|Category 5 major hurricane (SSHWS/NWS)|
|Formed||September 29, 2018|
|Dissipated||October 9, 2018|
|( Extratropical after October 6)|
|Highest winds|| 1-minute sustained:160 mph (260 km/h)|
|Lowest pressure||920 mbar (hPa); 27.17 inHg|
|Areas affected||Johnston Atoll, Northwestern Hawaiian Islands, Alaska, British Columbia|
|Part of the 2018 Pacific hurricane season|
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 Hawaiian language is a Polynesian language that takes its name from Hawaiʻi, the largest island in the tropical North Pacific archipelago where it developed. Hawaiian, along with English, is an official language of the State of Hawaii. King Kamehameha III established the first Hawaiian-language constitution in 1839 and 1840.
A Pacific hurricane is a mature tropical cyclone that develops within the eastern and central Pacific Ocean to the east of 180°W, north of the equator. For tropical cyclone warning purposes, the northern Pacific is divided into three regions: the eastern, central, and western, while the southern Pacific is divided into 2 sections, the Australian region and the southern Pacific basin between 160°E and 120°W. Identical phenomena in the western north Pacific are called typhoons. This separation between the two basins has a practical convenience, however, as tropical cyclones rarely form in the central north Pacific due to high vertical wind shear, and few cross the dateline.
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.
Johnston Atoll, also known as Kalama Atoll to Native Hawaiians, is an unincorporated territory of the United States currently administered by the United States Fish and Wildlife Service. Johnston Atoll is a National Wildlife Refuge and is closed to public entry. Limited access for management needs is only by Letter of Authorization from the U.S. Air Force and Special Use Permit from the U.S. Fish and Wildlife Service.
The Northwestern Hawaiian Islands or the Leeward Islands are the small islands and atolls in the Hawaiian island chain located northwest of the islands of Kauai and Niihau. Politically, they are all part of Honolulu County in the U.S. state of Hawaii, except Midway Atoll, which is a territory distinct from Hawaii and grouped as one of the United States Minor Outlying Islands. The United States Census Bureau defines this area, except Midway, as Census Tract 114.98 of Honolulu County. Its total land area is 3.1075 square miles (8.048 km2). All the islands except Nihoa are north of the Tropic of Cancer, making them the only islands in Hawaii that lie outside the tropics.
East Island was an island, formerly about 11-acre (4.5 ha)s in area, half a mile long and 400 feet wide. It was the second-largest in the French Frigate Shoals, and one of the Northwestern Hawaiian Islands, approximately 550 miles (890 km) northwest of Honolulu. It was largely washed away in 2018 by the storm surge from Hurricane Walaka. The remaining portion of the island above sea level consists of a sandy strip approximately 150 feet long.
Walaka originated from a trough of low pressure that the National Hurricane Center (NHC) first forecasted on September 22. The NHC forecasted a low-pressure area to form in the far western portion of the east North Pacific within a few days. 1,600 miles (2,575 km) south-southeast of Hilo, Hawaii. The NHC continued to monitor the disturbance for another day or so until it moved into the Central Pacific Hurricane Center's (CPHC) area of responsibility on September 26 at 12:00 UTC. The CPHC monitored the disturbance for another few days until the system organized into Tropical Storm Walaka on September 29 at 21:00 UTC. Environmental conditions, including low wind shear, high sea surface temperatures and ample moisture supported steady – perhaps even rapid – intensification into a powerful hurricane.Early on September 25, a trough of low pressure formed approximately
A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts. Troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.
The National Hurricane Center (NHC) is the division of the United States' National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean. The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida.
A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.
Over the next twelve hours, the system showed little change in intensity before quickly strengthening into a strong tropical storm, as a strong central dense overcast became established. mph (260 km/h) and a central pressure of 920 mbar (27.17 inHg). This made Walaka the second-most intense hurricane in the Central Pacific by pressure (behind Hurricane Ioke in 2006), and the second Category 5 hurricane recorded in the same year – Lane was the other storm of such intensity. Unrelated to Walaka, Typhoon Kong-rey developed and intensified into a Category 5-equivalent super typhoon around the same time Walaka reached its peak intensity, marking the first time since 2005 when two tropical cyclones of Category 5 strength existed simultaneously in the Northern Hemisphere.Over the next twelve hours, Walaka gradually strengthened, becoming a hurricane at 03:00 UTC on October 1. Explosive intensification then ensued as a small, well-defined eye formed, with Walaka reaching major hurricane status early that morning. Rapid intensification culminated at 00:00 UTC on October 2, when the storm peaked as a Category 5 hurricane, with 1-minute sustained winds of 160
The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity. Locating the center within the CDO can be a problem for strong tropical storms and with systems of minimal hurricane strength as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery.
Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center (NHC) defines rapid intensification as an increase in the maximum 1-min sustained winds of a tropical cyclone of at least 30 knots in a 24-hour period.
The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 km (20–40 miles) in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.
Soon afterward, Walaka began to undergo an eyewall replacement cycle, which subsequently caused weakening, and the eye became less defined. miles (1,740 km) north-northwest of Honolulu, Hawaii. Afterward, Walaka's extratropical remnant rapidly weakened while accelerating northeastward, reaching the Gulf of Alaska on October 8. On October 9, Walaka's remnant was absorbed by another frontal system over British Columbia.For the next day or so, Walaka remained a powerful hurricane as it turned northward, due to a ridge of high pressure to its northeast. However, as the hurricane moved into a less favorable environment on October 4, Walaka began to lose its intensity again. Later that day, Walaka fell below major hurricane status as it travelled north, away from the Northwestern Hawaiian Islands. At this time, it was noted that Walaka's low-level circulation center was exposed in the southwest quadrant due to strong wind shear. Weakening accelerated the next day as almost all deep convection was being sheared away, and Walaka weakened into a tropical storm on October 4. At 09:00 UTC on the next day, it was noted that Walaka was beginning to transition into an extratropical system as it continued north, under the influence of deep southwesterly flow. At 15:00 UTC on October 6, Walaka transitioned into an extratropical cyclone 1,085
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. 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.
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.
The Gulf of Alaska is an arm of the Pacific Ocean defined by the curve of the southern coast of Alaska, stretching from the Alaska Peninsula and Kodiak Island in the west to the Alexander Archipelago in the east, where Glacier Bay and the Inside Passage are found.
On September 30, a hurricane watch was issued for Johnston Atoll.Early on the next day, the hurricane watch was upgraded to a hurricane warning. A crew of four scientists on the isolated Johnston Atoll planned on riding out the storm in an evacuation shelter, until the United States Fish and Wildlife Service sought an emergency evacuation on October 1. On the next day, the United States Coast Guard flew a plane from Kalaeloa Airport to evacuate the personnel. The hurricane warning for Johnston Atoll was discontinued on October 3 as Walaka moved away.
The United States Fish and Wildlife Service is an agency of the US Federal Government within the US Department of the Interior dedicated to the management of fish, wildlife, and natural habitats. The mission of the agency is "working with others to conserve, protect, and enhance fish, wildlife, plants and their habitats for the continuing benefit of the American people."
The United States Coast Guard (USCG) is the coastal defense and maritime law enforcement branch of the United States Armed Forces and one of the country's seven uniformed services. The Coast Guard is a maritime, military, multi-mission service unique among the U.S. military branches for having a maritime law enforcement mission and a federal regulatory agency mission as part of its mission set. It operates under the U.S. Department of Homeland Security during peacetime, and can be transferred to the U.S. Department of the Navy by the U.S. President at any time, or by the U.S. Congress during times of war. This has happened twice: in 1917, during World War I, and in 1941, during World War II.
Kalaeloa Airport, also called John Rodgers Field and formerly Naval Air Station Barbers Point, is a joint civil-military regional airport of the State of Hawaiʻi established on July 1, 1999 to replace the Ford Island NALF facilities which closed on June 30 of the same year. Located on the site of the developing unincorporated town of Kalaeloa and nestled between the Honolulu communities of ʻEwa Beach, Kapolei and Campbell Industrial Park in West Oʻahu, most flights to Kalaeloa Airport originate from commuter airports on the other Hawaiian islands. While Kalaeloa Airport is primarily a commuter facility used by unscheduled air taxis, general aviation and transient and locally based military aircraft, the airport saw first-ever scheduled airline service begin on July 1, 2014, with Mokulele Airlines operating flights to Kahului Airport on Maui.
Early on October 2, a hurricane watch was issued for the Papahanaumokuakea Marine National Monument from Nihoa to French Frigate Shoals to Maro Reef.Late on the same day, the hurricane watch for Frigate Shoals to Maro Reef was upgraded to a hurricane warning. Additionally, a tropical storm warning was issued for Nihoa to French Frigate Shoals. Also on that day, seven researchers studying Hawaiian monk seals and green sea turtles on French Frigate Shoals were evacuated to Honolulu. On October 4, the hurricane warning for the Papahanaumokuakea Marine National Monument was lowered to a tropical storm warning, and all of them were eventually discontinued early on the next day, as Walaka weakened and moved away from the islands.
