SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Monday, 22 April 2019

MONITORING THE HURRICANE FACTOR --
THE 2018 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 2019 Pacific hurricane season defined by the National Hurricane Center will commence in a little more than three weeks (15 May, to be precise). In slightly more than five and a half weeks, the official 2019 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 begins on 1 June. All three hurricane seasons officially end on 30 November.

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 three 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 15 named tropical (and subtropical) cyclones having formed in the Atlantic basin, 2018 ranks as the thirteenth most active season since 1851, tied with 1932, 2001, 2007 and 2016. Eight of these 15 named tropical cyclones developed into hurricanes (with maximum sustained winds in excess of 74 mph). Two of these hurricanes (Florence and Michael) became major hurricanes (category-3 or higher on the Saffir-Simpson Hurricane Intensity Scale), with Florence reaching category 4 status and Michael becoming a category 5 hurricane. One tropical depression formed. (A subtropical cyclone is a hybrid system, having both tropical and extratropical characteristics.)

The first named tropical system of the 2018 Atlantic season was Subtropical Storm Alberto that formed on 25 April over the northwestern Caribbean Sea offshore of Yucatan Peninsula. The last tropical cyclone of this season was Hurricane Oscar, a category 2 hurricane that lost its tropical characteristics on 31 October as it was heading to the north-northeast approximately 540 miles to the southeast of Newfoundland.

In mid-September, four tropical cyclones (Hurricanes Florence and Helene, Tropical Storm Isaac and Subtropical Storm Joyce) were active simultaneously in the Atlantic basin, which is the first time in ten years since that many systems were active at one time.

Two hurricanes (Florence and Michael) made landfall along the Atlantic or Gulf coasts of the mainland United States in 2018. Hurricane Florence, which had reached category 4 status earlier, made landfall along the North Carolina coast as a category 1 hurricane on 14 September before taking a slow track toward the southwest and then northward, spreading torrential rains across the Carolinas and Virginia. Hurricane Michael became a category-5 hurricane over the Gulf of Mexico before making landfall along the Florida Peninsula between Panama City and Mexico Beach on 10 October. Catastrophic damage resulted across the Florida Panhandle. Michael was the fourth category 5 in history to make landfall along the U.S. coast and the first since 1992 when Andrew hit south Florida. Subtropical Storm Alberto and Tropical Storm Gordon also made landfall along the U.S. coast.

Because of the magnitude of their destruction, the names of Hurricanes Florence and Michael have been retired from the active list of names by the World Meteorological Organization (WMO) Region IV Hurricane Committee. The WMO committee has selected the names Francine and Milton to fill the list of storm names when it will be used again in 2024.

Additional information concerning the individual tropical cyclones during this season (with the exception of Florence, whose report is still being prepared) are available at https://www.nhc.noaa.gov/data/tcr/index.php?season=2018&basin=atl with a preliminary 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 2018 hurricane season had 21 named tropical cyclones, with 11 reaching hurricane status. Three tropical depressions were reported. Nine hurricanes became major (category-3 or greater) hurricanes, with seven becoming category-4 hurricanes and two reaching category-5 status (Hurricanes Lane and Willa).

The first named tropical cyclone in the eastern Pacific during 2018 was Tropical Storm Aletta, which became a tropical storm on 7 June and then a category 4 hurricane at least 400 miles to the west-southwest of Manzanillo, Mexico. The last tropical cyclone of the year was Tropical Storm Xavier, which formed slightly more than 200 miles south of El Salvador before making landfall along the coast of El Salvador on 6 November.

While most of the tropical systems remained well off the coasts of Mexico and Central America, three hurricanes (Bud, Sergio and Willa) and one tropical storm (Vicente) made landfall along the western coast of Mexico. Several of the long-lived tropical cyclones traveled westward and exited into the central Pacific basin (see below). Hurricane Olivia, a former category 4 hurricane while in the eastern Pacific, crossed the central Pacific basin and eventually made landfall on Maui and Lanai in the central Hawaiian Islands as a tropical storm.

