SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Monday, 23 April 2018

MONITORING THE HURRICANE FACTOR --
THE 2017 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 2018 Pacific hurricane season defined by the National Hurricane Center will commence three weeks from tomorrow (15 May, to be precise). In slightly more than five and a half weeks, the official 2018 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. Considering the period of record in the North Atlantic that extends back to 1851, the earliest recorded hurricane for any year was Hurricane Alex, which became a hurricane two years 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 on record (since 1949) 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 during modern times (since 1957) was Hurricane Pali on 11 January 2016. 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

With 17 named tropical cyclones (tropical storms or hurricanes) having formed in the Atlantic basin, 2017 ranks as the ninth most active season since 1851. Ten of these named tropical cyclones developed into hurricanes (with maximum sustained winds in excess of 74 mph). The ten consecutive tropical cyclones that became hurricanes was a record for the modern era. Six of these hurricanes became major hurricanes (category-3 or higher on the Saffir-Simpson Hurricane Intensity Scale), which is the most since seven formed in 2005. Furthermore, two hurricanes reaching category 5 status (Irma and Maria) in 2017, which marks only the sixth season with multiple category-5 hurricanes. In addition, these two category-5 hurricanes made landfall (Irma making landfall multiple times on several Caribbean islands and Maria making landfall on Dominica), marking only the second time for such an event. Hurricane Ophellia, which was a category 3 hurricane, was the easternmost hurricane of such strength on record, as it passed to the south and east of the Azores in mid-October.

The first named tropical system of the 2017 Atlantic season was Tropical Storm Arlene that formed in late April from a subtropical depression approximately 900 miles to the west-southwest of the Azores. The last tropical cyclone of this season was Tropical Storm Rina, which formed approximately 900 miles to the east of Bermuda in early November and finally becoming a post-tropical cyclone on 9 November approximately 350 miles to the east of Newfoundland.

Three hurricanes made landfall along the coast of the mainland United States in 2017. Hurricane Harvey came onshore along the lower Texas Gulf Coast near Port Aransas as a major category-4 hurricane on 25 August and then traveled to the northeast across coastal Texas, accompanied by torrential rain that created major flooding in the Houston metropolitan area. Hurricane Irma was a category-4 hurricane as it crossed the Florida Keys before making landfall along the southwestern Florida coast near Naples on 10 September. Irma continued to travel northward along the west coast of Florida, accompanied by torrential rain and strong winds. Hurricane Nate made landfall near Biloxi on the Mississippi coast as a category-1 hurricane in early October. Another major hurricane made landfall on US territory as Hurricane Maria, which crossed the Commonwealth of Puerto Rico after it brushed St. Croix in the US Virgin Islands as a category 5 hurricane on 20 September. Catastrophic damage resulted across these islands. With at least $188 billion in damage caused by Hurricanes Harvey, Irma and Maria, 2017 may be the costliest on record for the United States. Tropical Storms Cindy and Philippe also made landfall along the US coast.

The World Meteorological Organization (WMO) Region IV Hurricane Committee recently retired the names of Harvey, Irma, Maria and Nate from the internationally recognized Atlantic list of hurricane names because these hurricanes were very destructive or so deadly that the future use of the names would be insensitive. With the inclusion of these four names, this list now contains 86 retired names.

Additional information for each of the individual tropical cyclones during 2017 can be found at http://www.nhc.noaa.gov/data/tcr/index.php?season=2017&basin=atl with a map showing the tracks of these systems across the basin.

EASTERN NORTH PACIFIC BASIN

The 2017 hurricane season in the eastern North Pacific had 18 named tropical cyclones, with 9 reaching hurricane status. Four hurricanes became major (category-3 or greater) hurricanes, with two reaching category-4 status (Hurricanes Fernanda and Kenneth).

The first named tropical cyclone in the eastern Pacific during 2017 was Tropical Storm Adrian, which formed in early May approximately 360 miles to the southwest of El Salvador. The last tropical cyclone of the year was Tropical Storm Selma, which formed slightly more than 200 miles south of El Salvador before making landfall along the coast of El Salvador on 28 October.

While most of the tropical systems remained well off the coasts of Mexico and Central America, category-1 hurricane Max made landfall along the coast of southwestern Mexico in mid-September. Tropical Storms Beatriz, Calvin and Lidia made landfall along the western coast of Mexico, while Tropical Storm Selma reached the El Salvador coast.

For additional information concerning each of these tropical cyclones in 2017, along with two maps of their tracks across the basin, see http://www.nhc.noaa.gov/data/tcr/index.php?season=2017&basin=epac.

