2016 North Atlantic Hurricane Season Review

Transcription

1 2016 North Atlantic Hurricane Season Review RMS REPORT 1

2 Executive Summary The 2016 North Atlantic hurricane season will be remembered for several reasons. First and foremost, Major Hurricane Matthew, the first Category 5 storm in the Atlantic Basin since 2007, impacted the Caribbean and southeastern United States in early October. Although the U.S. was spared major wind damage from Hurricane Matthew, high winds and flooding, driven by storm surge and rainfall, impacted coastal areas from central Florida to North Carolina. Florida saw the first hurricane landfall in 11 years when Hurricane Hermine made landfall in the Big Bend region of Florida in September, followed in early October by a near miss from Major Hurricane Matthew, which came within 5 mi (8 km) of landfall as it tracked up the east coast of the state. With Matthew s miss, the wellpublicized hurricane drought, a term given to the lack of major hurricane landfalls in the U.S., was extended to an 11th season. Hurricane activity got off to an early start when Hurricane Alex spun up in mid-january, becoming the second earliest hurricane on record. In addition, the season ended late, as Hurricane Otto made landfall in Nicaragua in late November. The gap between the two events at 318 days broke the record for the most number of days between the first storm forming and the last storm dissipating. The rest of the hurricane season lived up to forecasters expectations with slightly above-average named storm activity, near-average hurricane and slightly above-average major hurricane activity, closing with fifteen named storms, seven hurricanes, and four major hurricanes. Early 2017 hurricane season forecasts by Tropical Storm Risk (TSR) predict the 2017 season will be similar to 2016, although forecast skill at this lead time is poor. Colorado State University (CSU) anticipates that activity in the 2017 season will be determined by the strength of the thermohaline circulation (THC) and Atlantic Multidecadal Oscillation (AMO), as well as the phase of El Niño-Southern Oscillation (ENSO). 1

3 Overview of the 2016 North Atlantic Hurricane Season The 2016 season was the most active since Fifteen named storms formed in 2016, which was above the average 1 (11.2 storms) and slightly above the average 2 (14.6 storms), as seen in Figure 1. Seven hurricanes formed in 2016, slightly above the average (6.2 storms) but slightly below the average (7.4 storms). There were four major hurricanes (Category 3 and above) during the 2016 season, 3 which lies above the average for (2.7 storms) and (3.4 storms) Average Average 2016 Season Number of Storms Named Storms Hurricanes Major Hurricanes Figure 1: Comparison of the 2016 North Atlantic hurricane season storms to the and averages. (Data from the National Oceanic and Atmospheric Administration Hurricane Research Division, 2017.) A total of five storms made landfall in the United States in 2016 three tropical storms (Bonnie, Colin, and Julia) and two hurricanes (Hermine and Matthew) though no major hurricanes made landfall. The 2016 season extended the longest period without a major hurricane landfall in the U.S. to 11 years, since records began in The last major hurricane to make landfall in the U.S. was Hurricane Wilma in Prior to the 2016 season, Wilma was also Florida s last hurricane landfall of any intensity. Hurricane Hermine broke this streak, making landfall in the Big Bend region of Florida on September 2 as a Category 1 hurricane. Major Hurricane Matthew was the strongest storm of the 2016 season, reaching peak intensity as a Category 5 hurricane in the Caribbean Sea. Matthew later made landfall over Haiti and Cuba as a strong Category 4 storm before weakening slightly and tracking through the Bahamas as a Category 3 storm. Matthew tracked offshore along Florida s east coast as a Category 3 storm and briefly made landfall as a Category 1 storm in South Carolina. 1 The historical database for land falling hurricanes worldwide is generally agreed to be complete since However, the record of hurricane activity in the Atlantic Basin alone is generally agreed to be complete only from 1950 onward, following the increases in aircraft reconnaissance and the onset of satellite technology. 2 It is widely recognized that the Atlantic Basin entered a period of elevated activity in 1995 compared to the long-term historical average, driven by a positive phase in the Atlantic Multidecadal Oscillation. 3 In its post-season assessment released on January 30, 2017, the National Hurricane Center (NHC) increased the intensity of Hurricane Otto to Category 3, making it the fourth major hurricane of the 2016 season. 2

4 The Accumulated Cyclone Energy (ACE) index, 4 which provides an alternative assessment of hurricane activity based on intensity and duration, was above both the long-term ( ) and recent ( ) averages at (10 4 kt 2 ) in The 2016 season ACE index ranks as the largest since 2010 at 38 percent above the long-term average of and 10 percent above the recent average of 125.5, as illustrated in Figure 2. Major Hurricane Matthew contributed approximately 35 percent of the total 2016 ACE index, breaking the record for the most ACE generated by a hurricane in the eastern Caribbean Sea, 5 and is ranked as eighth highest among Atlantic hurricanes in the satellite era. Figure 3 shows ACE value for all storms in the 2016 season ACE (10 4 kt 2 ) ACE Average Average Figure 2: North Atlantic hurricane season Accumulated Cyclone Energy (ACE) totals and and averages. (Data from WeatherBell Models, 2016.) [WeatherBell Models ACE values represent the operational advisories and not the post-season reports published by the National Hurricane Center and therefore these ACE values do not show the retrospective upgrade of Hurricane Otto] ACE (10 4 kt 2 ) Figure 3: Accumulated Cyclone Energy (ACE) values for storms in the 2016 season (TD = Tropical Depression, TS = Tropical Storm, H = Hurricane, MH = Major Hurricane). (Data from WeatherBell Models, 2016.) [WeatherBell Models ACE values represent the operational advisories and not the post-season reports published by the National Hurricane Center and therefore these ACE values do not show the retrospective upgrade of Hurricane Otto] 4 ACE is calculated as the square of the sum of the maximum sustained wind speed (in knots) at 6-hour intervals for the duration of the storm at tropical storm status or greater (sustained wind speeds of 35 knots or higher). The unit of ACE is 10 4 kt 2 5 Matthew broke the record of Hurricane David (1979). 3

