in Kirk (Figs 1 and 2, to be added in the final rendition) include the satellite-based Dvorak method from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). ADT satellite estimates from the University of Wisconsin CIMSS were also used. Kirk was also surveyed by various microwave data, along with scatterometer data, specifically the ASCAT. Most notably, the Advanced Microwave Sounding Unit (AMSU) was particularly useful in depicting the point at which Kirk had lost its tropical characteristics and become a post-tropical cyclone over the north Atlantic. This is estimated to have occurred around 1800 UTC 2 September, when the unit, along with satellite and water vapor imagery, indicated that Kirk had become fully involved with a frontal zone. At that time, ASCAT data and satellite animations suggested that the low-level circulation had become elongated in a southwest to northeast direction and well-separated from an isolated band of showers well to the north of the center.
The peak intensity of Kirk, estimated to be 90 kt, was reached on 0600 UTC 31 August. This was based on Dvorak estimates, as well as microwave data showing a well-defined eye surrounded by a ring of deep convection. It should be noted that some of the objective CIMSS-ADT estimates exceeded the major hurricane threshold during that time, but given the abrupt deterioration in the cloud pattern experienced thereafter, along with lower estimates elsewhere, Kirk's peak intensity is set at 90 kt as a compromise. It also interesting to note that the circulation of Kirk was extremely small, with scatterometer data indicating that 34-kt winds extenteded out no more than about 80 nmi from the center at the time Kirk was at its largest. Also, it is still a bit unclear as to how such a large tropical wave as the one that produced Kirk went on to generate such a small hurricane.
There were no ship or buoy reports of tropical storm or hurricane force winds in association with Kirk.
c. Casualty and Damage Statistics
There were no reports of casualities or damage association with Kirk.
d. Forecast and Warning Critique
Tropical Weather Outlooks (TWOs) were not generated for Kirk. Track and intensity forecasts were also unavailable. This was due to the author having been struck by Hurricane Isaac around the time genesis occurred, which caused widespread disruption to the regional electric grid and prevented the production of forecasts.
Infrared satellite image of Hurricane Kirk at peak intensity.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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