A complete A-Z reference for hurricane and tropical meteorology terminology — from ACE to wind radii.
A measure of the total energy released by a tropical cyclone over its lifetime. Calculated by summing the square of the maximum sustained wind speed (in knots) at each 6-hour interval when the storm is at tropical storm strength or higher. A highly active season typically has an ACE above 150.
An official statement issued by the National Hurricane Center (NHC) providing the current position, intensity, and forecast for a tropical cyclone. Advisories are issued every 6 hours for tropical storms and hurricanes, and every 12 hours for tropical depressions.
The period from June 1 through November 30 when tropical cyclone formation is most likely in the Atlantic basin. The peak of the season is September 10. Storms can and do form outside this window, but it is rare.
A geographic region of the ocean where tropical cyclones are tracked and named. The six main basins are: North Atlantic, Eastern Pacific, Central Pacific, Western Pacific, North Indian Ocean, and Southern Hemisphere.
The official NHC forecast track graphic showing the probable path of a tropical cyclone's center over the next 5 days. The cone represents the area within which the center is expected to remain about 60–70% of the time. The cone does NOT represent the size of the storm — dangerous conditions extend well outside the cone.
A 1–5 rating based on a hurricane's maximum sustained wind speed. Category 1: 74–95 mph. Category 2: 96–110 mph. Category 3 (major): 111–129 mph. Category 4 (major): 130–156 mph. Category 5 (major): 157+ mph. The scale does not account for storm surge, rainfall, or tornadoes.
A general term for a large-scale atmospheric circulation rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. In the context of tropical weather, 'cyclone' is the regional term used in the Indian Ocean and South Pacific for what the Atlantic calls a 'hurricane.'
A tropical cyclone with maximum sustained winds of 38 mph (33 knots) or less. Tropical depressions are numbered (e.g., Tropical Depression 1) rather than named. A depression that strengthens to 39 mph becomes a tropical storm and receives a name.
A region of dry, stable air that wraps into a tropical cyclone, often causing a temporary weakening or disruption of the storm's structure. Dry slots are visible on satellite imagery as a clear area cutting into the cloud shield.
The ring of towering thunderstorms surrounding the eye of a hurricane. The eyewall contains the storm's most intense winds, heaviest rainfall, and most severe weather. The highest wind speeds in a hurricane are found in the eyewall.
A natural process in intense hurricanes where an outer eyewall forms and contracts inward, eventually replacing the inner eyewall. During an ERC, the storm typically weakens temporarily before re-intensifying, often to a higher intensity. ERCs are common in Category 4 and 5 hurricanes.
The calm, clear center of a hurricane, typically 20–40 miles in diameter. The eye is surrounded by the eyewall. Conditions inside the eye can be deceptively calm — the most dangerous winds are in the eyewall just outside. Do not assume a storm has passed when the eye arrives.
The predicted path of a tropical cyclone's center over the next 3–5 days, as issued by the National Hurricane Center. The forecast track is the centerline of the cone of uncertainty. Track forecasts have improved significantly over the past 20 years; intensity forecasts remain more challenging.
The primary American global weather model, run by NOAA's National Centers for Environmental Prediction (NCEP). The GFS produces forecasts out to 16 days and is one of the key models used in hurricane track forecasting. Also called the 'American model.'
A specialized hurricane forecast model developed by NOAA specifically for tropical cyclone prediction. HWRF provides high-resolution forecasts of hurricane track and intensity, including wind structure. It is one of the primary models used by NHC forecasters.
A prediction of a tropical cyclone's future maximum sustained wind speed. Intensity forecasting is significantly more difficult than track forecasting due to the influence of small-scale atmospheric and oceanic processes. Rapid intensification events are particularly challenging to forecast.
A designation given by the National Hurricane Center to a tropical weather system being investigated for potential tropical cyclone development. An 'invest' is assigned a number (e.g., Invest 90L for Atlantic systems) and monitored by NHC. An invest is not yet a tropical cyclone.
The intersection of the center (eye) of a tropical cyclone with a coastline. Contrary to popular belief, a storm's most dangerous conditions often occur before landfall, as the storm surge, wind field, and outer rainbands can extend hundreds of miles from the center.
The atmospheric pressure at a tropical cyclone's center, measured in millibars (mb) or hectopascals (hPa). Lower pressure indicates a more intense storm. A typical hurricane has a central pressure of 950–970 mb; the most intense hurricanes can drop below 900 mb. Hurricane Wilma (2005) holds the Atlantic record at 882 mb.
The division of NOAA's National Weather Service responsible for tracking and forecasting tropical cyclones in the Atlantic and Eastern Pacific basins. Located in Miami, Florida. The NHC is the official source for all US hurricane watches, warnings, and advisories. Website: nhc.noaa.gov
The upper-level divergence of air away from a tropical cyclone. Strong, unimpeded outflow is necessary for a hurricane to intensify. Outflow channels are visible on satellite imagery as cirrus clouds spreading outward from the storm's top.
