Today, at the start of the 2026 Atlantic Hurricane Season, we are launching the TTWC Forecast Hub – an interactive 3-day outlook that breaks Trinidad and Tobago down into 35 zones and forecasts up to 14 distinct weather hazards for each of them.
At the heart of the Hub is a single idea: a color and a tier should tell you two things at once, how likely an impact is, and how bad it could be, in language you can act on without a meteorology degree.
This page explains the system behind those tiers. It is written for two audiences at once. If you are a member of the public, the first few sections should leave you knowing exactly what a High rainfall day is asking of you. If you work in emergency management, water, agriculture, energy, insurance, or the marine sector, the later sections give you the thresholds, the historical anchors, and the honest limits of the system, so you can judge it for yourself.
A note before we begin: the TTWC Forecast Hub does not replace official warnings. The Trinidad and Tobago Meteorological Service (TTMS) is the country’s official source of weather alerts, and the Office of Disaster Preparedness and Management (ODPM) is the authority on evacuations and emergency response. Our tiers are designed to complement those alerts with finer detail, not to compete with them. Always follow the official guidance.
The communication gap we set out to close
TTMS’s alert system follows the international Common Alerting Protocol (CAP), the same standard used by weather services worldwide. Operationally, it uses four colors:
- GREEN — all clear/cancellation
- YELLOW — be aware
- ORANGE — be prepared
- RED — take action
That is a clear, proven framework. But three active colors have to carry an enormous amount of weather. When we reviewed every alert TTMS issued between 2018 and 2026- more than 850 of them- one pattern stood out: a single YELLOW alert can mean very different things on the ground.
- A YELLOW Adverse Weather Alert can have 24-hour rainfall ranges of roughly 20 mm to 125 mm.
- YELLOW Hazardous Seas Alerts covered wave heights from a manageable 0.5 m up to a dangerous 4.0 m.
- YELLOW-Level alerts (from Adverse Weather Alerts to High Wind Alerts) that included wind as a hazard ranged from a breezy 40 km/h to a damaging 75 km/h.
A “be aware” afternoon and a genuinely disruptive day can wear the same color.
There is a second gap. A TTMS alert generally names one area and one hazard type at a time. Several hazards that matter a great deal in Trinidad and Tobago – Saharan dust, storm surge, landslides, tornadoes, and waterspouts – almost never get their own standalone early warning message. They live as a sentence inside an alert about something else, if they appear at all.
So the question we asked was simple: could we keep the trusted CAP backbone, but add resolution? Communicate likelihood and severity, per hazard, per zone, in plain language, and give the hazards that normally hide in the fine print a place of their own.
The answer is the six-tier system.
The six tiers, in plain language
Every hazard in the Forecast Hub is rated on the same six-step ladder. The wording changes from hazard to hazard, but the meaning of each rung does not.
| Tier | What it means | Likelihood of impact | What it asks of you |
|---|---|---|---|
| No Risk | Normal conditions; no concern. | Under 10% | Carry on as usual. |
| Low | Minor impacts possible. | Under 60% | Monitor and assess your surroundings. |
| Moderate | Moderate impacts likely. | 60–75% | Monitor and start preparing. |
| High | Severe impacts likely. | Over 75% | Prepare, and avoid affected areas. |
| Very High | Severe to extreme, potentially life-threatening, with high certainty. | Over 85% (or already observed) | Rush preparations; shelter or move as directed. |
| Extreme | Record-breaking or historically unprecedented. | Over 90% (or already observed) | Evacuate as directed by the ODPM/Local Government/other government authorities. |
Two design choices are worth calling out, because they explain a lot of what follows:
- Each step up the ladder is anchored to a real Trinidad and Tobago event. We did not want Very High to be an abstraction. Wherever possible it points to something the country has actually lived through.
- Extreme is deliberately rare. It is reserved for conditions on the scale of our worst events on record. We would rather it mean something the few times we use it than appear so often it becomes background noise.
Behind each tier sit the four standard CAP attributes: severity, certainty, urgency, and the recommended response – the same building blocks TTMS uses. You do not need to think about those to use the Hub. But they are what keeps our tiers translatable to the official system, which is the subject of the next section.
