SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Monday, 18 April 2016

MONITORING THE HURRICANE FACTOR --
THE 2015 HURRICANE SEASON


While many of us have just thought about spring cleaning, hurricane season is right around the corner. In the eastern North Pacific, covering a region extending from the western coast line of North America westward to a longitude of 140 degrees west, the official 2016 Pacific hurricane season defined by the National Hurricane Center will commence in four weeks (15 May, to be precise). In six weeks, the official 2016 Atlantic hurricane season will start on 1 June in the North Atlantic basin, which includes the Gulf of Mexico and the Caribbean. The hurricane season in the Central North Pacific basin (from 140 degrees west to the International Dateline) also starts on 1 June.

Although a hurricane or other tropical cyclone may develop before these official start dates, such an event would be rare. In the North Atlantic, the earliest recorded hurricane for the year was Hurricane Alex, which became a hurricane three months ago on 14 January 2016. Previously, an unnamed hurricane that formed on 7 March 1908 had been considered to be the earliest to form in a calendar year. The official end to the hurricane season is on 30 November, but the latest recorded hurricane for a year was the second Hurricane Alice of the year on 31 December 1954. Interestingly, Tropical Storm Zeta, formed on 31 December 2005 and continued into the new year. In the eastern North Pacific basin, 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, which coincides with the start of the hurricane season in that basin. On the other hand, the latest hurricane of the season was Hurricane Winnie, which became a Category-1 hurricane on 6 December 1983 and dissipated a day later. In the central North Pacific basin, the earliest hurricane to form was Hurricane Pali earlier this current year on 11 January. This hurricane was earlier than Hurricane Ekeka that formed on 28 January 1992. The latest central Pacific hurricane to have formed in a season during modern times was Hurricane Iwa, which reached hurricane strength on 23 November 1983.

ATLANTIC BASIN

In terms of the number of named tropical cyclones, the 2015 Atlantic hurricane season was slightly below average across the North Atlantic basin as eleven tropical cyclones became named systems in 2015. Four of these named tropical cyclones developed into hurricanes (with maximum sustained winds in excess of 74 mph). Two of these hurricanes became major hurricanes (category-3 or higher on the Saffir-Simpson Hurricane Intensity Scale), with Danny becoming a category-3 hurricane in mid-August and Joaquin reaching category-4 status in late September.

The first named tropical system of 2015 in the North Atlantic was Tropical Storm Ana that originally formed as a subtropical storm off South Carolina's Atlantic coast on 8 May before transforming to a tropical storm. The last tropical cyclone of 2015 was Hurricane Kate, which formed over the Bahamas from a tropical depression on 9 November. Kate became a category-1 hurricane on 11 November as it passed north of Bermuda and then lost its tropical characteristics over the North Atlantic to the southeast of Newfoundland on 12 November.

Two tropical storms made landfall along the coast of the United States in 2015. Tropical Storm Ana reached the South Carolina coast early on 10 May. Later, Tropical Storm Bill made landfall along the Texas Gulf Coast on 16 June.

Additional information concerning several of the individual tropical cyclones during this past season can be found at http://www.nhc.noaa.gov/data/tcr/index.php?season=2015&basin=atl with a map showing the tracks of these systems across the basin.

EASTERN NORTH PACIFIC BASIN

In the eastern North Pacific, the hurricane season runs from 15 May to 30 November. The 2015 hurricane season had 18 named tropical cyclones, with 13 reaching hurricane status. Nine hurricanes became major (category-3 or greater) hurricanes, with Hurricane Patricia reaching category 5 status. Strengthening from a tropical storm to a category 5 hurricane in slightly more than 24 hours, Patricia had the distinction of being the strongest hurricane on record for the eastern North Pacific basin as well as the Western Hemisphere. Maximum sustained surface winds surrounding Patricia reached 200 mph and the minimum central pressure fell to 880 mb (or 25.99 inches of mercury). The nine major hurricanes in 2015 represents the greatest number of major hurricanes since reliable records began in 1971. In addition, five unnamed tropical depressions formed in the basin.

The first tropical cyclone in the eastern Pacific during 2015 was category-4 Hurricane Andres, which formed in late May well off the western coast of Mexico, while the last tropical cyclone of the year was category-4 Hurricane Sandra, which formed during the third week of November and dissipated the following weekend. While most of the tropical systems remained well off the coasts of Mexico and Central America, category-4 hurricane Blanca and made landfall on Mexico's Baja California Peninsula, category-5 Patricia made landfall along the coast of western Mexico, and category-1 hurricane Carlos brushed along the central Mexican coast. Category-4 Hurricane Sandra became a post-tropical cyclone approximately 90 miles off the southern tip of Baja California. The remnants of several hurricanes (Blanca, Dolores and Patricia) spread much-needed rain across the parts of the Southwestern United States. Five named tropical cyclones that formed in the eastern North Pacific Basin eventually traveled westward and entered into the central North Pacific Basin.

