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Hurricane Forecast Terminology Reference for Non-Scientists
Ever wonder what those familiar terms and phrases used by the news media and others to describe forecasts and storms during hurricane season actually mean? Some of the terms are scientific in origin and are defined in the technical sources, while others, used more informally, are not. Official scientific definitions are available for some of these terms through the combined websites of the NHC, its parent agency the NWS, and the AMS. This list is meant to be comprehensive and to explain the meanings of the most frequently used hurricane terms in familiar language. If there is a term you do not see here that belongs here, please let us know through our Contact page.
Atlantic basin
Ocean basin astride the southern and eastern United States, in which Atlantic hurricanes form and develop. The Atlantic basin includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Hurricanes that form in the Atlantic basin are known as Atlantic hurricanes.
Cape Verde hurricane
Hurricane that has its origins in the extreme eastern Atlantic Ocean, in the vicinity of the Cape Verde Islands just off the west coast of Africa. These storms that form far out in the Atlantic are most common at the height of the hurricane season, when this distant region of the ocean becomes warm enough to support tropical cyclone formation. Historically, some of the most destructive hurricanes, most notably the Galveston 1900 Storm, were Cape Verde hurricanes.
center
The axis around which the hurricane winds revolve, the location at the physical center of the storm where atmospheric pressure is at its lowest.
central pressure
The atmospheric pressure, in millibars (mb), at the center of low pressure in a tropical cyclone. Minimum central pressure is closely related to maximum sustained wind speed in tropical cyclones and is likewise a reliable indicator of a storm's intensity. Minimum central pressure may range from 880 mb in the most intense hurricanes to just over 1000 mb for tropical depressions. For comparison, mean sea level pressure is 1013 mb.
concentric circles
One circle inside of another.
concentric eyewall cycle - see eyewall replacement cycle, below.
cone of uncertainty
Track forecast graphic containing the forecast positions of the center of a tropical cyclone for all the standard forecast times, with circles drawn around each position. The circles encompass two-thirds of the uncertainty in the forecast for each standard forecast time, based on forecast errors from the past five hurricane seasons.
contraction (opposite: expansion)
Decrease in the size of the storm, or in the size of one of its features (e.g., the eye).
Coordinated Universal Time (UTC), or Z-Time
A global standard for expressing clock times, used by meteorologists and other scientists. During daylight saving time, when Atlantic hurricanes are most active, UTC / Z-Time is five hours ahead of Central Daylight Time (CDT) and four hours ahead of Eastern Daylight Time (EDT).
decay (opposite: development)
Progressive decrease in a storm's intensity, often associated with its final phase, spin-down of a tropical cyclone. Hurricanes develop over the warm tropical oceans, which serve as their primary energy source, and begin to decay rapidly once they encounter a land mass and are deprived of that energy source.
de-intensification (opposite: intensification)
Decrease in a tropical cyclone's maximum one-minute sustained wind speed, decrease in Saffir-Simpson Hurricane Wind Scale category.
development (opposite: decay)
Progressive increase in a storm's intensity, often associated with the genesis phase, spin-up of a tropical cyclone. Hurricanes develop over the warm tropical oceans, which serve as their primary energy source, and begin to decay rapidly once they encounter a land mass and are deprived of that energy source.
diameter
Distance across, from edge to edge through the center. Used for communicating the width of the eye of the hurricane. To convey the size of the storm itself, radius is used.
disorganized (opposite: organized)
General term describing a storm that either has not yet acquired or has lost the structural attributes characteristic of well-developed, "healthy" tropical cyclones, i.e., deep convection (thunderstorms) in a roughly symmetrical arrangement around an identifiable center, as would be visible on satellite imagery. It is also sometimes used to describe the convection (thunderstorms) itself, as to whether or not it is arranging itself in the well-ordered spiral pattern characteristic of tropical cyclones.
dissipation (opposite: formation)
Disappearance of the closed wind circulation around the center of low pressure. The downward transition from tropical depression to remnant low, the end of the tropical cyclone.
easterly wave - see tropical wave, below.