5 The 2016 North Atlantic Storms and Their Impacts The 2016 North Atlantic hurricane season broke the record for the most number of days 318 between the first storm forming and the last storm dissipating. The period lasted from January 12 when Hurricane Alex formed to November 26 when Hurricane Otto dissipated, beating the previous record of 311 days set in The 2016 season was particularly active in terms of land falling storms 10 of the 15 named storms made landfall, while a further two tracked close enough to impact land. Five storms made landfall in the U.S.: Tropical Storm Bonnie, Tropical Storm Colin, Tropical Storm Julia, Hurricane Hermine, and Hurricane Matthew. Three storms made landfall over Mexico and Central America: Tropical Storm Danielle (Mexico), Hurricane Earl (Belize), and Major Hurricane Otto (Nicaragua). Major Hurricane Matthew impacted the Caribbean, with landfalls in Haiti, Cuba, and the Bahamas. Meanwhile Major Hurricane Nicole made landfall over Bermuda as a Category 2 storm and Hurricane Alex made landfall in the Azores. The 2016 North Atlantic storm activity is illustrated in Figures 4 and 5. Figure 4: The 2016 North Atlantic storm tracks and intensities. (Data from the National Hurricane Center, 2017.) 4

6 Named Storm H Alex TS Bonnie TS Colin TS Danielle H Earl TS Fiona MH Gaston TD Eight H Hermine TS Ian TS Julia TS Karl TS Lisa MH Matthew MH Nicole H Otto January May June July August September October November Figure 5: Timeline of the 2016 North Atlantic hurricane season (TD = Tropical Depression, TS = Tropical Storm, H = Hurricane, MH = Major Hurricane) with Accumulated Cyclone Energy (ACE) values for each storm. (Data for timeline from the National Hurricane Center, 2016; data for ACE values from WeatherBell Models, 2016.) [WeatherBell Models ACE values represent the operational advisories and not the post-season reports published by the National Hurricane Center and therefore these ACE values do not show the retrospective upgrade of Hurricane Otto] Hurricane Alex The first named storm of the 2016 season, Hurricane Alex, formed in the eastern North Atlantic on January 13, becoming the second earliest hurricane on record 6 and just the fourth named Atlantic storm to form in January since records began in Alex is also only the second hurricane to form in the Atlantic Basin east of 30 W and north of 30 N after Vince in Alex tracked north into increasingly cooler sea surface temperatures and weakened before making landfall on the island of Terceira on January 15 as a tropical storm, with maximum sustained wind speeds of 63 mph (101 km/hr). Alex caused minor damage in the Azores with some power outages and localized flooding reported. Tropical Storm Bonnie On May 28, a mid- to upper-level low that was cut off from the mid-latitude westerlies northeast of the Bahamas gained sufficient organization to become Bonnie, the second named storm of the 2016 Atlantic hurricane season. Bonnie made landfall near Charleston, South Carolina, on May 29 as a tropical depression. Bonnie remained over land for more than a day and produced rainfall accumulations of up to 10 in (250 mm) across southern and central parts of South Carolina and eastern Georgia, leading to localized flash flooding. While over land, deep convection slowly dissipated and on May 30 Bonnie was reclassified as a post-tropical remnant low. The system tracked east through May 31 and re-emerged into the Atlantic Ocean where it gradually weakened to a tropical depression and post-tropical remnant low by June 5. 6 The earliest hurricane on record formed on January 4,

