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Category 5 Hurricane Otis devastates Acapulco » Yale Climate Connections

In what the National Hurricane Center called a “nightmare scenario,” Hurricane Otis made landfall near Acapulco, Mexico, at 1:25 a.m. CDT on Wednesday, October 25, as a catastrophic Category 5 hurricane with 165 mph winds and a central pressure of 923 mb. Otis unexpectedly intensified from a tropical storm with 65 mph winds to a Category 5 storm with 165 mph winds — an astonishing 105 mph increase — in the 24 hours before landfall. Rapid intensification is extremely dangerous because it leaves people little time to prepare for strong storms. The phenomenon is expected to happen more often as the climate warms.

According to the National Hurricane Center, Otis’ peak intensification rate was 95 knots (110 mph) in 24 hours (ending at 11 p.m. EDT Tuesday), which is the second-highest such rate in the Western Hemisphere, behind a 120-mph increase by Hurricane Patricia off the Pacific coast of Mexico in 2015.

The Acapulco airport stopped transmitting data about four hours before the eyewall of Otis moved over, but Isla Roqueta, an island located just offshore of the west side of Acapulco, recorded sustained winds of 81 mph, gusting to 133 mph, at 12:45 a.m. CDT. A peak gust of 135 mph occurred at 12:30 a.m. The minimum pressure at the station was 957 mb at the time of the highest winds, so the station missed experiencing the eye, which passed just south of the island. This means that the most heavily developed areas of Acapulco (population just over 1 million) received the more powerful right-front winds of Otis, perhaps setting a record for the largest number of people ever to experience the eyewall of a Cat 5 storm. The only comparable case may be Hurricane Andrew of 1992, which hit South Florida as a Cat 5 with 165 mph winds.

The full extent of damage from Otis was still unclear as of midday Wednesday. There will likely be massive disruption for hours if not days in terms of power disruption, transportation, and the like. More intense destruction may be limited to certain parts of the Acapulco area, depending on where the storm’s highest winds and water arrived. But as explained in the Tweet below, wind damage may end up causing the main destruction from Otis. Acapulco is not particularly low-lying, and the deep water offshore is not conducive for generating large storm surges. In addition, Otis was small and was a hurricane for only about 12 hours before landfall, which limited its storm surge potential.

How much warning did Acapulco get?

It wouldn’t be entirely fair to say that Otis struck without warning. Almost all of the coast of Mexico’s Guerrera state, including Acapulco, was placed under a hurricane watch at 4 p.m. local time Monday and a hurricane warning at 4 a.m. local time Tuesday. What was unforeseen, in one of the biggest and most consequential forecast-model misses of recent years, was how quickly and how much Otis would intensify and how quickly it would make landfall — all of which led to an unprecedented and disastrous situation.

On Tuesday morning, just 16 hours before Otis would make landfall, the National Hurricane Center called for Otis to make landfall at Category 1 strength (top sustained winds of 90 mph) somewhere between 8 a.m. and 8 p.m. local time on Wednesday. Instead, Otis arrived as a monstrous Category 5 storm between 1 and 2 a.m. local time on Wednesday.

Among the factors hobbling accurate intensity forecasts of Otis on Tuesday: There was only one flight by the Hurricane Hunters (with just two penetrations) into Otis, and there was no Doppler weather radar available, either ground-based or from reconnaissance flights. Moreover, in the days leading up to Otis, forecast models struggled to determine whether Otis would make landfall at all. Some of the leading models kept Otis offshore for days, which matches typical hurricane behavior in this region much more closely than what actually happened.

Why did Otis intensify so rapidly?

Otis was a small system, with hurricane-force winds that extended out just 30 miles from its center. Small hurricanes are more easily able to intensify as well as to weaken, depending on the conditions at hand.

Moreover, Otis’ rapid intensification was aided by sea surface temperature of 30-31 degrees Celsius (86-88°F), which is about 1 degree Celsius (1.8°F) above the 1991-2020 average. These water temperatures were boosted by the warmest September temperatures on record for Mexico. These warm seas were partially because of the natural El Niño phenomenon, abetted by long-term human-caused global warming.