expansion (opposite: contraction)
Increase in the size of the storm, or in the size of one of its features (e.g., the eye).
extratropical cyclone / midlatitude cyclone
The type of storm characteristic of the middle latitudes: "extratropical" means "outside the tropics." These are the storms embedded in the jetstream that move from west to east across the United States, with their associated cold fronts. Both West Coast winter storms and Nor'easters are of this type. Like tropical cyclones, they have a closed circulation around low pressure and rotate counter-clockwise (northern hemisphere). However, they are on average less intense, much larger, and completely different in structure from their tropical counterparts. The most important difference is their respective energy sources. While tropical cyclones extract energy from the warm oceans, the energy source for extratropical cyclones is the temperature contrast between warm and cold air masses.
extratropical transition
Process by which a tropical cyclone loses its tropical characteristics as it enters the midlatitudes, changes its structure and primary energy source, and becomes an extratropical cyclone. Former tropical cyclones that have undergone this transition represent one subtype of post-tropical cyclone. It is possible for formerly tropical cyclones that have become extratropical to retain or re-gain wind speeds exceeding hurricane or tropical storm force.
eyewall replacement cycle / concentric eyewall cycle
Periodic tendency of a storm to develop a second (outer) eyewall around the first, which then contracts and replaces the original (inner) one. This commonly occurs in well-developed hurricanes and is often accompanied by short-term changes in storm intensity.
forecast error
Difference between the position and intensity of a storm that were forecast for one of the standard forecast times and those later observed. These errors are catalogued during the forecast verification process at the end of every hurricane season. Forecast errors are numerical values, expressed as distances (nautical miles) for the track forecast, or wind speeds (knots) for the intensity forecast. Errors generally are larger for longer forecast lead times. Taken together, past errors indicate the current state of the science of hurricane forecasting, as well as the amount of uncertainty in future forecasts.
forecast period / forecast lead time
The interval between the time a forecast is issued and the time the event is predicted to occur.
forecast position
The latitude/longitude coordinate representing where the center of the storm is forecast to be at one of the standard forecast times. Together these points, with the uncertainty factored in for each forecast time, make up the track forecast.
forecast uncertainty
The amount of error likely to exist in a forecast, the objective limit of forecast precision. The amount of uncertainty in any given year conveys the present-day state of scientific advancement, as it describes the level of forecasting skill currently possible. For any given forecast, the amount of uncertainty depends on how far out in time the forecast is issued. The longer the forecast lead time, the greater the uncertainty: a three-day forecast has more uncertainty than a two-day forecast, for example.
forecast verification
Process used by forecasters to determine the level of accuracy, or conversely, the amount of error, in their forecasts. This is done by comparing the forecast positions and intensities that were predicted for individual hurricanes for each of the standard forecast times to those later observed at those times. Every forecast for every tropical cyclone undergoes this comparison. Other types of forecasts are verified besides the track and intensity forecasts, but those are the two most important, because the forecast errors recorded during their verification are used to convey the amount of uncertainty in future forecasts to the public.
formation (opposite: dissipation)
Appearance of a closed wind circulation around an identifiable center of low pressure. The upward transition from tropical disturbance to tropical depression, the genesis of the tropical cyclone, also known as tropical cyclogenesis.
gale-force winds - see tropical storm-force winds, below.
growth
Increase in the size of a tropical cyclone. In the popular usage, the meaning of the term may be conflated with intensification or development, sometimes both; the term expansion may be a clearer way to express an increase in size alone.
hurricane
Cyclone of tropical origins and characteristics with one-minute maximum sustained winds of 74 mph or higher. In the popular usage, it is also commonly used as a general term to refer to this type of storm instead of the more technical-sounding tropical cyclone. In other parts of the world, this same phenomenon is known by different names, e.g., typhoon, etc.
hurricane conditions
Result of the intersection of the hurricane-force wind field of a tropical cyclone with a specific place on land or at sea. Winds of this force may extend out to distances of 100 miles or more from the center of a large storm.
