Tropical Cyclone Report: Hurricane Kirk (AL112012)

By: TropicalAnalystwx13 , 11:09 PM GMT on November 28, 2012

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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
Hurricane Kirk (AL112012)
Duration: 28 August – 2 September
By: TropicalAnalystwx13
Produced on: 28 November 2012

Kirk was a Category 2 (on the Saffir-Simpson Hurricane Wind Scale) over the eastern Atlantic that did not affect land as a tropical cyclone.

a. Synoptic history

A tropical wave, viewed with disorganized shower and thunderstorm activity and a defined area of cyclonic turning, exited the west coast of Africa and into the eastern Atlantic during the afternoon hours of 22 August. Initially, the wave slowly organized under a low wind shear, minimal dry air, and warm sea surface temperature environment, but a subsequent increase in vertical wind shear, due to a large upper-level low located roughly midway between the northern Leeward Islands and the Cape Verde Islands, led to the heavily sheared appearance on geostationary imagery by late on 23 August. As the wave rounded the southern periphery of an unusually strong ridge of high pressure in the northeastern Atlantic it continued to show a high level of low-level vorticity. A weakness in the ridge by 26 August allowed the wave to move more north-northwesterly into cooler waters and within a region of continued high wind shear. At the time, the prospects of tropical or subtropical development were lower than previously given, but a gradual decrease in wind shear on 28 August led to the formation of Tropical Depression Eleven at 1800z while centered about 1425 miles southwest of the Azores Islands. The best track positions and intensities are listed in Table 1 at the bottom of this entry (to be added).

Moderate southwesterly wind shear continued to impart on the depression shortly after formation due to the large upper-level low to its west over the Central Atlantic. The initial forecast was that this wind shear would prevent the depression from becoming anything more than a tropical storm, with absorption into a mid-latitude trough expected by five days out. However, a drop in wind shear, forecast only by the SHIPS and LGEM models, allowed for gradual intensification to occur. A tremendous burst of convection, with cloud tops cooler than -80 °C, occurred around 2100z, and satellite intensity estimates at 0000z the following day gave credence to the formation of Tropical Storm Kirk at that time. Wind shear initially kept the low-level center displaced slightly from the mid-level center, but by the pre-dawn hours of 30 August, a series of microwave passes indicated the development of an eyewall. At 1200z Kirk became a Category 1 hurricane on the Saffir-Simpson Hurricane Wind Scale, and by 1800z a well-defined eye was evident on geostationary imagery.

As cloud tops cooled in the eyewall and the inner-core temperature warmed, satellite intensity estimates increased enough to declare Kirk a Category 2 hurricane at 0000z on 31 August. By this time wind shear had finally decreased to conducive levels, as evidenced by upper-level low in the western quadrant of the system. Kirk reached its peak intensity of 95 knots, just below major hurricane intensity, at 0600z on 31 August before the storm began to enter cooler waters and a more stable air mass. The coldest cloud tops became displaced to the east of the low-level center and the inner-core structure dissipated; Kirk weakened back to a high-end Category 1 hurricane at 1200z before becoming a tropical storm once more at 1200z on 1 September. A small ball of convection fired the following morning as the tropical storm accelerated to the east-northeast across the open east-central Atlantic, but cooler sea surface temperatures and the influence of a frontal zone to the west of the cyclone cooled the core, transitioning Kirk into a post-tropical cyclone at 1800z on 2 September. The front imparting on the western side of the cyclone merged fully with the system over the course of the next few hours as Kirk continued over open seas, no threat to land.

b. Meteorological statistics

Observations used to determine the peak intensity of Hurricane Kirk include satellite intensity estimates from the University of Wisconsin's-Cooperative Institute for Meteorological Satellite Studies Advanced Dvorak Technique (UW-CIMSS), the Tropical Analysis and Forecasting Branch (TAFB), and the Satellite Analysis Branch (SAB). Data from the Advanced Microwave Sounding Unit (AMSU) and Advanced Scatterometer (ASCAT) passes were also useful in constructing the best track of Kirk.

The 95 knot peak intensity of Hurricane Florence on 31 August at 0600z is based on a satellite intensity estimate of T5.0/90-kts from SAB and TAFB, as well as T5.5/102 knots from UW-CIMSS, during the pre-dawn hours of that day. While it is possible Kirk may have briefly been a major hurricane there is not enough evidence for an upgrade.
There were no ship reports of winds of tropical storm force associated with Kirk.

c. Casualty and Damage statistics

There were no reports of damage or casualties associated with Kirk.

d. Forecasting and Warning Critique

The development of Hurricane Kirk was forecast well in advance due to extraordinary model support up to a week out. The precursor to the hurricane was first mentioned in the Tropical Weather Outlook late on 22 August, assessed with a low chance of tropical development, and was subsequently raised to a medium chance of tropical development by late on 24 August. These probabilities were once again raised to the high category by the early morning hours of 27 August as the disturbance developed a large area of symmetric, deep shower and thunderstorm activity. The outlook at 1200z on 28 August, six hours before genesis, stated that, “any further increase in organization could result in the formation of a tropical depression.”

Figure 1. Satellite imagery of Hurricane Kirk at peak intensity.

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2:22 AM GMT on December 01, 2012
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2:01 AM GMT on November 29, 2012
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About TropicalAnalystwx13

16-yr-old weather aficionado, with primary focus on tropical cyclones. High school and college student, working towards the National Hurricane Center.