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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:
  1. Tropical cyclones prior to issuance of watches and warnings, and
  2. Subtropical cyclones.
Air mass
A large body of air with relatively uniform characteristics such as temperature and humidity.
A special type of aneroid barometer used in airplanes to measure altitude.
An instrument for measuring the speed of the wind.
Aneroid barometer
A device to measure air pressure that uses an aneroid, which is a sealed, flexible metal bellows with some air removed that expands and contracts with air pressure changes.
Acronym for Automated Surface Observing Systems; a system designed to provide automated meteorological measurements of several parameters at selected airports.
The air surrounding Earth.
Atmospheric Wave
Any pattern with some roughly identifiable periodicity in time and/or space. In meteorology, waves in the horizontal flow pattern (e.g., Rossby wave, long wave, short wave).
AVN (AViatioN model)
One of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT.
Acronym for Advanced Weather Interactive Processing System; a computerized system that processes data received at a NWS Forecast Office from various weather observing systems.
A device for recording air pressure.
A device for measuring air pressure.
The strip of land between the river's edge and the levee.
The slope or shoulder of a levee.
Bermuda High
The semipermanent atmospheric subtropical anticyclone (high pressure system) over the North Atlantic Ocean, so name especially when it is located in the western part of the ocean, near Bermuda (near 30°N).
Bernoulli's Principle
Air flowing over an airfoil results in an increase in flow speed over the upper curved surface. Since a velocity increase in fluid flow results in a corresponding pressure decrease, the increased airflow over the upper surface of the airfoil produces a lift on the airfoil because of lower pressure exerted on the upper surface. Named for Daniel Bernoulli (1700 -1782), a Swiss physicist who discovered the effect.
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.
Borrow Pit
The excavation site from which clay is dug to build or raise a levee.
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 140°W and the International Dateline. The Central Pacific Hurricane Center (CPHC) in Honolulu, HI is responsible for tracking tropical cyclones in this region.
Climate Average weather of an area over a long time, usually 30 years.
Climate model
Mathematical model containing equations that describe climatic interactions.
Cold front
A warm-cold air boundary with the cold air advancing.
The change of a vapor to liquid.
Transfer of heat within a substance or from one substance to another by inter-molecular action.
Continental air mass
An air mass that forms over land, making it generally dry. It may be warm or cold.
Transfer of heat by the movement of the heated material. In meteorology, the up and down air motions caused by heat.
Coriolis Effect
The apparent curving motion of anything, such as wind, caused by Earth's rotation. It was first described in 1835 by French scientist Gustave-Gaspard Coriolis.
Deepening and/or straightening the bed of a river or stream.
The highest point of a flood or the measurement of a river or stream at any given point.
A break in a levee allowing the river or lake to rush through to the protected land.
The top of the levee.
An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Doppler radar
Radar that measures speed and direction of a moving object, such as wind.
Wind blasting downward through the air. It may be due to a thunderstorm or shower.
Drainage Valve
A valve used to control water that flows from outside flood protected areas to inside flood protected areas. In New Orleans, along both the river and the lake, underground drainage lines that empty directly into the waterway, instead of into a canal, are fitted with valves which can be dosed if rising river or lake water threatens to back up into the lines and flow into neighborhoods or downtown.
(verb) To excavate in water. (noun) The piece of machinery used to excavate in water.
Falling water drops with diameters less than .02".
Eastern North Pacific Basin
The region north of the Equator east of 140°W. The National Hurricane Center in Miami, FL is responsible for tracking tropical cyclones in this region.
El Niño
Linked ocean and atmospheric events that have worldwide effects, characterized by warming of water in the tropical Pacific from around the International Date Line to the coast of Peru.
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.
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.
Flash flood
Flooding with a rapid water rise.
Opening in a floodwall or levee to allow the passage of vehicles, boats or the normal tidal flow of waterways. Floodgates are closed to prevent flooding of protected houses and land during times of unusually high water.
A vertical concrete levee usually built along roadsides and around neighborhoods where there is not space for the massive side slopes of an earthen levee.
The measurement from the top of the highest expected flood to the top or crown of the levee.