A former tropical cyclone that has lost its tropical characteristics. Post-tropical cyclones can still produce dangerous winds, flooding, and storm surge, especially when they undergo extratropical transition. Sandy (2012) made landfall as a post-tropical cyclone but caused catastrophic damage.
A forecast of the expected amount of precipitation over a specific area and time period. For hurricanes, QPF is critical for predicting inland flooding risk, which has become the leading cause of hurricane-related deaths in the United States since the 1990s.
Defined by the NHC as an increase in maximum sustained winds of at least 35 mph (30 knots) within a 24-hour period. RI is one of the most dangerous aspects of hurricane forecasting because it can occur with little warning, turning a weaker storm into a major hurricane before evacuations can be completed. Warm sea surface temperatures and low wind shear are key RI enablers.
Aircraft that fly directly into tropical cyclones to measure wind speed, pressure, temperature, and other parameters. NOAA and the US Air Force Reserve operate hurricane hunter aircraft. Data from reconnaissance flights is critical for improving forecast accuracy.
A layer of very dry, dusty air that originates over the Sahara Desert and moves westward over the Atlantic Ocean. The SAL can suppress tropical cyclone development and intensification by introducing dry air into storm systems. The SAL is most active from late spring through mid-summer.
See 'Category.' The Saffir-Simpson Hurricane Wind Scale rates hurricanes from 1 to 5 based on maximum sustained wind speed. It is a wind-only scale and does not account for storm surge, rainfall, or tornadoes — all of which can be deadly regardless of category.
The temperature of the ocean's surface layer. Tropical cyclones require SSTs of at least 79°F (26°C) to form and maintain themselves. Warmer SSTs provide more energy for intensification. SST anomalies (departures from normal) are a key factor in seasonal hurricane forecasts.
An informal term for a display of multiple individual computer model track forecasts overlaid on a single map. The resulting image resembles a plate of spaghetti. Spaghetti models show the range of possible storm tracks; tight clustering indicates high model agreement, while wide spread indicates high uncertainty.
An abnormal rise in sea level caused by a tropical cyclone's winds and low pressure pushing water toward the shore. Storm surge is the leading cause of hurricane-related deaths in the United States. Surge heights can exceed 20 feet in extreme cases. The surge arrives as a wall of water, not a gradual rise.
A generic term for a non-frontal synoptic-scale low-pressure system originating over tropical or subtropical waters with organized convection and a closed surface wind circulation. Includes tropical depressions, tropical storms, hurricanes, typhoons, and cyclones.
A tropical cyclone with maximum sustained winds of 39–73 mph (34–63 knots). Tropical storms are named. A tropical storm that strengthens to 74 mph becomes a hurricane. Tropical storms can still cause significant damage from flooding and tornadoes.
An official NHC warning indicating that tropical storm conditions (sustained winds of 39–73 mph) are expected within a specified area within 36 hours. Tropical storm warnings are issued separately from hurricane warnings.
The regional term for a tropical cyclone in the Western Pacific basin. Typhoons are meteorologically identical to Atlantic hurricanes — the difference is only geographic. The Western Pacific is the most active tropical cyclone basin in the world.
The change in wind speed and/or direction with altitude in the upper atmosphere. High wind shear disrupts a tropical cyclone's vertical structure, inhibiting development and intensification. Low wind shear is a key ingredient for hurricane development and intensification.
A standardized report transmitted by hurricane hunter aircraft after completing a pass through a tropical cyclone's center. Vortex messages include the storm's position, central pressure, maximum winds, and other key parameters. This data is ingested directly into NHC forecast models.
A defining characteristic of tropical cyclones — the storm's center is warmer than the surrounding atmosphere at all levels. This warm core structure, maintained by the release of latent heat from condensation, is what distinguishes tropical cyclones from extratropical systems.
An official NHC notification that hurricane or tropical storm conditions are possible within a specified area within 48 hours. A watch means you should prepare now — do not wait for a warning. Watches are issued before warnings to give people more time to prepare.
An official NHC notification that hurricane or tropical storm conditions are expected within a specified area within 36 hours. A warning means conditions are imminent — complete preparations and follow evacuation orders immediately.
The distances from a tropical cyclone's center to which tropical storm force (34 kt), 50-knot, and hurricane force (64 kt) winds extend in each quadrant. Wind radii describe the size of a storm's wind field, which is distinct from its intensity. A large, weak storm can be more dangerous than a small, intense one.
Ready to track the next storm?
Use our live tracker, spaghetti models, and storm surge tool.