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.
Rainfall totals of up to 6in (15cm) are forecast, with 8in possible in some areas. Residents have also been warned of large ocean swells and life-threatening surf conditions for the next few days. "So far, the information we have is that (Polo) will not make a direct landfall on the coast," Mexico's President Claudia Sheinbaum said on Tuesday, but she warned residents along the Pacific coast to be vigilant.
Sheinbaum highlighted the states of Jalisco, Colima and Michoacan as being at risk. Data from the Hurricane Hunter aircraft indicated that Polo could still strengthen, because of what NOAA called a "very conducive" environment of extremely warm ocean waters feeding the storm. Scientists monitoring the ocean temperatures say the current El Niño climate phenomenon is now the strongest on record.
Daily sea surface temperatures in a key region of the central-eastern Pacific were 3.05C above average on 19 September - exceeding the previous record of 3.02C set during the very strong El Niño of 2015. Some longer-term measures of El Niño have not yet hit record levels but this is likely to change, with the peak not expected until later this year. Warmer ocean waters mean storms can pick up more energy, leading to higher wind speeds.
El Niño is a naturally-occurring pattern, which happens about every two to seven years and tends to last for around a year. Trade winds that typically blow from east to west across the Pacific Ocean can weaken or even reverse, allowing warm water to spread eastwards. Heat is then released from the ocean into the atmosphere which can boost global temperatures and drive changes in weather patterns around the world.
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