Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h (115 mph), or major hurricanes (Category 3 or above). When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a cyclone's eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.
The discovery of this process was partially responsible for the end of the U.S. government's hurricane modification experiment Project Stormfury. This project set out to seed clouds outside the eyewall, apparently causing a new eyewall to form and weakening the storm. When it was discovered that this was a natural process due to hurricane dynamics, the project was quickly abandoned.
Almost every intense hurricane undergoes at least one of these cycles during its existence. Recent studies have shown that nearly half of all tropical cyclones, and nearly all cyclones with sustained winds over 204 kilometres per hour (127 mph; 110 kn), undergo eyewall replacement cycles. Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 3 status on the Saffir-Simpson Hurricane Scale several times. Typhoon June (1975) was the first reported case of triple eyewalls, and Hurricane Juliette (2001) was a documented case of such.
The first tropical system to be observed with concentric eyewalls was Typhoon Sarah by Fortner in 1956, which he described as "an eye within an eye". The storm was observed by a reconnaissance aircraft to have an inner eyewall at 6 kilometres (3.7 mi) and an outer eyewall at 28 kilometres (17 mi). During a subsequent flight 8 hours later, the inner eyewall had disappeared, the outer eyewall had reduced to 16 kilometres (9.9 mi) and the maximum sustained winds and hurricane intensity had decreased. The next hurricane observed to have concentric eyewalls was Hurricane Donna in 1960. Radar from reconnaissance aircraft showed an inner eye that varied from 10 miles (16 km) at low altitude to 13 miles (21 km) near the tropopause. In between the two eyewalls was an area of clear skies that extended vertically from 3,000 feet (910 m) to 25,000 feet (7,600 m). The low-level clouds at around 3,000 feet (910 m) were described as stratocumulus with concentric horizontal rolls. The outer eyewall was reported to reach heights near 45,000 feet (14,000 m) while the inner eyewall only extended to 30,000 feet (9,100 m). 12 hours after identifying concentric eyewalls, the inner eyewall had dissipated.
Hurricane Beulah in 1967 was the first tropical cyclone to have its eyewall replacement cycle observed from beginning to end. Previous observations of concentric eyewalls were from aircraft-based platforms. Beulah was observed from the Puerto Rico land-based radar for 34 hours during which time a double eyewall formed and dissipated. It was noted that Beulah reached maximum intensity immediately prior to undergoing the eyewall replacement cycle, and that it was "probably more than a coincidence." Previous eyewall replacement cycles had been observed to decrease the intensity of the storm, but at this time the dynamics of why it occurred was not known.
As early as 1946 it was known that the introduction of carbon dioxide ice or silver iodide into clouds that contained supercooled water would convert some of the droplets into ice followed by the Bergeron–Findeisen process of growth of the ice particles at the expense of the droplets, the water of which would all end up in large ice particles. The increased rate of precipitation would result in dissipation of the storm. By early 1960, the working theory was that the eyewall of a hurricane was inertially unstable and that the clouds had a large amount of supercooled water. Therefore, seeding the storm outside the eyewall would release more latent heat and cause the eyewall to expand. The expansion of the eyewall would be accompanied with a decrease in the maximum wind speed through conservation of angular momentum.
The hypothesis was that the silver iodide would cause supercooled water in the storm to freeze, disrupting the inner structure of the hurricane. This led to the seeding of several Atlantic hurricanes. However, it was later shown that this hypothesis was incorrect. In reality, it was determined, most hurricanes do not contain enough supercooled water for cloud seeding to be effective. Additionally, researchers found that unseeded hurricanes often undergo the eyewall replacement cycles that were expected from seeded hurricanes. This finding called Stormfury's successes into question, as the changes reported now had a natural explanation.
The last experimental flight was flown in 1971, due to a lack of candidate storms and a changeover in NOAA's fleet. More than a decade after the last modification experiment, Project Stormfury was officially canceled. Although a failure in its goal of reducing the destructiveness of hurricanes, Project Stormfury was not without merit. The observational data and storm lifecycle research generated by Stormfury helped improve meteorologists' ability to forecast the movement and intensity of future hurricanes.
Secondary eyewalls were once considered a rare phenomenon. Since the advent of reconnaissance airplanes and microwave satellite data, it has been observed that over half of all major tropical cyclones develop at least one secondary eyewall. There have been many hypotheses that attempt to explain the formation of secondary eyewalls. The reason why hurricanes develop secondary eyewalls is not well understood.
Qualitatively identifying secondary eyewalls is easy for a hurricane analyst to do. It involves looking at satellite or radar imagery and seeing if there are two concentric rings of enhanced convection. The outer eyewall is generally almost circular and concentric with the inner eyewall. Quantitative analysis is more difficult since there exists no objective definition of what a secondary eyewall is. Kossin et al.. specified that the outer ring had to be visibly separated from the inner eye with at least 75% closed with a moat region clear of clouds.
