Tropical Storm Nolo strengthened to 60 mph (95 km/h) early Thursday, Sept. 24, and remained on a path that could bring hurricane conditions and extreme rainfall to Hawaii's Big Island, where a Hurricane Watch covers all of Hawaii County. In its latest public advisory on Nolo, the Central Pacific Hurricane Center said the storm could produce 15 to 20 inches of rain across the Big Island, with maximum amounts of 35 inches, enough to cause catastrophic, life-threatening flooding and mudslides in steep terrain.

At 5 a.m. HST Thursday, Nolo was stationary about 285 miles (460 km) south of South Point and 455 miles (735 km) south-southeast of Honolulu. Forecasters expected it to begin moving north or north-northeast later Thursday and through Friday, then make a sharp westward turn by Saturday in the vicinity of the Big Island. Nolo was forecast to become a hurricane by Thursday night and could become a major hurricane, meaning Category 3 or stronger, by the weekend. Hurricane conditions are possible in the watch area as early as Friday night.

Figures in this report come from Advisory 16, issued at 5 a.m. HST on Thursday, Sept. 24. Nolo's forecast is changing quickly, and residents should rely on the newest advisories from the National Weather Service and Hawaii County Civil Defense.

Hurricane Watch Covers All of Hawaii County

A Hurricane Watch means hurricane conditions, with sustained winds of 74 mph or higher, are possible within the watch area. In the Thursday morning NHC forecast discussion, forecasters said a Hurricane Warning would likely be required for the Big Island later that day and that Tropical Storm Watches may be needed for other islands Thursday or Friday. They urged Big Island residents to follow the advice of local officials, including evacuating if told to do so.

The National Weather Service in Honolulu has also posted a flood watch for Hawaii Island, Maui, Lanai, Molokai and Kahoolawe through Saturday afternoon, according to the Honolulu Star-Advertiser. In a bulletin for Hawaii County, the forecast office warned: "Efforts to protect life and property should now be rigorously underway."

Along the coast, the NHC projects storm surge of 1 to 3 feet above ground in Hawaii County if the peak surge arrives at high tide. Swells from Nolo were expected to reach exposed east- and south-facing Big Island shores by Friday, then spread west and build through the weekend, bringing life-threatening surf and rip currents.

Hawaii County Mayor Kimo Alameda has signed an emergency proclamation for Tropical Storm Nolo declaring a local state of emergency. Among other powers, it authorizes the county's Civil Defense Agency to take over public buildings and property for use as emergency shelters and staging sites for response crews.

Rainfall Threat Centers on Southeast Hawaii Island and Maui

The Hawaii Emergency Management Agency said the isolated 35-inch totals are most likely in southeast Hawaii Island and that catastrophic flooding and life-threatening landslides are possible on the island Friday and Saturday. The agency urged residents to consider evacuating to a safer shelter, especially in southeast Hawaii Island. Hawaii County said windward areas could see 10 to 30 inches of rain through Sunday morning.

Maui could see 5 to 10 inches of rain, with maximum amounts of 20 inches, and forecasters said life-threatening flooding and mudslides are possible there, especially in high terrain. Across the rest of the state, the NHC expects 1 to 3 inches, with maximum amounts of 6 inches and isolated flooding possible.

Recent storms raise the stakes. Nolo is the fourth tropical system to affect Hawaii this season, following Hurricane Lala, Tropical Storm Moke and Hurricane Lowell, according to The Weather Channel. Lala brought flooding, mudslides and strong winds to the Big Island and parts of Maui County in mid-August, and some areas only recently had power restored. Moke's rain caused flooding in part because the ground was still saturated from Lala.

Forecast Track Remains Uncertain Near the Turn

The central forecast question is how close Nolo comes to the Big Island before it turns west. "The big question is how close does Nolo get before making that turn," NHC forecaster Blake wrote Thursday morning, noting inconsistencies from one model run to the next and describing the forecast as more uncertain than normal.

