Glossary of Commonly Used Hurricane Terms
Best Track
A term used by the hurricane forecasters which represents 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.
Coastal Flood Warning
A warning issued by the National Weather Service that significant wind-forced
flooding is to be expected along low-lying coastal areas if weather patterns
develop as forecast.
Coastal Flood Watch
An alert issued by the National Weather Service that significant wind-forced
flooding is possible along low-lying coastal areas if weather patterns develop
as forecast.
Cyclone
An atmospheric closed circulation rotating counter-clockwise in the Northern
Hemisphere and clockwise in the South Hemisphere.
Emergency Alert System (EAS)
Formerly known as the Emergency Broadcast System (EBS). It is a system designed
to permit government officials to issue up-to-date and continuous emergency
information and instructions to the public, via radio and television, in case
of a threatening or actual emergency.
Evacuation Clearance Times
The lead time that a populated coastal jurisdiction must have to safely
relocate all residents of vulnerable areas to a safer area from an approaching
hurricane. This time can also be perceived as the necessary amount of time
between the issuance of the local official evacuation order and the projected
arrival of sustained tropical storm force winds (40 mph) and/or flooding.
Eye Wall/Wall Cloud
An organized band of clouds immediately surrounding the center of a tropical
cyclone. Eye wall and wall cloud are used synonymously.
Flood Warning
A warning issued by the National Weather Service which indicates the expected
severity of flooding (minor, moderate, or major), as well as where and when the
flooding will begin.
Forward Speed (Hurricane)
The rate of movement of the hurricane eye in miles per hour or knots. If you
track hurricanes and use official advisories for your information, be aware
that Public Advisories show wind speeds in miles per hour (mph) and Forecast
Advisories show wind speeds in knots (Kt). Generally 1 knot = 1.15 miles per
hour.
Gale Warning
A warning issued by the National Weather Service when sustained winds within
the range of 39- 54 miles per hour are either predicted or occurring. Gale
warnings are not normally issued during tropical storm situations.
High Wind Warning
A high wind warning is defined as 1-minute average surface winds of 40 mph or
greater lasting for 1 hour or longer, or winds gusting to 58 mph or greater
regardless of duration that are either expected or observed over land.
Hurricane
A type of tropical cyclone - the general term for all circulating weather
systems over tropical waters. The cyclone becomes a hurricane when the constant
surface wind speed reaches 74 mph or greater. These winds blow in a counter
clockwise spiral around the relatively calm center of an extremely low pressure
known as the eye of the hurricane. Around the eye wall or rim of the eye, wind
may gust to more than 200 miles per hour. The entire storm dominates the ocean
surface and lower atmosphere over tens of thousands of square miles. Hurricanes
are categorized by intensity from 1 - 5 with category 5 being the most intense.
Hurricane/Tropical Storm/Tropical Depression Advisories
Notices, issued by the National Hurricane Center, that are numbered
consecutively for each tropical depression, storm and hurricane, describing the
present and forecasted position and intensity of the storm. Advisories are
issued at six-hour intervals at midnight, 6:00 AM, noon, and 6:00 PM, Eastern
Daylight Time. Intermediate advisories are sometimes issued at three hour
intervals between regular advisories. Bulletins provide additional information.
Each advisory gives the name, eye position, intensity and movement of the
storm. Public Advisories give certain specific information for public use and
Forecast Advisories provide more detailed forecast information with the current
position and forecast positions at 12, 24, 36, 48 and 72 hours.
Hurricane Eye
The relatively calm area near the center of the storm. In this area, winds are
light and the sky is often only partly covered by clouds. The size of the eye
varies greatly with each storm.
Hurricane Landfall
The point in time when the eye, or physical center of the hurricane reaches the
coastline from the hurricane's approach over water. It is important to remember
that the most intense area of the storm will be up 50 miles to the right of the
eye and up to 20 miles to the left of the eye. Hurricane force winds may extend
up to 100 miles or great from the eye of the storm.
Hurricane Local Statement
A public release by the local National Weather Service office in or near a
threatened area giving specific details for its warning area on (1) weather
conditions, (2) evacuation decisions made by local officials, and (3) other
precautions necessary to protect life and property.
Hurricane Track
Line of movement of the eye of the hurricane through an area. It is important
to remember that hurricane advisories only provide the location of the center
or eye of the storm. Some hurricanes have hurricane for winds 100 miles or
greater and tropical storm force winds 200 miles or greater from the center of
the storm. When tracking a hurricane, you should also track the wind fields,
which are provided in hurricane advisories.
Hurricane Season
The portion of the year having a relatively high incidence of hurricane
activity. In the Atlantic, Caribbean and Gulf of Mexico, hurricane season
begins June 1 and ends November 30.
Hurricane Warning
A warning issued by the National Hurricane Center for a specific area when
hurricane conditions are expected within 24 hours. Hurricane warnings identify
coastal areas where winds of at least 74 miles per hour are expected to occur.
A warning may also describe coastal areas where dangerously high water or
exceptionally high waves are expected, even though winds may be less that
hurricane force. Hurricane warnings are added to hurricane advisories and list
the areas where the warning is in affect.