For additional information about these tropical cyclones including a preliminary map of their tracks across the basin, see https://www.nhc.noaa.gov/data/tcr/index.php?season=2018&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 2018 season, five hurricanes entered the central Pacific basin from the eastern Pacific basin in August and September. In addition, Hurricane Walaka formed within the basin and intensified to become a category 5 hurricane as it passed to the southwest and west of the western Hawaiian Islands. Three of the hurricanes coming from the eastern Pacific were major hurricanes or became major hurricanes that continued to travel long distances across the central Pacific, passing the Hawaiian Islands. Hector was a category 4 that passed to the south of Hawaii's Big Island. Lane became a category 5 hurricane before passing to the south of the Hawaiian Islands. Norman passed to the northeast of the Hawaiian Islands as a category 3 hurricane. Olivia, which had become a category 4 hurricane in the eastern Pacific basin, crossed the central Hawaiian Islands as a tropical storm. For additional information concerning tropical cyclones in the Central North Pacific, see http://www.prh.noaa.gov/cphc/tcpages/archive.php.

PERSPECTIVE

The fifteen named tropical cyclones that formed during the 2018 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 eight hurricanes in 2018 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. However, the two major hurricanes in 2018 were less than the long-term average of three hurricanes that annually reach at least category-3 status. For comparison, the record active 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. Interestingly, a record number of subtropical storms formed in 2018, with six eventually becoming tropical storms or hurricanes; only Subtropical Storm Alberto did not make a transition.

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 2018, Klotzbach's team released its initial forecast of the 2018 North Atlantic hurricane season that indicated a slightly above-average season in terms of the number of tropical cyclones. In this initial forecast, fourteen named tropical cyclones were envisioned, which included seven hurricanes. Of these hurricanes, the forecasters foresaw three major hurricanes. A slightly above-average probability was anticipated for major hurricanes making landfall along the United States coastline and in the Caribbean in 2018. The team had based their initial outlook upon the transition of the weak La Niña to ENSO-neutral conditions occurring over the summer. However, subsequent outlooks prepared in June and July began to favor an average then slightly-below average hurricane season because of cooler than average sea surface temperatures across the eastern North Atlantic and an increased chance of weak El Niño conditions developing during the hurricane season. Klotzbach issued a final updated forecast in August that called for a total of twelve named tropical cyclones for the entire 2018 season, with five potentially becoming hurricanes. One major hurricane was also envisioned.

Earlier this month of April 2019, the Colorado State University forecast team released its initial "Extended Range Forecast of Atlantic Seasonal Hurricane Activity and Landfall Strike Probability for 2019" that provides projected estimates of the number of named tropical cyclones during the upcoming hurricane season. The team foresees slightly below-average tropical cyclone activity in 2019. The team's initial April forecast envisions thirteen named tropical cyclones, which include five hurricanes. A typical North Atlantic hurricane season would have slightly less than eleven named systems, based upon long-term averages running from 1931 to 2010. The average number of hurricanes is six per annum. A slightly below-average probability is anticipated for major hurricanes making landfall along the United States coastline and in the Caribbean in 2019. The team bases their outlook on the likelihood that the current weak El Niño conditions will continue through spring and possibly strengthen this upcoming summer and autumn during the peak in the Atlantic hurricane season. El Niño conditions tend to suppress tropical cyclone activity in the North Atlantic. Additionally, the waters of the tropical Atlantic are slightly below average, while the far North Atlantic are anomalously cool. Consequently, this pattern indicates that the Atlantic Multi-Decadal Oscillation index is below its long-term average. [The Tropical Meteorology Project]

With 22 named systems, the 2018 season in the eastern North Pacific tied 1985 for the second most active season in the 70-year period of record, and is more than the long-term (1966-2012) average of slightly more than 15 named tropical cyclones. The twelve hurricanes in 2018 was also above the long-term average of slightly more than eight hurricanes per annum. The ten major hurricanes in 2018 were more than twice the long-term average of slightly more than four in a year. Furthermore, the six landfalling systems in 2018 tied 1971 for the highest number of tropical cyclone landfalls in a single season. In May 2018 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 seven to twelve hurricanes. Between three and seven major hurricanes were also envisioned. This outlook was based upon the expectation of ENSO-neutral or weak El Niño conditions, along with above-average sea-surface temperatures across the eastern and central tropical North Pacific.
CPC forecasters should provide their outlook for the 2019 eastern Pacific hurricane season by mid-May.

The 2018 season in the Central North Pacific basin was very active as compared to the long-term average as six named tropical cyclones traversed sections of the basin. Over the last 60 years, slightly more than three named tropical cyclones either developed or entered this basin from the east on average; in addition, an average of one hurricane typically forms only once in two years. An outlook for the 2019 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.


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2019, The American Meteorological Society.