7 Tropical Storm Colin Tropical Storm Colin developed from a broad area of low pressure in the western Caribbean Sea, near the Yucatan Peninsula, in early June. Conditions were favorable for development as the system tracked north into the Gulf of Mexico. Circulation and structure became sufficient to classify the system as a tropical storm on June 5, making it the Atlantic Basin s earliest third named storm in a calendar year on record. Colin tracked northeast toward the northwestern coast of Florida and made landfall in Taylor County, Florida, on Tuesday, June 7. Tropical storm conditions impacted much of northwestern Florida and southeastern Georgia, although no significant damage was reported. Colin rapidly tracked across northern Florida and southeastern Georgia, weakening in the process, and emerged into the Atlantic on June 7 as a weak tropical storm. Although deep convection persisted, the system gradually weakened as it tracked northeast into the Atlantic Ocean and was reclassified as a post-tropical cyclone. Tropical Storm Danielle Danielle developed from a low pressure system over the southwestern Gulf of Mexico. The system tracked west and entered a region of reduced vertical wind shear, reaching tropical storm strength on Monday, June 20, to become the Atlantic Basin s earliest fourth named storm in a calendar year on record. Danielle made landfall just north of Tuxpan, Mexico, on Monday, June 20, as a weak tropical storm, bringing rainfall accumulations of up to 6 in (152 mm) to northern Veracruz over a period of 72 hours, causing localized flash flooding. Danielle rapidly weakened as the system encountered the mountainous terrain of eastern Mexico, dissipating over southwestern Mexico on Tuesday, June 21. Hurricane Earl After an active start, the 2016 hurricane season quieted down with no named storms forming in the month of July, though a tropical wave was observed in the eastern Atlantic on Monday, July 25, which developed into Earl on Tuesday, August 2, in the Caribbean Sea. Earl tracked rapidly west and intensified to hurricane status on Wednesday, August 3, making landfall the following day near Belize City as a Category 1 storm, with maximum sustained wind speeds of around 75 mph (120 km/hr). Earl was the first hurricane to affect Belize since Hurricane Ernesto almost exactly four years earlier (August 7, 2012). Hurricane force conditions were experienced primarily over northern Belize, where buildings were damaged or destroyed, power lines downed, and trees uprooted. A storm surge of up to 5 ft (2 m) left up to 80 percent of the houses in Belize District under water. Shortly after landfall, Earl was downgraded to a tropical storm over the northern Guatemalan province of Peten, though it retained its tropical storm status during its transit over Guatemala and the Campeche Province of Mexico. Earl made a second landfall as a tropical storm on Saturday, August 6, approximately 40 mi (65 km) southeast of Veracruz City, Mexico. Widespread rainfall accumulations of up to 8 in (200 mm) were recorded across eastern Mexico, with maximum accumulations of up to 18 in (450 mm) in mountainous regions. The heavy rains led to multiple mudslides in eastern Mexico, where an entire hill collapsed onto a nearby village near Huauchinango in Puebla, killing up to 39 people. The mountainous terrain of eastern Mexico disrupted the storm s circulation, causing it to dissipate on Saturday, August 6. The remnants of Earl continued westward and interacted with an area of disturbed weather along the Pacific coast of Mexico, reintensifying into a new named system in the eastern Pacific: Tropical Storm Javier. There are just 11 cases of tropical storms or hurricanes making the transition between the Atlantic Basin and the eastern 6

8 Pacific Basin in records dating back to Tropical Storm Fiona The sixth named storm of the 2016 season, Tropical Storm Fiona was declared a tropical storm on Wednesday, August 17, approximately 800 mi (1,300 km) west of the Cape Verde islands. The system tracked west-northwest to the south of a subtropical high over the eastern Atlantic. Fiona reached peak intensity over the Atlantic on Saturday, August 20, with maximum sustained wind speeds of 52 mph (84 km/hr). The system did not impact land and the remnants tracked northwest, merging with a decaying frontal zone south of Bermuda to form Tropical Depression Eight. Major Hurricane Gaston Gaston increased to tropical storm strength early UTC on Tuesday, August 23. Gaston briefly strengthened to a Category 1 hurricane on Thursday, August 25, with maximum sustained wind speeds of 75 mph (120 km/hr) before weakening back to a tropical storm 12 hours later. The system reintensified on Sunday, August 28, over the open waters of the central Atlantic quickly reaching Category 3 strength, becoming the first major hurricane of the 2016 season. The system weakened to a tropical storm on Friday, September 2, and passed about 20 mi (30 km) south of Flores, the westernmost island in the Azores. Although tropical storm conditions were reported, there were no reports of damage. The system degenerated to a post-tropical cyclone 24 hours later on Saturday, September 3, northwest of the Azores. Tropical Depression Eight The remnants of Tropical Storm Fiona merged with a decaying stationary frontal zone south of Bermuda to form Tropical Depression Eight on Sunday, August 28, around 400 mi (650 km) southeast of Cape Hatteras, North Carolina. The depression failed to intensify in an environment of moderate shear and dry air as it tracked westnorthwest and came within about 70 mi (111 km) of Cape Hatteras, though no significant wind, rain, or surf was experienced along the coastline. The system became embedded within the mid-latitude flow and dissipated by Thursday, September 1, with the remnants absorbed into a frontal system the following day. Hurricane Hermine The precursor to Hermine was an African easterly wave designated as Invest 99L by the National Hurricane Center (NHC) that tracked for 10 days across the tropical Atlantic. Dry and stable Saharan air initially inhibited development over the central tropical Atlantic and moderate-to-high wind shear disrupted the development as it tracked north of Puerto Rico and through the southern Bahamas. The system was declared Tropical Storm Hermine on Wednesday, August 31. Hermine rapidly strengthened as it tracked northeast over warm waters in the Gulf of Mexico and was upgraded to a hurricane late on Thursday, September 1, approximately 175 mi (280 km) south-southwest of Tallahassee. The storm made landfall approximately 10 mi (16 km) east of St. Marks and around 28 mi (44 km) southeast of Tallahassee early local time on Friday, September 2, as a Category 1 storm with maximum sustained winds of 80 mph (130 km/hr). Hermine was the first hurricane to hit Florida since 2005 when Hurricane Wilma made landfall as a Category 3 storm. Hurricane Hermine also broke a streak of no hurricanes in the Gulf of Mexico, which had stretched 1,082 days back to Hurricane Ingrid in At landfall, tropical storm conditions extended across much of northwestern Florida. Rainfall accumulations of up to 5 in (125 mm) were recorded in Tallahassee, while the outer rain bands of Hermine produced up to 22 in (560 mm) in Pinellas County, Florida, in the 72 hours up to Friday, September 2. The United States Geological Survey (USGS) observed high-water marks indicating that the Florida panhandle region experienced a storm surge of between 7.9 and 10.3 ft ( m) above North American Vertical Datum (NAVD 88). The combination of heavy 7