Hurricane Watch
An alert issued by the National Hurricane Center for a specific area when
hurricane conditions are a real possibility, usually within 36 hours. When a
hurricane watch is issued, everyone in the watch area should listen for further
advisories and be prepared to act quickly if hurricane warnings or other
official orders are issued.
NOAA Weather Radio
A twenty-four hour continuous broadcast of existing and forecasted weather
conditions. In this area, the broadcasts are issued by the Mobile, Alabama
Office of the National Weather Service and the transmitter is located in Santa
Rosa County. Some weather radios have the ability to be placed in an "alert"
mode and will only activate or turn on when a severe weather alert is issued or
the user depresses a button to listen to the broadcast.
Severe Thunderstorm Warning
A warning issued for a specific by the National Weather Service when a severe
thunderstorm has been sighted or indicated on radar. These storms are capable
of producing damaging winds greater than 55 miles per hour, frequent lightning,
heavy rainfall and/or hail.
Severe Thunderstorm Watch
An alert issued by the National Weather Service when conditions are favorable
for development of severe thunderstorms. These storms are capable of producing
damaging winds greater than 55 miles per hour, frequent lightning, heavy
rainfall and/or hail.
SLOSH (Sea, Lake and Overland Surges from Hurricanes)
A computerized model which is able to estimate the overland tidal surge heights
that result from hypothetical hurricanes with selected characteristics in
pressure, size, forward speed, track and wind speed. The resultant tidal surge
action is then applied to a specific locale's shoreline configuration, while
also incorporating the unique bay and river configurations, water depths,
bridges, roads, and other physical features. The model then estimates open
coastline heights as well as surge heights over land, thus predicting the areas
that will be inundated by water during a specific category of hurricane. This
data is then used to produce storm surge maps for each region along the coast.
Small Craft Advisory
A warning issued by the National Weather Service for winds from 20 - 33 knots
inclusive or for sea conditions either forecasted or occurring which are
considered potentially hazardous to small boats in coastal waters.
Special Marine Warning
A warning issued by the National Weather Service for hazardous weather
conditions, usually of short duration, not adequately covered by existing
marine warning. Such weather conditions include sustained winds or gusts of 35
knots or greater.
Storm Surge
The large dome of water, often 50 to 100 miles wide, that sweeps across the
coastline near where a hurricane makes landfall. The surge of high water,
topped by waves is devastating. The stronger the hurricane and the shallower
the offshore water, the higher the storm surge will be. In the United States,
most deaths from hurricane are as a result of storm surge. In this area, a
storm surge of over 20 feet is possible with a category 4 or 5 hurricane.
Storm Tide
If the storm surge arrives at the same time as the high tide, the water height
will be even greater. The storm tide is the combination of the storm surge and
the normal astronomical tide.
Sustained Wind
The surface wind obtained by averaging observed value over a one minute period.
Tornado
A relatively shore-lived local storm, that is composed of violently rotating
columns of air that descend in the familiar funnel shape from a thunderstorm
system.
Tornado Warning
A warning issued by the National Weather Service when a tornado has been
sighted or has been detected on radar. Warnings will give the location of the
tornado, the area immediately affected by the warning and the direction of
travel. In this area, tornadoes sometimes occur quickly and dissipate before a
warning can be issued.
Tornado Watch
An alert issued by the National Weather Service when weather conditions are
favorable for a tornado to develop and that the sky should be monitored.
Tropical Cyclone
A circulating weather system, generally originating over tropical oceans,
distinguished by torrential rains and damaging winds that reach a maximum very
near the low pressure center. These systems circulate counter clockwise around
a low pressure center in the Atlantic, Caribbean, Gulf of Mexico and Eastern
Pacific are called a Tropical Depression, Tropical Storm or Hurricane, based on
wind speed.
Tropical Depression
A tropical cyclone in which the maximum sustained surface wind is 38 miles per
hour or less. When the cyclone reaches this stage, it is given a number (i.e.
Tropical Depression #4).
Tropical Disturbance
A moving area of thunderstorms over tropical waters that maintains its identity
for at least 24 hours.
Tropical Storm
A warm core tropical cyclone in which the maximum sustained surface wind is in
the range of 39 - 73 miles per hour. When the cyclone reaches this stage, it is
given a name.
Tropical Storm Warning
A warning issued by the National Hurricane Center when tropical storm
conditions (sustained winds between 39 and 73 miles per hour) are expected to
affect a specified coastal area, within 24 hours.
Tropical Storm Watch
An alert for a specified coastal area that tropical storm conditions (sustained
winds between 39 and 73 miles per hour) are possible, usually within 36 hours.
A tropical storm watch is not normally issued if the system is forecast to
attain hurricane strength.
Tropical Wave
A westward moving trough of low pressure embedded in the deep easterly current.
It tends to organize low level circulation and may travel thousands of miles
with little change in shape, sometimes producing significant shower and
thunderstorm activity along it path.
Water Spout
A relatively small tornado over a body of water with winds rarely exceeding 50
miles per hour.
Weather Advisory
Information issued regularly by the National Weather Service to alert the
public to possible hazardous conditions that may be caused by severe weather.