Friday 30 October 2015

Hurricane Patricia: The most severe storm ever recorded causes widespread flooding but relatively few casualties.

Hurricane Patricia formed as a tropical depression over the eastern Pacific Basin in mid-October 2015, before increasing rapidly in intensity on 22 October, increasing to an Category 5 hurricane (a hurricane with sustained winds of more than 252 kilometers per hour) within 24 hours. On 23 October the storm was recorded as having sustained winds of 325 kilometers per hour and a low pressure of 879 millibars shortly before making landfall in Jalisco State, Mexico, on 23 October. This was the fastest drop in pressure and growth in wind-speed ever recorded for a tropical storm, and matched the highest sustained wind speed ever recorded for such a storm, set by Typhoon Nancy in 1961 (a record thought likely to be inaccurate as the methods used to record wind-speeds in the early 1960s tended to produce over-estimations). 

 Animation of Hurricane Patricia making landfall on the southern coast of Mexico on 23 October 2015, based upon images from the GOES-15 Satellite. Rosenstiel School of Marine and Atmospheric Science/University of Miami/National Oceanic and Atmospheric Administration.
The extreme severity of Hurricane Patricia prompted authorities in Mexico to evacuate around 50 000 people from coastal areas in Colima, Jalisco, and Nayarit states. Fortunately the hurricane dropped in intensity as rapidly as it grew, and while still a Category 5 storm when it made landfall (the first Category 5 storm ever to make landfall on the southern coast of Mexico), it has dropped back to a tropical depression (storm with sustained winds of less than 61 kilometers per hour) within 24 hours of making landfall.

 Damage caused by Hurricane Patricia at Chamela in Jalisco State, Mexico. AP.

The hurricane made landfall in an area of Jalisco with a low population density, which combined with the program of evacuations has been credited with a very low loss of life for a storm of this size. Six fatalities were reported in the state, two people killed by a falling tree and four more in an automobile accident associated with the storm. However damage caused by the storm was still severe with over 9000 homes destroyed, including the entire town of Chamela, and extensive damage to crops across much of the state.

Damage caused by Hurricane Patricia in Barra de Navidad in Jalisco State, Mexico. Omar Torres/AFP/Getty Images.

In neighbourig Colima State around 400 homes were destroyed, and severe damage has been recorded to infrastructure, with damage recorded to schools, medical centers and roads, as well as flooding again causing extensive damage to crops and agricultural land. Extensive crop damages were also recorded in Michoacán and Nyarit states, while Quintana Roo state received a record breaking 502 mm of rain within 24 hours and suffering extensive flood damage, particularly in the city of Chetumal, where about 1500 homes were affected. Flooding was also recorded in Veracruz, Tabasco, Oaxaca, Chiapas, Guerrero and Tapachula States.

A banana plantation destroyed by Hurricane Patricia in Colima State, Mexico. Getty Images.

In Central America flooding was recorded in Guatemala, El Salvador, Honduras and Nicaragua, with numerous landslides, and extensive damage to homes and crops. Four people died in flooding in El Salvador and one in Guatemala, while in Nicaragua one person was killed and three injured in a landslide at a mine.

 Flooding in El Progreso, Honduras. Bomberos de Honduras.

In the United States flooding caused by Hurricane Patricia affected many parts of southern Texas, causing severe disruption around Houston. In Navarro County in the north of the State a flash flood washed away rail tracks causing a freight train to derail, though fortunately nobody was hurt in the incident. Heavy rains associated with the remnants of the storm caused a levee to breach south of New Orleans in Louisiana, and caused disruption up the east coast of North America as far as Quebec, where power outages were attributed to the effects of the storm.

Train derailment in Navarro County, Texas, caused by Hurricane Patricia. KGO-TV.

Tropical storms are caused by the warming effect of the Sun over tropical seas. As the air warms it expands, causing a drop in air pressure, and rises, causing air from outside the area to rush in to replace it. If this happens over a sufficiently wide area then the inrushing winds will be affected by centrifugal forces caused by the Earth's rotation (the Coriolis effect). This means that winds will be deflected clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere, eventually creating a large, rotating Tropical Storm. They have different names in different parts of the world, with those in the northwest Atlantic being referred to as hurricanes.

 Photograph of Hurricane Patricia crossing the Mexican coast taken from the International Space Station. Scott Kelly/NASA.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm serge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.

 Vehicle swept from a road during a flash flood in Texas associated with Hurricane Patricia. International Business Times.

It has been suggested that the exceptional severity of Hurricane Patricia may be the beginning of a much wetter climate phase associated with this years El Niño weather system over the southern Pacific Ocean, a phenomenon that typically brings high levels of rainfall to the southwest United States, though if this is the case then the weather has arrived dramatically early, as the increase in storm activity associated with El Niño does not usually begin in the Americas until December.

 Hotel rooms exposed after Hurricane Patricia ripped the roof off the building in Morelos State, Mexico. Rebecca Blackwell/AP.

The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.

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