While secondary eyewalls have been seen as a tropical cyclone is nearing land, none have been observed while the eye is not over the ocean. July offers the best background environmental conditions for development of a secondary eyewall. Changes in the intensity of strong hurricanes such as Katrina, Ophelia, and Rita occurred simultaneously with eyewall replacement cycles and comprised interactions between the eyewalls, rainbands and outside environments. Eyewall replacement cycles, such as occurred in Rita as it approached the Gulf Coast of the United States, can greatly increase the size of tropical cyclones while simultaneously decreasing in strength.
During the period from 1997–2006, 45 eyewall replacement cycles were observed in the tropical North Atlantic Ocean, 12 in the Eastern North Pacific and 2 in the Western North Pacific. 12% of all Atlantic storms and 5% of storms in the Pacific underwent eyewall replacement during this time period. In the North Atlantic, 70% of major hurricanes had at least one eyewall replacement, compared to 33% of all storms. In the Pacific, 33% of major hurricanes and 16% of all hurricanes had an eyewall replacement cycle. Stronger storms have a higher probability of forming a secondary eyewall, with 60% of category 5 hurricanes undergoing an eyewall replacement cycle within 12 hours.
During the years 1969-1971, 93 storms reached tropical storm strength or greater in the Pacific Ocean. 8 of the 15 that reached super typhoon strength (65 m/s), 11 of the 49 storms that reached typhoon strength (33 m/s), and none of the 29 tropical storms (<33 m/s) developed concentric eyewalls. The authors note that because the reconnaissance aircraft were not specifically looking for double eyewall features, these numbers are likely underestimates.
During the years 1949-1983, 1268 typhoons were observed in the Western Pacific. 76 of these had concentric eyewalls. Of all the typhoons that underwent eyewall replacement, around 60% did so only once; 40% had more than one eyewall replacement cycle, with two of the typhoons each experiencing five eyewall replacements. The number of storms with eyewall replacement cycles was strongly correlated with the strength of the storm. Stronger typhoons were much more likely to have concentric eyewalls. There were no cases of double eyewalls where the maximum sustained wind was less than 45 m/s or the minimum pressure was higher than 970 hPa. More than three-quarters of the typhoons that had pressures lower than 970 hPa developed the double eyewall feature. The majority of Western and Central Pacific typhoons that experience double eyewalls do so in the vicinity of Guam.
Since eyewall replacement cycles were discovered to be natural, there has been a strong interest in trying to identify what causes them. There have been many hypotheses put forth that are now abandoned. In 1980, Hurricane Allen crossed the mountainous region of Haiti and simultaneously developed a secondary eyewall. Hawkins noted this and hypothesized that the secondary eyewall may have been caused by topographic forcing. Willoughby suggested that a resonance between the inertial period and asymmetric friction may be the cause of secondary eyewalls. Later modeling studies and observations have shown that outer eyewalls may develop in areas uninfluenced by land processes.
There have been many hypotheses suggesting a link between synoptic scale features and secondary eyewall replacement. It has been observed that radially inward traveling wave-like disturbances have preceded the rapid development of tropical disturbances to tropical cyclones. It has been hypothesized that this synoptic scale internal forcing could lead to a secondary eyewall. Rapid deepening of the tropical low in connection with synoptic scale forcing has been observed in multiple storms, but has been shown to not be a necessary condition for the formation of a secondary eyewall. The wind-induced surface heat exchange (WISHE) is a positive feedback mechanism between the ocean and atmosphere in which a stronger ocean-to-atmosphere heat flux results in a stronger atmospheric circulation, which results in a strong heat flux. WISHE has been proposed as a method of generating secondary eyewalls. Later work has shown that while WISHE is a necessary condition to amplify disturbances, it is not needed to generate them.
In the vortex Rossby wave hypothesis, the waves travel radially outward from the inner vortex. The waves amplify angular momentum at a radius that is dependent on the radial velocity matching that of the outside flow. At this point, the two are phase-locked and allow the coalescence of the waves to form a secondary eyewall.
In a fluid system, β (beta) is the spatial, usually horizontal, change in the environmental vertical vorticity. β is maximized in the eyewall of a tropical cyclone. The β-skirt axisymmetrization (BSA) assumes that a tropical cyclone about to develop a secondary eye will have a decreasing, but non-negative β that extends from the eyewall to approximately 50 kilometres (30 mi) to 100 kilometres (60 mi) from the eyewall. In this region, there is a small, but important β. This area is called the β-skirt. Outward of the skirt, β is effectively zero.
Convective available potential energy (CAPE) is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. The higher the CAPE, the more likely there will be convection. If areas of high CAPE exist in the β-skirt, the deep convection that forms would act as a source of vorticity and turbulence kinetic energy. This small-scale energy will upscale into a jet around the storm. The low-level jet focuses the stochastic energy a nearly axisymmetric ring around the eye. Once this low-level jet forms, a positive feedback cycle such as WISHE can amplify the initial perturbations into a secondary eyewall.
After the secondary eyewall totally surrounds the inner eyewall, it begins to affect the tropical cyclone dynamics. Hurricanes are fueled by the high ocean temperature. Sea surface temperatures immediately underneath a tropical cyclone can be several degrees cooler than those at the periphery of a storm, and therefore cyclones are dependent upon receiving the energy from the ocean from the inward spiraling winds. When an outer eyewall is formed, the moisture and angular momentum necessary for the maintenance of the inner eyewall is now being used to sustain the outer eyewall, causing the inner eye to weaken and dissipate, leaving the tropical cyclone with one eye that is larger in diameter than the previous eye.