For additional information concerning some of these tropical cyclones including a map of their tracks across the basin, see http://www.nhc.noaa.gov/data/tcr/index.php?season=2015&basin=epac. More details can be found 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 2015, eight named tropical cyclones formed in the central North Pacific basin, which included three hurricanes (Kilo, Loke and Oho). Six named systems that included five Hurricanes (Guillermo, Hilda, Ignacio, Jimena and Olaf) and a tropical Storm (Nora) entered the central Pacific basin from the eastern Pacific, while Olaf had the distinction of returning to the eastern North Pacific basin after traveling across the central Pacific for nearly one week. The strongest of the hurricanes that formed in the central Pacific basin was category 4 Hurricane Kilo. Hurricanes Guillermo, Hilda, Ignacio, Jimena, Niala and Oho approached the Hawaiian Islands, but none made landfall on any of the islands. For additional information concerning tropical cyclones in the Central North Pacific, see http://www.prh.noaa.gov/cphc/tcpages/archive.php.

PERSPECTIVE

The eleven named tropical cyclones that formed during the 2015 North Atlantic hurricane season meant that the season was slightly more active than average, as a typical North Atlantic hurricane season would have slightly more than ten named systems, based upon long-term averages running from 1931 to 2010. However, the four hurricanes in 2015 were below the average number of six per annum for the last 80 years, while the seven tropical storms were more than the seasonal average of nearly five tropical storms. By comparison, the record 2005 season had 28 named systems, with 13 classified as hurricanes and eight tropical storms; seven of the hurricanes during that year became major hurricanes. Conversely, the least active season in recent history was 1983 when only four named tropical cyclones were reported (three hurricanes and one tropical storm). Only two hurricanes formed in 1982 and 2013, which represent the two years with the fewest hurricanes since 1931.

Hurricane forecasters suggest that the reason for the below average 2015 Atlantic hurricane season was due to the development of one of the strongest El Niño events during the fall and early winter season. which resulted in strong vertical wind shear (or rapid changes in wind speed and/or direction with altitude). Increased atmospheric stability, greater sinking motion and drier air across the basin due to the El Niño were also viewed as contributors.

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 Niño. For additional information on these forecasts, refer to http://tropical.atmos.colostate.edu/forecasts/. This past week, the team released their forecast of the 2016 North Atlantic hurricane season. They foresee that this upcoming hurricane season should have approximately average tropical cyclone activity. Although Hurricane Alex formed in January 2016, their initial April forecast envisions twelve named tropical cyclones following Hurricane Alex, along with five additional hurricanes. Of these hurricanes, the forecasters foresee two major hurricanes. A near-average probability is anticipated for major hurricanes making landfall along the United States coastline and in the Caribbean. The team bases their outlook on the continued weakening of the current El Niño event this late spring in the Northern Hemisphere, followed by a transition to ENSO-neutral conditions or possibly La Niña conditions by the peak of the Atlantic hurricane season in early to mid September. They are also considering the relatively warm waters of the tropical Atlantic along with a quite cold far North Atlantic, which is indicative of a of a negative phase of the Atlantic Multi-Decadal Oscillation.
In mid-May, forecasters with NOAA's Climate Prediction Center (CPC) also should provide their outlook for the 2016 Atlantic hurricane season.

The 2015 season in the eastern North Pacific with 18 named systems was more active than the long-term (1966-2012) average of slightly more than 15 named tropical cyclones. The 13 hurricanes in 2015 ranks in fourth place behind 1990 (with 16 hurricanes) over the last 50 years, well above the long-term average of slightly more than eight hurricanes per annum. The ten major hurricanes in 2015 far exceeded the average of slightly more than four in a year. The El Niño event enhanced tropical cyclone activity across the region. CPC forecasters should provide their outlook for the 2016 eastern Pacific hurricane season in mid-May.

The 2015 season in Central North Pacific basin with fourteen named tropical cyclones was relatively active compared to 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. The eight hurricanes that traversed the Central Pacific in 2015 was well above the average of one hurricane that typically forms only once in two years. The above average activity for the basin was attributed to the presence of the El Niño event. An outlook for the central Pacific basin should be provided by CPC in May.

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 2013 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@aos.wisc.edu
© Copyright, 2016, The American Meteorological Society.