The key factor in Otis’s rapid intensification, though, was the presence of a strong band of winds, known as a jet streak, embedded in the fast-flowing current of jet stream air to the north of Otis. As Otis pulled warm, moist air from the surface and pushed it aloft in its eyewall, the strong winds of this jet streak acted to ventilate the hurricane. This is a process similar to when one ventilates a fire, except in the case of a hurricane, the extra “oxygen” it gets comes in the form of moist surface air. This provides heat energy to the hurricane when the moisture condenses and thus releases the latent heat that was taken up by the water vapor when it evaporated. The ventilation of Otis from the outflow jet also helped to foster the upward motion that supported thunderstorm development around the center of Otis.

One reason it was not obvious Otis would intensify at such a breakneck pace was wind shear. The SHIPS model was diagnosing a moderate 10-15 knots of shear out of the southeast. However, Otis ended up moving from southeast to northwest more rapidly than expected (about eight to 10 mph or seven to eight knots), and this likely reduced the impact of the shear on the hurricane. Otis’s rapid motion not only helped it to maintain its peak strength all the way up to landfall, but it also allowed precious little time for locals to prepare for a middle-of-the-night storm that was far stronger than predicted.

Figure 1. Intensity forecasts from a variety of models used routinely to predict hurricane behavior. The official National Hurricane Center forecast is shown in black. All of these forecasts were produced on Monday night, October 23, from data available at 0Z Tuesday (8 p.m. Monday), or about 31 hours before Otis made landfall. (Image credit: Tomer Burg)

Extreme rapid intensification rates just before landfall expected to become more common

In a 2016 study, “Will Global Warming Make Hurricane Forecasting More Difficult?” from the Bulletin of the American Meteorological Society, MIT hurricane scientist Kerry Emanuel showed using the theory of thermodynamics that warmer oceans should be expected to spawn hurricanes that intensify more rapidly. To study this effect, he used a computer model that generated a set of 22,000 landfalling U.S. hurricanes between 1979 and 2005, then compared their intensification rates to a similar set of hurricanes for the year 2100 generated using an extreme scenario for global warming (RCP8.5).

Emanuel found that the odds of a hurricane intensifying by 70 mph or more in the 24 hours just before landfall were about once every 100 years in the climate of the late 20th century. But in the climate of the year 2100, these odds increased to once every five to 10 years.

What’s more, 24-hour pre-landfall intensifications of 115 mph or more, essentially nonexistent in the late 20th-century climate, would occur as often as once every 100 years by the year 2100. Emanuel found that major metropolitan areas most at risk for extreme intensification rates just before landfall included Houston, New Orleans, Tampa/St. Petersburg, and Miami. While global warming is unlikely to follow this extreme trajectory if the goals of the Paris climate accord are met, the research underscores that rapid intensification is likely to occur much more often in a future warmer climate. Increased rates of rapid intensification have already been observed in the Atlantic in recent decades, according to a 2023 study, Observed increases in North Atlantic tropical cyclone peak intensification rates, and according to a 2019 study, Recent increases in tropical cyclone intensification rates.

Acapulco’s skimpy hurricane history

Part of what makes Otis such a historic hurricane is the improbability of getting a direct strike on Acapulco. Although the city is situated firmly in the tropics at latitude 17 degrees north, adjacent to very warm water, the southern Pacific coast of Mexico does not see many hurricanes moving inland. Instead, the easterly steering currents that prevail at this latitude tend to drive hurricanes toward the west-northwest, roughly parallel to the coast (see Figure 1). The odds of a hurricane recurving to the north and northeast are greatest late in the season, when the strong troughs of low pressure needed to provide those steering currents penetrate farther to the south.

maps of 1) all hurricanes with 100 miles of Acapulco and 2) all hurricanes and tropical storms within 50 miles of Acapulco show that these storms usually remain offshore.
Figure 2. All hurricanes recorded within 100 miles of Acapulco (top) and all hurricanes and tropical storms within 50 miles (bottom), according to historical data and analyses archived by NOAA. The database goes back to 1851, but most entries are from the reliable satellite-based record, which goes back to the early 1970s. (Image credit: NOAA Historical Hurricane Tracks)