In the moat region between the inner and outer eyewall, observations by dropsondes have shown high temperatures and dewpoint depressions. The eyewall contracts because of inertial instability. Contraction of the eyewall occurs if the area of convection occurs outside the radius of maximum winds. After the outer eyewall forms, subsidence increases rapidly in the moat region.
Once the inner eyewall dissipates, the storm weakens; the central pressure increases and the maximum sustained windspeed decreases. Rapid changes in the intensity of tropical cyclones is a typical characteristic of eyewall replacement cycles. Compared to the processes involved with the formation of the secondary eyewall, the death of the inner eyewall is fairly well understood.
Some tropical cyclones with extremely large outer eyewalls do not experience the contraction of the outer eye and subsequent dissipation of the inner eye. Typhoon Winnie (1997) developed an outer eyewall with a diameter of 200 nautical miles (370 km) that did not dissipate until it reached the shoreline. The time required for the eyewall to collapse is inversely related to the diameter of the eyewall which is mostly because inward directed wind decreases asymptotically to zero with distance from the radius of maximum winds, but also due to the distance required to collapse the eyewall.
Throughout the entire vertical layer of the moat, there is dry descending air. The dynamics of the moat region are similar to the eye, while the outer eyewall takes on the dynamics of the primary eyewall. The vertical structure of the eye has two layers. The largest layer is that from the top of the tropopause to a capping layer around 700 hPa which is described by descending warm air. Below the capping layer, the air is moist and has convection with the presence of stratocumulus clouds. The moat gradually takes on the characteristics of the eye, upon which the inner eyewall can only dissipate in strength as the majority of the inflow is now being used to maintain the outer eyewall. The inner eye is eventually evaporated as it is warmed by the surrounding dry air in the moat and eye. Models and observations show that once the outer eyewall completely surrounds the inner eye, it takes less than 12 hours for the complete dissipation of the inner eyewall. The inner eyewall feeds mostly upon the moist air in the lower portion of the eye before evaporating.
Annular hurricanes have a single eyewall that is larger and circularly symmetric. Observations show that an eyewall replacement cycle can lead to the development of an annular hurricane. While some hurricanes develop into annular hurricanes without an eyewall replacement, it has been hypothesized that the dynamics leading to the formation of a secondary eyewall may be similar to those needed for development of an annular eye. Hurricane Daniel (2006) and Typhoon Winnie (1997) were examples where a storm had an eyewall replacement cycle and then turned into an annular hurricane. Annular hurricanes have been simulated that have gone through the life cycle of an eyewall replacement. The simulations show that the major rainbands will grow such that the arms will overlap, and then it spiral into itself to form a concentric eyewall. The inner eyewall dissipates, leaving a hurricane with a singular large eye with no rainbands.
Intense Tropical Cyclone Funso was a powerful tropical cyclone which produced flooding in Mozambique and Malawi in January 2012. It was the eighth tropical cyclone, the sixth named storm and the second tropical cyclone to form during the 2011–12 South-West Indian Ocean cyclone season. Funso was also the first intense tropical cyclone since Gelane in 2010 and the first storm to affect Mozambique since Jokwe in 2008.Cyclone Giovanna
Intense Tropical Cyclone Giovanna was a powerful tropical cyclone that affected Madagascar. Giovanna was the ninth tropical depression, seventh named storm and third tropical cyclone of the 2011–12 South-West Indian Ocean cyclone season. Giovanna is still blamed for 33 deaths along the Madagascar coast, and it is the first intense tropical cyclone to impact Madagascar, since Cyclone Bingiza in February 2011.Cyclone Hary
Very Intense Tropical Cyclone Hary was the strongest tropical cyclone in the 2001–02 South-West Indian Ocean cyclone season and the strongest storm worldwide in 2002. Developing on March 5 from the monsoon trough, the storm initially moved generally to the west and gradually intensified. With favorable conditions, Hary quickly intensified on March 7, developing an eye and well-defined outflow. After reaching an initial peak, the cyclone briefly weakened due to an eyewall replacement cycle, by which time the storm turned southwestward toward Madagascar. Hary re-intensified and attained peak winds of 220 km/h (140 mph) on March 10 just offshore eastern Madagascar, which made it the first very intense tropical cyclone since 2000.