hurricane-force wind field / hurricane-force winds The spatial distribution of 74 mph or higher winds around the center of low pressure in a hurricane. Outside it is the tropical storm-force wind field (39 mph or higher winds), with the two wind fields arranged as concentric circles. A small hurricane-force wind field, by Atlantic basin standards, would have radius of approximately 29 miles (25 nautical miles) or less. A medium-sized hurricane-force wind field would have approximate radius greater than 29 miles (25 nautical miles) but less than about 63 miles (55 nautical miles). A large hurricane-force wind field would have a radius of approximately 63 miles (55 nautical miles) or greater.
hurricane warning
An announcement by the National Hurricane Center that hurricane conditions are expected somewhere within the specified coastal area. Because the arrival of tropical storm-force winds may be enough to block some coastal evacuation routes, the hurricane warning is posted 36 hours in advance of the anticipated onset of the tropical storm-force winds.
hurricane watch
An announcement by the National Hurricane Center that hurricane conditions are possible somewhere within the specified coastal area. Because the arrival of tropical storm-force winds may be enough to block some coastal evacuation routes, the hurricane watch is posted 48 hours in advance of the anticipated onset of the tropical storm-force winds.
intensification (opposite: de-intensification)
Increase in the maximum one-minute sustained wind speed in a tropical cyclone, increase in Saffir-Simpson Hurricane Wind Scale category.
intensity
Maximum one-minute sustained wind speed in a tropical cyclone, used for classification as a tropical depression, tropical storm, or hurricane of Category 1, 2, 3, 4, or 5 on the Saffir-Simpson Hurricane Wind Scale. Previously, central pressure was used as the primary indicator of a storm's intensity, because of the difficulty that existed in directly measuring the wind speed, especially over the open ocean.
knot (kt) / nautical mile per hour
Alternate unit of measurement for speed, shorthand for "nautical mile per hour." It is used by mariners and aviation interests, and by operational meteorologists, instead of miles per hour. Conversion: Multiply the knots by 1.15 to get miles per hour.
landfall
The intersection of the center of a tropical cyclone with a coastline. There is no forecast explicitly for landfall.
large storm
A large Atlantic hurricane or tropical storm has a tropical storm-force wind field with outermost radius of approximately 201 miles (175 nautical miles) or greater. It is the reach of the tropical storm-force wind field that indicates the total size of both hurricanes and tropical storms, with sizes expressed as nautical miles.
medium-sized storm
A medium-sized Atlantic hurricane or tropical storm has a tropical storm-force wind field with outermost approximate radius greater than 132 miles (115 nautical miles) but less than about 201 miles (175 nautical miles). It is the reach of the tropical storm-force wind field that indicates the total size of both hurricanes and tropical storms, with sizes expressed as nautical miles.
midlatitude cyclone - see extratropical cyclone, above.
millibar (mb)
Standard unit of atmospheric pressure measurement. Standard pressure at sea level is 1013 mb.
nautical mile (nm)
Alternate unit of measurement for distance, based on the spherical geometry of the Earth's surface, used by mariners and aviation interests, and by operational meteorologists, instead of miles. Conversion: Multiply the nautical miles by 1.15 to get miles.
nautical mile per hour - see knot (kt), above.
organized (opposite: disorganized)
General term describing a storm that has either acquired or is in the process of acquiring the structural attributes characteristic of well-developed, "healthy" tropical cyclones, i.e., deep convection (thunderstorms) in a roughly symmetrical arrangement around an identifiable center, as would be visible on satellite imagery. It is also sometimes used to describe the convection (thunderstorms) itself, as to whether or not it is arranging itself in the well-ordered spiral pattern characteristic of tropical cyclones.
position
The single latitude/longitude coordinate representing the current location of the center of the storm.
post-tropical cyclone
A former tropical cyclone, as it no longer has sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds. Former tropical cyclones that have become fully extratropical, as well as remnant lows, are two subtypes of post-tropical cyclones. A former tropical cyclone that no longer meets the requirements of a tropical cyclone but has not yet completed the transition into either of those two subtypes is simply described as post-tropical.
