This coming Thursday (30 November 2017) signals the end of the
official 2017 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 from the 140th meridian to the International Dateline). While a named tropical cyclone (hurricane
or tropical storm) may develop in these basins after 30 November, such an event
is rare. Considering the period of record in the North Atlantic that extends back to 1851, the latest recorded hurricane was the second Hurricane Alice of the year 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 any season was Hurricane
Alex, which formed on 14 January 2016.
In the eastern North Pacific basin,
the latest hurricane of the season of record (since 1949) was Hurricane Winnie,
which became a category-1 hurricane (on the
Saffir-Simpson Scale) 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 that formed on 12 May 1990 and reached hurricane
status on the 15th.
In the central North Pacific basin, the latest hurricane to have formed in a season during modern times (since 1957) was Hurricane Iwa, which reached hurricane strength on 23 November 1983. On the other hand, the earliest hurricane to form in the central Pacific was Hurricane Pali that formed on 11 January.
With 17 named tropical cyclones having formed in the Atlantic basin as of late November 2017, this current year 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). 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 Mariamaking 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. As of this writing, 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.
Additional information concerning several of the individual tropical cyclones
during this season will be forthcoming at http://www.nhc.noaa.gov/data/tcr/index.php?season=2017&basin=atl with a preliminary map showing the tracks of these systems across the basin. More details should appear in the next few weeks, as the final version of the
North Atlantic summary is prepared.
In the eastern North Pacific, the hurricane season runs from
15 May to 30 November. As of the last week of November, the 2017
hurricane season
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 (as of the time of this writing) was Tropical Storm Selma, which formed slight 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 some
of these tropical cyclones including a preliminary map of their tracks across the basin, see http://www.nhc.noaa.gov/data/tcr/index.php?season=2017&basin=epac.
More details should be forthcoming in the Eastern North Pacific
summary.
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.
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.
The late Professor William Gray and 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. In this initial forecast, eleven named tropical cyclones were envisioned, which included four hurricanes. Of these hurricanes, the forecasters foresaw two major hurricanes. A below-average probability was anticipated for major hurricanes making landfall along the United States coastline and in the Caribbean. 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 the summer, subsequent outlooks prepared in June and July began to favor a near average hurricane season. Klotzbach issued a final updated forecast in August that 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.
Forecasters with NOAA's Climate Prediction
Center (CPC) also generated an outlook for the 2017 Atlantic hurricane
season in mid-May. They foresaw an above-normal hurricane season, giving a 70-percent
chance that the basin could experience 11 to 17 named tropical cyclones. The forecasters also felt that five to nine tropical cyclones could become hurricanes, with as many as four of these hurricanes possibly becoming major hurricanes. The forecasters
anticipated weak El Niño conditions during the summer, near- or above-average sea-surface temperatures across the tropical Atlantic Ocean and Caribbean Sea, and average or weaker-than-average vertical wind shear in the basin.
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.
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.
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.