After peaking, Hary weakened due to land interaction, and it struck Madagascar southeast of Antalaha. It was turning south over land, and as a result it quickly moved offshore. There were three deaths in the country, one of which from electrocution. There was locally heavy crop damage, and four bridges were destroyed. However, the damage was considered minimal, given the intensity of the storm. After affecting Madagascar, Hary accelerated to the southeast, and the eastern periphery of the circulation moved over Réunion. On the mountain peaks of the island, rainfall reached 1,344 mm (52.9 in), although it was much less near the coast. The rainfall caused flooding, killing one person, and 20,000 people were left without power. Hary became extratropical on March 13, although its remnants continued for several days as a powerful mid-latitude storm.Cyclone Kalunde
Intense Tropical Cyclone Kalunde was the strongest storm of the 2002–03 South-West Indian Ocean cyclone season. The eleventh named storm and sixth cyclone of the season, Kalunde formed on March 4 from an area of disturbed weather east-southeast of Diego Garcia. The storm steadily strengthened and attained severe tropical storm intensity on March 6. After starting a phase of rapid deepening, Kalunde attained cyclone intensity the next day. Kalunde attained its peak intensity on March 8, as an intense tropical cyclone. It maintained its peak strength for a day; shortly thereafter, the system began to weaken. After undergoing an eyewall replacement cycle, the storm brushed Rodrigues. Shortly after doing so, Kalunde weakened into a tropical cyclone and later a severe tropical storm. Two days later, on March 16, the cyclone transitioned into an extratropical cyclone and dissipated the next day.
Cyclone Kalunde brought US$3.15 million in damage to Rodrigues Island. A total of 1,600 homes and 40 boats were damaged. Severe coastal damage took place across the island; many roads were washed out. Power outages also occurred across the island, delaying residents access to information pertaining to Kalunde. About 80 percent of the drinking water was contaminated and the entire food crop was destroyed. However, no deaths were reported.Cyclone Kenneth
Intense Tropical Cyclone Kenneth was the strongest tropical cyclone to make landfall in Mozambique since modern records began. The cyclone also caused significant damage in the Comoro Islands and Tanzania. The fourteenth tropical storm, record-breaking tenth tropical cyclone, and tenth intense tropical cyclone of the 2018–19 South-West Indian Ocean cyclone season, Kenneth formed from a vortex that the Météo-France office on La Réunion (MFR) first mentioned on 17 April. The MFR monitored the system over the next several days, before designating it as Tropical Disturbance 14 on 21 April. The disturbance was located in a favorable environment to the north of Madagascar, which allowed it to strengthen into a tropical depression and later a tropical storm, both on the next day. The storm then began a period of rapid intensification, ultimately peaking as an intense tropical cyclone with 10-minute sustained winds of 215 km/h (130 mph) and a minimum central pressure of 934 hPa (27.58 inHg). At that time, Kenneth began to undergo an eyewall replacement cycle and weakened slightly, before making landfall later that day as an intense tropical cyclone. As a result of land interaction, Kenneth became disorganised as it made landfall and rapidly degenerated thereafter. The storm then shifted southward, with the MFR cancelling all major warnings for inland cities. Kenneth was reclassified as an overland depression after landfall, with the MFR issuing its warning at midnight UTC on 26 April. Thunderstorm activity developed off the coast of Mozambique on 27 April as the system began drifting northward. Kenneth re-emerged off the coast of northern Mozambique on 28 April, before dissipating on the next day.
Prior to Kenneth's landfall, local authorities evacuated over 30,000 people in the path of the storm in northern Mozambique. Kenneth killed at least 52 people in total; in the country of Comoros; Kenneth's wind and rainfall caused at least seven deaths, while 45 people were killed in Mozambique. Damage is currently estimated at $100 million (2019 USD).Hurricane Hector (2018)
Hurricane Hector was a powerful and long-lived tropical cyclone that was the first to traverse all three North Pacific basins since Genevieve in 2014. The eighth named storm, fourth hurricane, and third major hurricane of the 2018 Pacific hurricane season, Hector originated from an area of low pressure that formed a couple hundred miles west-southwest of Mexico on July 28. Amid favorable weather conditions, a tropical depression formed a few days later on July 31. The depression continued strengthening and became Tropical Storm Hector on the next day. Hector became a hurricane on August 2, and rapidly intensified into a strong Category 2 hurricane later in the day. After weakening while undergoing an eyewall replacement cycle, Hector quickly strengthened into a Category 4 hurricane late on August 5. Over the next week, Hector fluctuated in intensity multiple times due to eyewall replacement cycles and shifting wind shear. Hector achieved its peak intensity on August 6, as a high-end Category 4 hurricane with winds of 155 mph (250 km/h). On the following day, the hurricane bypassed Hawaii approximately 200 mi (320 km) to the south. Increasing wind shear resulted in steady weakening of the storm, beginning on August 11. At that time, Hector accumulated the longest continuous stretch of time as a major hurricane in the northeastern Pacific since reliable records began. Eroding convection and dissipation of its eye marked its degradation to a tropical storm on August 13. The storm subsequently traversed the International Dateline that day. Hector later weakened into a tropical depression on August 15, before dissipating late on August 16.