9 rainfall and storm surge at the time of the system s landfall had produced localized flash, river, and storm surge flooding along parts of Florida s Gulf Coast. Up to 3,000 properties were damaged to varying degrees across northwestern Florida and approximately 325,000 power outages were reported at the peak of the storm. Shortly after landfall, Hermine weakened back to a tropical storm as it tracked northeast across northwestern Florida, Georgia, South Carolina, and North Carolina, with tropical storm force conditions experienced across the regions. No major structural damage was reported in Florida, though minor flash and river flooding caused power cuts to over 107,000 customers. A tornado damaged 22 homes on Skidaway Island in Georgia. More than 1,500 homes were flooded or sustained minor wind damage in the Carolinas, and up to 100,000 people were without power during the storm. Hermine re-entered the North Atlantic approximately 55 mi (85 km) south of Elizabeth City, North Carolina, on Saturday, September 3, and shortly after was reclassified as a post-tropical low, though it continued to produce tropical storm strength winds. Figure 6 illustrates Hermine s 1-minute sustained wind speed footprint in the southeastern U.S. RMS does not expect insured losses from wind and surge damage associated with Hurricane Hermine to exceed US$400 million, with residential insured losses representing approximately percent of the total insured loss and approximately 30 percent associated with coastal flooding. Losses associated with inland flooding are not included in this estimate. Figure 6: Hurricane Hermine s 1-minute sustained wind speed footprint in southeastern U.S. (Image from RMS HWind, 2016.) 8

10 Tropical Storm Ian The ninth named storm of the 2016 season, Tropical Storm Ian, formed on Monday, September 12, from a tropical wave off the western coast of Africa. Over the storm s life span, Ian tracked north and then northeast over the open Atlantic, strengthening slowly over warm sea surface temperatures before tracking over cooler sea surface temperatures and into an environment of high wind shear. Ian did not impact any land masses before deep convection gradually dissipated as cold air wrapped into the center of the storm and the system was reclassified as a post-tropical low on Friday, September 16. Tropical Storm Julia On Tuesday, September 13, an area of low pressure developed persistent and sufficient deep convection to be classified as a tropical storm. At the time of classification the center of the system was over land, becoming the first storm on record to form over land in Florida. Other tropical storms that formed over land include Tropical Storm Beryl in Louisiana in 1988, Tropical Storm Danny, which strengthened from a depression over North Carolina in 1997, and tropical cyclones Paul and Carlos in Australia in 2010 and The system slowly tracked north along Florida s east coast and into Georgia, producing bands of showers and thunderstorms across the southeastern United States. Julia produced rainfall totals up to 4 in (100 mm) in Charleston, South Carolina, causing some minor localized flooding in Georgia and South Carolina. The system generated strong surf conditions and rip currents. Julia tracked northeast offshore of Georgia through Wednesday, September 14, and weakened to a tropical depression as it meandered off the southeastern coast of the U.S, transitioning to a post-tropical cyclone on Sunday, September 18. Tropical Storm Karl The eleventh named storm of the 2016 season, Tropical Storm Karl, was declared on Friday, September 16. Karl tracked west over the central tropical Atlantic, although strong to moderate wind shear and a dry environment prevented intensification. In fact, strong wind shear weakened the system back to a tropical depression as it curved northwest around the subtropical high. Karl re-strengthened to a tropical storm due to reduced shear as the system approached Bermuda. Karl bypassed within 50 miles of Bermuda on Saturday, September 24, as a tropical storm, with a maximum intensity of 63 mph (102 km/hr). Minimal damage was reported on the island although heavy rains associated with the storm brought some flooding in low-lying areas. Karl was absorbed by a larger extratropical storm over the North Atlantic on September 26. Tropical Storm Lisa The twelfth named storm of the 2016 season, Tropical Storm Lisa, developed from a tropical wave that moved off the western coast of Africa in mid-september. The system tracked northwest under the influence of a weak midtropospheric ridge and strengthened as it moved into favorable environmental conditions of light shear and warm sea surface temperatures in the eastern Atlantic. Lisa remained at tropical storm strength until Friday, September 23, when strong shear weakened the system to a tropical depression. The system continued to weaken and transitioned to a remnant low on Saturday, September 24. Major Hurricane Matthew Matthew, the thirteenth named storm of the 2016 hurricane season, was named on Wednesday, September 28, before intensifying to become the fifth hurricane of the 2016 season on Thursday, September 29, while over the 9