How the tiers map (and don’t map) to TTMS alerts
Because both systems are built on CAP, they line up. But they do not line up one-to-one, and it is important to be honest about exactly where they diverge.
| TTWC Tier | Closest TTMS Color | What’s going on |
|---|---|---|
| No Risk | (no alert) | — |
| Low | (no alert) | TTWC warns earlier here. There is no TTMS equivalent. |
| Moderate | YELLOW (lower end) | TTMS YELLOW begins around here… |
| High | YELLOW (core) | …and most YELLOW alerts land here. |
| Very High | YELLOW (upper)/ORANGE | ORANGE is used sparingly outside tropical-cyclone situations. |
| Extreme | RED | TTMS issues RED only about 1.5 times a year on average. |
Three takeaways:
We sometimes flag risk before an official alert exists. Our Low tier has no TTMS counterpart. That is by design. It lets you start watching a developing situation early, with the clear understanding that it means possible minor impacts, not imminent danger.
We subdivide the big YELLOW band. What TTMS communicates as a single YELLOW, we split across Moderate, High, and sometimes Very High. A 30 mm afternoon and an 110 mm afternoon are both YELLOW to TTMS; to us, they are clearly different days.
Extreme is our RED, and like RED, it should be rare. Across eight years, TTMS issued just 12 genuine RED alerts. We hold our top tier to the same standard.
This is not a translation table to be applied mechanically. Our system is finer-grained, hazard-by-hazard and zone-by-zone, while a TTMS alert is broader and official. If TTMS has issued an alert, that alert is the authoritative word for the area and hazard it covers.
How the tiers were derived
The tiers are not arbitrary. They rest on three foundations.
First, eight years of local practice. We reviewed every TTMS alert from 2018 through 2026 to see what thresholds the official service has actually used. From what rainfall, wind, and wave numbers tend to trigger YELLOW, ORANGE, and RED specifically in Trinidad and Tobago. It is also important to note that the criteria (if any exist) have not been made public by the TTMS, which is the basis for some of these alerts, particularly the catch-all Adverse Weather Alert. However, based on the extensive analysis of over 850 early warning messages, our breakpoints mirror real local practice rather than borrowing numbers from elsewhere. Our rainfall steps of 25/50/75/100 mm, for instance, track the thresholds that recur throughout TTMS’s own adverse-weather alerts.
Second, historical anchors for the top of the ladder. Extreme is tied to events the country remembers:
- The October 2018 floods: the benchmark for an Extreme rainfall and riverine-flooding event. Widespread totals of 100–150 mm in 24 hours, three-day accumulations above 250 mm at Piarco, roughly 80% of Trinidad affected, and an estimated 100,000–150,000 people impacted. It remains the defining non-cyclone RED in TTMS history.
- Hurricane Flora (1963): the anchor for an Extreme wind and surge event on Tobago, with sustained winds around 165 km/h that damaged or destroyed an estimated 6,250 of the island’s 7,500 houses.
- Hurricane Beryl (2024): a Category 4 passing within about 90 km of Tobago, the most recent reminder of what a major hurricane near the islands looks like.
- The 1933 Trinidad Hurricane: the only modern landfall on Trinidad itself, with winds near 137 km/h.
Third, international standards where we have no local precedent. For hazards that Trinidad and Tobago has rarely or never experienced at the high end, we anchor to recognized external authorities, such as the Pacific Tsunami Warning Center’s Caribbean guidance, Trinidad and Tobago’s Tsunami Warning Information Dissemination Protocol (updated in February 2020), and NOAA’s Saharan dust products. Regional comparators inform our heat tiers, including recent Caribbean heat-index readings as high as 56 °C in Cuba.
Tying all of this together is one principle we apply with discipline: our top tier should correspond to roughly a one-in-ten-year event or rarer. If we ever found ourselves reaching for Extreme on an ordinary stormy afternoon, that would be a sign the system was drifting, and the fix would be to tighten our use of the tier, not to loosen its meaning.
The two ways a hazard reaches “Extreme.”
Most of the time, a hazard climbs the ladder on its own merits – more rain, stronger wind, bigger waves. But our worst weather is rarely one thing at a time. So there are two distinct, transparent pathways to the top tier.
Pathway 1: Single-hazard threshold. One hazard reaches a record-class level on its own. Rainfall above 100 mm over a wide area; sustained winds above 120 km/h; nearshore waves above 3.5 m. October 2018’s riverine flooding reached Extreme this way.
Pathway 2: Compound severity. Several hazards are dangerous at once during a high-confidence tropical-cyclone approach, and their combined severity justifies Extreme even if no single one has hit its individual record. Tropical Storm Karen in September 2019 is an example: damaging gusts, heavy rain, and a very high landslide risk arriving together.