Hector prompted several islands in the Papahānaumokuākea Marine National Monument to issue tropical storm watches after the close pass by in Hawaii that warranted the issuance of a tropical storm warning for Hawaii County. Despite Hector having passed a couple hundred miles to the south of Hawaii, it still brought numerous adverse weather effects to Hawaii County and the surrounding islands.Hurricane John (2006)
Hurricane John was the eleventh named storm, seventh hurricane, and fifth major hurricane of the 2006 Pacific hurricane season. Hurricane John developed on August 28 from a tropical wave to the south of Mexico. Favorable conditions allowed the storm to intensify quickly, and it attained peak winds of 130 mph (210 km/h) on August 30. Eyewall replacement cycles and land interaction with western Mexico weakened the hurricane, and John made landfall on southeastern Baja California Sur with winds of 110 mph (175 km/h) on September 1. It slowly weakened as it moved northwestward through the Baja California peninsula, and dissipated on September 4. Moisture from the remnants of the storm entered the southwest United States.
The hurricane threatened large portions of the western coastline of Mexico, resulting in the evacuation of tens of thousands of people. In coastal portions of western Mexico, strong winds downed trees, while heavy rain resulted in mudslides. Hurricane John caused moderate damage on the Baja California peninsula, including the destruction of more than 200 houses and thousands of flimsy shacks. The hurricane killed five people in Mexico, and damage totaled $663 million (2006 MXN, $60.8 million 2006 USD). In the southwest United States, moisture from the remnants of John produced heavy rainfall. The rainfall aided drought conditions in portions of northern Texas, although it was detrimental in locations that had received above-normal rainfall throughout the year.Hurricane Norbert (2008)
Hurricane Norbert is tied with Hurricane Jimena as the strongest tropical cyclone to strike the west coast of Baja California Sur in recorded history. The fifteenth named storm, seventh hurricane, and second major hurricane of the 2008 hurricane season, Norbert originated as a tropical depression from a tropical wave south of Acapulco on October 3. Strong wind shear initially prevented much development, but the cyclone encountered a more favorable environment as it moved westward. On October 5, the National Hurricane Center (NHC) upgraded the depression to Tropical Storm Norbert, and the system intensified further to attain hurricane intensity by October 6. After undergoing a period of rapid deepening, Norbert reached its peak intensity as a Category 4 on the Saffir–Simpson hurricane wind scale, with maximum sustained winds of 135 mph (215 km/h) and a minimum barometric pressure of 945 mbar (hPa; 27.91 inHg). As the cyclone rounded the western periphery of a subtropical ridge over Mexico, it began an eyewall replacement cycle which led to steady weakening. Completing this cycle and briefly reintensifying into a major hurricane, a Category 3 or higher on the Saffir–Simpson hurricane wind scale, Norbert moved ashore Baja California Sur as a Category 2 hurricane late on October 11. After a second landfall at a weaker intensity the following day, the system quickly weakened over land and dissipated that afternoon.
In preparation for the cyclone, the NHC issued hurricane warnings for coastal regions of the Baja California Peninsula. Residents living in low-lying areas and flood-prone regions were advised to evacuate, and beach-goers were warned to stay out of the water. Upon landfall, the hurricane produced waves in excess of 13 ft (4.0 m). Though the strongest winds observed were less than hurricane intensity, heavy rainfall as a result of Norbert lead to substantial damage. In Baja California Sur, roughly 5,000 homes sustained major damage; the heaviest-impacted municipality, Comondú, reported 16,000 homes affected. Thousands of people were forced into shelters, and many trees were blown down by strong winds. A total of 25 fatalities occurred in Sonora, 5 of which occurred in Álamos, where excessive rainfall caused a majority of the town to become flooded; homes were submerged to their roofs and many trees were downed. The hurricane severely impacted the fishing industry in Sinaloa, while thousands of residents were left homeless. In the United States, Norbert produced minimal rainfall. In the aftermath of the storm, many towns and municipalities were declared disaster areas. Overall, Norbert inflicted $98.5 million (2008 USD) in damage.Hurricane Sergio (2018)
Hurricane Sergio was a powerful and long-lived tropical cyclone that affected the Baja California Peninsula as a tropical storm. Sergio became the eighth Category 4 hurricane in the East Pacific for 2018, breaking the old record of seven set in 2015. The twentieth named storm, eleventh hurricane, and ninth major hurricane of the season, Sergio originated from a system that was located over northwestern South America on September 24. The National Hurricane Center monitored the disturbance for several days as the system organized into a tropical storm on September 29. Sergio gradually strengthened for the next couple of days as it traveled west-southwestward, becoming a hurricane on October 2. The storm then turned towards the northwest as it underwent rapid intensification and an eyewall replacement cycle, before peaking as a Category 4 hurricane on October 4, with maximum sustained winds of 140 mph (220 km/h). The hurricane maintained peak intensity for 12 hours before undergoing a second eyewall replacement and turning towards the southwest. The system then began another period of intensification, achieving a secondary peak with winds of 125 mph (205 km/h) on October 6. The next day, Sergio began a third eyewall replacement cycle, falling below major hurricane strength. At the same time, the system unexpectedly assumed some annular characteristics. Over the next few days, the cyclone curved from the southwest to the northeast, weakening into a tropical storm on October 9. Sergio made landfall as a tropical storm on October 12 on the Baja California Peninsula, and later in northwestern Mexico as a tropical depression before dissipating early on October 13.