11 Caribbean Sea. Over the next 36 hours, Matthew rapidly intensified from a Category 1 storm and briefly reached Category 5 strength on Saturday, October 1, with a peak intensity of 160 mph (260 km/hr). At this point Matthew became the second major hurricane of the 2016 season and the first Category 5 hurricane in the Atlantic Basin since Hurricane Felix in 2007, as well as the southernmost Category 5 storm in the Atlantic Basin s history. After briefly reaching Category 5 strength, Matthew weakened to a Category 4 storm by 18:00 UTC on Saturday, October 1, and took a sharp right turn toward Jamaica, western Haiti, and eastern Cuba, making landfall over Haiti s southern Tiburon Peninsula on Tuesday, October 4, as a strong Category 4 storm. Matthew was the strongest storm to hit the country in 53 years, since Cleo in In Haiti, Matthew impacted more than 370,000 structures and over 1,000 fatalities were confirmed. The storm is thought to have affected at least 2.1 million people, at least 1.4 million of whom required humanitarian assistance. Following the event, the United Nations said the country was facing its largest humanitarian event since the earthquake in Matthew weakened slightly after interacting with the mountainous terrain of western Haiti, though remained at Category 4 strength as it made landfall along the extreme eastern coast of Cuba on Wednesday, October 5. The municipality of Baracoa was the worst affected, with significant wind damage to hundreds of properties and roof damage to hundreds of other buildings. Matthew briefly weakened to a Category 3 storm shortly after passing over the mountainous terrain of eastern Cuba and curved to the northwest into the Bahamas, re-strengthening to a Category 4 storm on Thursday, October 6. RMS HWind observations revealed maximum 1-minute sustained wind speeds over water of 123 mph (198 km/h) as Matthew passed over Grand Bahama Island (see Figure 7). RMS reconnaissance teams observed widespread wind damage on the islands of New Providence and Paradise Island, particularly on the south and west sides of the island. Damage was generally more severe and widespread on Grand Bahama, including large-scale structural roof failures in both residential and commercial buildings, damage to Grand Bahama International Airport, some storm surge impacts, and downed trees and power lines. On October 6-7, Matthew underwent an eyewall replacement cycle 7 and slowly weakened to a Category 3 hurricane when the system was located approximately 40 mi (65 km) off the east coast of Florida at 06:00 UTC on Friday, October 7. Operational model guidance indicated that the possibility of Matthew making landfall along Florida s east coast was extremely sensitive to changes in the location and extent of a subtropical ridge located off the southeastern coast of the U.S., with slight variations between models bringing the storm on land or keeping it offshore. As Matthew approached Florida, the ridge progressed east and steered the storm further offshore. Matthew made its closest pass to land when the western eyewall came within 5 mi (8 km) of Cape Canaveral shortly after 12:00 UTC on Friday, October 7. Hurricane force winds impacted a 185 mi (300 km) stretch of eastern Florida between Vero Beach and Ponte Vedra Beach, extending around 8 mi (13 km) inland. Much of eastern Florida experienced tropical storm force winds, though southern and central Florida was mostly spared major wind damage from Matthew. RMS reconnaissance identified minor damage to buildings in Florida, particularly to older homes and to properties in Daytona Beach. Storm surge effects were more significant than wind effects north of St. Augustine Beach. Flood heights reached up to approximately 2 ft (0.6 m) on Anastasia Island, causing damage to many small residential and commercial buildings. Downtown Jacksonville exhibited few signs of visible damage, though there was notable flood damage to properties surrounding the Amelia River. In Georgia, RMS reconnaissance revealed few signs of direct wind damage to buildings in and around Savannah. Up to 3 ft (0.9 m) of storm surge inundated a confined, low-lying portion of downtown Savannah near the Savannah 7 Intense tropical cyclones can sometimes develop two concentric eyewalls. Eyewall replacement is the process where the outer eyewall slowly contracts and replaces the inner eyewall. This phenomenon generally weakens the intensity of the storm, though intensification can occur after completion of the process. 10

12 River, leading to some property damage. Matthew weakened to a Category 2 storm off the coast of Jacksonville early UTC on Saturday, October 8, and continued to weaken as the system passed offshore of Georgia where river or surge flooding inundated hundreds of homes and more than 250,000 people were left without power. Matthew then made a brief landfall as a Category 1 storm near McClellanville in Charleston County, South Carolina, roughly 35 mi (55 km) northeast of Charleston between 15:00 and 15:20 UTC on Saturday, October 8. Matthew became the first hurricane since Hazel in 1954 to make landfall in the United States north of Florida in the month of October. Matthew caused major flooding across North Carolina in the days after passing over the state, with several rivers surpassing major flood stage. Rainfall totals of up to 14 in (355 mm) fell during the event, leading to widespread severe flash and river flooding. Princeville, Lumberton, and Fair Bluff remained under water for several days. North Carolina officials estimated that around 100,000 homes, businesses, and government buildings were damaged due to inland flooding. County-level states of emergency were declared in 40 of North Carolina s 100 counties. Matthew caused 32 fatalities across South and North Carolina. RMS reconnaissance teams noted limited wind damage along the South and North Carolina coastline. Coastal properties with raised foundations prevented significant and widespread flooding damages. In Virginia, rainfall of up to 10 in (250 mm) and tidal flooding caused some minor localized flooding, while wind damage was confined to the Norfolk and Virginia Beach areas. Matthew tracked into the open waters of the Atlantic and was classified as a post-tropical cyclone on Sunday, October 9, before dissipating and merging with another low the following day. RMS estimates that Matthew will be the costliest Atlantic hurricane for the (re)insurance industry since Sandy in 2012, with insured loss from Major Hurricane Matthew in the U.S. expected to be between US$1.5 billion and US$5 billion. Residential lines contribute approximately 70 percent of the loss, and coastal flooding caused by storm surge contributes around 30 percent of the all-lines loss. In the Caribbean, RMS estimates that insured losses will fall between US$1 billion and US$3 billion, with a large majority of the loss originating from the Bahamas. The RMS best-estimate losses exclude damage associated with inland flooding. 11

13 Figure 7: Major Hurricane Matthew s 1-minute sustained wind speed footprint in the Caribbean and the southeastern U.S. coast. (Image from RMS HWind.) 12