probability
Percent chance that the forecasted event will occur or begin within a certain amount of time.
quadrant
One of four regions within a tropical cyclone, relative to its center. There are two ways of expressing quadrant, used for two completely different purposes. In the size forecasts, the quadrant is expressed as one of four compass directions from the storm center. The northeast (NE) quadrant, for example, is that quarter of the storm's wind field located northeast of the center. In addressing the likely severity of storm surge or wind damage, quadrants are expressed as the quarters of the storm located respectively to the left and ahead of the center, to the left and behind the center, to the right and ahead of the center, and to the right and behind the center, relative to the storm's direction of motion. This is important because the fastest winds and highest storm surge potential are usually in the storm's right front quadrant (RFQ), the quarter of the storm ahead of and to the right of the center. For a storm moving in a westward direction, its NW quadrant will be the right front quadrant. For a storm moving to the north, its NE quadrant will be the right front quadrant.
radius (plural: radii) Distance from center.
radius of maximum winds
The distance from the calm center of a tropical cyclone to the location of its fastest winds. This is a relatively small number; 30 miles from center might be a typical value. From that point outward, the winds progressively decrease in speed. In well-developed hurricanes, the radius of maximum winds is usually found at the inner edge of the eyewall.
rapid intensification / rapid deepening
Dramatic increase in the intensity of a tropical cyclone in a short period of time. Defined alternately as a sharp increase in maximum sustained wind speed or a sharp decrease in central pressure (deepening of the low). It is a 30 mph (formerly 35 mph) increase in the storm's one-minute maximum sustained wind speed, or, alternatively, a 42 mb drop in its central pressure, within a 24-hour period. The two definitions allude to the same phenomenon, as both wind speed increase and central pressure drop are indicators of a storm's intensification.
recurvature
Tendency of hurricanes to turn toward the northeast as they enter the midlatitudes, and eventually curve back out to sea. Storms recurve as they are drawn into the prevailing westerlies, or into the winds of other weather features (such as high and low pressure systems) they may encounter as they gain latitude.
remnant low
A type of post-tropical cyclone that no longer possesses the convective organization (thunderstorms) required even of a tropical depression, and that has maximum sustained winds below tropical storm force (less than 39 mph). Remnant lows may pose a major flooding hazard to communities far from the coast, as they may be capable of releasing very large volumes of rain on the same area, especially if stalled out or moving slowly.
Saffir-Simpson Hurricane Wind Scale
Classification of the one-minute maximum sustained wind speeds of hurricanes into intensity categories, numbered 1 through 5. Formerly known as the Saffir-Simpson Hurricane Scale, as it previously had attached to it estimated ranges for central pressure and storm surge likely to accompany each wind speed category.
size
Outermost geographical extent or radius of tropical storm-force winds (39 mph or higher sustained) in a hurricane or tropical storm, described by the distance from the center of the storm to the outermost edge where winds of those speeds are observed in any quadrant of the storm. Atlantic hurricanes are on average about 40 percent smaller than their counterparts in the western Pacific Ocean (East Asian typhoons).
small storm
A small Atlantic hurricane or tropical storm has a tropical storm-force wind field with outermost radius of approximately 132 miles (115 nautical miles) or less. It is the reach of the tropical storm-force wind field that indicates the total size of both hurricanes and tropical storms, with sizes expressed as nautical miles.
standard forecast times
Hours or days into the future for which hurricane forecasts are issued. For the Atlantic basin, there are 12h, 24h (1-day), 36h, 48h (2-day), 72h (3-day), 96h (4-day), and 120h (5-day) forecasts. The entire set is issued every six hours, more often as a storm approaches land. In addition, there is a zero-hour (0h) estimate of the storm's current position, intensity, and size, which serves as an initial value. With each successive forecast time, the level of precision in the forecast decreases and the amount of uncertainty increases.
statute mile (mi)
Alternate name for mile. This name is used by mariners, aviation interests, meteorologists, and others who use nautical miles by default, to distinguish any occasional reference to regular miles from their usual use of nautical miles.
