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Glossary of NHC Terms
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- Advisory:
- Official 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.
- Center:
- 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.
- Cyclone:
- 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 hit, strike.
- 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.
- Eye:
- 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.
- Extratropical:
- 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 storm force.
- 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.
- 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 hurricane conditions
(sustained winds of 74 mph or higher) are expected somewhere within
the specified area. Because hurricane preparedness activities
become difficult once winds reach tropical storm force, the hurricane
warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.
- Hurricane Watch:
- An announcement that hurricane conditions
(sustained winds of 74 mph or higher) are possible within the specified
area. 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.
- Invest:
-
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.
- 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. Compare
direct hit,
indirect hit, and
strike.
- Major Hurricane:
- A hurricane that is classified as Category 3 or higher.
- 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.
-
Rapid Intensification:
-
An increase in the maximum sustained winds of a
tropical cyclone
of at least 30 kt in a 24-h period.
- Relocated:
- 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:
Category |
Wind Speed (mph) |
Damage |
1 |
74 -
95 |
Very
dangerous winds will produce some damage |
2 |
96 -
110 |
Extremely
dangerous winds will cause extensive damage |
3 |
111
- 130 |
Devastating
damage will occur |
4 |
131
- 155 |
Catastrophic
damage will occur |
5 |
>
155 |
Catastrophic
damage will occur |
A detailed description of the Saffir-Simpson Hurricane Wind Scale, which was revised in early 2010, is available at http://www.nhc.noaa.gov/sshws.shtml.
- 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.
- Strike:
-
For 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 tropical storm conditions (sustained winds of
39 to 73 mph) are expected somewhere within the specified
area within 36 hours.
- Tropical Storm Watch:
- An announcement that tropical storm conditions (sustained winds
of 39 to 73 mph) are possible within the specified
area within 48 hours.
- Tropical Wave:
- A trough or cyclonic curvature maximum in the trade-wind
easterlies. The wave may reach maximum amplitude in the lower middle
troposphere.
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