SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Monday, 2 December 2013

MONITORING THE HURRICANE FACTOR --
THE 2013 HURRICANE SEASON


Saturday (30 November 2013) signaled the end of the official 2013 hurricane season in the North Atlantic basin (which includes the Gulf of Mexico and the Caribbean), the Eastern North Pacific basin (a region extending from the western coast line of North America westward to a longitude of 140 degrees west) and the Central North Pacific basin (westward to the International Dateline). While a hurricane or other tropical cyclone may develop after 30 November, such an event is rare. In the North Atlantic, the latest recorded hurricane was on 31 December 1954, while on 31 December 2005 Tropical Storm Zeta formed and continued into the new year. The earliest recorded Atlantic hurricane for a season was an unnamed hurricane on 7 March 1908. In the eastern North Pacific basin, the latest hurricane of the season was Hurricane Winnie, which became a Category-1 hurricane on 6 December 1983 and dissipated a day later, while the earliest hurricane of the season for that basin was Hurricane Alma, a category 1 hurricane on the Saffir-Simpson Scale that formed on 12 May 1990 and reached hurricane status on the 15th.

ATLANTIC BASIN

In terms of the number of named tropical cyclones, the just-completed Atlantic hurricane season had a moderate season across the North Atlantic basin as 13 tropical cyclones became named systems in 2013. However, these systems were not exceptionally strong, as only two of these 13-named tropical cyclones developed into hurricanes (with maximum sustained winds in excess of 74 mph). Furthermore, neither of these hurricanes became a major hurricane (Category 3 or higher on the Saffir-Simpson Hurricane Intensity Scale).

The first named tropical system of the 2013 Atlantic season was Tropical Storm Andrea that formed in the east central Gulf of Mexico approximately 300 off Florida's central Gulf coast on 15 May. As of this writing, the last tropical cyclone was Tropical Storm Melissa, which formed initially as a subtropical storm on 18 November over the central Atlantic and then became a remnant low pressure system to the north-northwest of the Azores on 21 November. The first hurricane of the season was Hurricane Humberto, which formed over the far eastern Atlantic on 11 September, a relatively late date for the first Atlantic hurricane. Tropical Storm Andrea was the only named tropical cyclone to make landfall along the continental United States in 2013, although Tropical Storms Dorian and Karen approached the US coast. Andrea made landfall in northern Florida and then traveled to the northeast along coastal sections of the Southeast in late May.

Additional information concerning several of the individual tropical cyclones during this season will be found at http://www.nhc.noaa.gov/2013atlan.shtml. More details should appear late next week, as the final version of the North Atlantic summary is prepared.

EASTERN NORTH PACIFIC BASIN

In the eastern North Pacific, the hurricane season runs from 15 May to 30 November. The 2013 hurricane season had 18 named tropical cyclones, with eight reaching hurricane status. Only one hurricane (Raymond) became a major category 3 hurricane. The first tropical cyclone in the eastern Pacific during 2013 was Tropical Storm Alvin, which formed in mid May, while the last tropical cyclone was Tropical Storm Sonia dissipated during the first week of November. While most of the tropical systems remained well off the coasts of Mexico and Central America, Hurricane Barbara made landfall along the coast of southwestern Mexico, while Hurricane Manuel and Tropical Storms Octave and Sonia made landfall along the central and northern Mexican coast. For additional information concerning some of these tropical cyclones, see http://www.nhc.noaa.gov/2013epac.shtml. More details should be forthcoming in the Eastern North Pacific summary.

CENTRAL NORTH PACIFIC BASIN

The region of the North Pacific Ocean lying between 140 degrees west and the International Dateline (180 degrees longitude) is identified as the Central North Pacific Basin. The hurricane season for this basin officially begins on 1 June and ends on 30 November. The region is monitored by the National Weather Service Forecast Office in Honolulu, which activates the Central Pacific Hurricane Center (CPHC) when tropical cyclone activity becomes imminent in this basin.

During the 2013 season, five named tropical cyclones moved across the central North Pacific. Tropical Storm Flossie and former Hurricanes Gil and Henriette entered the central Pacific basin from the eastern Pacific. Tropical Storms Pewa and Unala formed in the central Pacific. In addition, a tropical depression formed in the basin. For additional information concerning tropical cyclones in the Central North Pacific, see http://www.prh.noaa.gov/cphc/tcpages/archive.php.