Sergio's approach warranted the issuance of tropical storm watches and warnings along the western and eastern coasts of Baja California from October 10–11. The cyclone made landfall in western Baja California Sur and Sonora on October 13 as a tropical storm and tropical depression, respectively, causing over US$2 million1 in damage, over a thousand school closures, and a few hundred evacuations due to severe flooding. Sergio's remnants brought heavy rainfall to Arizona, resulting in the closure of its state fair. Multiple tornadoes also spawned in Texas as a result of the increased moisture. About US$548,000 in damage occurred throughout both states. No injuries or deaths were reported in association with the hurricane or its remnants.Hurricane Walaka
Hurricane Walaka ( ua-la-ka; Hawaiian: ʻwalaka meaning "ruler of the army") was one of the most intense Pacific hurricanes on record. By minimum pressure, Walaka is the second-strongest tropical cyclone in central Pacific, alongside Hurricane Gilma in 1994, and is only surpassed by Hurricane Ioke in 2006. The nineteenth named storm, twelfth hurricane, eighth major hurricane, and second Category 5 hurricane of the 2018 Pacific hurricane season, Walaka originated from an area of low pressure that formed over a thousand miles south-southeast of Hawaii on September 25. The National Hurricane Center tracked the disturbance for another day or so before it moved into the Central Pacific Basin. The Central Pacific Hurricane Center monitored the disturbance from that time until September 29, when the system organized into Tropical Storm Walaka. Walaka gradually strengthened, becoming a hurricane on October 1. Walaka then began to rapidly intensify, reaching Category 5 intensity by early on October 2. An eyewall replacement cycle caused some weakening of the hurricane, though it remained a powerful storm for the next day or so. Afterward, less favorable conditions caused a steady weakening of the hurricane, and Walaka became extratropical on October 6, well to the north of the Hawaiian Islands. The storm's remnants then accelerated northeastward, before dissipating on October 9.
Although the hurricane did not impact any major landmasses, it passed very close to the unpopulated Johnston Atoll as a strong Category 4 hurricane, where a hurricane warning was issued in advance of the storm. Four scientists there intended to ride out the storm on the island, but were then evacuated before the storm hit. Walaka then neared the far Northwestern Hawaiian Islands, but weakened considerably as it did so. East Island in the French Frigate Shoals suffered a direct hit and was completely destroyed.Hurricane Willa
Hurricane Willa was the strongest tropical cyclone to make landfall in the Mexican state of Sinaloa since Lane in 2006. The twenty-second named storm, thirteenth hurricane, tenth major hurricane, and record-tying third Category 5 hurricane of the 2018 Pacific hurricane season, Willa originated from a tropical wave that the National Hurricane Center (NHC) first began monitoring for tropical cyclogenesis in the southwestern Caribbean Sea, on October 14. The system subsequently crossed over Central America into the East Pacific, without significant organization. The NHC continued to track the disturbance until it developed into a tropical depression on October 20, off the coast of southwestern Mexico. Later in the day, the system became a tropical storm as it began to rapidly intensify. On October 21, Willa became a Category 4 major hurricane, before strengthening further to Category 5 intensity on the next day. Afterward, a combination of an eyewall replacement cycle and increasing wind shear weakened the hurricane, and early on October 24, Willa made landfall as a marginal Category 3 hurricane, in Sinaloa of the northwestern Mexico. Following landfall, Willa rapidly weakened, dissipating later on the same day over northeastern Mexico.
Up to its landfall, Willa prompted the issuance of hurricane and tropical storm watches and warnings for western Mexico. The hurricane killed six people, and caused $536.8 million (2018 USD) in damages, mostly around the area where it moved ashore.Meteorological history of Hurricane Ivan
The meteorological history of Hurricane Ivan, the longest tracked tropical cyclone of the 2004 Atlantic hurricane season, lasted from late August through late September. The hurricane developed from a tropical wave that moved off the coast of Africa on August 31. Tracking westward due to a ridge, favorable conditions allowed it to develop into Tropical Depression Nine on September 2 in the deep tropical Atlantic Ocean. The cyclone gradually intensified until September 5, when it underwent rapid deepening and reached Category 4 status on the Saffir-Simpson Hurricane Scale; at the time Ivan was the southernmost major North Atlantic hurricane on record.
Ivan quickly weakened due to dry air, but it gradually reorganized, passing just south of Grenada as a major hurricane on September 7. The hurricane attained Category 5 status in the central Caribbean Sea. Over the subsequent days its intensity fluctuated largely due to eyewall replacement cycles, and Ivan passed just south of Jamaica, the Cayman Islands, and western Cuba with winds at or slightly below Category 5 status. Turning northward and encountering unfavorable conditions, Ivan gradually weakened before making landfall just west of Gulf Shores, Alabama on September 16 with winds of 120 mph (195 km/h). The cyclone quickly weakened to tropical depression status as it turned to the northeast, and Ivan transitioned into an extratropical cyclone on September 18.
