Tropical Definitions

TROPICAL DEFINITIONS

 

Tropical Wave
An inverted trough (an elongated area of relatively low pressure) or cyclonic curvature maximum moving east to west across the tropics. These can lead to the formation of a tropical cyclone. Also known as an easterly wave.

 

Tropical Disturbance
A tropical weather system with organized convection (generally 100-300 miles in diameter) originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or longer. It may or may not be associated with a detectable perturbation of the wind field.

 

 

Tropical Cyclone
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).

 

Potential Tropical Cyclone (PTC)
A term used in NWS advisory products to describe a disturbance that is not yet a tropical cyclone, BUT which poses the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours.

 

Invest
A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets  and/or running model guidance. The designation of a system as an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone.

 

Extratropical Cyclone
A cyclone (of any intensity) for which the primary energy source is baroclinic (i.e., results from the temperature contrast between warm & cold air masses).

 

Post-Tropical Cyclone
A cyclone that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue to carry heavy rains and high winds. Note: former tropical cyclones that become extratropical and remnant lows are 2 specific classes of post-tropical cyclones.

 

Remnant Low
A post-tropical cyclone that no longer possesses the convective organization required of a tropical cyclone and has maximum sustained winds of less than 34 knots.

 

Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. Subtropical cyclones originate over tropical or subtropical waters and have a closed circulation about a well-defined center. In comparison to tropical cyclones, the maximum winds occur relatively far from the center (greater than 60 nautical miles) and have a less symmetric wind field and distribution of convection.

 

Tropical Depression
A tropical cyclone that has maximum sustained surface winds (one-minute average) of 38 mph (33 knots) or less.

 

Tropical Storm
A tropical cyclone that has maximum sustained surface winds ranging from 39-73 mph (34 to 63 knots).

 

Hurricane
A hurricane is a tropical cyclone that has maximum sustained surface winds of 74 mph or greater (64 knots or greater).

 

Major Hurricane
A hurricane that is classified as Category 3 or higher.

 

NOTE: The number of Tropical Storms and Hurricanes increases substantially in August, peaks in mid-September and decreases towards a minimum by early November.

 

 

Eye Wall
An organized band of cumulonimbus clouds immediately surrounding the center of the tropical cyclone.

 

Rapid Intensification
An increase in the maximum sustained winds of a tropical cyclone of at least 30 knots in a 24 hour period.

 

Landfall
The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water.

 

Storm Surge
An abnormal rise in sea level accompanying a tropical cyclone. This is the difference in height between observed level of the sea surface and the level that would have occurred in the absence of the storm. Storm surge is usually estimated by subtracting the normal or astronomical tide from the observed storm tide.

 

Storm Tide
The actual level of sea water resulting from the astronomic tide combined with the storm surge.

 

Inundation
The flooding of normally dry land, primarily caused by events along the coasts, estuaries, and adjoining rivers. The winds drive large waves and storm surge on shore, and heavy rains raise rivers. 

 

 

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Tropical Storm Watch
A Tropical Storm Watch is issued when Tropical Storm conditions, including winds of 39-73 mph, pose a POSSIBLE threat to a specified coastal area within 48 hours.

 

Tropical Storm Warning
A Tropical Storm Warning is issued when Tropical Storm conditions, including winds of 39-73 mph, are EXPECTED in a specified coastal area within 36 hours or less.

 

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Hurricane Watch
A Hurricane Watch is issued when sustained winds of 74 mph or higher are POSSIBLE within the specified area of the Watch. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the Watch is issued 48 hours in advance of the onset of tropical storm force winds.

 

Hurricane Warning
A Hurricane Warning is issued when sustained winds of 74 mph or higher are EXPECTED somewhere within the specified area of the Warning. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the Warning is issued 36 hours in advance of the onset of tropical storm force winds.

 

NOTE: A Hurricane Warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continues...even if the winds have subsided below hurricane intensity.

 

Extreme Wind Warning
Extreme Wind Warning (EWW) is issued for sustained surface winds of 115 mph or greater from a MAJOR Hurricane (Category 3, 4, or 5) are expected within the next hour. This is usually associated with the eyewall of a major hurricane and is considered your final call to shelter in place.

 

 

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Storm Surge Watch
The POSSIBILITY of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area, generally within 48 hours, in association with an ongoing or potential tropical cyclones, a subtropical cyclone or a post-tropical cyclone. The watch may be issued earlier when other conditions, such as the onset of tropical-storm-force winds are expected to limit the time available to take protective actions for surge (e.g., evacuations). The warning may also be issued for locations not expected to receive life-threatening inundation but which could potentially be isolated by inundation in adjacent areas.

 

Storm Surge Warning
The DANGER of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area, generally within 36 hours, in association with an ongoing or potential tropical cyclones, a subtropical cyclone or a post-tropical cyclone. The warning may be issued earlier when other conditions, such as the onset of tropical storm force winds are expected to limit the time available to take protective actions for surge (e.g., evacuations). The warning may also be issued for locations not expected to receive life-threatening inundation but which could potentially be isolated by inundation in adjacent areas.