14 Major Hurricane Nicole Nicole was declared a tropical storm on Tuesday, October 4, and strengthened into a hurricane on Thursday, October 6. The system rapidly intensified to a Category 2 storm the following day, reaching an initial peak intensity of 104 mph (167 km/hr). On Thursday, October 7, both Hurricane Matthew and Hurricane Nicole were active in the Atlantic, marking the first time since 1964 that two Category 2 or stronger hurricanes occurred simultaneously in the Atlantic Basin west of 65 W. The National Hurricane Center (NHC) noted that in 1964 Hurricanes Dora and Ethel were located in similar positions to Matthew and Nicole. Nicole s intensity fluctuated somewhat, though the system strengthened to a Category 2 storm on Wednesday, October 12, while tracking northwest and then north. Nicole curved to the northeast into the mid-latitude westerly flow and accelerated toward Bermuda. Nicole impacted Bermuda as a Category 3 hurricane when the center of the storm passed approximately 10 mi (16 km) to the east of Bermuda. The strongest winds stayed offshore to the east, although the northwest and southwest eyewall passed over Bermuda, exposing the island to peak wind gusts of up to 122 mph (196 km/hr). Hurricane force winds were experienced across the whole island. The storm brought heavy rain, damaging winds, and large swells to Bermuda, causing some minor damage to property, localized flooding, power outages, and transport disruption. It is rare for a major hurricane to impact Bermuda. According to the NHC s Atlantic hurricane database, which goes back to 1851, there have only been seven major hurricanes that have passed within 46 mi (74 km) of Bermuda. Nicole was the first major hurricane to impact Bermuda since Hurricane Fabian bypassed to the west of the island as a Category 3 hurricane in September Nicole was also the first hurricane to make landfall in the small British Overseas Territory since Hurricanes Fay and Gonzalo made landfall as Category 1 and 2 storms, respectively, in October Major Hurricane Otto Otto, the fifteenth and final named storm of the 2016 North Atlantic hurricane season formed on Monday, November 21. Otto strengthened over the following 24 hours to become the seventh hurricane of the 2016 season on Tuesday, November 22, breaking the record previously set by Hurricane Martha in 1969 for the latest hurricane to develop in the Caribbean Sea. Otto continued to set several more records. The system rapidly intensified to a Category 3 storm 8 shortly before making landfall in a remote region of southeastern Nicaragua on Thursday, November 24, with maximum sustained wind speeds of around 115 mph (185 km/hr). Otto was the fourth major hurricane of the 2016 season and became the southernmost land falling hurricane in Central America on record, surpassing Hurricane Irene in 1971 which made landfall in southern Nicaragua around 35 miles north of Otto. Around 900 properties sustained wind damage in Nicaragua and more than 12,000 people were displaced. The storm brought up to 7 in (178 mm) of rainfall to Bluefields, Nicaragua, with localized flash and river flooding. Otto rapidly weakened as it tracked across the mountainous terrain of southern Nicaragua, weakening to a Category 1 storm shortly before crossing the border into northwestern Costa Rica on Friday, November 25. Otto is the first known hurricane to move over Costa Rica. While in Costa Rica, Otto damaged at least 100 structures, and flash river flooding or mudslides impacted several municipalities. Although weakened, Otto tracked into the eastern Pacific as a tropical storm and while doing so became the first 8 In its post-season assessment released on January 30, 2017, the National Hurricane Center (NHC) increased the peak intensity of Hurricane Otto from Category 2 to Category 3. 13

15 storm to retain its name while moving from the North Atlantic to the northeastern Pacific. 9 The system continued tracking west-southwest and gradually weakened as it encountered increased shear, dry air, and cooler sea surface temperatures. The system deteriorated and dissipated on Saturday, November 26. Review of the 2016 Season Forecasts The 2016 season fell within the range of published forecasts of named storms, 4-10 hurricanes, and 1-4 major hurricanes, and an ACE value ranging from 94 to 179. Table 1 shows the 2016 seasonal forecast from the three main forecasting groups Colorado State University (CSU), Tropical Storm Risk (TSU), and National Oceanic and Atmospheric Administration Climate Prediction Center (NOAA CPC) at the time the RMS North Atlantic Hurricane Season Outlook was released in June Activity averages for and are also presented. Forecast Group Named Storms Hurricanes Major Hurricanes ACE Index (10 4 kt 2 ) CSU (June 2016) TSR (May 2016) 17 (±4) 9 (±3) 4 (±2) 130 (±49) NOAA CPC (May 2016) Average Average Season Table 1: Summary of the 2016 North Atlantic Basin seasonal forecasts, average activity, and 2016 storm totals. The agencies anticipated a near-average to above-average season in the Atlantic Basin in 2016 due to a combination of contrasting atmospheric and oceanic conditions, including the possible evolution of the El Niño- Southern Oscillation (ENSO) into La Niña conditions and the possibility for colder-than-average sea surface temperatures (SSTs) in the North Atlantic. La Niña conditions, in isolation, would be expected to result in greaterthan-average Atlantic hurricane activity, while colder-than-average SSTs could inhibit hurricane formation. During the peak of the Atlantic hurricane season, sea surface temperatures in the equatorial Pacific were near climatological average, in line with predictions from the main forecasting groups, which had anticipated either cool neutral or weak La Niña conditions. Observed North Atlantic SSTs throughout the hurricane season were slightly above normal, especially in the tropical Atlantic, defying some forecasts of below-average SSTs. Role of the Ocean and Atmosphere in the 2016 Season Activity The 2016 North Atlantic hurricane season was influenced by several different conflicting conditions, resulting in slightly above-average activity. The main influencing factors were: Anomalously dry air at mid-levels in the tropical Atlantic and Caribbean Sea, which can inhibit the formation and intensification of tropical cyclones Lower-than-average wind shear in the Atlantic main development region and southeastern U.S., with above-average shear in the far western Caribbean Sea and Central America, which offers a preferential environment for cyclone formation and intensification Above-average sea surface temperatures in the tropical Atlantic throughout the season, which can provide a source of energy for cyclone formation Slightly below-average sea level pressures across the tropical Atlantic, which can lead to enhanced tropical convection and cyclone formation 9 In 1996, Hurricane Cesar retained its tropical cyclone status while crossing from the Atlantic to the northeastern Pacific but was renamed as Hurricane Douglas. 14