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 km/hr) to 47 kt (54 mph or 87 km/hr) inclusive, either predicted or occurring not directly associated with tropical cyclones.
Geostationary Operational Environmental Satellite, a U.S. weather satellite in an orbit that keeps it above the same place on the equator.
Greenwich Mean Time (GMT)
global time scale based on the local time observed on the Prime Meridian; also called Zulu (Z) time or Universal Coordinated Time (UTC)
Gulf Stream
A warm ocean current that flows from the Gulf of Mexico across the Atlantic to the European coast. It helps warm Western Europe.
An area of relatively high-atmospheric pressure, also called an anticyclone.
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.
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 km/hr) 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 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
A warning that sustained winds 64 kt (74 mph or 119 km/hr) 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.
Intertropical Convergence Zone (ITCZ)
The axis dividing the southeast trades from the northeast trades, toward which the surface winds tend to converge.
Stable air condition in which air near the ground is cooler than air at a higher altitude.
Jet stream
A narrow band of wind in the upper atmosphere with speeds greater than 57 mph.
A unit of speed most often used by marine interests in which one nautical mile per hour is achieved. One knot equals approximately 1.15 miles per hour.
Latent heat
Energy stored when water evaporates into vapor or ice melts into liquid. It is released as heat when water vapor condenses or water freezes.
The distance on Earth's surface measured in degrees north and south of the equator.
An embankment raised to prevent a river from overflowing or to protect land from other rising water. In New Orleans, levees are usually huge, sloping mounds of earth running along the Mississippi River, the outfall canals, and Lake Pontchartrain.
The distance on Earth's surface measured in degrees east and west from the prime meridian.
An area of low-atmospheric pressure.
Maritime air mass
An air mass that forms over an ocean, making it humid. It may be warm or cold.
Meridional flow
A north to south to north flow of high-altitude winds.
A rotating, upward moving column of air in a thunderstorm that can spawn tornadoes.
In meteorology, weather systems and events up to about 250 miles in diameter.
Meteorological bomb
An extratropical cyclone in which the center pressure drops an average of one millibar an hour for 24 hours. Usually refers to storms off the U.S. East Coast.
Mid latitudes
The region of Earth between the polar and tropical regions, between latitudes 23.5° and 66.5°.
A metric unit of air pressure measurement. The average atmospheric pressure at sea level is 1013 millibars.
Mississippi River Commission
A five member board overseeing flood protection and assuring navigable waterways along the Mississippi River and its tributaries. Made up of some civilian and some military personnel, the commission is chaired by the District Engineer of the U.S. Army Corps of Engineers of the Lower Mississippi Valley District at Vicksburg, Mississippi.
Moist adiabatic lapse rate
The variable rate at which rising air cools or sinking air warms when water is changing phases in the air.
Persistent, widespread, winds that seasonally reverse directions. Usually summer winds from the ocean bring rain, while winter winds from the land are dry.
National Climatic Data Center (NCDC)
The National Oceanic and Atmospheric Administration office in Asheville, NC, that keeps climate records.
National Hurricane Center (NHC)
National Weather Service office in Coral Gables, FL, that tracks and forecasts hurricanes and other weather in the Atlantic, Gulf of Mexico, Caribbean Sea, and parts of the Pacific.
National Meteorological Center (NMC)
National Weather Service center in Camp Springs, MD, that prepares worldwide computer forecasts. Hurricane and Severe Storms centers are part of NMC.
National Severe Storms Forecast Center
National Weather Service center in Kansas City, MO, that issues watches for severe thunderstorms and tornadoes across the nation.
National Severe Storms Laboratory (NSSL)
National Oceanic and Atmospheric Administration Laboratory in Norman, OK, that studies severe thunderstorms.
National Weather Service
Federal agency that observes and forecasts weather. Formerly the U.S. Weather Bureau, it is part of the National Oceanic and Atmospheric Administration, which is part of the Department of Commerce.
Numerical forecasting or prediction
Use of computers to solve mathematical equations and produce weather forecasts.
100-year flood
Water level that, on average, should occur once a century. This is the same as a water level with a 100 to 1 chance of occurring in any single year.
