1978 January subtropical storm

Subtropical Storm One in the 1978 Atlantic hurricane season is the only Atlantic subtropical cyclone on record to form during the month of January. The formation of Subtropical Storm One marked the second-earliest start to an Atlantic hurricane season, after a hurricane in 1938 which formed on January 3. It formed on January 18 well to the east-northeast of the Lesser Antilles, and throughout its duration it maintained a general westward track. The storm attained peak winds of about 45 mph (75 km/h), and only briefly threatened the northeastern Caribbean islands before dissipating on January 23. There have been only three other tropical cyclones in January – Hurricane Alice in 1955 and Tropical Storm Zeta in 2006 – both of which formed in the end of December and lasted into January; and Hurricane One in 1938.

Meteorological history
In the middle of January, an upper-level trough in the westerlies spawned a surface low pressure area to the east-northeast of the Lesser Antilles. The low developed south of the subtropical ridge, and as such was isolated from the detrimental effects of the westerlies. Its formation was non-tropical in nature, and initially it intensified under a baroclinic energy source, or one that derives energy from the interaction of cold and warm air. Convection increased slightly in association with the low despite cool sea surface temperatures of around 75° F (24° C). At 1200 UTC on January 18, it organized into a subtropical depression about 1725 mi (2790 km) east-northeast of Puerto Rico.

The National Hurricane Center (NHC) initiated Dvorak classifications on the cyclone at 0000 UTC on January 19, assessing a Dvorak number of T2.5; at that time, the storm was exhibiting both tropical and subtropical characteristics. On the same day, the pressure gradient between the storm and the subtropical ridge to its north produced gale force winds in association with the storm. It maintained a general westward track throughout its duration, initially moving west-northwestward. By early on January 20, the storm maintained minimal convection near its center, with its primary rainband revolved cyclonically around its well-defined center. Later that day, its banding features improved, and the storm strengthened slightly to attain peak winds of 45 mph (75 km/h) and it became a subtropical storm. Its intensity was supported by both ship reports and the Hurricane Hunters.

Late on January 21, the outer rainbands to the south and east of the center began diminishing, which began a weakening trend after the storm maintained peak winds for about 36 hours. At around that time, the cyclone was moving west-southwestward, and within 72 hours was forecast by one hurricane forecast model to be located over Hispaniola. By midday on January 22, the winds decreased to below gale force after the convection dissipated near the center. Subsequently it turned more to the west away from land, and by January 23 the circulation degenerated into a remnant trough about 185 mi (300 km) north of the Lesser Antilles.

Impact and records
The storm was the first Atlantic subtropical cyclone on record to develop in the month of January. Throughout the entire Atlantic hurricane database, the storm remains only the second tropical or subtropical cyclone to form during the month, after Hurricane One in 1938. There have also been two storms that formed in late December and persisted into January. In 1954, Hurricane Alice developed in the same region as the subtropical storm, which moved through the Lesser Antilles in early January 1955 as a minimal hurricane. The other storm was Tropical Storm Zeta, which lasted from December 2005 to January 2006 over the open Atlantic Ocean. The NHC did later note, however, that "subtropical cyclones have only been tracked since 1968 (tropical cyclone records go back to 1851) and it is possible some systems that were designated extratropical prior to 1968 could have been subtropical."