16 These conditions individually created favorable and unfavorable environments for cyclone formation and intensification, with the conflicting influences resulting in slightly above-average activity. The following sections discuss the conditions generally used for seasonal forecasts, as well as the factors that contributed to the slightly above-average activity observed in Atlantic Sea Surface Temperatures Some forecasting agencies had anticipated cooler-than-average sea surface temperatures (SSTs) in the tropical North Atlantic in 2016, which typically inhibits hurricane formation. However, the 2016 season was characterized by slightly warmer-than-average temperatures in the tropical North Atlantic, with very little change observed throughout the season. This behavior stands in contrast to recent years, where significant changes have been observed during the season associated with intra-seasonal changes to the thermohaline circulation (THC) and the North Atlantic Oscillation (NAO). Warmer conditions in the main development region of the central tropical Atlantic are conducive to above-average storm activity. Cooler SST anomalies observed in the eastern tropical North Atlantic gradually reduced through the 2016 season, tempering a key inhibitor of tropical cyclone formation (see Figure 8). Figure 8: Anomalous sea surface temperatures ( K) across the North Atlantic Basin for the period from June to November [Image from National Oceanic and Atmospheric Administration Hurricane Research Division Earth System Research Laboratory (NOAA ESRL).] Tropical Atlantic Sea Level Pressure Tropical cyclone activity in the North Atlantic Basin can be enhanced by lower sea level pressure in the tropical Atlantic due to increased instability and increased low-level moisture. Conversely, higher sea level pressure in the tropical Atlantic can reduce cyclone activity. 15

17 In general, the 2016 hurricane season experienced slightly below-average sea level pressures across the tropical Atlantic, while much of the main development region, the Caribbean Sea, and off the southeastern U.S. coastline observed lower-than-average sea level pressures, which was one of the favorable factors for increased activity in 2016 (see Figure 9). Figure 9: June to November 2016 anomalous changes in sea level pressure (mb) across the North Atlantic Basin. (Image from NOAA ESRL.) El Niño-Southern Oscillation There is a strong relationship between the El Niño-Southern Oscillation (ENSO) and hurricane activity (Gray, 1984). El Niño events, warm phases of ENSO in the Pacific Basin, inhibit Atlantic hurricane activity due to teleconnections that increase vertical wind shear over the main development region of the tropical Atlantic, with the reverse true for La Niña events. Heading into the 2016 hurricane season, one of the strongest El Niño years on record was starting to dissipate. One of the key factors considered by the forecasts in how the season would play out was the timing of the transition to neutral and La Niña conditions (see Figure 10). El Niño conditions dissipated through the spring of 2016, replaced by cool neutral conditions during the summer and a weak La Niña phase by September, validating the predictions of many main forecasting groups. La Niña typically promotes an active hurricane season in the Atlantic, with generally weaker vertical wind shear and trade winds and less atmospheric stability. Most of the main development region and southeastern U.S. observed lower-than-average wind shear through the 2016 season, which is consistent with a La Niña phase of the ENSO. 16

18 Oceanic Niño Index (ONI) El Niño Threshold DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND La Niña Threshold Figure 10: The 2016 Oceanic Niño Index (ONI) ( C) [three-month running mean of ERSST.v4 SST anomalies in the Niño 3.4 region (5 N-5 S, W)], based on centered 30-year base periods updated every five years. Months of the year are abbreviated with first letter (e.g., A = April or August). (Data from the NOAA CPC, 2016.) El Niño/La Niña conditions are defined by six or more consecutive months above/below the (-)0.5 C threshold. (Data from the National Weather Service Climate Prediction Center, 2016.) Tropical Atlantic Vertical Wind Shear Most of the main development region and southeastern U.S. observed lower-than-average wind shear through the 2016 season, contributing to the above-average development and intensification of tropical cyclones in these regions. In contrast, the far western Caribbean Sea and Central America recorded above-average shear, which likely suppressed activity locally. Strong vertical wind shear dominated the Caribbean Sea and central tropical Atlantic in previous seasons, limiting cyclone formation and intensification in the region in 2014 and Madden-Julian Oscillation The Madden-Julian Oscillation (MJO) is an intra-seasonal oscillation of tropical rainfall that often cycles around the tropics every days. It is characterized by an eastward progression of a large region of both enhanced and suppressed convection and tropical rainfall. There is evidence to show that the MJO modulates tropical cyclone activity by providing a large-scale environment that influences Atlantic hurricane development. When the MJO produces enhanced convection, precipitation, and divergent upper-level winds, Gulf of Mexico and Caribbean Sea hurricanes are four times more likely to occur (Klotzbach, 2010). Contrarily, when upper-level winds are convergent and convection and precipitation is suppressed, tropical cyclone activity is decreased. The MJO was relatively weak during the early and late stages of the 2016 season, having no significant impact on tropical storm development. 17

