Hurricane Kilo

Tropical Storm Kilo intensifying on August 28, 2015.

The tropical disturbance that would become Kilo first developed on August 17, about 1,150 mi (1,850 km) southeast of the Big Island of Hawaii, within a broad, low-level trough. The disturbance began to move slowly northeast out of the trough on August 19 while remaining poorly organized, producing intermittent bursts of convection. Steered to the northwest by a mid-level ridge to its northeast that produced easterly wind shear over the system, it initially remained poorly organized. However, a brief surge in convection and weak low-level circulation center (LLCC) developed over the system on August 20, though the two features separated later that day. As a ridge to the disturbance's north steered it more quickly to the west-northwest, wind shear relaxed over the system, allowing its LLCC to re-consolidate. The disturbance developed into a tropical depression by 6:00 UTC on August 22.[1][lower-alpha 1]

The newly-formed depression continued to the west-northwest over the following days, initially hindered by continued easterly shear. The storm turned more northward on August 24 and slowed as it neared the edge of the ridge. The slow-moving storm gradually became more organized as a new ridge strengthened to its northwest, and wind shear began to lessen. Turning southwestwards, the depression strengthened into Tropical Storm Kilo at 18:00 UTC[lower-alpha 2] on August 26 as it drifted towards Johnston Atoll. Kilo continued to intensify as it moved southwestwards, passing northwest of the territory as a strong tropical storm. Turning west under the influence of another ridge to its north, Kilo achieved hurricane status at 6:00 UTC on August 29.[1][2]

Hurricanes Kilo (left), Ignacio (center), and Jimena (right), all at major hurricane intensity, spanning the Central and Eastern Pacific basins on August 30, 2015.

Kilo was situated within an environment of warm waters and low vertical wind shear when it reached hurricane status. Within these favorable environmental conditions, the storm began a significant bout of rapid intensification. In a 12-hour period from 6:00 to 18:00 UTC on August 29, Kilo intensified from a low-end, 75 mph (120 km/h) Category 1 hurricane to a high-end, 125 mph (200 km/h) Category 3 major hurricane. The hurricane continued to intensify as it turned more west-northwestward, and the cyclone achieved peak intensity as a powerful Category 4 hurricane at 6:00 UTC on August 30, with maximum sustained winds of 140 mph (225 km/h) and a minimum barometric pressure of 940 mbar (28 inHg).[1] Kilo weakened slightly later that day, though its intensity plateaued at 125 mph (200 km/h) by August 31. A break in the mid-level ridge steering Kilo allowed it to turn northward as it approached the International Dateline. By 18:00 UTC, Kilo had crossed the dateline and entered the Western Pacific basin, at which time it was re-classified as a typhoon.[1][2]

While Kilo was traversing the Central Pacific as a major hurricane, hurricanes Ignacio and Jimena were also active as Category 4 hurricanes. This marked the first time on record that three concurrent major hurricanes were active east of the International Date Line, as well as the first time three Category 4 hurricanes were simultaneously active in the Eastern/Central Pacific. Additionally, Ignacio was active within the Central Pacific basin alongside Kilo, marking the first time two simultaneous major hurricanes were active within the Central Pacific. Kilo also became the third tropical cyclone of the 2015 season to cross the Date Line, surpassing the previous mark of two crossing the dateline in 1997.[1]

Kilo to the southeast of Japan as a weakening tropical cyclone on September 10, with the extratropical remnants of Severe Tropical Storm Etau to its northwest.

Kilo began to weaken as it crossed into the Western Pacific, encountering cooler waters and higher wind shear. The storm weakened to a Category 2-equivalent typhoon on September 1 as it entered the basin. Kilo maintained its intensity for another day as its motion slowed and the storm turned westward. Late on September 3, Kilo turned to the southwest again, and underwent another brief weakening trend, bottoming out as an 85 mph (135 km/h), Category 1-equivalent typhoon on September 5, due to increased southwesterly wind shear.[3] Kilo turned westward around this time, along the Tropic of Cancer, and began a new intensification phase on September 6. The storm reached its secondary peak intensity by 00:00 UTC on September 7, with maximum sustained winds of 105 mph (170 km/h), near the intersection of the Tropic of Cancer and the 170th meridian east.[2][4] Kilo began to weaken again later that day as its eye became irregularly shaped, and convection eroded over the southern semicircle of the storm.[5]

Embarking on a more northwestward course that day, the storm began another weakening phase as it was steered by a ridge to its north. Deep convection associated with Kilo decreased in intensity and coverage through September 8 as drier air wrapped into the core of the typhoon.[6] Kilo only weakened gradually, however, with its winds slowly diminishing to severe-tropical-storm-force by 12:00 UTC on September 9, as its center became exposed from its convective activity within an environment of even stronger shear.[7][8] Kilo accelerated northwestward, with a gradual northward bend as it became caught up in the middle latitude westerlies east of Japan. The storm flat-lined in intensity on September 10 as it passed the country, bending further in direction and turning to the northeast as it began to undergo extratropical transition.[9] Kilo then resumed weakening on September 11, before transitioning to an extratropical cyclone at 12:00 UTC that day, near the Kuril Islands of Russia,[2] over 4,000 mi (6,400 km) from its genesis point.[1] Kilo also slightly strengthened as it became extratropical, with winds of 60 mph (95 km/h).[4] Kilo's extratropical remnant continued to race northeastward, crossing into the Sea of Okhotsk on September 12 before dissipating over the Russian Far East on September 15.[2]