A powerful storm surge accompanied the hurricane as it traversed the French Frigate Shoals. The small, low-lying East Island suffered a direct hit and was completely destroyed, with sediment scattered across coral reefs to the north. The island served as one of the major nesting locations for the endangered green sea turtles, and critically endangered Hawaiian monk seals. percent of 2018's sea turtle nests on the island were lost; however, all adult females tending the nests left before the storm. Approximately half of Hawaii's green sea turtles nested on the island, and Charles Littnan—director of the National Oceanic and Atmospheric Administration's protected species division—stated it would take years for the implications of the island's loss to be fully understood.An estimated 19
Hurricane John, also known as Typhoon John, was both the longest-lasting and the farthest-traveling tropical cyclone ever observed. John formed during the 1994 Pacific hurricane season, which had above-average activity due to the El Niño of 1994–95, and peaked as a Category 5 hurricane on the Saffir–Simpson hurricane wind scale, the highest categorization for hurricanes.
The 2007 Pacific hurricane season was a below-average Pacific hurricane season, featuring one major hurricane. The season officially started on May 15 in the eastern Pacific and on June 1 in the central Pacific, and ended on November 30; these dates conventionally delimit the period during which most tropical cyclones form in the region. The first tropical cyclone of the season, Alvin, developed on May 27, while the final system of the year, Kiko, dissipated on October 23. Due to unusually strong wind shear, activity fell short of the long-term average, with a total of 11 named storms, 4 hurricanes, and 1 major hurricane. At the time, 2007 featured the second-lowest value of the Accumulated cyclone energy (ACE) index since reliable records began in 1971. Two tropical cyclones – Cosme and Flossie – crossed into the central Pacific basin during the year, activity below the average of 4 to 5 systems.
Hurricane Dora was one of few tropical cyclones to track across all three north Pacific basins and the first since Hurricane John in 1994. The fourth named storm, third hurricane, and second major hurricane of the 1999 Pacific hurricane season, Dora developed on August 6 from a tropical wave to the south of Mexico. Forming as a tropical depression, the system gradually strengthened and was upgraded to Tropical Storm Dora later that day. Thereafter Dora began heading in a steadily westward direction, before becoming a hurricane on August 8. Amid warm sea surface temperatures and low wind shear, the storm continued to intensify, eventually peaking as a 140 mph (220 km/h) Category 4 hurricane on August 12.
Hurricane Emilia was, at the time, the strongest tropical cyclone on record in the Central Pacific Ocean, and the second of such to be classified as a Category 5 hurricane – the highest rating on the Saffir–Simpson hurricane wind scale. However, hurricanes Gilma later that year, Ioke in 2006, and hurricanes Lane and Walaka in 2018 later reached lower barometric pressures in the Central Pacific. The fifth named storm and the first of three Category 5 hurricanes of the 1994 hurricane season, Emilia developed from an area of low pressure southeast of Hawaii on July 16. Tracking westward, the initial tropical depression intensified into a tropical storm several hours after tropical cyclogenesis. Subsequently, Emilia entered the Central Pacific Ocean and moved into the area of responsibility of the Central Pacific Hurricane Center (CPHC).
The 2009 Pacific hurricane season was the most active Pacific hurricane season since 1994. The season officially started on May 15 in the East Pacific Ocean, and on June 1 in the Central Pacific; they both ended on November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Pacific basin. However, the formation of tropical cyclones is possible at any time of the year.
The 2016 Pacific hurricane season was tied as the fifth-most active season on record, alongside the 2014 season. Throughout the course of the year, a total of 22 named storms, 13 hurricanes and six major hurricanes were observed within the basin. Although the season was very active, it was considerably less active than the previous season, with large gaps of inactivity at the beginning and towards the end of the season. It officially started on May 15 in the eastern Pacific, and on June 1 in the central Pacific; they both ended on November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Pacific basin. However, as illustrated by Hurricane Pali, which became the earliest Central Pacific tropical cyclone on record, the formation of tropical cyclones is possible at any time of the year. After Pali, however, the active season had a slow start, becoming the first season since 2011 in which no tropical cyclones occurred in May, and also the first since 2007 in which no named storms formed in the month of June.