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.
Prevailing Westerlies
The westerly winds that dominant in middle latitudes.
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.
Pressure gradient force
Force acting on air caused by air pressure differences.
Quantitative Precipitation Forecast (QPF)
A forecast of precipitation amount specifically in depth units (e.g., inches).
Falling water drops with a diameter greater than .02".
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.
Relative humidity
The ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor the air can hold at its current temperature and pressure. This is expressed as a percentage.
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.
A facing of concrete or masonry used to support a levee.
An elongated area of high atmospheric pressure, running generally north-south, at the surface or aloft.
Rip Rap
Broken stone and concrete piled together. Commonly used as a breakwater to prevent erosion along a shoreline.
Saffir-Simpson Hurricane Damage Potential Scale
A 1-5 scale, developed by Robert Simpson and Herbert Saffir that measures hurricane intensity.
Sand Boil
A spot on the land side of a levee where water bubbles up signaling a breech in the levee's integrity.
Scour Hole
A place in a stream or river bed that is sharply deepened by action of the current or weakness of the underlying soil. Scour holes are especially common in the outside bend of a river.
Sea breeze
Winds blowing inland from any body of water.
The process of moving a flood control levee back away from a river or waterway to lessen the erosion or other damage caused by the water. Setbacks may also be used to give the levee a more stable foundation to prevent its collapse.
Sheet Piling
Huge sheets of corrugated metal driven vertically into the ground to stabilize shifting soil and increase levee strength. Sheet piles are also used to keep water from seeping through porous soil to protected land. They are commonly encased in concrete embedded in a levee slope to protect the metal from corrosion.
The bottom of the floodgate opening.
Small Craft Advisory
A marine advisory generally issued for winds greater than 20 knots but less than 33 knots.
A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location; used extensively in weather forecasting.
The structure through which a controlled amount of floodwater is diverted to another waterway. This diversion is necessary to relieve the enormous pressures exerted by rising water on the flood control levees downstream of the spillway.
Stable air
Air in which temperature and moisture discourage formation of updrafts and downdrafts. Clouds will be low and flat Any precipitation will be steady.
Stationary front
A warm-cold air boundary with neither cold nor warm air advancing.
Still Basin
The area directly behind a spillway containing structures used to dissipate the energy of the water flowing through the spillway opening. Without a still basin, the tremendous force of the water cascading through the spillway would erode the foundation of the spillway, and cause its collapse.
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.
The layer of the atmosphere from about 7 to 30 miles above the earth's surface.
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 km/hr) or less.
Subtropical High
A semi-permanent high pressure region near 30 degrees latitude.
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 scale
Large-scale weather events and systems, generally more than 200 miles across.
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.
Trade winds
The wind system, occupying most of the tropics, which are northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.
Release of water vapor into the air by plants.
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 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
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 km/hr) 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.
The region of Earth between latitude 23.5 degrees north--the Tropic of Cancer--southward across the equator to latitude 23.5 degrees south--the Tropic of Capricorn.
The upper boundary of the troposphere, usually characterized by an abrupt change in how the temperature changes with height; below the tropopause temperature generally decreases with height; above the tropopause temperature generally increases with height.
The portion of the atmosphere which extends outward about 10 to 20 km from the earth's surface, and in which generally temperature decreases rapidly with altitude, clouds form, and convection is active.
A hurricane which occurs in the region of the Philippines or the China Sea.
U.S. Army Corps of Engineers
That branch of the U. S. Army directly responsible for the design of levees and flood control structures, navigation, hazardous waste cleanup, hydropower, military encampments and airfields.
The "bringing up of water" from deeper water towards the surface. A hurricane can "upwell" colder water to the surface as it moves along and churns up the ocean. This can actually weaken a hurricane if it becomes stationary and thus upwells colder water underneath itself.
Warm Front
A warm-cold air boundary with warm air advancing.
Water Vapor
Water in a gaseous state.
Wind Direction
The location from which the wind is blowing FROM.
Zulu Time (Z)
Global time scale based on the local time observed on the Prime Meridian; also called Greenwich Mean Time (GMT) or Universal Coordinated Time (UTC).

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