Tropical weather comes with its own vocabulary. Here are the terms you'll hear most, in plain language — so the forecasts make sense when it matters.
The systems
- Tropical cyclone — the general term for any rotating tropical storm system. “Hurricane,” “typhoon,” and “cyclone” are all tropical cyclones, named differently by region.
- Tropical disturbance — a disorganized cluster of thunderstorms that might develop.
- Tropical depression — an organized system with a closed circulation and winds up to 38 mph.
- Tropical storm — winds of 39-73 mph; this is when it earns a name.
- Hurricane — winds of 74 mph or higher.
- Major hurricane — Category 3 or stronger (111 mph and up).
- Invest — a system NHC has flagged to investigate for development; not yet a depression.
- Potential Tropical Cyclone (PTC) — a disturbance that isn't a tropical cyclone yet but is expected to bring tropical-storm or hurricane conditions to land within 72 hours, so NHC can issue advisories, watches, and warnings early.
- Post-tropical / remnant low — a former tropical cyclone that has lost tropical characteristics but can still bring dangerous rain and wind.
The hazards
- Storm surge — the abnormal rise of seawater pushed ashore by the storm; historically the deadliest hazard.
- Storm tide — storm surge plus the normal astronomical tide.
- Freshwater (inland) flooding — flooding from the storm's rainfall, now the leading cause of U.S. hurricane deaths.
- Rip current — a narrow channel of water rushing away from the beach; deadly even from a distant storm.
The forecast
- The cone — the probable path of the storm's center; not a map of who gets hit.
- Watch — the hazard is possible; get ready (issued about 48 hours out).
- Warning — the hazard is expected; act now (issued about 36 hours out).
- Landfall — when the storm's center crosses the coast. Serious impacts arrive before, and reach well outside, the landfall point.
- Eye / eyewall — the calm center, ringed by the eyewall where the strongest winds are.
- Rapid intensification — a jump of at least 35 mph in 24 hours; it can turn a modest storm dangerous fast.
The science
- Wind shear — a change in wind with height that can tear a storm apart.
- Sustained winds — the averaged wind speed used to classify a storm (gusts are higher).
- Tropical Weather Outlook (TWO) — NHC's forecast of where new systems might form over the next 2 and 7 days.
- Saharan Air Layer (SAL) — a mass of hot, dry, dusty air off the Sahara that drifts over the tropical Atlantic in summer and suppresses storms with dry air, added wind shear, and a stabilizing warm layer.
When a term isn't clear in a forecast, that's exactly the moment to look it up — understanding the words is the first step to acting on them.