Tropical meteorology has its own language: invests, rapid intensification, eyewall replacement cycles, the cone. This page walks the full ladder from tropical disturbance to Category 5 hurricane and decodes the NHC products and phenomena around landfalling systems. When the tropics wake up, Tropical Coverage runs live in the Xtreme Weather Discord (XWD).
A tropical cyclone is the generic term for a warm-core low-pressure system with organized convection and a closed surface circulation. The classification by maximum sustained winds: Tropical Depression (<39 mph) → Tropical Storm (39–73 mph) → Hurricane (74+ mph) → Major Hurricane (111+ mph).
A tropical disturbance is a discrete area of organized thunderstorm activity in the tropics with minimal rotation — the very earliest stage of potential tropical cyclone development. NHC monitors disturbances and assigns formation probabilities in the Graphical Tropical Weather Outlook. Most disturbances dissipate; a fraction develop into named storms.
A Tropical Wave (African Easterly Wave / AEW) is a westward-propagating disturbance embedded in the trade winds, originating off Africa's west coast. They're the primary "seeds" of Atlantic hurricanes — the majority of significant Atlantic tropical cyclones can be traced back to an AEW that crossed the coast from Senegal/Guinea.
An Invest (Investigative Area) is a designation by the NHC for a tropical disturbance being monitored for development potential. Numbered 90–99 (Atlantic) or 90–99W (Pacific) with a suffix. The NHC issues development probabilities for invests in 2-day and 7-day outlooks.
A Tropical Depression (TD) is an organized tropical cyclone with sustained winds 38 mph or less. Gets a number designation (e.g. TD Three) but not a name. Upgrades to Tropical Storm at 39 mph.
A Tropical Storm (TS) has sustained winds of 39–73 mph. Receives a name from the seasonal list. Upgrades to Hurricane status at 74 mph.
A subtropical storm is a hybrid cyclone with both tropical and extratropical characteristics — deriving energy from both a warm ocean surface (tropical) and horizontal temperature contrasts (extratropical). They're named on the Atlantic list but often have a broader wind field and asymmetric structure compared to purely tropical cyclones.
A hurricane is a tropical cyclone in the Atlantic or eastern Pacific with sustained winds of 74+ mph. Called typhoon in the western Pacific and cyclone in the Indian Ocean — same storm type, different names by region.
A Major Hurricane is Category 3 or higher — sustained winds 111+ mph. The threshold for catastrophic structural damage and the basis for many evacuation triggers.
The Saffir-Simpson Hurricane Wind Scale rates hurricanes by sustained wind: Cat 1 (74–95 mph), Cat 2 (96–110), Cat 3 (111–129, major), Cat 4 (130–156), Cat 5 (157+). Does not directly account for surge, rainfall, or size.
Category 1: 74–95 mph sustained winds. Some damage to roofs, siding, and trees. Power outages common. Minimum threshold for XWD Tropical Coverage Mode.
Category 2: 96–110 mph. Extensive damage to roofs and siding, snapped/uprooted trees, near-total power loss for days to weeks.
Category 3: 111–129 mph. The threshold for major hurricane. Devastating damage — well-built homes lose roofs/gable ends, water and electricity unavailable for days to weeks.
Category 4: 130–156 mph. Catastrophic damage — most trees snapped, severe structural damage to well-built homes. Most of area uninhabitable for weeks to months.
Category 5: 157+ mph. The highest category. Catastrophic damage — total roof failure of many residences, most of area uninhabitable for weeks to months.
Rapid Intensification (RI) is defined as a 35 mph or greater increase in maximum sustained winds in 24 hours. It's one of the most dangerous and difficult-to-forecast tropical phenomena. RI can transform a Cat 1 into a Cat 4 before coastal populations have time to evacuate — examples include Hurricane Michael (2018), Ida (2021), and Ian (2022).
A post-tropical cyclone is a former tropical or subtropical storm that has lost its tropical characteristics — either by weakening completely (remnant low) or transitioning to an extratropical cyclone. Still dangerous: Hurricane Sandy's most destructive landfall in New Jersey occurred after it had been classified post-tropical. Hazards persist regardless of classification.
The eye is the calm, often clear center of a mature tropical cyclone. Surrounded by the eyewall — the most intense ring of convection. A well-defined eye usually means a strong, organized storm.
The eyewall is the ring of intense thunderstorms surrounding the eye, where a hurricane's strongest winds and heaviest rain occur. Landfall in the eyewall is when the worst damage happens.
During an Eyewall Replacement Cycle (ERC), an outer rainband contracts inward, forming a concentric eyewall that chokes off the inner eyewall. The storm weakens as the inner wall collapses, then often re-intensifies with a larger eye. ERCs can cause a storm to go from peak intensity to weakening in hours, then re-strengthen.
An Eyewall Replacement Cycle (ERC) is when an outer ring of convection contracts inward, replacing the original eyewall. Causes temporary weakening followed by re-intensification, often expanding the wind field.
Rainbands are spiral bands of thunderstorms extending outward from a tropical cyclone's center. They can produce tornadoes (especially on the right-front quadrant) and heavy rain hundreds of miles from the storm core.
Storm surge is the abnormal rise in seawater driven onshore by a tropical cyclone's winds. The deadliest hurricane hazard — responsible for ~half of all U.S. tropical cyclone fatalities historically.
A Storm Surge Warning means life-threatening inundation (3+ feet above ground) is expected within 36 hours from a tropical system. A Storm Surge Watch means conditions are possible within 48 hours. Introduced in 2017, these products are independent of wind-based hurricane warnings and target the deadliest aspect of a hurricane.
A Tropical Storm Warning means sustained winds of 39–73 mph are expected within 36 hours. A Tropical Storm Watch means conditions are possible within 48 hours. These are issued by the NHC for coastal areas and trigger protective actions even if a hurricane warning is also in effect.
The National Hurricane Center (NHC) is the NWS national center in Miami responsible for tracking and forecasting tropical cyclones in the Atlantic and eastern Pacific basins.
The Tropical Weather Outlook is issued by the NHC and provides 2-day and 7-day probabilities of tropical cyclone formation for disturbances in the Atlantic and eastern Pacific basins. Color-coded: yellow (low, <40%), orange (medium, 40–60%), red (high, 60%+). Issued four times daily.
The Cone of Uncertainty is the NHC's forecast track graphic showing where the storm's center has a ~66% probability of falling. Critical misconception: the cone does NOT show the storm's size or hazard extent. Deadly storm surge, flooding rain, and tropical storm-force winds regularly impact areas far outside the cone. Never assume you're safe because you're outside it.
Hurricane Hunters are NOAA and Air Force aircraft that fly directly into tropical cyclones to gather data unavailable from satellites. They release dropsondes through the eyewall to measure pressure, wind, and temperature in 3D.
A dropsonde is an instrument package dropped from an aircraft (typically a hurricane hunter) that descends by parachute and transmits temperature, humidity, pressure, and wind data in real time back to forecasters.
Sea Surface Temperature (SST) is the water temperature at the ocean surface. SSTs of 26°C (79°F) or higher generally support tropical cyclone development and maintenance. Anomalously warm SSTs (driven by La Niña, the Loop Current, etc.) fuel rapid intensification. The Gulf of Mexico's Loop Current is a particularly potent energy source.
The Loop Current is a warm ocean current in the Gulf of Mexico that loops northward from the Yucatan Channel, arcs toward the Florida Straits, and sheds warm-core eddies. These eddies contain very warm, deep water that fuels dramatic rapid intensification in Gulf hurricanes — the reason storms like Katrina and Ida exploded before landfall.