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Zeta was the 27th and final named storm in the Atlantic during 2005, establishing the
record for the most named storms in one year in that basin. It was the second-latest tropical
storm to form in the Atlantic basin, only six hours earlier than Hurricane Alice (1954) that also
became a tropical storm on 30 December. Zeta and Alice are the only two Atlantic tropical
cyclones on record to cross from one calendar year to the next. Zeta spent about one week
moving erratically in a general westward direction over the subtropical waters of the central
Atlantic, and it did not affect land.
a. Synoptic History
The origins of Tropical Storm Zeta can be traced back to the interaction between a
weakening frontal boundary and an upper-tropospheric trough. By 28 December, the upper-level
trough had cut off and evolved into an upper-level low centered about 650 n mi west-northwest
of the Cape Verde Islands. A surface trough, the remains of a weakening front, was oriented
from southwest to northeast and lay beneath the upper-level low. On 29 December, just
northwest of the center of the upper-level low in an area of upper-level difluence, a closed low
formed along the surface trough about 675 n mi northwest of the Cape Verde Islands. Late on
the 29th, thunderstorm activity increased near the center of this low, and it is estimated that the
system gained sufficient organization to be designated a tropical depression at 0000 UTC 30
December. 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. The best track positions and
intensities are listed in Table 1.
Convective banding quickly developed and wrapped around the low-level center early on
30 December, and Zeta became a 40-kt tropical storm by 0600 UTC. Initially, Zeta moved
slowly northwestward around a mid-level low to its southwest. Situated beneath weak
anticyclonic flow aloft, the tropical storm strengthened to 45 kt by 1200 UTC that day. A weak
low- to mid-level ridge to its north forced Zeta to turn westward on 31 December, but upperlevel
westerlies slowed its forward motion to a 2-kt crawl. Although Zeta reached an intensity of
50 kt early that day, westerly shear later stripped Zeta of nearly all its deep convection in the
hours just before the new year. Convection soon rebounded, however, and although westerly
shear persisted, Zeta was located beneath an upper-level diffluent region with stronger shear to
the north and south. Zeta also was probably too shallow of a system to be substantially
weakened by shear associated with strong winds in the upper troposphere. These factors allowed
Zeta to resume a slow strengthening trend as it turned toward the southwest on 1 January, still