Assessing Tropical Cyclone Risk in Hawaii Through the Lens of Hurricane Iniki

Hurricane Risk in the Central Pacific Basin

Twenty years ago, the most powerful storm to strike the Hawaiian Islands in recorded history made landfall on the island of Kauai on Sept. 11, 1992. With sustained winds of 140 miles per hour, Hurricane Iniki pummeled Kauai and seriously damaged beachfront buildings and other structures on Oahu’s west coast. In all, Iniki inflicted billions of dollars of property damage on the state of Hawaii, leaving over 14,000 homes damaged or destroyed.

Hurricanes seldom affect Hawaii, and strong ones like Iniki are even rarer. But given the strength of Hurricane Iniki and the scope of the damage it inflicted, how concerned should residents and insurance companies be about hurricane risk in Hawaii?

Tropical cyclones that can potentially affect Hawaii only form when vertical wind shear is low and sea surface temperatures of the equatorial Pacific Ocean are warm, usually between June and November. These warm Pacific seas, including coastal waters near Mexico and Central America, can spawn tropical cyclones, which are propelled northwest by the trade winds into the Central Pacific. But, just how common are these “right” conditions for generating tropical cyclones that can affect Hawaii?

The tropical Pacific shifts between the two phases of the El Nino-Southern Oscillation (ENSO) — La Nina and El Nino — every three to four years, leading to an average of two or three periods of enhanced hurricane risk in Hawaii per decade during El Nino phases. Very few tropical cyclones, however, are known to have affected Hawaii. Hawaii is a very small “target” within the vastness of the Pacific Ocean, which reduces the probability of landfall.

Hurricanes have been causing damage in Hawaii since well before the advent of official meteorological records. For example, several tropical cyclones reportedly struck Hawaii during the 1800s; however, available information about these storms is limited to anecdotes in contemporary newspapers. Since reliable record keeping began in the 1950s, Hawaii has, on average, been affected by just one damaging tropical cyclone per decade. The tracks and Saffir-Simpson categories of the five most significant tropical cyclones to affect Hawaii since 1950 are shown in Figure 1 (note that the colors show how each storm intensified or weakened throughout its lifespan).

Hurricane Iniki: Characteristics and Effects

On Sept. 5, 1992, the tropical depression that would become Hurricane Iniki took shape about 1,600 miles southwest of Baja California. The system intensified as it tracked westward across the Central Pacific, reaching hurricane strength on Sept. 9, about 470 miles southeast of the island of Hawaii (see Figure 1). Over the course of the following day, an increased southerly flow of local winds abruptly turned Hurricane Iniki toward the north, putting the storm on a direct path for Kauai. When Hurricane Iniki made landfall on Kauai on Sept. 11, 1992, this small but intense storm exhibited sustained winds of 140 miles per hour and gusts of up to 175 miles per hour, making Iniki the most powerful storm to strike Hawaii in the officially recorded history, according to the National Hurricane Center.

Although the south shore of Kauai experienced the worst damage from both wind and storm surge, damage was extensive over the whole island. Hurricane Iniki damaged or destroyed more than 14,000 homes on Kauai; of these, more than 5,000 suffered major damage and almost 1,500 residences were completely flattened. In all, nearly 90 perccent of structures on Kauai were damaged. Hurricane Iniki also downed over 35 percent of the power lines on Kauai, inflicting a blackout over the entire island and wiping out nearly all communication systems. Sugar cane, papaya, and banana crops were also significantly damaged by Iniki’s high winds and heavy rain.

In fact, if Hurricane Iniki were to strike today, catastrophe modeling company AIR Worldwide estimates that insured losses would exceed $3 billion.

Could a Storm Like Iniki Strike Honolulu?

Oahu, where Hawaii’s capital city of Honolulu is located, is the economic and cultural hub of Hawaii. A direct hurricane strike on this island would have far-reaching effects on the state as a whole. Honolulu hosts many government offices and tourist attractions, and is home to more than 300,000 residents. Indeed, according to AIR Worldwide, over 70 percent of the exposure value on the Hawaiian Islands is located within Honolulu. Waikiki, a major beach destination in Hawaii, is also located in the island of Oahu. Many economically important crops are grown on Oahu, including pineapples, coffee and macadamia nuts.

About one quarter of homes in Honolulu are characterized by single-wall construction, with lightweight metal roofs, “tofu block” foundation, and light outer cladding. (Homes with a “tofu block” foundation are supported by short wooden posts embedded in concrete blocks, rather than resting directly on a full concrete slab.)

Single-wall homes are likely to experience the worst damage in a hurricane because their weak connections between adjacent structural elements make these homes notably vulnerable to high winds. Although stronger, more wind-resistant conventional (double-wall) timber frame structures make up about 75 percent of residences in Honolulu, these houses would not be exempt from damage inflicted by an Iniki-like storm. For example, all buildings with large windows and garage doors — regardless of wall structure –were notably vulnerable to damage during Iniki.

The effects of an Iniki-like storm on Honolulu and the entire island of Oahu would certainly be significant and far-reaching. But is such a scenario realistic? Information from several 19th century Hawaiian hurricanes contradicts the popular belief that Kauai is the only Hawaiian island vulnerable to tropical cyclones. In addition, using reports of these Hawaiian tropical cyclones from the 1800s, AIR researchers have concluded that Iniki could have made landfall on Maui or Oahu, instead of Kauai, under only slightly different steering conditions. Furthermore, Iniki would have been just as intense had it struck Maui or Oahu, instead of Kauai. The storm’s track — and which island it eventually struck — depended mainly on when the storm made its sharp turn toward the north, not on the storm’s strength or other parameters. In fact, from a meteorological perspective, the annual probability of an Iniki-like event striking Kauai is similar to the annual probability of an Iniki-like event striking Oahu. Therefore, a powerful Iniki-like storm striking Oahu or any of the other Hawaiian Islands is a realistic, if remote, scenario.

Modeling the Impact

To estimate the effect of a major tropical cyclone on the Honolulu region, AIR Worldwide has simulated a storm identical to Hurricane Iniki in its meteorological parameters — including central pressure, radius of maximum winds, and forward speed — but with a track that takes it to Oahu, about 100 miles east of Kauai (see Figure 2).

Analysis of this simulation shows that insured losses caused by a direct hit on Honolulu by an Iniki-like event would approach $37 billion — over 11 times higher than the estimated losses from a recurrence of Hurricane Iniki on Kauai. In addition, if a more powerful hurricane — one with a lower central pressure, a larger radius of maximum winds or a slower forward speed — were to follow the track shown in Figure 2 and strike Oahu, this tropical cyclone would cause even greater losses. The extremely high losses inflicted by these simulated, though realistic, storms lend insight into the devastation that even rare tropical cyclones may cause in Hawaii.

Although hurricanes seldom strike the Hawaiian Islands, residents of all of the islands and insurers should still understand the risks posed by these events. AIR estimates that significant insured losses in Hawaii from natural catastrophes are quite possible, and with tools such as catastrophe models, the insurance industy is better prepared to weather these types of events.

Topics Catastrophe Natural Disasters Hurricane Hawaii