steering currents
The surrounding wind flow in which the tropical cyclone is embedded, and which carry it along its path. This includes prevailing winds (trade winds, westerlies) as well as high and low pressure systems (the semi-permanent Bermuda High Pressure System off the Atlantic, or passing midlatitude cyclones that move from west to east, respectively). If the hurricane is caught in the winds of any of these systems, its direction of movement will be altered accordingly.
storm
Generic name for any type of weather disturbance surrounding an area of low pressure, including tropical cyclones of any intensity.
storm surge
Inundation of coastal areas by seawater, caused by the high winds of a tropical cyclone forcing it ashore.
storm tide
The combined effect of the storm surge and the astronomical high or low tide.
storm track
Actual trajectory of the hurricane.
strength
Average wind speed in the storm. This is different from the maximum sustained wind speed, which describes storm intensity.
track forecast
The series of forecast positions, in co-ordinates of latitude and longitude, with the forecast uncertainty taken into account, at which the center of the storm is predicted to be at each of the standard forecast times.
tropical cyclone
Generic name for the hurricane-type storm, the cyclone that originates over the tropical oceans. In the Atlantic basin, a tropical cyclone is either a hurricane, a tropical storm, or a tropical depression. At minimum it must be of tropical origin with organized deep convection (thunderstorms) and a closed circulation around an identifiable center of low pressure to be characterized as a tropical cyclone.
tropical depression
Tropical cyclone with a closed wind circulation and one-minute maximum sustained winds of less than 39 mph.
tropical disturbance
A recognizable tropical weather system with organized convection (thunderstorms) that has maintained its identity for 24 hours or more, with the potential to develop into a tropical cyclone.
tropical flooding
Submersion of a land area, regardless of distance from the coast, in fresh water, caused by the rain from a tropical cyclone or remnant low.
tropical storm
Tropical cyclone with one-minute maximum sustained winds of at least 39 mph but less than 74 mph.
tropical storm conditions
Result of the intersection of the tropical storm-force wind field of a tropical cyclone with a specific area on land or at sea. Winds of this force may extend out to distances of 200-300 miles from the center of a large storm.
tropical storm-force wind field / tropical storm-force winds
The spatial distribution of the 39 mph or higher winds around the center of low pressure in a hurricane or tropical storm, with its radius serving as a measure of the storm's overall size. In hurricanes only, there is a hurricane-force wind field (74 mph or higher winds) inside it, smaller and closer to the center, with the two wind fields arranged as concentric circles.
tropical storm warning
An announcement by the National Hurricane Center that tropical storm conditions are expected somewhere within the specified coastal area within 36 hours.
tropical storm watch
An announcement by the National Hurricane Center that tropical storm conditions are possible somewhere within the specified coastal area within 48 hours.
tropical wave / easterly wave
Low pressure wave in the trade winds prevailing in the tropical latitudes, moving in a westward direction, with the potential to develop into a tropical cyclone. Common origin of Cape Verde hurricanes.
wind field
The spatial distribution of the winds of different speeds that flow around the center of low pressure. The different wind speeds appear as concentric circles, with the highest wind speeds close to the center of the storm (at the radius of maximum winds) and from there progressively decreasing with distance from the center. Predictions of the extent (radius) of the hurricane-force wind field and the tropical storm-force wind field for each of the standard forecast times are contained in the size forecast.
wind radii
These are the numbers that appear in the size forecast, technically known as the wind radii forecast, with the distances expressed in nautical miles for each of the storm's four quadrants. There is a separate number for each quadrant (NE, SE, SW, NW) of the storm. For a hurricane, the size forecast contains two sets of four radii, representing outermost extent of both the tropical storm-force winds and hurricane-force winds in each quadrant of the storm. For a tropical storm, which by definition does not have winds of hurricane force, there is only one set of radii in the forecast. Because wind speeds in tropical cyclones decrease with distance from center, the two wind fields appear as concentric circles, with the tropical storm-force wind field the larger of the two and representing the overall size of the storm.
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