Hurricane Polo, one of the strongest storms of the Pacific season, is swirling off Mexico's southwestern coast and is expected to remain offshore, the US National Hurricane Center said, though it warned of heavy rain and dangerous surf along the shoreline.

The storm underwent extreme rapid intensification, with winds leaping from 45 mph to 150 mph in about 24 hours between September 21-22. The centre called the transformation "truly remarkable". Its central pressure made it the second-strongest hurricane on record in the Eastern Pacific, behind 2015's Hurricane Patricia, while its wind speeds tied 2009's Hurricane Rick as the third-strongest.

Polo's intensity has fluctuated. A NOAA Hurricane Hunter aircraft recorded sustained winds near 180 mph, and the storm was downgraded from Category 5 to Category 4 on September 23 before reaching Category 5 again early on September 24. Forecasters expect it to remain a powerful major hurricane for the next few days, and some near-term strengthening is possible.

As of early today, its core was about 175 miles off Zihuatanejo, in Guerrero state, moving north-west at 9 mph. Mexico issued a tropical storm warning from Tecpan de Galeana, in Guerrero, to Manzanillo, in Colima, and a watch from Punta San Telmo to Playa Perula, in Jalisco. Rainfall of 4 to 6 inches is forecast for parts of Guerrero, with up to 8 inches possible, and the centre warned of life-threatening flooding and mudslides in steep terrain.

Mexican President Claudia Sheinbaum said the storm was not expected to make landfall but urged vigilance, naming Jalisco, Colima and Michoacan as states at risk.

Scientists say the current El Niño is now the strongest on record. Daily sea surface temperatures in a key Pacific region were 3.05C above average on September 19, beating the 3.02C set in 2015. Warmer waters give storms more energy. Elsewhere in the Pacific, Tropical Storm Nolo has triggered a hurricane watch for Hawaii's Big Island.