Hurricane Polo more than doubled its wind speed in less than a day, climbing from a 40 mph tropical storm late Sunday to an 85 mph hurricane by Monday afternoon, and the National Hurricane Center expects it to become a major hurricane on Tuesday while it churns off some of Mexico's most visited Pacific beach towns. In its public advisory on Hurricane Polo, Advisory 5, issued at 3 p.m. CST (5 p.m. EDT) on Monday, Sept. 21, the NHC said a NOAA Hurricane Hunter aircraft located the center about 195 miles south-southwest of Zihuatanejo and 290 miles south-southeast of Manzanillo.

The same advisory placed the heaviest rain threat along the Guerrero and Michoacán coast. Forecasters project 4 to 8 inches of rain, with isolated totals up to 12 inches, across coastal portions of both states through Thursday. Coastal Oaxaca, Colima and Jalisco could see 3 to 6 inches. Earlier in the storm's life, forecasts had focused the heavy rain threat farther west, on Michoacán, Colima and Jalisco.

For residents of steep coastal towns and for travelers booked into Ixtapa-Zihuatanejo, Manzanillo or Puerto Vallarta this week, the immediate concerns are flooding, mudslides and dangerous surf. The NHC expects Polo's core to stay offshore, but it has cautioned that it is too soon to determine the exact location or magnitude of coastal impacts.

An 18-Hour Leap from Tropical Storm to Hurricane

The NHC defines rapid intensification as an increase in maximum sustained winds of at least 35 mph (30 knots) within 24 hours. Polo cleared that bar with room to spare. It carried 40 mph winds at 9 p.m. CST Sunday, reached 65 mph by 9 a.m. Monday and hit 85 mph at 3 p.m. Monday, a gain of about 45 mph in roughly 18 hours. Its minimum central pressure fell to 974 millibars.

The environment favors more strengthening. In its forecast discussion for Advisory 5, the NHC cited very warm water, light wind shear and a moist atmosphere, and forecaster Blake wrote that "a high-end rapid intensification event is expected." Aircraft data showed the storm in the early stages of an eyewall replacement cycle, which could slightly slow the climb, although the discussion noted that the old inner eyewall was already fading.

The official forecast brings Polo to about 145 mph, Category 4 strength, by midday Tuesday, then to 160 mph early Wednesday and 165 mph by Wednesday afternoon, above the 157 mph threshold for Category 5. Polo is a compact storm, with hurricane-force winds extending only about 25 miles from the center and tropical-storm-force winds reaching about 70 miles. Yale Climate Connections meteorologists Bob Henson and Jeff Masters and forecasters at AccuWeather have also said Polo could approach or reach Category 5 by midweek. These are forecasts, not certainties, and the NHC said increasing easterly wind shear could cause some weakening later in the forecast period.

Rain, Mudslides and Surf Along the Guerrero and Michoacán Coast

A Tropical Storm Watch was in effect from Tecpan de Galeana to Punta San Telmo as of Advisory 5, a stretch that includes the Zihuatanejo and Lázaro Cárdenas areas. The NHC said tropical storm conditions are possible there by Tuesday night or early Wednesday and that the watch would likely be expanded westward later Monday. Readers should check the latest advisory, since that expansion may already have happened.

Rain is the most widespread threat, and the terrain makes it worse. The Sierra Madre del Sur rises steeply behind much of this coastline, and the NHC warned that the heaviest totals could produce life-threatening flooding and mudslides, especially in steep terrain. Hillside neighborhoods, river valleys and the coastal highway corridor face the greatest exposure. Swells from Polo were expected to begin reaching the coast on Tuesday, bringing life-threatening surf and rip currents for beachgoers and small-boat fishermen.

Forecasters do not all place the heaviest rain in the same spot. AccuWeather's Polo forecast has emphasized flooding rain for Jalisco, Colima and Michoacán beginning Tuesday and noted that a track closer to the coast would increase rainfall, winds and wave action. Mexico's Servicio Meteorológico Nacional has forecast its heaviest totals for Oaxaca and Guerrero. The differences largely reflect uncertainty in the track, and a small shift in Polo's path could change local rainfall significantly.

