Rapid deepening

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Hurricane Charley nearing landfall after its rapid deepening phase

Rapid deepening is a condition that occurs when the minimum sea-level atmospheric pressure of a tropical cyclone decreases drastically in a short period of time. The National Weather Service describes rapid deepening as a decrease of 42 millibars in less than 24 hours. ^[1] However, this phrase is liberally applied to most storms undergoing rapid intensification.

1 Necessary conditions
2 Specific instances
3 Explosive intensification
4 References
5 See also

[edit] Necessary conditions

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.^{[citation needed]}

[edit] Specific instances

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.^[2]

In 2005, the minimum central pressure of Hurricane Wilma dropped from 981 to 882 mbar in 24 hours, including an incredible drop of 53 mbar in less than 6 hours. This is by far the fastest intensification of any Atlantic hurricane, and possibly the fastest intensification for any system in recorded history.^[3] Typhoon Forrest in 1983 may have strengthened faster, deepening from 976 to 876 mbar in just under 24 hours, but pressure estimates for this storm were much less accurate. ^[4]

In 2006, Typhoon Chebi in the West Pacific was upgraded from a 65 mph tropical storm to a Category 4 equivalent typhoon in one advisory.

[edit] Explosive intensification

Explosive intensification is a more extreme case of rapid deepening that involves a tropical cyclone deepening at a rate of at least 2.5 mbar per hour for a minimum of 12 hours. Explosive intensification may occur only if conditions are near perfect, and therefore is rarely observed in the Western hemisphere. However, explosive intensification occurs regularly in the West Pacific basin, with the greatest frequency off the north coast of Australia where water temperatures can attain 30+ degrees C during mid-season.^{[citation needed]}