The WeatherMax Hurricane Glossary
For an excellent FAQ on hurricanes and tropical systems, please go to this site on the National Hurricane Center server.
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 path, versus a precise and very erratic fix-to-fix path, used to represent
tropical cyclone movement. It is based on an assessment of all available data.
Center:
The vertical axis or core of a tropical cyclone. It is usually determined by cloud vorticity
patterns, wind, and/or pressure distributions.
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, HI is respobsible 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.
Dvorak Technique:
A statistical system using observed satellite cloud pattern and a set of rules to produce a current estimate of a cyclone's strength. Estimates can be made from both visible and infrared satellite imagery. The Dvorak technique is the main tool for determining tropical cyclone strength when it is out of range of reconnaissance aircraft.
Eastern North Pacific Basin:
The region north of the Equator east of 140W. The National Hurricane Center in Miami, FL is
responsible for tracking tropical cyclones in this region.
Eye:
The relatively calm center of the tropical cyclone that is more than one half surrounded by wall
cloud.
Eye Wall/Wall Cloud:
An organized band of cumuliform clouds immediately surrounding the center of a tropical cyclone. Eye
wall and wall cloud are used synonymously.
Explosive Deepening:
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12
hours or 5 mb/hr for at least six hours.
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) proceses. It is important to note that cyclones
can become extratropical and still retain winds of hurricane or tropical storm force.
Fujiwhara Effect:
A binary interaction where tropical cyclones within a certain distance (300-750 nm depending on the
sizes of the cyclones) of each other begin to rotate about a common midpoint.
Gale Warning:
A warning of 1-minute sustained surface winds in the range 34 kt (39 mph or 63 kph) to 47 kt (54 mph
or 87 kph) inclusive, either predicted or occurring 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 kph) or
greater lasting for 1 hour or longer, or winds gusting to 50 kt (58 mph or 93 kph) or greater
regardless of duration that are either expected or observed over land.
Hurricane/Typhoon:
A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute
average) is 64 kt (74 mph or 119 kph) or more. The term hurricane is used for Northern Hemisphere cyclones
east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific 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 indidence 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:
A warning that sustained winds 64 kt (74 mph or 119 kph) or higher associated with a hurricane are
expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when
dangerously high water or a combination of dangerously high water and exceptionally high waves continue,
even though winds may be less than hurricane force.
Hurricane Watch:
An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a
possible threat, generally within 36 hours.
Post-storm Report:
A report issued by a local National Weather Service office summarizing the impact of a tropical
cyclone on it's forecast area. These reports include information on observed winds, pressures, storm
surges, rainfall, tornadoes, damage and casualties.
Preliminary 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 preliminary 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.
Probability of Tropical Cyclone Conditions:
The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75
miles to the left of the listed location within the indicated time period when looking at the coast in the
direction of the cyclone's movement.
Rapid Deepening:
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24
hours.
Relocated:
A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the
latest know position is not necessarily a reasonable representation of the cyclone's movement.
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 kph) or greater, either predicted
or occurring, not directly associated with tropical cyclones.
Subtropical Cyclone:
A low pressure system that develops over subtropical waters that initially has a non-tropical
circulation but in which some elements of tropical cyclone cloud structure are present. Subtropical
cyclones can evolve into tropical cyclones. Subtropical cyclones are generally of two types:
(1) An upper level cold low with circulation extending to the surface and maximum sustained winds generally
occurring at a radius of about 100 miles or more from the pressure center.
(2) A mesoscale cyclone originating in or near a frontolyzing zone of horizontal wind shear, with radius
of maximum sustained winds generally less than 30 miles. The entire circulation sometimes encompasses
an area initially no more than 100 miles in diameter. These generally short-lived, marine cyclones may
vary in structure from cold to warm core.
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 kph) 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 kph) 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, nonfrontal low pressure system of synoptic scale that develops over tropical or
subtropical waters and has a definite organized surface circulation.
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 kph) 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 kph) to 63 kt (73 mph or 118 kph).
Tropical Storm Warning:
A warning for tropical storm conditions including sustained winds within the range of 34 to 63 kt
(39 to 73 mph or 63 to 118 kph) that are expected in a specified coastal area within 24 hours or less.
Tropical Storm Watch:
An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal
areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is
forecast to attain hurricane strength.
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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