19 Tropical Atlantic Moisture Moist air in the low and mid-levels of the atmosphere is critical for tropical cyclone development. Dry air hinders development by increasing the energy barrier that must be overcome to allow the dry air to rise from the water s surface. In the three-month period from August to October, the tropical Atlantic and Caribbean was anomalously dry at mid-levels, as illustrated in Figure 11, similar to the pattern observed in the three previous years. Lower moisture availability can inhibit the formation and intensification of tropical cyclones. From mid-june to the end of July, there was a particularly inactive period of the 2016 season, with no tropical storms or hurricanes forming in the Atlantic. During this period, the downward Madden-Julian Oscillation (MJO) phase combined with the very dry air of the Saharan Air Layer across the tropical Atlantic and the main development region to produce an atmosphere unsuitable for development or intensification of cyclones. Figure 11: August to October mb relative humidity (%) moisture anomalies. (Image from NOAA ESRL.) Steering Currents Steering currents in the troposphere, resulting from atmospheric pressure patterns across the Northern Hemisphere, determine the movement of storm systems, ultimately determining which storms end up making landfall or are pushed out to sea. For several years, an elongated region of relatively low atmospheric pressure, known as anomalous troughing, dominated the southeastern U.S. coast. This had resulted in a predominant steering flow that prevented storms from progressing toward the U.S. East Coast. In 2016, this pattern was not present. Instead, much of the North Atlantic experienced above-average tropospheric pressures, including the southeastern U.S., resulting in a predominant steering flow toward the Caribbean Sea, the Gulf of Mexico, and the U.S. East Coast (see Figure 12). 18

20 Figure 12: 600mb geopotential height (m) anomalies across the North Atlantic Basin from June to November (Image from NOAA ESRL.) 19

21 Outlook for the 2017 North Atlantic Hurricane Season As many of the factors that influence hurricane activity cannot be predicted at this lead time, it is difficult to forecast what the 2017 North Atlantic hurricane season will bring. However, Tropical Storm Risk (TSR) and Colorado State University (CSU) issued extended forecasts in December 2016 to provide a glimpse as to what may be in store. TSR issued its first forecast for the 2017 hurricane season on December 13, predicting 14 (±4) tropical storms, 6 (±3) hurricanes, and 3 (±2) major hurricanes. TSR forecast that the 2016 ACE index will reach 101 (±58), with a 27 percent probability that the index will be above the average, a 35 percent probability that it will be near average, and a 38 percent probability that the index will be below average. TSR s main predictor of this extended forecast is the July-September trade wind speed over the Caribbean Sea and the tropical Atlantic, which influences cyclonic vorticity and wind shear in the main development region. TSR noted a large degree of uncertainty associated with its December forecast, which the group attributed to large uncertainties in ENSO and in the North Atlantic and Caribbean Sea surface temperatures forecast at this extended range. TSR reports that the precision of its forecast based on activity between 1980 and 2016 is low, where the average forecast skill at this lead time is 1 percent for tropical storms, 1 percent for hurricanes, 7 percent for major hurricanes, and 12 percent for ACE. On December 1, CSU issued a qualitative discussion of features likely to affect the 2017 Atlantic hurricane season, rather than a specific quantitative forecast. Per CSU, the 2017 Atlantic hurricane season will be determined by the strength of the thermohaline circulation (THC) and Atlantic Multidecadal Oscillation (AMO), as well as the phase of ENSO. Several forecast models are suggesting a transition from weak La Niña conditions back to warm neutral or El Niño conditions by next summer. It would be unusual to return to El Niño conditions just two years after the strong El Niño of 2015, with CSU noting that of the 20 moderate-to-strong El Niño summer events since 1871, only one returned to El Niño conditions two years later. As in previous years, NOAA does not release a seasonal forecast prior to late May. The number of storms that will make landfall on an Atlantic coastline or over the Caribbean in 2017 will be determined by the weather patterns and steering currents throughout the season, which cannot be predicted this far in advance. RMS will provide an in-depth review of the seasonal forecasts and the oceanic and atmospheric conditions for the 2017 hurricane season in June 2017, along with a detailed overview of RMS Event Response offerings for the 2017 season. 20

22 References Gray, W. M. (1984). Atlantic seasonal hurricane frequency. Part I: El Niño and the 30 mb quasi-biennial oscillation influences. Monthly Weather Review, 112, Klotzbach, P. J. (2010). On the Madden-Julian Oscillation Atlantic hurricane relationship. Journal of Climate, 23, Retrieved from pdf Klotzbach, P. J. (2016a). Summary of 2016 Atlantic tropical cyclone activity and verification of author s seasonal and two-week forecasts. Department of Atmospheric Science, Colorado State University. Retrieved from webcms.colostate.edu/tropical/media/sites/111/2016/11/ pdf Klotzbach, P. J. (2016b). Qualitative discussion of Atlantic Basin seasonal hurricane activity for Department of Atmospheric Science, Colorado State University. Retrieved from sites/111/2016/12/ pdf National Hurricane Center (NHC). (2016). Retrieved December 2016 from National Hurricane Center (NHC). (2017). Retrieved February 2017 from National Oceanic and Atmospheric Administration Earth System Research Laboratory (NOAA ESRL). (2016). Retrieved December 2016 from National Oceanic and Atmospheric Administration Hurricane Research Division (NOAA HRD). (2016). Retrieved December 2016 from National Weather Service Climate Prediction Center (NOAA CPC). (2016). Retrieved December 2016 from RMS HWind. (2016). Real-time hurricane impact data. Retrieved December 2016 from hwind/ Saunders, M., & Lea, A. (2016). Extended range forecast for Atlantic hurricane activity in Tropical Storm Risk. Issued December 13, Retrieved from WeatherBell Models. (2016). Dr. Ryan N. Maue. Global tropical cyclone activity. Retrieved December 2016 from 21