The compound pathway is not a license to inflate. We apply it only when all four of these hold at once:
- a tropical cyclone is the driver,
- forecast certainty is 80% or higher,
- at least three independent hazards are already at High or Very High in the same zone, and
- the impact window is within 24 hours.
When that bar is met, the escalation is communicated as compound-driven, so it’s clear we are flagging a dangerous combination rather than one extreme number.
The hazards, family by family
The Forecast Hub rates 14 hazards. This section groups them into families. For each, we give a worked example, the threshold tables, and, where TTMS does issue an alert for that hazard, a look at how far a single official color has to stretch.
Throughout, watch how often one TTMS YELLOW has to cover two, three, or four of our tiers. That spread is the whole reason this system exists.
Rain and flooding
Rainfall · Street & flash flooding · Riverine flooding
Over three days, parts of Trinidad recorded more rain than they normally see in a season’s worth of heavy days: four-hour rates of 100–168 mm around Greenvale, three-day totals above 250 mm at Piarco, and over 350 mm in parts of south Trinidad. The Caroni Basin overflowed, around 80% of Trinidad was affected, and well over 100,000 people were impacted. This is what our rainfall and riverine-flooding Extreme tier is anchored to.
How TTMS handles rainfall: A single YELLOW Adverse Weather alert in our 2018–2026 dataset covered roughly 20–125 mm in 24 hours – a six-fold spread. The two most common YELLOW triggers were “in excess of 25 mm” and “in excess of 50 mm”; ORANGE has cited 75 mm. In TTWC terms, that one yellow band runs from Moderate through High and into Very High.
Rainfall (24-hour accumulation):
| Tier | Threshold | What to expect |
|---|---|---|
| No Risk | none | No rainfall expected. |
| Low | under 10 mm, isolated higher totals | Brief, isolated showers possible. |
| Moderate | 10–25 mm, localized higher totals | Isolated heavy rainfall likely; localized ponding possible in flood-prone areas. |
| High | 25–50 mm, isolated higher totals | Heavy rainfall expected; localized to sporadic street flooding likely. |
| Very High | 50–100 mm, isolated totals above 100 mm | Prolonged heavy rainfall; significant flooding likely across multiple areas. |
| Extreme | above 100 mm widespread (isolated totals above 150 mm), or a 4-hour rate above 100 mm | Catastrophic rainfall on the scale of October 2018, when Piarco recorded 250 mm over three days — a 1-in-50-year event in records dating to 1946. |
Street and flash flooding is rated on rainfall intensity over time – how much, how fast – rather than the daily total, because a short, violent cell can flood streets without a large 24-hour figure.
Street/Flash Flooding:
| Tier | Threshold | What to expect |
|---|---|---|
| No Risk | under 10 mm/hr or no convective rain | Drainage capacity not expected to be challenged. |
| Low | 10–25 mm in under 1 hr; brief cells | Slight risk of ponding on flood-prone roads; short-lived street flooding possible. |
| Moderate | ~25 mm in under 1 hr; localized | Ponding in flood-prone roads; short-lived street flooding in urban areas; flash flooding possible in one or two areas. |
| High | 25–75 mm in 1–3 hrs; localized to sporadic | Significant street/flash flooding likely across a few towns; avoid floodwaters; roads may be temporarily impassable. |
| Very High | 50–100 mm in 1–3 hrs, or 75 mm/24h on saturated soils | Dangerous flash flooding; multiple roadways impassable; seek higher ground if water is rising; travel strongly discouraged. |
| Extreme | over 75 mm in under 24 hr widespread, or over 100 mm in 4 hr; multiple basins | Catastrophic flash flooding on the scale of October 2018; do not drive through floodwaters; potential loss of life from rapidly rising water. |
Riverine flooding tracks the major at-risk basins: Caroni, Caparo, North and South Oropouche, to specifically name a few, but primarily across Trinidad. Unlike flash flooding, riverine flooding occurs when a watercourse overtops its banks and remains outside for days. In Tobago, due to the island’s terrain and size, riverine flooding isn’t common, but severe flash flooding does occur.
| Tier | Threshold | What to expect |
|---|---|---|
| No Risk | Levels normal | River levels well below threshold. |
| Low | Minor watercourses elevated | Minor watercourses may rise; prolonged flooding not expected. |
| Moderate | Major watercourses approaching threshold | Smaller watercourses overtop; low-lying areas near rivers likely flooded. |
| High | Multiple watercourses overwhelmed | Major and minor watercourses overwhelmed, flooding several communities; access may be restricted. |
| Very High | Threshold exceedance imminent or in progress | Major river flooding across multiple basins lasting days; evacuations may be necessary. |
| Extreme | October-2018-class, widespread and prolonged | Record-class flooding across multiple basins, comparable to or exceeding October 2018; potential loss of life. |
A transparency note on rivers: where a Water Resources Agency gauge has a defined bank-full level, we can read riverine risk straight from the gauge as a percentage of bank-full. Under 50% is normal flow; at or above 100% is Very High, and a gauge sitting at or above bank-full for three consecutive days is Extreme. The worst gauge governs its zone.
