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Storms

Fortunate outcome of a monster storm

Category 5 hurricanes that make landfall usually result in catastrophes. Not Hurricane Patricia: a powerful but small storm on the Pacific coast.

02.03.2016

The North Pacific experienced an unusually active hurricane season in 2015, with Hurricane Patricia setting new records on 23 October. When it made landfall, it was the most powerful hurricane in the eastern Pacific since records began, and one of the strongest hurricanes ever registered worldwide. However, thanks to a combination of fortunate circumstances, damage was moderate. Patricia was able to draw its energy from the waters off the coast of Mexico, which had warmed significantly due to the prevailing El Niño conditions. Small differences in wind speed between sea level and higher levels in the atmosphere also favoured the development of tropical hurricanes in the region. As a result, the ten category 3, 4 and 5 hurricanes that developed in the eastern Pacific in 2015 easily exceeded the long-term annual average of 4.1 between 1981 and 2010. Prior to Patricia, the last time that a category 5 hurricane formed in this ocean basin and made landfall was back in 1959.

Peak gusts of 400 km/h

Patricia originated on 20 October 2015 with the development of a tropical depression roughly 300 km south of the Gulf of Tehuantepec in southern Mexico. The low pressure area moved in a west-northwesterly direction parallel to the coast, and by 22 October had already intensified to a category 1 hurricane. Over the next 15 hours, Patricia underwent an explosive intensification, and in the night of 23 October wind speeds for a category 5 hurricane (the highest category) were recorded. Because of the exceptionally warm ocean temperatures of 31°C and the weak wind shear, the storm quickly gathered strength over the next twelve hours, to the extent that peak gusts of around 400 km/h are likely to have been reached during this period. The maximum one-minute sustained wind speed was estimated at a record 325 km/h. Approximately 24 hours later, a slightly weakened Patricia made landfall close to Cuixmala in the Mexican state of Jalisco. The US National Hurricane Center estimated the hurricane’s peak wind speed at 270 km/h (one-minute sustained), with gusts of up to 340 km/h in the Chamela-Cuixmala Biosphere Reserve. The hurricane quickly weakened due to the influence of the near-coastal mountains, and dissipated within 24 hours over the mountains of central Mexico. The remnants of the storm briefly intensified a rain system over the south of the USA, but without any major impact.

Little damage despite category 5

The reason Patricia caused so little damage in Mexico despite its record wind speeds is primarily because it was relatively small in size. It may even be the least damaging category 5 tropical storm ever to make landfall in the western hemisphere. The diameter of the total wind field with at least hurricane wind speed was only around 200 km. Similarly, the diameter of Patricia’s eye, on whose wall the strongest winds and therefore the greatest damage occurred, was extremely small at less than 20 km. There was also the fact that Patricia moved at approximately 23 km/h, an advance speed that is above the average for these latitudes. This reduced the time in which the hurricane could develop its maximum destructive potential. The peak wind speeds that were found in Patricia’s rear eyewall lasted for only 17 minutes (Fig. 1). The precipitation field also passed rapidly by, so that there was very little flooding. Despite this, the Mexican National Water Commission did record daily rainfall of 300 mm in some places.

Category 5 hurricanes that make landfall usually result in catastrophes. Not Hurricane Patricia: a powerful but small storm on the Pacific coast.

Patricia’s narrow wind field passed over an area that was relatively sparsely populated, sparing almost entirely the tourist city of Puerto Vallarta to the north, and the port of Manzanillo to the south (Fig. 2). Another reason the feared catastrophe did not materialise was because the government had ordered evacuations at an early stage and residents had been brought to safety. In the regions affected, for example in the community of Emiliano Zapata, there was the usual pattern of severe wind damage: houses collapsed, roofs were torn off, concrete power poles snapped, and trees were uprooted or split in two. The insured loss was US$ 25m with an overall loss of US$ 550m.

Category 5 hurricanes that make landfall usually result in catastrophes. Not Hurricane Patricia: a powerful but small storm on the Pacific coast.

Storm size often more important than intensity

Patricia is a good example to illustrate that it is not just the maximum wind speed or the category that is important when assessing a hurricane. If factors like the size of the storm and the dimensions of the eye are ignored, a false picture of the actual risk situation can quickly result. Wide-area storms, such as Ike, for example, which struck Texas in 2008 as a category 2 hurricane, and Sandy, which was only just at hurricane force as it passed over New York in 2012, caused many times the amount of damage that Patricia left behind. The influence of coastal topography is another aspect that should not be forgotten. In the cases of Ike, Sandy, and also with Katrina (2005), a large part of the damage came from the storm surge that formed as a result of the hurricane. Along the coast of Mexico, on the other hand, the ocean floor falls away sharply, preventing the development of a high storm surge, and the enormous waves that resulted from Patricia’s extreme wind speed smashed harmlessly against the steeply rising coastline.

It could have been much worse

The small and relatively sparsely populated area, the rapid passage of the storm, and the unfavourable conditions for a storm surge clearly prevented much higher losses occurring. This is illustrated by a comparison with Hurricane Odile, which made landfall in 2014 in Baja California. Odile was only category 3, but struck a region with a large number of luxury resorts, causing insured losses of over US$ 1.2bn. From this, we can only imagine what destruction a hurricane of Patricia’s strength could have wreaked in Puerto Vallarta, one of Mexico’s leading tourist centres. In short, we can say that Mexico had a lucky escape with Hurricane Patricia.

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