Hurricane watches are in effect for the Big Island and Maui.
“Dangerous, life-threatening surf will arrive ahead of the hurricane on Saturday,” the National Hurricane Center wrote. "Heavy rain and increasing winds are possible on the Big Island starting Saturday night, and could quickly spread up the chain Sunday. "
As of late Friday morning Hawaii time, Douglas had weakened slightly, its peak winds down to 115 mph (Category 3 intensity), after reaching 130 mph on Thursday night. Satellite imagery showed its eye becoming briefly swallowed by hulking thunderstorms, an indication that its organization was becoming slightly disrupted. But the Hurricane Center warned: “Douglas remains a powerful hurricane.”
As of late Friday morning Hawaii time, Douglas was moving west at around 18 mph. Douglas’s core of hurricane-force winds only extends outward 25 miles from its center, making it a rather compact cyclone. Subsequently, any gentle jaunts in its path could have significant bearings on the impacts realized on the various islands of Hawaii.
The National Hurricane Center forecasts winds of 65 to 75 mph surrounding Douglas’s immediate center during its transit of the Hawaii islands. However, the Hurricane Center’s forecast track includes the possibility of a direct hit or a graze on the north side of the islands. The center of the latest track forecast takes the storm just north of the Big Island and Maui before making landfall on Oahu, although shifts in the predicted course are inevitable in the next 48 hours.
“The most likely arrival time of tropical storm force wind speeds based on this forecast is early Sunday morning for the eastern end of the state and late Sunday into Sunday night for the western end,” writes the National Weather Service in Honolulu.
Before making its closest approach, Douglas is expected to diminish from a major hurricane as it outruns warmer waters and encounters a less favorable atmosphere. Douglas will run into slightly drier air, eroding the periphery of the cyclone, while moving into a more “sheared” environment.
Wind shear — or a change of wind speed/direction with height — can act to shred a developing system and disturb ongoing cyclones by sort of knocking them off-kilter.
Nevertheless, significant impacts over the islands are anticipated if the current track forecast does not shift. According to the National Weather Service in Honolulu, they include: “strong winds capable of damaging infrastructure, downing trees and causing power outages, heavy rainfall that could lead to flash flooding, and warning-level surf for coastal areas exposed to Douglas moving through.”
It added: “A combination of the winds, large seas, and higher than predicted water levels could translate to coastal flooding issues in the vulnerable low-lying areas due to surge.”
Heavy rain potential
The Hurricane Center projects widespread rainfall totals of 6 to 10 inches with isolated amounts to 15 inches, especially in the high terrain, between Saturday night and Monday.
“The potentially close passage of this cyclone to the state may result in very heavy rainfall and flash flooding,” wrote the National Weather Service in Honolulu.
Heavy rains could begin as early as Saturday evening on the Big Island, progressing westward up the chain and arriving later Sunday in Kauai.
Tropical cyclones are notorious for interacting with the mountainous topography of Hawaii to yield exorbitant rainfall totals, often defying the limits of prediction. Even comparatively weak tropical storms have dropped upward of 30 inches of more of rainfall in Hawaii, keeping forecasters on the edge of their seats any time a cyclone nears the state.
In August 2018, Hurricane Lane collapsed upon approach to Hawaii, dumping upward of 52 inches on Mountain View on the Big Island, a community 15 miles south-southwest of Hilo. It beat out a state record set by Hurricane Hiki in 1950, which produced 52 inches of rain.
It also vied for a national record, edged out only by Tropical Storm Harvey. 60.58 inches came down in Nederland, Tex., part of the same setup that flooded Houston beneath feet of water.
In the month after Lane hit Hawaii, the state was also grazed by Norman and Olivia, the trio of storms contributing to the Pacific’s most active hurricane season on record.
Hurricane frequency in Hawaii
Direct hurricane hits are rare in Hawaii. The island chain is frequently sideswiped or grazed by cyclones, but it has been years since a hurricane struck.
In fact, the most recent direct hurricane landfall came on Sept. 12, 1992. That was also Hawaii’s worst hurricane strike on record, when Iniki roared through Kauai as a Category 4 tempest with 130 mph wind gusts.