The remnant low of Ivan turned to the south and southwest, and after crossing Florida on September 21 it began to reacquire tropical characteristics. It became a tropical depression again on September 22 to the southeast of Louisiana, and Ivan reached winds of 60 mph (95 km/h) before weakening and moving ashore along southwestern Louisiana as a tropical depression; the circulation of Ivan dissipated after crossing into Texas on September 25. The cyclone broke several intensity records, and its duration was the tenth-longest on record for an Atlantic hurricane.Meteorological history of Hurricane Wilma
Hurricane Wilma was the most intense tropical cyclone in the Atlantic basin on record, with an atmospheric pressure of 882 hPa (mbar, 26.05 inHg). Wilma's destructive journey began in the second week of October 2005. A large area of disturbed weather developed across much of the Caribbean Sea and gradually organized to the southeast of Jamaica. By late on October 15, the system was sufficiently organized for the National Hurricane Center to designate it as Tropical Depression Twenty-Four.
The depression drifted southwestward, and under favorable conditions, it strengthened into Tropical Storm Wilma on October 17. Initially, development was slow due to its large size, though convection steadily organized. From October 18, and through the following day, Wilma underwent explosive deepening over the open waters of the Caribbean; in a 30-hour period, the system's central atmospheric pressure dropped from 982 mbar (29.00 inHg) to the record-low value of 882 mbar (26.05 inHg), while the winds increased to 185 mph (298 km/h). At its peak intensity, the eye of Wilma was about 2.3 miles (3.7 km) in diameter, the smallest known eye in an Atlantic hurricane. After the inner eye dissipated due to an eyewall replacement cycle, Hurricane Wilma weakened to Category 4 status, and on October 21, it made landfall on Cozumel and on the Mexican mainland with winds of about 150 mph (240 km/h).
Wilma weakened over the Yucatán Peninsula, and reached the southern Gulf of Mexico before accelerating northeastward. Despite increasing amounts of vertical wind shear, the hurricane re-strengthened to hit Cape Romano, Florida, as a major hurricane. Wilma weakened as it quickly crossed the state, and entered the Atlantic Ocean near Jupiter, Florida. The hurricane again re-intensified before cold air and wind shear penetrated the inner core of convection. By October 26, it transitioned into an extratropical cyclone, and the next day, the remnants of Wilma were absorbed by another extratropical storm over Atlantic Canada.Typhoon Choi-wan (2009)
Typhoon Choi-wan was a powerful typhoon that became the first Category 5 equivalent-super typhoon to form during the 2009 Pacific typhoon season. Forming on September 11, 2009, about 1100 km (700 mi) to the east of Guam, the initial disturbance rapidly organized into a tropical depression. By September 12, the depression intensified into a tropical storm, at which time it was given the name Choi-wan. The following day, rapid intensification took place through September 14. Choi-wan attained its peak intensity on September 15, as it moved through the Northern Mariana Islands with the Japan Meteorological Agency reporting peak windspeeds of 195 km/h (120 mph 10-minute sustained). Additionally, the Joint Typhoon Warning Center reported the storm to have attained winds of 260 km/h (160 mph 1-minute sustained). The typhoon remained very powerful until September 17 when the storm's outflow weakened. The typhoon underwent an eyewall replacement cycle, leading to intensity fluctuations. By September 19, Choi-wan rapidly weakened as strong wind shear caused convection to diminish. The following day, the system transitioned into an extratropical cyclone and dissipated several hours later over open waters.
Despite the intensity of Choi-wan when it passed through the Northern Mariana Islands, no casualties were reported. However, following the storm, the United States Navy deemed that the island of Alamagan was uninhabitable, with all but one of the structures completely destroyed and most of the islands' trees downed. In response to this, all residents on the island were evacuated to nearby Saipan.Typhoon Halong (2014)
Typhoon Halong, known in the Philippines as Typhoon Jose, was a very powerful tropical cyclone in the Western Pacific basin in August 2014. It was the twelfth named storm and the fifth typhoon of the 2014 Pacific typhoon season. The storm reached its maximum intensity as a Category 5 super typhoon, making it the fifth strongest storm of the season, surpassed by Genevieve, Vongfong, Nuri and Hagupit.Typhoon Lekima (2019)
Typhoon Lekima, known in the Philippines as the Typhoon Hanna, was the second-costliest typhoon in Chinese history, only behind Fitow in 2013. The ninth named storm of the 2019 Pacific typhoon season, Lekima originated from a tropical depression that formed east of the Philippines on July 30. It gradually organized, became a tropical storm and was named on August 4. Lekima intensified under favourable environmental conditions and peaked as a Category 4–equivalent super typhoon. However, an eyewall replacement cycle caused the typhoon to weaken before it made landfall in Zhejiang late on August 9, as a Category 2–equivalent typhoon. Lekima weakened subsequently while moving across the East China, and made its second landfall in Shandong on August 11.