Hurricane Uleki, also referred as Typhoon Uleki, was a long-lived tropical cyclone in August–September 1988 that had minimal effects on land. Originating from a disturbance in the Intertropical Convergence Zone in late-August, Uleki was identified as a tropical depression well to the southeast of Hawaii on August 28. Steady organization ensued as it moved west, becoming a tropical storm on August 30 and a hurricane on August 31. Rapid intensification took place thereafter and the storm reached its peak intensity on September 2 as a Category 3 on the Saffir–Simpson hurricane wind scale. Hurricane Hunters investigating the cyclone found peak winds of 125 mph (205 km/h) and a barometric pressure of 957 mbar. Thereafter, Uleki stalled for two days to the southwest of Hawaii, resulting in heavy surf across the state. The dangerous swells killed two people on Oahu.
Tropical Storm Flossie yielded stormy weather to Hawaii in late July 2013. The sixth tropical cyclone and named storm of the annual hurricane season, Flossie originated from a tropical wave that emerged off the western coast of Africa on July 9. Tracking westward across the Atlantic with little development, it passed over Central America and into the eastern Pacific Ocean on July 18, where favorable environmental conditions promoted steady organization. By 0600 UTC on July 25, the wave acquired enough organization to be deemed a tropical depression; it intensified into a tropical storm six hours later. Continuing westward, Flossie attained peak winds of 70 mph (110 km/h) on July 27 before entering the central Pacific Ocean. There, unfavorable upper-level winds established a weakening trend; on July 30, Flossie weakened to a tropical depression, and by 1200 UTC that same day, the storm degenerated into a remnant low, northeast of Kauai.
Hurricane Raymond was the only major hurricane in the eastern Pacific in 2013 and briefly threatened the southwestern coast of Mexico before recurving back out to sea. The seventeenth named storm and eighth hurricane of the annual cyclone season, Raymond developed from a tropical wave on October 20 south of Acapulco, Mexico. Within favorable conditions for tropical cyclone development, Raymond quickly intensified, attaining tropical storm intensity and later hurricane intensity within a day of cyclogenesis. On October 21, the hurricane reached its peak intensity with winds of 125 mph (205 km/h). A blocking ridge forced the hurricane to the southwest, while at the same time Raymond began to quickly weaken due to wind shear. The following day, the tropical cyclone weakened to tropical storm status. After tracking westward, Raymond reentered more favorable conditions, allowing it to intensify back to hurricane strength on October 27 while curving northward. The hurricane reached a secondary peak intensity with winds of 105 mph (165 km/h) several hours later. Deteriorating atmospheric conditions resulted in Raymond weakening for a final time, and on October 30, the National Hurricane Center (NHC) declared the tropical cyclone to have dissipated.
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Hurricane Iselle was the strongest tropical cyclone to make landfall on the Big Island of Hawaii in recorded history. The eleventh named storm of the annual hurricane season, Iselle developed from an area of disturbed weather southwest of Mexico on July 31, 2014. Assuming a west-northwest course that it would maintain throughout its existence, generally favorable atmospheric conditions allowed for gradual strengthening, with the cyclone attaining hurricane status a day after formation. Continued strengthening progressed for several days up until August 4, when Iselle reached peak intensity with maximum sustained winds of 140 mph (220 km/h) and a minimum barometric pressure of 947 mbar, making it a Category 4 hurricane. Thereafter, Iselle encountered hostile environmental conditions and quickly weakened before making landfall on the Big Island on August 8 as a moderate tropical storm. Its passage over the island disrupted the cyclone, and Iselle later dissipated on August 9.
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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.
Hurricane Lane was the wettest tropical cyclone on record in Hawaii, with rainfall accumulations of 52.02 inches (1,321 mm) in Mountain View. It also ranked as the second-wettest tropical cyclone in the United States, after Hurricane Harvey of 2017. The first Category 5 Pacific hurricane since Patricia in 2015, Lane was the twelfth named storm, sixth hurricane, and fourth major hurricane of the 2018 Pacific hurricane season. It originated from a tropical wave that moved off the coast of Africa on July 31, and was eventually monitored for tropical cyclogenesis several hundred miles off the southern coast of Mexico on August 11. Over the next four days, the disturbance gradually strengthened amid favorable atmospheric and thermodynamic conditions and became a tropical storm on August 15. Steady intensification occurred and Lane reached hurricane status by August 17, followed by rapid intensification that brought Lane to its initial peak intensity as a Category 4 hurricane on August 18. On August 19, Lane crossed into the Central Pacific basin, where increased wind shear weakened it. However, on August 20, Lane re-intensified into a Category 4 hurricane, and reached its peak intensity as a Category 5 hurricane early on August 22. As Lane approached the Hawaiian Islands, it began to weaken as vertical wind shear once again increased, dropping below hurricane status on August 25. Over the next few days, Lane followed a westwards course away from the Hawaiian Islands as influence from the easterly trade winds increased as Lane weakened. On August 29, Lane became a remnant low, and dissipated shortly afterward.
