Nolo reaches Category 3 at 115 mph and aims at the date line, not Honolulu

Hurricane Nolo strengthened to Category 3 on Saturday in the remote central Pacific, with maximum sustained winds of 115 mph, about 185 kilometres an hour. The National Hurricane Center in Miami placed the storm about 200 miles, or 320 kilometres, south-southwest of Lisianski Island, moving west at 20 mph. Forecasters expected it to cross the international date line on Sunday. No populated island was under a threat forecast.

A tropical storm warning covered a stretch of the Papahanaumokuakea Marine National Monument from Maro Reef to Lisianski Island. The center said large surf tied to Nolo would likely inundate some of the low-lying atolls. The monument is a federal protected area of islands, reefs and shoals northwest of the main Hawaiian chain. It has research camps and wildlife, not cities. The warning is for that polygon, not for Oahu.

The intensity path is the useful part of the advisory. Nolo passed near Hawaii as a hurricane, weakened to a tropical storm, regained hurricane strength on Friday, and reached major-hurricane strength on Saturday. An earlier advisory had it southeast of Maro Reef with 90 mph winds and said rapid intensification was possible. The evening Category 3 figure confirmed that the strengthening had continued. A storm that weakens and then returns to Category 3 inside two days is a forecast problem, because the second peak can arrive after the watches for the first pass have been dropped.

Category 3 begins at 111 mph on the Saffir-Simpson scale and runs to 129 mph. Nolo at 115 sits just inside that band. The next step, Category 4, starts at 130 mph. The evening advisory did not put that step in the forecast. The forward speed, 20 mph, is fast for a storm still intensifying. Fast motion shortens the time any one atoll spends in the core and spreads the surf ahead of the center. That is why the warning is about inundation of low atolls rather than a long battering of a harbour.

Honolulu and the main islands are not in this advisory. The center was explicit. The relevant coast is the monument, hundreds of miles northwest of Kauai. A reader in Hilo does not need to board a window for Nolo's Sunday position. A field crew on Lisianski does. The distinction is the whole of the public warning. Lisianski is a low coral island in the monument. Maro Reef, at the other end of the warning, is a largely submerged reef. Neither has a resident town. Both can take surf that covers the dry ground that does exist.

A second Pacific storm, Category 1 Rachel, was moving west away from Mexico. The hurricane center said dangerous swells would continue through the weekend for west-central Mexico and through Monday for the Baja California peninsula. Rachel and Nolo are not the same system. They are two basins producing surf on opposite sides of the ocean in the same weekend. Category 1 starts at 74 mph. Rachel is a surf event for Mexico. Nolo is a major hurricane aimed at empty atolls and then at the date line.

The date line crossing is an administrative fact as well as a geographic one. Once Nolo passes 180 degrees longitude, responsibility shifts from the Central Pacific Hurricane Center's patch toward the western Pacific warning agencies. The Sunday forecast said that crossing was expected. A storm that is Category 3 at the line is someone else's major hurricane by Monday, still the same vortex. The name stays. The office that writes the advisory changes.

Papahanaumokuakea was established as a marine national monument and later expanded. It is one of the largest protected areas on earth by water area. A tropical storm warning inside it does not evacuate a city. It tells the small number of people who are allowed on those islands, researchers and wildlife crews, to get off or to shelter. The center's inundation line is the operational sentence for them.

What Saturday's advisory fixes is a position, a speed, a wind, and a warning polygon that stops at Lisianski. The open item is whether the atolls inside that polygon take the inundation the center called likely, and whether the second intensification holds after the date line. The main Hawaiian islands are outside both questions. The number that would pull them back in is a turn. The evening track did not have one.