Guidance has shifted in both directions. The Weather Channel reported early Thursday that some model runs had pulled the track farther south. The NHC's 5 a.m. HST discussion, issued shortly afterward, said the bulk of the models had shifted slightly northward and that the official forecast moved with them. The forecast cone shows the likely path of the storm's center, not the full reach of its impacts, and heavy rain, wind and surf can extend well beyond it.

Nolo was forming an inner core over warm water with low wind shear, and forecasters said conditions appeared to be coming together for rapid intensification. The official forecast showed Nolo reaching about 115 mph by early Saturday. A track closer to the Big Island could substantially increase the threat of damaging winds, particularly over higher terrain, along exposed ridges and in areas downwind of ridges, the NHC said.

The Weather Channel noted that only one other major hurricane, Dot in 1959, has tracked within 150 miles of the Big Island in NOAA's hurricane database.

Shelters, Sandbags and Water Supplies

Hawaii County opened public shelters for Tropical Storm Nolo at 6 p.m. Wednesday, and it provided a free shuttle from Naalehu Gym to Robert Herkes Gym in Pahala for residents of Waiohinu and Naalehu. Shelters are pet-friendly, but pets must be kept in a carrier, and evacuees should bring their own food, water, and medicines. "If your home flooded before, or is at risk, consider moving to safer shelter," Alameda said.

Sand piles are available around the island, but residents must supply their own bags. The county Department of Water Supply urged customers to store an emergency supply of drinking water now, warning that severe weather and power disruptions could interrupt water service.

Standard flood safety applies. Never try to cross moving water on foot or in a vehicle, stay away from streams and steep slopes during heavy rain, and treat downed power lines as live. Heavy rain falling uphill can cause flooding even where it is not raining. Households still repairing damage from Lala may want to photograph their homes and belongings before the storm and check whether their insurance covers flooding, since standard homeowners policies generally do not.

The key changes to watch are an upgrade to a Hurricane Warning for the Big Island and any new watches for Maui County and other islands.

Nolo Timeline So Far

Sept. 24, 5 a.m. HST: Nolo strengthens to 60 mph, and the NHC says a Hurricane Warning will likely be required for the Big Island later in the day.

Sept. 23, 6 p.m. HST: Hawaii County opens public shelters.

Sept. 23, 2 p.m. HST: Nolo strengthens slightly to 45 mph while the Hurricane Watch remains in effect.

Sept. 23, 11 a.m. HST: Tropical Depression Fifteen-E becomes Tropical Storm Nolo about 310 miles south of South Point with 40 mph winds, and a Hurricane Watch is issued for Hawaii County.

Nolo was still a tropical storm Thursday morning, but forecasters expected it to strengthen quickly and bring extreme rain to the Big Island and Maui regardless of its exact track. Residents should finish preparations and follow Hawaii County Civil Defense and National Weather Service updates.

What Readers Want to Know

Is Nolo a hurricane yet?

As of the 5 a.m. HST advisory on Thursday, Sept. 24, Nolo was a tropical storm with 60 mph winds. The Central Pacific Hurricane Center expected it to become a hurricane by Thursday night and possibly a major hurricane by the weekend.

Which areas are under a Hurricane Watch?

All of Hawaii County, which covers the Big Island. Forecasters said a Hurricane Warning would likely be issued for the Big Island later Thursday, and other islands could see Tropical Storm Watches.

How much rain could fall?

The Big Island could see 15 to 20 inches, with maximum amounts of 35 inches, most likely in the southeast. Maui could see 5 to 10 inches, with up to 20 inches in places, and the other islands 1 to 3 inches, with up to 6 inches.

When will conditions worsen?

Heavy rain could begin over the Big Island late Thursday and increase Friday. Hurricane conditions are possible in the watch area as early as Friday night, with the closest approach expected Saturday.

Who faces the greatest risk?

Residents near streams and steep slopes on the Big Island and Maui, people in southeast Hawaii Island, coastal homes on exposed east- and south-facing shores, and communities still recovering from Hurricane Lala face the highest flood and landslide risk.

Where can residents find shelters and official updates?

Hawaii County opened public shelters Wednesday evening, and Civil Defense can be reached at (808) 935-0031. The Hawaii Emergency Management Agency and the National Weather Service in Honolulu post updated alerts.