Hurricane Olivia was the first tropical cyclone to make landfall on Maui and Lanai in recorded history. The fifteenth named storm, ninth hurricane, and sixth major hurricane of the 2018 Pacific hurricane season, Olivia formed southwest of Mexico on September 1. The depression slowly organized and strengthened into Tropical Storm Olivia on the next day. Olivia then began a period of rapid intensification on September 3, reaching its initial peak on September 5. Soon after, Olivia began a weakening trend, before re-intensifying on September 6. On the next day, Olivia peaked as a Category 4 hurricane, with winds of 130 mph and a minimum central pressure of 951 mbar. Six hours later, Olivia began another weakening trend that resulted in the hurricane being downgraded to Category 1 status on September 8, east of the 140th meridian west. On September 9, Olivia entered the Central Pacific Basin. Over the next couple of days, Olivia prompted the issuance of Tropical Storm Watches and Warnings for Hawaii County, Oahu, Maui County, and Kauai County. Olivia weakened into a tropical storm on September 11, before making brief landfalls in northwest Maui and Lanai on the next day, becoming the first tropical cyclone to impact the islands in recorded history. Tropical storm-force winds mainly affected Maui County and Oahu. Torrential rains affected the same area from September 11 to 13, causing flash flooding. Olivia caused a total of US$25 million in damages. Olivia was downgraded to a tropical depression on September 13 while continuing to head west. Due to wind shear disrupting Olivia's convection, the system weakened into a remnant low on September 14. Olivia crossed into the West Pacific Basin on September 19 as a remnant low, before dissipating later that day.
Hurricane Rosa was the first tropical cyclone to make landfall in the Mexican state of Baja California since Nora in 1997. The seventeenth named storm, tenth hurricane, and seventh major hurricane of the 2018 Pacific hurricane season, Rosa originated from a broad area of low pressure that the National Hurricane Center began monitoring on September 22. The disturbance moved westward and then west-northwestward for a few days, before developing into a tropical depression on September 25. Later that day, the depression strengthened into Tropical Storm Rosa. One day later, Rosa became a hurricane. On September 27, Rosa began a period of rapid intensification, ultimately peaking as a Category 4 hurricane with maximum sustained winds of 150 mph and a minimum central pressure of 936 mbar on the next day. Over the next couple of days, Rosa turned towards the northeast. By September 29, Rosa had weakened into a Category 2 hurricane due to ongoing structural changes and less favorable conditions. Later on the same day, Rosa re-intensified slightly. On September 30, Rosa resumed weakening as its core structure eroded. Early on October 1, Rosa weakened into a tropical storm. On October 2, Rosa weakened to a tropical depression and made landfall in Baja California. Later in the day, Rosa's remnants crossed into the Gulf of California, with its surface and mid-level remnants later separating entirely. The mid-level remnants of Rosa continued to travel north, reaching northeast Arizona late in the day. On October 3, Rosa's remnants were absorbed into an upper-level low situated off the coast of California.
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 which was set in 2015. The twentieth named storm, eleventh hurricane, and ninth major hurricane of the season, Sergio originated from a broad area of low pressure that formed a few hundred miles south-southeast of the southern coast of Mexico on September 26. The National Hurricane Center monitored the disturbance for a few days until it organized into a tropical storm, after which it was assigned the name Sergio. The system gradually strengthened for the next couple of days, becoming a hurricane on October 2. Sergio then began a period of rapid intensification, becoming a major hurricane later that day. Intensification then halted for about twelve hours before resuming on October 3. The next day, Sergio peaked as a Category 4 hurricane with maximum sustained winds of 140 mph and a minimum central pressure of 942 mbar. Sergio maintained peak intensity for six hours before beginning to weaken. On October 5, the system bottomed out as a low-end Category 3 hurricane. Sergio then began another period of intensification, achieving a secondary peak on October 6. The next day, Sergio began to weaken again, falling below major hurricane strength. At the same time, Sergio unexpectedly assumed the structure of an annular tropical cyclone. By October 9, Sergio had weakened into a tropical storm. On October 12, Sergio made landfall as a tropical storm on the Baja California Peninsula, and later in northwestern Mexico as a tropical depression before dissipating early on October 13.
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