Lekima's precursor enhanced the southwestern monsoon in the Philippines, which brought heavy rain to the country. The rains caused three boats to sink and 31 people died in this accident. Lekima brought catastrophic damage in mainland China, with a death toll of 89 people and more than CN¥53.7 billion (US$7.6 billion) in damages. The system also caused minor damage in Ryukyu Islands and Taiwan.Typhoon Maria (2018)
Typhoon Maria, known in the Philippines as Typhoon Gardo, was a powerful tropical cyclone that affected Guam, Taiwan, and China. Developing into the eighth named tropical storm of the northwest Pacific Ocean and impacting the Mariana Islands on July 4, Maria strengthened into the fourth typhoon of this season and underwent extremely rapid intensification the next day, due to favorable environmental conditions. The typhoon reached its first peak intensity on July 6; subsequently, Maria weakened due to an eyewall replacement cycle, but it re-intensified and reached its second peak intensity on July 8. Later, it started to gradually weaken due to colder sea surface temperatures. After hitting the Yaeyama Islands and affecting Taiwan on July 10, Maria ultimately made landfall over Fujian, China, early on July 11, before dissipating on the next day.Typhoon Wutip (2019)
Typhoon Wutip, known in the Philippines as Tropical Depression Betty, was the most powerful February typhoon on record, surpassing Typhoon Higos of 2015. The third tropical cyclone, second tropical storm, and the first typhoon of the 2019 Pacific typhoon season, Wutip originated from a low-pressure area on February 16, 2019. The disturbance moved westward, passing just south of the Federated States of Micronesia, before later organizing into Tropical Depression 02W on February 18, 2019. On February 20, 2019, the tropical depression intensified into a tropical storm and was named Wutip, before strengthening further into a typhoon on the next day. Wutip underwent rapid intensification, and on February 23, Wutip reached its initial peak intensity, with 10-minute sustained winds of 185 km/h (115 mph), 1-minute sustained winds of 250 km/h (155 mph), and a minimal pressure of 925 millibars (27.3 inHg) while passing to the southwest of Guam, becoming the strongest February typhoon on record as it did so.
Wutip underwent an eyewall replacement cycle shortly afterward, which caused the storm to weaken as it turned to the northwest. Wutip finished its eyewall replacement cycle on February 24, which allowed Wutip to restrengthen, with the typhoon rapidly intensifying once again. On February 25, Wutip reached its peak intensity with 10-minute sustained winds of 195 km/h (120 mph), 1-minute sustained winds of 260 km/h (160 mph), and a minimum central pressure of 920 millibars (27 inHg), becoming the first Category 5-equivalent super typhoon recorded in the month of February. Afterward, Wutip weakened on February 26, due to encountering strong wind shear. Wutip rapidly weakened as it moved northwestward, before dissipating on March 2.
Wutip caused at least $3.3 million (2019 USD) in damages in Guam and Micronesia.Typhoon Yutu
Typhoon Yutu, known in the Philippines as Typhoon Rosita, was an extremely powerful tropical cyclone that caused catastrophic destruction on the islands of Tinian and Saipan in the Northern Mariana Islands, and later impacted the Philippines. It is the strongest typhoon ever recorded to impact the Mariana Islands, as well as the second-strongest tropical cyclone to strike the United States and its unincorporated territories by both wind speed and barometric pressure; the storm is tied with Hurricane Camille of 1969 for the latter record. Yutu was also the most powerful tropical cyclone worldwide in 2018. The fortieth tropical depression, twenty-sixth named storm, twelfth typhoon, and the seventh super typhoon of the 2018 Pacific typhoon season, Yutu originated from a low-pressure area that formed in the western Pacific Ocean on October 15. The disturbance organized into a tropical depression on the same day, as ocean sea-surface heat content increased. Shortly after becoming a tropical depression, the Joint Typhoon Warning Center (JTWC) assigned the system the identifier 31W. The system continued to strengthen, becoming a tropical storm several hours later, with the Japan Meteorological Agency (JMA) naming the system Yutu. Increasingly favorable conditions allowed Yutu to explosively intensify, as the system maintained deep convection and subsequently became a severe tropical storm and then a typhoon.
Through October 23, Yutu continued to explosively intensify, quickly reaching Category 5 super typhoon intensity on October 24. On October 25, Yutu made landfall on the island of Tinian and the southern part of Saipan at its peak intensity, with a minimum central pressure of 900 millibars (27 inHg), 10-minute sustained winds of 215 km/h (130 mph), 1-minute sustained winds of 285 km/h (180 mph), and gusts of up to 305 km/h (190 mph). This made it the most powerful tropical cyclone worldwide in 2018. Immediately after making landfall, Yutu underwent an eyewall replacement cycle, causing it to momentarily weaken as it completed the process. Maintaining super typhoon status, Yutu continued to move westward towards the Philippines, entering the Philippine Area of Responsibility (PAR) whereupon it was assigned the local name Rosita. Intrusions of dry air and lower sea surface temperatures, however, caused Yutu to weaken significantly through October 28, though it remained a strong typhoon. Late on October 29, Yutu made landfall in the Filipino province of Isabela, with 10-minute sustained winds of 100 mph (155 km/h). The JTWC estimated 1-minute winds to be 165 km/h (105 mph) at that time.
The storm wrought catastrophic damage across Tinian and Saipan, destroying numerous homes and killing two people. Violent winds destroyed concrete structures in southern Saipan and stripped areas of vegetation. In the Philippines, landslides and flooding killed at least 27 people, while in Hong Kong, one person was killed by high surf.