Hurricane Polo, a large storm off the coast of Mexico, has been downgraded to category 4 but remains “very powerful”, the US National Hurricane Center said.

The Pacific hurricane, one of the strongest of the season, was projected to head towards the coast of south-western Mexico, bringing intense wind and rain, though was not expected to make landfall.

A Hurricane Hunter aircraft, operated by the US National Oceanic and Atmospheric Administration (Noaa), flew into the storm on Tuesday and recorded sustained wind speeds near 180 mph (285 km/h), with stronger gusts.

The slow-moving storm is being fuelled by extremely warm waters caused by the El Niño climate phenomenon.

It was previously a category 5 storm – the highest level on the scale – but was downgraded to category 4 on Wednesday morning.

As it slowly drifts northwest, Polo will drive torrential rainfall in parts of Mexico’s south-west coast, and “may produce life-threatening flooding and mudslides, especially in areas of steep terrain,” the forecaster said.