Walaka (2018)

Walaka formed from a surface trough that organized into a tropical depression on 29 September 2018 about 600–700 nautical miles south-southeast of the Island of Hawaiʻi. It moved generally westward then northwestward under a subtropical ridge, became a named tropical storm late on 29 September, reached hurricane strength by 30 September, and moved northward before becoming a post-tropical low on 6 October. The system persisted through 6 October and dissipated over the open central North Pacific on 7 October.

The hurricane’s core passed across parts of the Papahānaumokuākea Marine National Monument on 4 October. The eastern eyewall moved across French Frigate Shoals and the eye passed near Brooks Banks (about 31 nmi WNW of French Frigate Shoals) around 0620 UTC 4 October. There were no reported mainland U.S. landfalls; the primary impacts were to small islands and atolls in the monument and to Johnston Atoll where warnings were issued and personnel evacuated.

Walaka’s peak intensity was estimated at 140 knots (161 mph) sustained winds with a minimum central pressure of 921 mb, reached around 0000–0600 UTC 2 October. That peak made Walaka a Category 5 hurricane on the Saffir–Simpson scale. By the time it crossed the Papahānaumokuākea area on 4 October it was about 110 knots (Category 3) and then steadily weakened as it moved into cooler water and higher wind shear.

Storm surge combined with very large waves produced substantial erosion on Tern Island and East Island at French Frigate Shoals; East Island’s sand was almost completely removed. The report does not list specific measured surge heights at tide gauges for populated places, and there were no reports of heavy rainfall totals for cities or counties in the main Hawaiian Islands—most direct effects were on the small islands, reefs, and beaches of the atolls and shoals.

There were no reported human casualties associated with Walaka. The most significant impacts were ecological and physical: near-complete loss of East Island (an important green sea turtle nesting site), major sediment and land loss on Tern Island, and widespread damage to coral reefs and reef fish around French Frigate Shoals, Lisianski Island, and Pearl and Hermes Atoll. Forecasts and watches performed well: genesis was anticipated days in advance and National Weather Service/CPHC track forecasts were generally more accurate than recent averages, with timely watches and warnings that allowed evacuations (for example, researchers and personnel from Johnston Atoll and the monument were evacuated).