Olivia (2018)

A tropical depression formed about 350 nautical miles southwest of Manzanillo, Mexico at 0000 UTC 1 September 2018 and became Tropical Storm Olivia on 2 September. Olivia moved generally west to west‑northwest, underwent rapid strengthening from 3–5 September, and reached hurricane strength on 4 September. After an initial weakening, it unexpectedly re‑intensified and reached its peak over the eastern Pacific on 0000 UTC 7 September, then crossed 140°W into the central Pacific just before 0000 UTC 9 September. It gradually weakened as it approached Hawaii, became a tropical storm on 11 September, and degenerated to a remnant low by 0600 UTC 14 September.

Olivia made two recorded landfalls in Hawaii on 12 September 2018. The first landfall was about 10 nautical miles northwest of Kahului, Maui at 1910 UTC (9:10 AM HST) on 12 September as a tropical storm with estimated sustained winds of 40 kt (46 mph). The second landfall was about 6 nautical miles north‑northwest of Lāna‘i City at 1954 UTC (9:54 AM HST) on 12 September, also with estimated sustained winds of 40 kt (46 mph). The system moved just north of the Big Island (closest approach ~50 n mi) and caused most direct impacts across Maui County and windward slopes of other islands.

The storm’s maximum intensity was estimated at 115 kt (132 mph) with a minimum central pressure of 951 mb at 0000 UTC 7 September, making it a Category 4 hurricane at peak in the eastern Pacific. In the central North Pacific before reaching Hawaii, Olivia’s maximum observed intensity was 75 kt (86 mph) with a lowest aircraft‑measured pressure of 980 mb while approaching the islands. At Hawaiian landfall the cyclone was a minimal tropical storm with surface pressures around 1006–1007 mb.

Olivia produced very heavy rainfall and flash flooding across windward sections of Maui, Moloka‘i, and Oʻahu. Peak 48‑hour rainfall totals for the period 6 AM HST 11 September–6 AM HST 13 September were generally in the 11–13 inch range on windward slopes. Specific station totals in the report include 12.03 in at Puu Alii, 10.31 in at Manoa Lyon Arboretum, 7.81 in at Molokai 1, and 7.41 in Nuuanu Upper; many west‑Maui and Napili/Waiheʻe River basin locations saw severe flash flooding. Observed sustained tropical‑storm‑force winds were limited; the strongest sustained wind reported on land was 34 kt (39 mph) at Lāna‘i City Airport with a gust to 48 kt (55 mph). Some harbors and north/east shores experienced high surf, with breaking waves up to about 20 ft reported.

There were no reported fatalities associated with Olivia. Damage was concentrated on Maui County where flash floods swept away vehicles and damaged or flooded homes, a taro farm was devastated, numerous trees fell, landslides occurred, and thousands lost power (about 4,500 customers in Makawao). Road closures occurred across multiple islands, and roughly 32,000 gallons of raw sewage overflowed into Kapalama Stream and Honolulu Harbor on Oʻahu. No comprehensive monetary damage estimate was provided in the report.

Noteworthy aspects include Olivia’s two phases of rapid intensification (one in early September producing a Category 4 peak) and an unexpected re‑intensification after an initial weakening. Forecast track guidance performed well overall: NHC and the Central Pacific Hurricane Center had track errors much lower than five‑year means and the 120‑hour NHC track error was the lowest on record (for storms with sufficient forecasts) in the eastern Pacific. Intensity forecasts exhibited larger errors during the two intensification periods, with official intensity forecasts tending to be biased low during those times.