Track Uncertainty and the Odalys Wild Card

Polo was drifting east-southeast near 6 mph on Monday afternoon. A mid-level ridge of high pressure building over Mexico is expected to force a sharp turn toward the northwest and west-northwest late Tuesday into Wednesday, carrying the core near or south of the southwestern Mexican coast. At longer range, the NHC expects troughing over the eastern Pacific to bend the storm more northwestward toward the Baja California peninsula.

Confidence in that path is limited. The NHC said a potential interaction with Tropical Storm Odalys is contributing to a large north-south spread in the computer models and that uncertainty is high at and beyond 72 hours. Readers should look at the full forecast cone graphic rather than the center line alone. The cone shows where the center is likely to track, not the size of the storm or where impacts will stop.

Residents and visitors in Colima, Jalisco and southern Baja California Sur should monitor updates even though the heaviest rain forecast currently sits farther east. A track only slightly closer to the coast would raise wind, surf and rain risks along a much longer stretch of shoreline. Forecasters have also flagged the possibility that moisture from Polo could reach northwest Mexico and the U.S. Southwest later in its life, adding to flood concerns there.

An El Niño Split Screen as the Atlantic Stays Quiet

Polo is strengthening during a season with a stark contrast between ocean basins. In their Yale Climate Connections analysis, Henson and Masters wrote that the gap between the Atlantic and Pacific seasons, which they attributed to the powerful El Niño now underway, would "grow even starker this week." Atlantic Tropical Storm Fay was already weakening on Monday.

El Niño typically warms the eastern Pacific and increases upper-level wind shear over the Atlantic, which tilts the odds toward busier Pacific seasons and quieter Atlantic ones. That is a seasonal pattern, not a finding that El Niño caused any single storm. Nature World News reported last week that the Atlantic had gone without a hurricane through Sept. 14, passing the satellite-era record for the latest first hurricane, and NOAA's August update put the odds of a below-normal Atlantic season at 75%.

What Travelers and Residents Should Do

Travelers should confirm flight and hotel status directly, follow instructions from Mexico's Servicio Meteorológico Nacional and state civil protection agencies, and consider enrolling in the U.S. State Department's Smart Traveler Enrollment Program for alerts. Stay off beaches during high surf, and never drive or walk through floodwater, since mountain runoff can rise quickly after dark. Evacuation and shelter decisions belong to local authorities.

The NHC issues full advisories every six hours, with intermediate updates every three hours while coastal watches are in effect. The figures in this report come from Advisory 5, and newer advisories supersede them.

The bottom line is that Polo is strengthening fast and is forecast to become an extremely powerful hurricane close to the coast, but its core is currently expected to stay at sea. Flooding rain in Guerrero and Michoacán is the clearest near-term risk, while the track after Wednesday remains the central uncertainty.

What Readers Want to Know

How strong is Hurricane Polo right now?

As of NHC Advisory 5 at 3 p.m. CST on Sept. 21, Polo had maximum sustained winds near 85 mph. The NHC forecast it to become a major hurricane by Tuesday, so check the newest advisory for current strength.

Will Polo make landfall in Mexico?

The NHC's forecast keeps the core near or south of the southwestern Mexican coast rather than over land. However, the agency says it is too soon to determine the exact location or magnitude of impacts, and a small track shift could bring worse conditions ashore.

Which areas face the heaviest rain?

The NHC forecasts 4 to 8 inches, with isolated totals up to 12 inches, across coastal Guerrero and Michoacán through Thursday. Coastal Oaxaca, Colima and Jalisco could see 3 to 6 inches.

What does rapid intensification mean?

It means a tropical cyclone's maximum sustained winds increase by at least 35 mph in 24 hours. Polo gained about 45 mph in roughly 18 hours between Sunday night and Monday afternoon.

Should I cancel a trip to Puerto Vallarta, Manzanillo or Zihuatanejo?

Nature World News cannot make that call for you. Check with your airline and hotel, monitor NHC and Mexican government advisories, and follow local officials' instructions if you are already there.

Why is the Pacific so active while the Atlantic is quiet?

El Niño tends to warm eastern Pacific waters and raise wind shear over the Atlantic. This year's strong El Niño has coincided with a record-late start to Atlantic hurricane formation and a busy eastern Pacific.

When is the next update?

The NHC issues full advisories every six hours, at 3 a.m., 9 a.m., 3 p.m., and 9 p.m. CST, with intermediate advisories in between while watches are in effect.