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 2017 season, two tropical cyclones from the eastern Pacific basin entered the central Pacific basin in July. One of these was Tropical Storm Fernanda, which had been a category-5 hurricane earlier, and the other was Tropical Depression Greg. These two systems dissipated quickly after entry into 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 seventeen named tropical cyclones that formed during the 2017 North Atlantic hurricane season meant that the season was 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. Furthermore, the ten hurricanes in 2017 were more than the average number of six per annum for the last 80 years, as were the seven tropical storms that were more than the long-term annual average of nearly five tropical storms. The six major hurricanes in 2017 were twice as many as the long-term average of three hurricanes that are at least category-3 hurricanes. 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.

Another way of estimating the intensity or strength of a hurricane season has been to calculate the "Accumulated Cyclone Energy" (or ACE) for the season, an index that combines the numbers of systems, how long they existed and how intense they became. ACE is calculated by squaring the maximum sustained surface wind in the system every six hours that the cyclone is a named tropical cyclone and summing it for the season. The 2017 Atlantic hurricane season had the seventh highest ACE since 1851, ranking behind 2005, which had the second largest ACE as major Hurricanes Katrina and Wilma slammed the Gulf Coast and South Florida.

The late Professor William Gray and his associate Philip Klotzbach, hurricane experts from Colorado State University in Fort Collins, have been issuing long-range Atlantic 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/.
In early April 2017, Klotzbach's team released its initial forecast of the 2017 North Atlantic hurricane season that envisioned eleven named tropical cyclones, which included four hurricanes. Of these hurricanes, the forecasters foresaw two major hurricanes. The team had based their initial outlook on the likelihood that either weak or moderate El Niño conditions would have evolved by early autumn during the peak in the Atlantic hurricane season. However, as the chances for an El Niño conditions waned over last summer, subsequent outlooks prepared in June and July began to favor a near average hurricane season. A final updated forecast in August called for a total of 16 named tropical cyclones for the entire 2017 season, with eight potentially becoming hurricanes. Three major hurricanes were also envisioned.

Earlier this month, the Colorado State University forecast team released its initial "Extended Range Forecast of Atlantic Seasonal Hurricane Activity and Landfall Strike Probability for 2018" that provides projected estimates of the number of named tropical cyclones during the upcoming hurricane season. The team foresees slightly above-average tropical cyclone activity in 2018, with fourteen named tropical cyclones, which include seven hurricanes. Of these hurricanes, the forecasters foresee three major hurricanes (category 3 or higher). A slightly above-average probability is anticipated for major hurricanes making landfall along the United States coastline and in the Caribbean in 2018. The team bases their outlook on the likelihood that the current weak La Niña conditions will evolve into ENSO-neutral conditions during the next several months. They do not foresee any development of an El Niño event by late summer and early autumn during the peak in the Atlantic hurricane season. However, the waters of the western tropical Atlantic are anomalously warm, while the eastern tropical Atlantic and far North Atlantic are anomalously cool over the past month, which indicates that the Atlantic Multi-Decadal Oscillation index is close to the long-term average. [The Tropical Meteorology Project]

The 2017 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 nine hurricanes in 2017 was also a little above the long-term average of slightly more than eight hurricanes per annum. The four major hurricanes in 2017 was essentially the same as the long-term average of slightly more than four in a year. In May 2017 CPC forecasters had issued an outlook for the eastern North Pacific basin that called for a near- to above-normal hurricane season with a 70-percent chance of the formation of between 14 and 20 named tropical cyclones, with six to eleven hurricanes. Between three and seven major hurricanes were also envisioned. Weak El Niño conditions were anticipated. CPC forecasters should provide their outlook for the 2018 eastern Pacific hurricane season by mid-May.

The 2017 season in Central North Pacific basin with two named tropical cyclones was less active as 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 and an average of one hurricane that typically forms only once in two years. CPC forecasters in May 2017 had issued an outlook for the central North Pacific basin with activity ranging from near- to above-average. Five to eight tropical cyclones were expected to affect the central North Pacific in 2017, either forming within the basin or entering it from the Eastern Pacific. At the time, the forecasters had anticipated a possible transition to a weak El Niño during the hurricane season, along with near- or above-average ocean temperatures in the main hurricane formation. However, below average ocean temperatures were found through the fall, along with a transition to La Niña conditions. An outlook for the 2018 season in 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 that is available for sale, a free 164-page pdf file is available online. 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. A compilation of individual tropical cyclones in the central Pacific is also available by year extending back to 1957, along with notable systems dating back into the 19th century.


Return to RealTime Weather Portal
Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.