Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24 hour period.
In order for rapid deepening to occur, several conditions must be in place. Water temperatures must be extremely warm (near or above 30 °C, 86 °F), and water of this temperature must be sufficiently deep such that waves do not churn deeper cooler waters up to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well — for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.
The United States National Hurricane Center previously defined a tropical cyclone as having rapidly intensified, when the minimum central pressure decreased by 42 hectopascals (1.240 inHg) over a 24 hour period. However it is now defined as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24 hour period.
In 1983, Super Typhoon Forrest underwent one of the most explosive deepening events ever recorded with the pressure dropping from 975 to 885 millibars as it went from a tropical storm to a category 5 super typhoon in 24 hours.
In 1992, Hurricane Andrew was just a minimum hurricane a day before making landfall in the Bahamas when it began to rapidly deepen into a category 5 hurricane right up to its landfall at peak strength. It weakened significantly over the island chain, but it soon hit open water again and rapidly intensified back into a category 5 hurricane just hours before making a second landfall at category 5 strength in Florida.
On September 10, 1997, Hurricane Linda was just a tropical storm. The next day, Linda became a minimal hurricane. By 6AM on September 12, however, Linda's wind speeds increased to a record 185 mph, the pressure dropping at an average rate of 3.38 millibars per hour.
In 2004, Hurricane Charley was approaching the west coast of Florida as a category two storm on the Saffir-Simpson scale of hurricane strength. When just off the coast, however, its sustained winds rapidly increased from 110 to 150 mph (along with a pressure drop from 965 to 941 mbar) in only three hours. Charley caused unprecedented destruction in the Punta Gorda area, and inflicted major damage across the state of Florida.
During October 2005, Hurricane Wilma sustained winds rapidly intensified, from 60 knots (70 mph; 110 km/h) to 150 knots (175 mph; 280 km/h) within 24 hours. As a result Wilma intensified from being a tropical storm to a category 5 hurricane on the Saffir-Simpson hurricane scale. Over the same time span the pressure dropped from 982 hectopascals (29.00 inHg) to 892 hectopascals (26.34 inHg).
In 2006, the minimum central pressure of Typhoon Chebi in the Western Pacific dropped 75 mbar in 24 hours, including a 60 mbar pressure drop in 6 hours, as it intensified from a tropical storm to a category four equivalent typhoon in one advisory.
In 2013, Typhoon Usagi underwent a 55 mb drop as it had intensified from a category 1 to a category 4 super typhoon in just 24 hours. Typhoon Lekima rapidly intensified from a category 1 to category 5 in 24 hours, and most notably Typhoon Haiyan deepened from 35 mb within 12 hours, and, 965 mb to 905 mb within a day.
A few instances have been recorded in the North Indian Ocean, such as the 1999 Odisha Cyclone, the strongest storm on record in the basin, undergoing a 28 millibar pressure drop in 6 hours and Cyclone Giri undergoing 24 millibar drop in 6 hours.
In May 2014, Hurricane Amanda rapidly intensified from a tropical storm to a Category 4 in 24 hours. Over the span of 30 hours, its central pressure decreased by 62 mb (hPa; 1.83 inHg) while its winds increased by 95 mph (153 km/h).
In August 2014, Hurricane Genevieve underwent explosive intensification while crossing the international date line from a tropical storm to a Category 5 typhoon in 30 hours with a wind speed rise of 90 knots.
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