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Glossary of NHC Terms
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information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations,
intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings
and (b) subtropical cyclones.
- Best Track:
- A subjectively-smoothed representation of a tropical cyclone's location and intensity
over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and
intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion
implied by connecting individual center fix positions.
- Generally speaking, the vertical axis of a tropical cyclone, usually defined by the location
of minimum wind or minimum pressure. The cyclone center position can vary with altitude. In advisory products, refers to the center position at the surface.
- Center / Vortex Fix:
- The location of the center of a tropical or subtropical cyclone obtained
by reconnaissance aircraft penetration, satellite, radar, or synoptic data.
- Central North Pacific Basin:
- The region north of the Equator between 140W and the International Dateline. The Central
Pacific Hurricane Center (CPHC) in Honolulu, Hawaii is responsible for tracking tropical cyclones in this region.
- An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- Direct Hit:
- A close approach of a tropical cyclone to a particular location. For locations
on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius
of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind. Compare indirect
- Eastern North Pacific Basin:
- The portion of the North Pacific Ocean east of 140W. The National Hurricane Center in Miami,
Florida is responsible for tracking tropical cyclones in this region.
- The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone.
The eye is either completely or partially surrounded by the eyewall cloud.
- Eyewall / Wall Cloud:
- An organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center of a tropical
cyclone. Eyewall and wall cloud are used synonymously.
- A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics.
The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature
contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical
- Extratropical Cyclone:
- A cyclone of any intensity for which the primary energy source is baroclinic, that is, results from the temperature
contrast between warm and cold air masses.
- Flash Flood Warning:
- A weather warning issued when a flash flood is imminent and immediate action is necessary.
- Flash Flood Watch:
- A weather watch issued when there is a potential for flash flooding in a given area.
- Fujiwhara Effect:
- The tendency of two nearby tropical cyclones to rotate cyclonically about each other.
- Gale Warning:
- A warning of 1-minute sustained surface winds in the range 34 kt (39 mph or 63 km/hr) to 47 kt (54 mph or
87 km/hr) inclusive, either predicted or occurring and not directly associated with tropical cyclones.
- High Wind Warning:
- A high wind warning is defined as 1-minute average surface winds of 35 kt (40 mph or 64 km/hr) or greater
lasting for 1 hour or longer, or winds gusting to 50 kt (58 mph or 93 km/hr) or greater regardless of duration that are either expected or observed over land.
- Hurricane / Typhoon:
- A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute
average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term
typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.
- Hurricane Local Statement:
- A public release prepared by local National Weather Service offices in or
near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary
to protect life and property.
- Hurricane Season:
- The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean,
and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central
Pacific basin runs from June 1 to November 30.
- Hurricane Warning:
- An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are expected somewhere within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds. The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force.
- Hurricane Watch:
- An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are possible within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical storm force winds.
- Indirect Hit:
- Generally refers to locations that do not experience a direct hit from a tropical
cyclone, but do experience hurricane force winds (either sustained or gusts) or tides of at least 4 feet above normal.
- A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data
sets (e.g., microwave imagery) and/or running model guidance. Once a system has been designated as an invest, data collection and processing is initiated on a number of government and academic
web sites, including the Naval Research Laboratory (NRL) and the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS). The designation of a system as
an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone; operational products such as the Tropical Weather Outlook or the JTWC/TCFA should
be consulted for this purpose.
- 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. Compare direct hit, indirect
hit, and strike.
- Major Hurricane:
- A hurricane that is classified as Category 3 or higher.
- Numerical computer models which attempt to forecast the state of the atmosphere and future storm intensity/movement.
- National Geodetic Vertical Datum of 1929 [NGVD 1929]:
- A fixed reference adopted as a standard geodetic datum for elevations
determined by leveling. The datum was derived for surveys from a general adjustment of the first-order leveling nets of both the United States and Canada. In the adjustment, mean sea level
was held fixed as observed at 21 tide stations in the United States and 5 in Canada. The year indicates the time of the general adjustment. A synonym for Sea-level Datum of 1929. The geodetic
datum is fixed and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because the geodetic datum represents a best fit over
a broad area, the relationship between the geodetic datum and local mean sea level is not consistent from one location to another in either time or space. For this reason, the National Geodetic
Vertical Datum should not be confused with mean sea level.