Wind
Gusty winds (or high winds)
Trinidad and Tobago’s wind extremes are bracketed by three storms: the 1933 hurricane, the only modern system to strike Trinidad directly (winds near 137 km/h); Flora in 1963, which devastated Tobago at around 165 km/h; and Beryl in 2024, a Category 4 that passed close to Tobago. Our Extreme wind tier – destructive winds above 120 km/h – is anchored to these.
How TTMS handles wind: A YELLOW wind alert (Adverse Weather/High Wind/Tropical Storm) has historically been issued for wind speeds of roughly 40–75 km/h. The single most common YELLOW trigger in our dataset was a gust “in excess of 55 km/h” (91 of 134 mentions); ORANGE has cited 55, 60, and 65 km/h; and the 2019 Tropical Storm Warning (RED Level) cited 65 km/h. We set High at 65 km/h specifically to align with how TTMS actually uses ORANGE.
| Tier | Gust speed | What to expect |
|---|---|---|
| No Risk | under 30 km/h | Normal trade-wind conditions. |
| Low | up to 50 km/h | Secure loose outdoor objects. |
| Moderate | over 50–65 km/h | Minor impacts on poorly secured items; secure loose objects. |
| High | over 65–90 km/h | Structural impacts possible on older or poorly secured structures; branches and loose materials become hazards. |
| Very High | over 90–120 km/h | Damaging winds; structural damage likely, trees uprooted, widespread power outages possible. Avoid travel. |
| Extreme | over 120 km/h | Destructive winds and major structural damage, comparable to Beryl (2024), Flora (1963), and the 1933 Trinidad hurricane. |
Severe convective weather
Thunderstorms · Funnel clouds · Tornadoes & Waterspouts
Most of our severe-weather days are convective: heat and moisture build through the afternoon and erupt into thunderstorms, occasionally spinning up a waterspout offshore or a funnel cloud inland. These hazards are rated by how widespread and intense the activity is, rather than by a single number.
How TTMS handles these: thunderstorms are folded into Adverse Weather Alerts, but funnel clouds, tornadoes, and waterspouts get no standalone TTMS alert at all. In the Forecast Hub, they are first-class hazards, so a heightened risk of tornadoes or waterspouts can be flagged on its own.
Thunderstorms:
| Tier | Coverage | What to expect |
|---|---|---|
| No Risk | none | Stable; no development expected. |
| Low | isolated possible | Isolated storms with occasional lightning and brief gusts near rainfall. |
| Moderate | scattered likely | Scattered storms with lightning, gusty winds, and street/flash flooding. |
| High | numerous expected | Numerous storms, frequent lightning, gusty winds; flooding where heavy rain falls. |
| Very High | widespread severe | Widespread severe storms with damaging gusts, frequent lightning, and heavy rain. |
| Extreme | persistent severe episode | Intense, persistent activity with damaging winds and flooding rain; significant property damage possible. |
Funnel clouds and tornadoes/waterspouts climb the same ladder, escalating from a marginal risk near showers up to (at the very rare Extreme end) strong tornadoes or waterspouts that warrant immediate shelter.
Marine hazards
Hazardous seas (nearshore and open water) · Storm surge
How TTMS handles the sea: YELLOW Hazardous Seas alerts are the widest band of all, issued anywhere from a calm 0.5 m in sheltered waters up to a dangerous 4.0 m off Tobago, for both ordinary wind-waves and long-period swell. In eight years, there were only three ORANGE marine alerts (around 3.5 m breaking waves) and no RED. One YELLOW therefore stretches across nearly our entire nearshore ladder.
Hazardous seas – nearshore (within ~5 km of the coast):
| Tier | Wave height | What to expect |
|---|---|---|
| No Risk | under 0.5 m | Calm to slight. |
| Low | 0.5–1.0 m | Normal small-craft caution. |
| Moderate | 1.0–1.5 m | Small-craft caution; beach users alert to currents. |
| High | 1.5–2.5 m | Small craft use extreme caution; beach activities discouraged on exposed coasts. |
| Very High | 2.5–3.5 m | Coastal flooding likely on low-lying coasts at high tide; small craft seek harbor. |
| Extreme | above 3.5 m | Breakers near or above 4 m on north-exposed Tobago coasts; major coastal flooding; evacuation of low-lying coastal areas may be advised. |
Hazardous seas – open water (offshore Atlantic, deep Caribbean, outer Gulf). Conditions offshore differ from those at the coast, so open water uses a separate, higher scale – a distinction TTMS does not draw.
| Tier | Wave height | What to expect |
|---|---|---|
| No Risk | under 1.5 m | Calm to slight. |
| Low | 1.5–2.5 m | Moderate seas; small-craft caution. |
| Moderate | 2.5–3.5 m | Rough seas; all marine interests use extreme caution. |
| High | 3.5–5.0 m | Rough to very rough; consider deferring offshore operations. |
| Very High | 5.0–7.0 m | Very rough; all marine traffic seek safe harbor. |
| Extreme | above 7.0 m | Phenomenal seas, consistent with a major hurricane’s proximity; all traffic in harbor. |
Storm surge – water pushed above the normal tide level is activated when a tropical cyclone threatens, sitting with the storm-surge hazard in the Hub.
Heat and air quality
Extreme heat · Saharan dust
Our hottest, most dangerous days are usually not just heat alone. They come when light winds and high humidity arrive together, pushing the feels-like temperature well beyond the actual air temperature.
How TTMS handles heat: Historically, Hot Spell Alerts have been YELLOW only. It has never been escalated to ORANGE or RED. It triggers around “near to or greater than 34.0 °C” ambient (a little lower for Tobago) with a feels-like range of 34–44 °C. That covers our Low and Moderate tiers, with no official pathway above it. We extend the ladder higher to capture higher-end, increasingly common heat events, with regional comparators (Cuba has recorded a 56 °C heat index) showing that the upper range is real.
Extreme heat — either trigger, the heat index (“feels like”) or the actual air temperature, can raise the tier:
| Tier | Heat index | Air temperature | What to expect |
|---|---|---|---|
| No Risk | under 33 °C | under 28 °C | Stay hydrated. |
| Low | 33–38 °C | 28–31 °C | Hydrate; limit strenuous activity at peak hours. |
| Moderate | 38–42 °C | 31–34 °C | Limit prolonged outdoor activity; vulnerable groups take care. |
| High | 42–48 °C | 34–37 °C | Heat illness likely with prolonged exposure; avoid exertion at peak. |
| Very High | 48–52 °C | 37–39 °C | Heat-stroke risk; minimize exposure and seek cooling. |
| Extreme | above 52 °C | above 39 °C | Life-threatening; remain indoors. |
Saharan dust (peaks May–September). TTMS issues no standalone dust alert; we rate it by haze and visibility:
| Tier | Threshold | What to expect |
|---|---|---|
| No Risk | background | Normal concentrations; no impacts. |
| Low | Slight haze | AQI (EMA): Good | Minimal air-quality impact. |
| Moderate | Hazy | AQI (EMA): Moderate | Air quality reduced; sensitive groups (asthmatics, elderly, very young) may be affected. |
| High | Visibility under 10 km | AQI (EMA): Unhealthy for sensitive groups | High concentrations; sensitive groups limit exposure. |
| Very High | Visibility under 5 km | AQI (EMA): Unhealthy | Unhealthy air; general public limit exposure, sensitive groups indoors. |
| Extreme | Visibility under 3 km | AQI (EMA): Very Unhealthy to Hazardous | Major outbreak; very unhealthy to hazardous. Remain indoors with windows closed; air filtration recommended. |
Coastal and geophysical hazards
Landslides · Tsunamis (nearshore and open water)
Trinidad and Tobago carries a medium tsunami hazard classification, with a better-than-one-in-ten chance of a damaging tsunami over 50 years, with confirmed events in 1795, 1825, 1831, 1991, and 1997. The threats are regional: the Kick’em Jenny submarine volcano north of Grenada and the seismic zones along the Lesser Antilles and Venezuela.