Reliable satellite-based records of tropical cyclones extend back only to the early 1970s for the Eastern Pacific, but it is clear that Acapulco does not get hit often. The last hurricane of any strength to make landfall in the immediate vicinity of the city was an unnamed storm in 1951, which passed from southeast to northwest across the city with top sustained winds estimated at 75 knots or 85 mph (Category 1 strength). Acapulco’s population at that point was only about 30,000, as this preceded the mid-century boom in tourism. In 1973, Tropical Storm Claudia came ashore roughly 30 miles east of Acapulco, dropping widespread heavy rain across southern Mexico. Otherwise, no tropical storms or hurricanes are on record as having made landfall within 50 miles of Acapulco.

In 1997, catastrophic Hurricane Pauline (which peaked at Category 4 strength offshore) made landfall more than 200 miles east of Acapulco as a Cat 2, then took a classic coast-hugging track inland, weakening but dumping massive amounts of rain along the way. Pauline passed just north of Acapulco while it was near minimal hurricane strength. Some 5,000 homes were destroyed by flash flooding and mudslides in the Acapulco area, mostly in poorer areas outside of downtown, and most Acapulco residents lost power and water. Pauline took up to 500 lives across southern Mexico.

Two top-7 Pacific hurricane landfalls for Mexico this month

Mexico’s Pacific coast typically sees about four landfalling hurricanes every three years, but there have been three such landfalls during this October alone. Otis comes only two weeks after Hurricane Lidia hit the Pacific coast of Mexico, about 35 miles south-southwest of Puerto Vallarta, as a Category 4 storm with 140 mph winds. Lidia was also a rapid intensifier, increasing its winds by 65 mph in the 24 hours up to landfall. At the time, Lidia was tied as Mexico’s third-strongest Pacific hurricane on record, so Mexico has now had two of its top-seven landfalling Pacific hurricanes on record this month:

1) Hurricane Otis (2023): 165 mph
2) Hurricane Patricia (2015): 150 mph
3) Hurricane Madeline (1976): 145 mph
4) Hurricane Lidia (2023): 140 mph
4) Hurricane 12 (1957): 140 mph
4) Hurricane Kenna (2002): 140 mph
4) Mexico Hurricane (1959): 140 mph

So far in 2023, the northeast Pacific has had eight Category 3 or stronger hurricanes – nearly double the average of 4.6 expected for this point in the season – including five Category 4 hurricanes and two Category 5 hurricanes. The basin’s accumulated cyclone energy, or ACE, of 158 makes 2023 the most active hurricane season since 2018, which had an ACE index of 318.

Tammy makes a run at major-hurricane strength

Hurricane Tammy picked up steam overnight, approaching the threshold of Category 3 strength as it spun over the open waters of the Atlantic north of the Leeward Islands. As of 11 a.m. EDT Wednesday, Tammy was packing top sustained winds of 105 mph as it moved northeast at 13 mph. Wind shear is increasing over the storm and will cap its intensification on Wednesday, likely before Tammy’s peak winds reach Cat 3 strength. Despite unusually warm waters, the wind shear and dry air will erode Tammy’s showers and thunderstorms, and the National Hurricane Center predicts Tammy to become a gradually weakening post-tropical cyclone by Thursday. Tammy will likely stall in an area of weak steering currents southeast of Bermuda this weekend, but it may still be packing an extensive area of gale-force winds that could affect the islands. As of Wednesday morning, there was a 30-40% chance that tropical storm-force winds would reach Bermuda by Monday.

Hamoon reaches Bangladesh

Cyclone Hamoon came ashore on Tuesday night local time on the coast of Bangladesh south of Chittaging at category 1 strength, with top sustained winds of around 85 mph. Hamoon has since dissipated while moving inland. The storm damaged or destroyed some of the extensive shelters at the camps that house tens of thousands of Rohingya refugees at Cox’s Bazar, near the border of Bangladesh and Myanmar.

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