Potential Tropical Cyclone One-C update for Thursday: Heavy rainfall forecast, with increasing risk of wind, wave impacts

Potential Tropical Cyclone One-C was about 775 miles east-southeast of Hilo as of 11 p.m. Aug. 12 and 985 miles east-southeast of Honolulu, still churning west at 10 mph with maximum sustained winds of 45 mph.

A turn toward the west-northwest is expected today (Aug. 13).

It still remains too soon to specify the exact location and magnitude of impacts from the storm system, forecasters at Central Pacific Hurricane Center advised residents to continue closely monitoring forecast updates and ensure they have their preparedness plans in place.

A hurricane or tropical storm watch is still likely necessary to be issued for a portion of the state by early today.

Heavy rainfall associated with the potential tropical cyclone will begin to impact the Big Island on Aug. 14 and then spread west across the island chain during the weekend. This rainfall could produce life-threatening flooding and mudslides, especially in areas of steep terrain.

Rainfall totals of 5 to 10 inches is forecast throughout much of the island chain, with maximum totals of 20 inches possible in some locations.

There is an increasing risk of wind and wave impacts regardless of the exact track and intensity of the system.

Wind speed probabilities show sustained winds of up to between 50 and 60 mph are possible for much of the Big Island. Swells generated by the disturbance are expected to reach east-facing shores of the state on Friday then spread west and build through the weekend, likely causing life-threatening surf and rip currents.

U.S. Coast Guard port captain in Honolulu is increasing port heavy weather conditions for commercial ports of City and County of Honolulu as well as Kaua’i, Maui and Hawai’i counties because of One-C.

Changes are:

• Whiskey for commercial ports of City and County of Honolulu and Kaua‘i County 8 p.m. Wednesday. Condition Whiskey is set when National Weather Service advisories indicate sustained gale force winds of 39 to 54 mph are predicted within 72 hours.
• X-ray for commercial ports of Maui and Hawai‘i counties effective at 8 p.m. Wednesday. Condition X-ray is set when weather advisories indicate sustained gale force winds are predicted within 48 hours.
• X-ray for City and County of Honolulu commercial ports effective at 8 a.m. today.

Click here for additional information about port conditions. Follow the Coast Guard on Facebook, X and Instagram for updates.

Several events and activities also were canceled or postponed Wednesday because of the threat of the potential cyclone.

Canceled events included the Puʻukoholā Heiau National Historic Site 54th annual Hoʻokuʻikahi i Puʻukoholā, its annual Establishment Day Hawaiian Cultural Festival, and Hawai‘i Island Agrifood Summit, which were originally slated for this weekend.

Nā Mākua Hana Noʻeau Hawaiian Arts Festival was postponed to Aug. 22.

Shipping company Young Brothers is closely working with U.S. Coast Guard and state departments to monitor the incoming tropical cyclone. The port of Honolulu will not accept refrigerated cargo, hazardous material or livestock for all sailings departing today to Nāwiliwili on Kaua‘i, Kahului on Mau and Kawaihae on the Big Island.

The firm also anticipates changes to its sailing schedule as well as cargo acceptance and availability as the storm continues to develop.

Hawaiian Electric advises its customers to review their emergency plans and ensure they have necessary supplies on hand. Preparation and safety tips can be found at the electric utility’s website.

Hawai‘i County Civil Defense also urges residents to prepare for high surf, flooding rains and damaging winds, making sure they have food, water and fuel as well as signing up for alert messages.

The potential tropical storm had changed little in organization from the Wednesday evening update and remains forecast to bring heavy rainfall, high winds and dangerous surf as early as Friday night to the Hawaiian Islands — starting with the Big Island.

One-C by the 11 p.m. update Wednesday still lacked sufficient organized shower and thunderstorm activity to be considered a tropical storm, but it is expected to become a tropical storm by tonight with additional strengthening forecast thereafter.

Tropical storm force winds extended outward up to 275 miles, mainly to the northeast of the system’s center.

Some data suggest the circulation of the disturbance has become more circular throughout the night; however, the strongest winds are still located almost about 230 miles northeast of the center.

A large high pressure system north of the Hawaiian Islands should steer the system generally west-northwestward for the next several days. Guidance is in general agreement with that scenario, but there was a slight shift to the south since the previous advisory Wednesday evening.

Several models continue to put the center near the north edge of the guidance envelope. Reformations of the center also could occur as the disturbance organizes, so confidence in the track forecast, including how close the center will come to Hawai’i, remains low until a better-defined center forms.

The environment looks generally favorable for intensification during the next 2 days. However, Central Pacific Hurricane Center forecasters report the ongoing sprawling structure suggests strengthening will be gradual.

Some southwesterly shear, interaction with land on the Big Island and dry air entrainment will likely slow or even stop intensification as the system approaches the islands, but if One-C tracks somewhat south, it would remain in favorable conditions to strengthen more than forecast.