How A Tsunami Warning Is Issued in T&T
- Within 8 minutes of a large earthquake, the Pacific Tsunami Warning Center (PTWC) will analyze the seismic event and, if criteria are met, issue a Tsunami Information Statement (no risk) or a Tsunami Threat Message, which is received by the Trinidad and Tobago Meteorological Service (T&T’s Tsunami Warning Focal Point). The UWI Seismic Research Center and information from the United States Geological Survey will also flow to the TTMS
- The Office of Disaster Preparedness and Management is Trinidad and Tobago’s National Tsunami Warning Center and is the organization responsible for issuing applicable alert levels. A Tsunami Bulletin is issued if the PTWC issues an Information Statement (no risk) or there is a Tsunami Threat Message, and T&T is not under any threat. A Tsunami Warning is issued if there is a potential tsunami threat to T&T and/or there is verified information of local tsunami signs being observed (for earthquakes close to T&T and there is a local tsunami threat).
- Once a warning is issued, it will be shared by the ODPM through all its channels and by the TTMS through the CAP. Once confirmed by the ODPM/TTMS, the warning will be visible on the Forecast Hub.
What each tier asks of you – by sector
The same tier carries different practical weight depending on what you do. Here is how to read the ladder for the sectors that rely on it most.
- Agriculture. Moderate and above for rainfall or flooding is the cue to protect low-lying plots, secure inputs, and hold off on activities that saturated soil will ruin. High wind tiers threaten tree crops and structures; extreme heat and dust stress livestock and field workers.
- Water management. The riverine and rainfall tiers map directly onto reservoir and watercourse decisions; the gauge-based riverine reading is designed to feed operational thresholds.
- Health. The heat and dust tiers are public-health tools first. At High and above, respiratory-sensitive people, the elderly, and the very young should limit exposure; at Very High and Extreme are when those groups should stay indoors.
- Energy and infrastructure. Very High and Extreme wind tiers are the planning signal for outage preparation and crew positioning; flooding tiers flag risk to low-lying assets.
- Marine and fishing. Read the swell period alongside the wave height and respect the open-water-versus-nearshore distinction, they can differ sharply on the same day.
In every case, Monitor at Low, Prepare at High, and follow official direction at Very High and Extreme.
Honest limitations, and how to use this well
A system is only as useful as its limits are clear.
- Forecasts carry uncertainty. The probability bands in Section 2 are part of the message, not fine print. High means severe impacts are likely, not certain.
- This complements official alerts; it does not replace them. TTMS is the official source of warnings, and the ODPM is the authority on evacuation and emergency response. When an official alert is in force, it governs.
- Why per-hazard – and why so many colors? A grid of fourteen hazards across six tiers can look, at first glance, like a wall of color. Three things keep it from being one. First, you read your row, not the whole grid: a fisherman watches seas and swell, a farmer watches rain, wind, and heat, someone with asthma watches dust. What looks like a wall is really a set of targeted signals, only a few of which are ever meant for you. Second, this isn’t a new language — it’s the same GREEN-to-RED family TTMS already uses, just finer-grained and split by hazard. And third, the colors aren’t the thing you act on; the headline is. Each zone carries one top-line tier – its worst active hazard – and the per-hazard breakdown sits beneath it for those who need the detail. The deeper point is that correlated hazards are not redundant ones. Rainfall and flooding usually rise together, but not always, and the exceptions are exactly what a single blended color would erase: a short, violent cell can flood streets without a large daily total, and a river can keep cresting for a day after the rain has stopped and the sky has cleared. A low rainfall tier sitting beside a high riverine tier isn’t a contradiction; it’s the most useful thing on the page. We hold the tiers to a standard of coherence rather than sameness: related hazards should tell one consistent physical story, and wherever they diverge, there’s a reason we can name. Finally, the system is built to stay quiet. Most hazards sit at No Risk on most days, and Extreme is reserved for roughly a one-in-ten-year event, so on an ordinary day the grid is mostly green with one or two cells lit — the opposite of an alert system where everything shouts at once.
- The system is versioned and open to revision. These definitions are the first version, calibrated against more than 850 TTMS alerts. As we gather more data, including by verifying our own forecasts against what actually happens, the thresholds may be refined. We will say so when they are.
- Sargassum is handled separately. Sargassum is a slow-onset, multi-week phenomenon with its own five-tier framework (anchored to NOAA’s Sargassum Inundation Risk product, and with no Extreme tier). It has its own outlook: see the TTWC Sargassum Outlook.
If you take one thing from this page, let it be this: a tier is a sentence compressed into a color. High is not just a darker shade of yellow, it is severe impacts are likely, so prepare and stay clear. Read it that way, and the Forecast Hub does what we built it to do.