Science2 publishers3 min readPublished
Hurricane Polo climbed to Category 5 in under 24 hours over record-warm Pacific water
The Met Office's Julian Heming credits this year's record El Nino for the ocean heat beneath the storm. Rapid intensification happens several times a year worldwide, and the two published accounts differ on Polo's peak winds.
The Scientist · Science desk
What happened
- Live Science reported that Polo went from a tropical storm to a Category 5 hurricane in less than 24 hours on Tuesday, 22 September, with winds reaching 180 mph.
- AFP's account of the same storm puts the climb at Category 1 to Category 5 inside a day, with sustained winds of 160 mph and further strengthening still forecast.
- By Wednesday, 23 September, the National Hurricane Center had downgraded Polo to Category 4 while still calling it "a very powerful major hurricane".
- The NHC does not predict a direct landfall, and warned that heavy rain may produce life-threatening flooding and mudslides along with extremely hazardous ocean conditions.
- Mexican authorities suspended classes in Pacific coast states including Guerrero and Michoacan, the first in line for the heavy rain.
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Why it matters
- contradiction Anyone reconstructing Polo's intensification rate from the coverage gets a different answer depending on the publisher, because the accounts differ both on the peak wind and on the category the storm started from.
- constraint A single storm cannot carry the El Nino attribution when rapid intensification already happens several times a year worldwide; the test needs event counts compared across El Nino and neutral seasons.
- exposure An offshore core spares Mexico's Pacific coast an eyewall but not the rainfall, so the flooding and mudslide exposure holds even if the forecast track never touches land.
- precedent If the WMO is right that this El Nino runs to at least February 2027, the warm background under the eastern Pacific stays in place well past the end of this hurricane season.
Julian Heming, an environmental scientist at the UK Met Office, gave Live Science the ingredient list by email. "The main requirements are high sea temperatures, low wind shear and upper air conditions which allow air to escape quickly from the top of the hurricane," he said [10]. Polo had all of them. Heming said the unusually warm ocean supplied abundant heat and moisture, while very moist air and weak winds higher in the atmosphere let thunderstorms organize and strengthen without being disrupted [13]. The storm also built a compact, well-defined core that concentrated its energy into a relatively small area [14].
On cause he was less guarded. Polo's rapid and sudden growth is testament to the dominance of this year's so-called super El Nino, Heming said, an El Nino that set a record on Monday as the most extreme on record and has amplified a number of extreme weather events globally [15]. In the same email he put the global rate of rapid intensification at "several times per year across the globe" [11]. One storm over hot water is consistent with the El Nino explanation and also consistent with that base rate. Separating the two takes a count of rapid intensification events in El Nino seasons against neutral ones.
NOAA identifies El Nino conditions when the eastern tropical Pacific sits at least 0.5 degrees Celsius above average, alongside matching changes in winds, surface pressure and rainfall [16]. "Super El Nino" is not a scientific term; it describes warming above 2 C [17], four times the level at which NOAA declares an event [1].
The two accounts of Polo disagree on more than wording. The 20 mph gap between the peak wind figures is consistent with readings taken hours apart, since AFP reported the storm was still forecast to strengthen [2]. The starting point is a harder problem: a climb from tropical storm strength and a climb from Category 1 are not the same rate across the same 24 hours.
For an emergency manager the useful measure is the gap between the hour the NHC first forecast Category 5 and the hour Polo got there, and neither account reports it. Local alerting did begin before the jump. On Monday, authorities in Acapulco used megaphones on the beaches and avenues to broadcast warnings of possible thunderstorms [7], urging people to take "preventive measures at home or at work, checking safety conditions in your home and locating your nearest temporary shelter" [8]. Acapulco was devastated by Hurricane Otis in 2023 [9].
Where the storm goes is still open. "There is currently a lot of uncertainty in the long term as to whether Polo will move further out to sea or turn north-eastwards towards Mexico," Heming said [20].
What to watch
- A post-season best track from the National Hurricane Center would fix Polo's peak intensity and the category the 24-hour climb began from.
- Measured rain totals and mudslide reports along Mexico's southwest coast over the coming days.
- Whether the system tracks far enough north to bring rain, rip currents and large swells to Southern California.