User:Hurricane Noah/Hurricane Walaka

Origins, development, and peak intensity
The National Hurricane Center (NHC) first forecasted that a low-pressure area would form around 130–140° west on September 22, 2018. Two days later, a trough – an elongated region of low atmospheric pressure – formed around 1,600 mi (2,575 km) south-southeast of Hilo, Hawaii. The disturbance entered the central Pacific Ocean on September 26 as a mixture of low-level clouds and larger cumulus clouds. A subtropical ridge located north of the Hawaiian Islands caused the system to track westward over the next few days. A surface low formed by 12:00 UTC on September 27 as the system was located 805 mi (1,295 km) southeast of Hilo, Hawaii. The system became Tropical Depression One-C around 12:00 UTC on September 29 while it was around 690 mi (1,110 km) south of Honolulu, Hawaii. Convection or thunderstorm activity formed near the system's low-level circulation center, and a banding feature – significantly elongated, curved bands of rain clouds – became established over the southern and eastern portions of the depression. Six hours later, the system strengthened into a tropical storm, receiving the name Walaka from the Central Pacific Hurricane Center (CPHC).

The nascent tropical storm was located in an environment of warm 29–30 C sea surface temperatures, low vertical wind shear, and humid air. After forming, the cyclone's banding features degraded, although its convection persisted. Meanwhile, the ridge continued to steer Walaka westward. Around this time, Walaka began a stint of rapid intensification. Convection became more abundant around the storm's low-level center during the morning of September 30. Walaka's cloud tops cooled to around -80 to -90 C; the tropical cyclone intensified into a hurricane by 18:00 UTC. A cloud-filled eye emerged on visible satellite imagery by early October 1 as Walaka continued to strengthen. Walaka became a Category 3 major hurricane around 12:00 UTC on October 1, the fourth storm to do so in the central Pacific in 2018. At that time, the hurricane possessed a prominent eye surrounded by a sizeable ring of cold, -70 to -85 C clouds. Walaka had also turned towards the west-northwest as it moved around the southwestern edge of the ridge. Walaka peaked as a Category 5 hurricane with maximum sustained winds of 160 mph (260 km/h) and a minimum central pressure of 921 mbar at 00:00 UTC on October 2. Walaka's peak intensity made it the second Category 5 hurricane of the 2018 Pacific hurricane season. Around that time, the cyclone had a clear, 23-mile (37 km) wide eye surrounded by cool cloud tops. Meanwhile, microwave imagery and ASCAT data showed the formation of concentric eyewalls, signaling that an eyewall replacement cycle had commenced. Additionally, a strengthening low-pressure system located north of Walaka was causing the hurricane to begin a more northward track.

Weakening, impact to the Northwestern Hawaiian Islands, and demise


Hurricane Walaka maintained its peak intensity for six hours before beginning to decay as a result of the eyewall replacement cycle. the cyclone continued to track northward under the influence of a Pacific upper-level low. The hurricane weakened to a minimal Category 4 hurricane by 00:00 UTC on October 3. By that time, Walaka's eye had degraded on satellite imagery; the eye had become cloud-filled, and the clouds making up the eyewall and central dense overcast had warmed. After completing the eyewall cycle, Walaka reintensified slightly, reaching a secondary peak of 145 mph (230 km/h) around 12:00 UTC on October 3. The cyclone's eye became increasingly delineated as the clouds comprising the eyewall cooled. Although the storm had restrengthened, increasing wind shear was thinning the northwestern eyewall. Soon after, Walaka began to weaken once more as it advanced north-northeastward. Later on October 3, the western and southwestern also eroded as a result of the wind shear. At the same time, upper-level cirrus outflow was disrupted in the southwestern and northeastern portions of the storm. The already strong wind shear increased even further, peaking at 54 mph (87 km/h) around 00:00 UTC. Walaka's eye disappeared from visible satellite imagery and the southwestern portion of the low-level center became uncovered.

As Walaka approached the Northwestern Hawaiian Islands, a hurricane watch and warning were issued for the Johnston Atoll on September 30 and October 1, respectively. Early on October 2, a hurricane watch was issued for Nihoa to French Frigate Shoals to Maro Reef. Later in the day, a hurricane warning was issued for French Frigate Shoals to Maro Reef and a tropical storm warning was issued for Nihoa to French Frigate Shoals. Over the next couple of days, the watches and warnings were gradually discontinued. A crew of four scientists on the isolated Johnston Atoll had 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. The United States Coast Guard flew a plane from Kalaeloa Airport to evacuate the personnel on the next day. Also on October 2, seven researchers studying Hawaiian monk seals and green sea turtles on French Frigate Shoals were evacuated to Honolulu.

Walaka made landfall as a high-end Category 3 hurricane approximately 35 mi (55 km) west-northwest of French Frigate Shoals at 06:20 UTC on October 4. 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 had served as one of the major nesting locations for the endangered green sea turtles, and critically endangered Hawaiian monk seals. An estimated 19 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. Coral reefs near the French Frigate Shoals, Lisianski Island, and the Pearl and Hermes Atoll were substantially damaged, displacing the native fish population.

After accelerating away from the islands, Walaka's eye disappeared entirely. Environmental conditions deteriorated even further on October 4 as sea surface temperatures fell below 27 C and ocean heat content decreased. This caused Walaka to rapidly weaken; the hurricane fell below major hurricane intensity around 12:00 UTC and was a minimal Category 1 hurricane by 00:00 UTC on October 5. Early on October 5, Walaka turned towards the northwest as it traced the northern boundary of the upper-level low. Convection associated with the storm continued to dissipate; the remaining thunderstorm activity was displaced northeast of the storm's low-level center. The wind shear abated later on October 5, however, sea surface temperatures along the remainder of the tropical storm's track were cooler than 25 C. As a result of the decreased shear, Walaka's low-level center was temporarily recovered by convection and the weakening trend slowed as the storm continued north-northwest. By late October 5, the low-level center was completely exposed once more and the remaining convection had all but dissipated. Walaka had also turned towards the northeast by this time. Walaka transitioned into an extratropical cyclone around 12:00 UTC on October 6 after having been deprived of thunderstorm activity. The extratropical cyclone tracked over the open sea and dissipated by 18:00 UTC on October 7.