- Post-storm Report:
- A report issued by a local National Weather Service office summarizing the impact of a tropical
cyclone on its forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.
- Post-tropical Cyclone:
- A former tropical cyclone. This generic term describes a cyclone that no longer possesses sufficient tropical characteristics
to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds. Note that former tropical cyclones that have become fully extratropical...as
well as remnant lows...are two classes of post-tropical cyclones.
- Preliminary Report:
- Now known as the "Tropical Cyclone Report". A report summarizing the life history and effects of an Atlantic
or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best
track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on
forecasts and warnings associated with the cyclone. NHC writes a report on every tropical cyclone in its area of responsibility.
- Present Movement:
- The best estimate of the movement of the center of a tropical cyclone at
a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.
- Radius of Maximum Winds:
- The distance from the center of a tropical cyclone to
the location of the cyclone's maximum winds. In well-developed hurricanes, the radius of maximum winds is generally found at the inner edge of the eyewall.
- An increase in the maximum sustained winds of a tropical cyclone of at least 30 kt in a 24-h period.
- A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the
latest known position is not necessarily a reasonable representation of the cyclone's movement.
- 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. The term is most commonly applied to the nearly deep-convection-free swirls of stratocumulus in the eastern North Pacific.
- Saffir-Simpson Hurricane Wind Scale:
- The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's
intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. The following table shows
the scale broken down by winds:
||Wind Speed (mph)
||74 - 95
||Very dangerous winds will produce some damage
||96 - 110
||Extremely dangerous winds will cause extensive damage
||111 - 129
||Devastating damage will occur
||130 - 156
||Catastrophic damage will occur
||Catastrophic damage will occur
For a detailed description of the Saffir-Simpson Hurricane Wind Scale, click here.
- Storm Surge:
- An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is
the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal
or astronomic high tide from the observed storm tide.
- Storm Tide:
- The actual level of sea water resulting from the astronomic tide combined with the storm surge.
- Storm Warning:
- A warning of 1-minute sustained surface winds of 48 kt (55 mph or 88 km/hr) or greater, either predicted or occurring, not
directly associated with tropical cyclones.
any particular location, a hurricane strike occurs if that location passes within the hurricane's strike circle, a circle of 125 n mi diameter, centered 12.5 n mi to the
right of the hurricane center (looking in the direction of motion). This circle is meant to depict the typical extent of hurricane force winds, which are approximately
75 n mi to the right of the center and 50 n mi to the left.
- Subtropical Cyclone:
- A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system
is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center.
In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric
wind field and distribution of convection.
- Subtropical Depression:
- A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S.
1-minute average) is 33 kt (38 mph or 62 km/hr) or less.
- Subtropical Storm:
- A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute
average) is 34 kt (39 mph or 63 km/hr) or more.
- Synoptic Track:
- Weather reconnaissance mission flown to provide vital meteorological information in
data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical
cyclone development and movement.
- 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).
- Tropical Cyclone Plan of the Day:
- A coordinated mission plan that tasks operational weather reconnaissance requirements
during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance
requirements for the succeeding 24-hour period.
- Tropical Depression:
- A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute
average) is 33 kt (38 mph or 62 km/hr) or less.
- Tropical Disturbance:
- A discrete tropical weather system of apparently organized convection -- generally 100 to 300 nmi in diameter
-- originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation
of the wind field.
- Tropical Storm:
- A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute
average) ranges from 34 kt (39 mph or 63 km/hr) to 63 kt (73 mph or 118 km/hr).
- Tropical Storm Warning:
- An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are expected somewhere within the specified area within 36 hours in association with a tropical, subtropical, or post-tropical cyclone.
- Tropical Storm Watch:
- An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are possible within the specified area within 48 hours in association with a tropical, subtropical, or post-tropical cyclone.
- Tropical Wave:
- A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle