NHC Adds Unnamed Subtropical Storm to 2013's Atlantic Tally

The National Hurricane Center (NHC) announced last week that the hurricane season of 2013 had one more storm that should have been named--a short-lived low that developed south of the Azores during early December, which became a subtropical storm on December 5. "Should-Have-Been-Named-Subtropical-Storm-Nestor" reached top sustained winds of 45 mph. The storm formed over unusually cool waters of 22°C (72°F), and brought sustained 10-minute winds of 37 mph with a gust to 54 mph near 00 UTC December 7 to Santa Maria in the southeastern Azores. With this addition, the 2013 Atlantic season ended with 14 tropical and subtropical storms. Two, Ingrid and Humberto, became hurricanes, but neither became a major hurricane.


Figure 1. True-color MODIS satellite image of Unnamed Subtropical Storm Fourteen at 15:30 UTC December 6, 2013, as it approached the Azores Islands. At the time, "Should-Have-Been-Named-Subtropical-Storm-Nestor" had 45 mph sustained winds. Image credit: NASA.

December named storms are rare
The unnamed 2013 subtropical storm is the Atlantic's first December tropical or subtropical storm since Tropical Storm Olga of 2007. There have been eighteen Atlantic tropical or subtropical storms that have formed in the month of December since record keeping began in 1851. Only four have hit land. Eight of the eighteen storms have occurred since 1995. Eight have been hurricanes, with a Category 2 hurricane in 1925 being the only December storm to hit the mainland U.S.

December named storms have higher than usual odds of being subtropical in nature, since the ocean temperatures required to create a fully tropical system are typically lacking. The NHC began naming subtropical storms in 2002. Between 1968 and 2001, subtropical storms were simply given numbers ("One", "Two", etc). Before 1968, subtropical storms were never classified as such, but were sometimes called "Unnamed storm". A landmark study performed by Herbert and Poteat (1975) led to a substantial increase in the identification and naming of subtropical storms in the late 1970s and early 1980s. However, when Bob Sheets became director of the National Hurricane Center between 1987 and 1995, he declared that subtropical storms should not be recognized, and very few subtropical storms were classified during this period. Prior to 1968, there are many systems that were subtropical in the Atlantic that should have been included in the official HURDAT database. I've seen estimates that 5-10 storms were missed in the 1950s, and ten between 1969 and 1999. A reanalysis effort is underway to include these "missed" storms into the database. However, it will be several years before this process is complete. Here are all of the tropical and subtropical storms that have formed in December in the Atlantic since 1995:

2013: Unnamed Subtropical Storm Fourteen, December 5
2007: Tropical Storm Olga, December 11
2005: Tropical Storm Zeta, December 30
2003: Tropical Storm Odette, December 4
2003: Tropical Storm Peter, December 7

Dr. Jim Kossin of the University of Wisconsin published a 2008 paper in Geophysical Research Letters titled, "Is the North Atlantic hurricane season getting longer?" He concluded that yes, there is an "apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high".

Mexican National Hurricane Agency formed
The president of Mexico announced in mid-January the creation of a new National Hurricane Agency. The new institution may begin operations as soon as this summer, and will study how to generate and communicate hurricane and severe weather forecasts.

Jeff Masters