PERSPECTIVE

The 19 named tropical cyclones that formed so far during the 2013 North Atlantic hurricane season were tied with 1887, 1995, 2010, 2011 and 2012 for the third most active season since 1850. Only the record 2005 season with 28 named tropical cyclones could be considered the most active. Using long-term averages running from 1931 to 2010, a typical North Atlantic hurricane season would have slightly more than ten named systems, with nearly six hurricanes and slightly more than four tropical storms. In the record 2005 season with 28 named systems, 13 were classified as hurricanes and eight tropical storms; seven of the hurricanes during that year became major hurricanes. The least active in recent history was 1983 when only four named tropical cyclones were reported (three hurricanes and one tropical storm).

Hurricane forecasters suggest that the reason for the active 2013 Atlantic hurricane season was a combination of several factors that included above average tropical Atlantic sea surface temperatures during the hurricane season, the absence of an El Niño event and anomalously low tropical Atlantic sea level pressures. However, a very dry mid-level air combined with a region of mid-tropospheric subsidence and relatively stable atmospheric conditions combined to significantly suppress hurricane development in the 2013.

Hurricane experts Professor William Gray and Philip Klotzbach from Colorado State University in Fort Collins have been issuing long-range hurricane forecasts for more than two decades. Their forecasts are based upon several factors that include analysis of the wind field at several levels, the rainfall over West Africa and the effects of El Niņo. For additional information on these forecasts, refer to http://typhoon.atmos.colostate.edu/forecasts/. In early April 2013, this team released their forecast of the 2013 North Atlantic hurricane season. At the time, they thought that the season would be more active than average due in part to to an anomalous warming of the tropical Atlantic in during the previous several months and the anticipation that El Niño conditions would not develop through the upcoming autumn season in the Northern Hemisphere. In their April forecast, the forecasters called for 18 named tropical cyclones, with nine potentially becoming hurricanes. As many as four of these hurricanes had been expected to become severe, reaching category 3 or higher on the Saffir-Simpson Scale. They also anticipated an above average probability of a major Atlantic hurricane making landfall somewhere along the coast of the coterminous US and in the Caribbean.
Their final August forecast called for 18 named storms and eight hurricanes, three of which would have been major hurricanes. In mid-May forecasters with NOAA's Climate Prediction Center also generated an outlook for the 2013 Atlantic hurricane season. They foresaw an active hurricane season, giving a 70 percent chance that the basin could experience 13 to 20 named tropical cyclones, with seven to eleven hurricanes. They felt that three to six hurricanes could become major hurricanes.

The 2013 season in the eastern North Pacific was more active than average, as the year's eighteen named systems were above the long-term (1966-2010) average of slightly more than 15 named tropical cyclones. However, the eight hurricanes in 2013 equalled the number of tropical cyclones that typically reach hurricane status over the last 45 years. The ENSO-neutral conditions may have affected the tropical cyclone activity in the eastern North Pacific.

The 2013 season in Central North Pacific basin with only five named tropical cyclones was above the long-term average. Over the last 60 years, nearly three named tropical cyclones either develop or enter this basin from the east. On average, one hurricane forms once in two years.

LOOKING AT LONG-TERM TROPICAL CYCLONE CLIMATOLOGIES

The National Hurricane Center (NHC) has an updated and revised edition of its "Tropical Cyclones of the North Atlantic Ocean, 1851-2006." While a paper copy of this book is available for a cost from NHC, a 243-pg pdf file of this edition can be downloaded for free. NHC also released the first edition of "Tropical Cyclones of the Eastern North Pacific Ocean, 1949-2006." In addition to a paper copy is available for sale, a free 164-page pdf file is available. Both of these climatologies have numerous graphics that show long-term changes in tropical cyclone frequency in the two basins.
A climatology of tropical cyclones in the central North Pacific from the 1950s to 2008 is available from the CPHC climatology website maintained by the Central Pacific Hurricane Center (CPHC) in Honolulu, HI.


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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2013, The American Meteorological Society.