Tropical Cyclone Report (TCR): Hurricane Gordon
Tropical Cyclone Report
15-20 August 2012
Gordon impacted the Azores as a rapidly-accelerating tropical storm. It attained category 2 status prior to that point. Gordon was the first tropical cyclone to directly impact the Azores since the 2006 incarnation of the same storm.
a. Synoptic History
A tropical wave and attendant surface low that moved off the coast of Africa early on 10 August appears to be the catalyst responsible for spawning Gordon. While the wave soon lost most of the associated shower activity due to relatively cool sea surface temperatures, satellite pictures suggested a well-marked cyclonic circulation accompanied the tropical wave at the mid-levels as the wave moved west-northwestward. The wave became quite disorganized over the next day or two, with the circulation and associated low-level wind field becoming more asymmetric; this could have been due to dry air entrainment, as the wave was embedded in a highly subsident airmass at that time. Convection began to reignite along and to the west of the wave axis near 0600 UTC 14 August when the system was centered about 1000 miles southwest of the Azores. Convection continued to increase and slowly organize into bands, while scatterometer and microwave data (not shown) suggested that the surface circulation was becoming better defined; during this time, the system began a turn to the northwest and then north as the subtropical ridge to the north weakened with the approach of an upper-level trough over the western Atlantic.
Based on the continued organization of the cloud pattern, the system is estimated to have transformed into a tropical depression around 1800 UTC 15 August while located around 600 miles east of Bermuda. 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 provided in the final rendition). The best track positions and intensities are listed in Table 1 (to be provided in the final rendition). The cyclone became a tropical storm near 1200 UTC 16 August as deep convection increased around the center in cyclonic bands. Gordon turned to the northeast after becoming a tropical storm as it became enraptured in deep-layer southwesterly flow associated with the aforementioned trough, which had effectively moved into the central Atlantic by that time. Gordon was slow to intensify, with the cloud pattern becoming unimpressive at times. Nonetheless, a large and ragged eye began to become evident in satellite pictures beginning around 0600 UTC 18 August as Gordon was accelerating eastward in mid-latitude westerly flow on the north side of the Atlantic subtropical high, and it is during this time that the storm is believed to have reached hurricane status. At that time, Gordon was located approximately 450 miles southwest of Flores in the northwestern Azores.
The hurricane continued to strengthen, attaining category 2 strength just six hours later while continuing to move at a quick pace. Based on satellite pictures, Gordon is estimated to have reached its peak strength of 95 kt near 1900 UTC 18 August while centered about 300 miles southwest of Flores. Gordon turned to the east-northeast and gradually weakened due to increasing southwesterly shear and cooler waters. Gordon made landfall at approximately 0600 UTC 20 August on Santa Maria Island in the southeastern Azores as a 60-kt tropical storm. The tropical cyclone consisted of an asymmetric cloud pattern at landfall, with virtually all of the associated convection in a band about 75 miles north of the center. The satellite appearance of Gordon continued to degrade, and AMSU microwave data (shown below in Fig 5) indicated that the horizontal temperature gradient between Gordon and the surrounding environment was becoming less contrasting, suggesting that Gordon lost tropical characteristics near 1800 UTC 20 August while accelerating rapidly northeastward away from the Azores.
b. Meteorological Statistics
Observations in Gordon (Figs 2 and 3, to be provided in the final rendition)) include the Dvorak satellite intensity technique, which was used throughout the entirety of the cyclone's life cycle since areal reconnaissance coverage was unavailable. Various microwave data were also used, most prominently the SSMI/S and AMSUB units. The Advanced Microwave Sounding Unit (AMSU) was particularly useful in determining when Gordon had lost its warm core and become a non-tropical system over the north Atlantic. Scatterometer data was also useful in determining the strength and position of Gordon, and for determining the existence of hurricane-force winds within the cyclone circulation as it approached the Azores.
Gordon's estimated peak intensity of 95 kt near 1900 UTC 18 August was based on a blend of Dvorak satellite estimates from TAFB and SAB. While no microwave data could be obtained from when Gordon assumed peak intensity, microwave images before and after the tabulated peak intensity suggest that Gordon was a well-organized hurricane, with a 25-mile wide eye surrounded by deep convection, particularly for a high latitude system. It is interesting to note that Gordon's intensity was harder to estimate than usual, especially early in the cyclone's life before a well-defined eye had developed; this was because there was a rather strong dichotomy between geostationary satellite images and available microwave data.
The highest 1-minute sustained wind speed associated with Gordon was 42 kt with a gust to 45 kt on Santa Maria Island. However, data within this island, and the archipelago as a whole, is scarce and relatively spatial, suggesting that higher winds likely occurred between observational time periods, especially in areas of higher elevation.
c. Casualty and Damage Statistics
No fatalities were reported with Gordon, and no significant damage occurred. Some minor flooding and power outages were reported on Sao Miguel Island, while strong winds blew down doors and windows. Overall damages were minimal, and were confined to the far eastern portion of the island chain.
d. Forecast and Warning Critique
The genesis of Gordon was perhaps over-anticipated. As the initial tropical wave moved off the coast of Africa early on 10 August, the system was immediately assigned a medium (30 to 50%) chance of developing into a tropical cyclone within 48 hours. The wave lost organization later that day, at which time development probabilities were downgraded to a low (under 30%) chance. Around 0200 UTC 15 August, forecasts reached the "high" (greater than 50%) category with explicit mention of a tropical depression. This was about 16 hours before the time of genesis as denoted in post-storm analysis.
Watches and warnings associated with Gordon are given in Table 6 (to be added in the final rendition). A tropical storm watch was issued for the central and eastern Azores at 2100 UTC 17 August, roughly two days before landfall. The watch was replaced with a warning at 0900 UTC 18 August, shortly after Gordon became a hurricane. A hurricane warning was issued for the same area at 1500 UTC that day.
Figure 4. Visible satellite image of Hurricane Gordon at peak intensity on 1900 UTC 18 August. At this time, Gordon was a category 2 hurricane with 95-kt winds.
Figure 5. Homogeneous comparison of two separate AMSU microwave images of Gordon at 1413Z and 1737Z 20 August, respectively. Notice that the former still denotes a (weakening) warm core, with a temperature difference of 3.5 to 4.0C relative to the surrounding environment. In the latter, the horizontal temperature gradient is somewhat less, which when used with satellite imagery, suggests that Gordon lost tropical characteristics around that time.