Tropical Cyclone Classifications

Hurricane Ivan just west of Grenada. north of Trinidad and Tobago, in the Caribbean Sea on September 7, 2004. At the time, Ivan’s tropical cyclone classification at the time was Category Three, with maximum sustained winds of 120 mi/h (195 km/h). At the time, Ivan was located at about 11.9° N, 61.2° W. Photo as taken from a satellite with the GOES-12 1 km visible imagery.

Globally, tropical cyclones are classified in different ways, based on the location, structure, and intensity. In the North Atlantic Basin, tropical cyclones are classified as tropical depressions, tropical storms, hurricanes, and major hurricanes.

Content:
The Saffir-Simpson Hurricane Wind Scale (SSHWS)
Tropical Disturbances
What is an INVEST?
Potential Tropical Cyclones
Tropical Depressions
Tropical Storms
Subtropical Depressions & Storms
Category 1 Hurricane
Category 2 Hurricane
Category 3 Hurricane
Category 4 Hurricane
Category 5 Hurricane
Post-Tropical Cyclones
Remnant Low
Extratropical Cyclones

The Saffir Simpson Hurricane Wind Scale

The Saffir-Simpson Hurricane Wind Scale (SSHWS), formerly the Saffir-Simpson Hurricane Scale (SSHS) classifies tropical cyclones in the Western Hemisphere into five categories based on the intensities of their sustained winds.

The goal of the SSHWS is to make the risk associated with these powerful low-pressure systems straightforward and simple to understand. However, the category or classification of the system can falter when looking at the impacts – rainfall, storm surge, etc., as well as periphery impacts.

Tropical Disturbances

A tropical disturbance east of Trinidad and Tobago on August 15th, 2018 during the 2018 Hurricane Season. This disturbance was designated Invest 99L, which eventually dissipated as it traversed the Southern Windwards. Image: NASA Worldview (Visible Infrared Imaging Radiometer Suite (VIIRS))
A tropical disturbance east of Trinidad and Tobago on August 15th, 2018 during the 2018 Hurricane Season. This disturbance was designated Invest 99L, which eventually dissipated as it traversed the Southern Windwards. Image: NASA Worldview/Visible Infrared Imaging Radiometer Suite (VIIRS)

A tropical disturbance is a discrete tropical weather system with apparently organized convection (generally 100 to 300 miles in diameter) originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more.

A disturbance can be a perturbation within the Intertropical Convergence Zone, a tropical wave, an outflow boundary from an organized thunderstorm system, or any other low-level feature with sufficient spin (vorticity) and convergence to begin tropical cyclogenesis.

Without a low-level to surface focus, even with optimal upper-level conditions and atmospheric instability, no organized convection and surface low-pressure system will be able to form.

When the National Hurricane Center begins to monitor a tropical disturbance, it is sometimes called an INVEST.

What is an INVEST?

INVEST 99L, a tropical disturbance north of Tobago which developed into Tropical Storm Karen three hours after this advisory on the 22nd September 2019. Image: TTWC/NHC
INVEST 99L, a tropical disturbance north of Tobago which developed into Tropical Storm Karen three hours after this advisory on the 22nd September 2019. Image: TTWC/NHC

In Meteorology, an INVEST (short for investigative area, also called area of interest) is a designated area of disturbed weather that is being monitored for potential tropical cyclone formation.

In the Atlantic Basin, these INVESTs are designated by the United States National Hurricane Center, numbered from 90 to 99 followed by the suffix L. These suffixes change for each basin, with “E” and “C” in the Eastern and Central Pacific basins (respectively), or “W” in the Western Pacific basin. Numbers are rotated within the season and are re-used as necessary (the next invest after 99 would be numbered 90).

According to the National Hurricane Center, by designating a tropical weather system as an “Invest,” the collection of specialized data sets and computer model guidance on the area of interest can begin. This collection and processing of data is shown on several government and academic websites for analyzing.

The designation of a system as an invest does not necessarily correspond to any particular likelihood of development of the system into a tropical cyclone.

Potential Tropical Cyclones

The first Potential Tropical Cyclone ever, PTC-2, southeast of Trinidad and Tobago on June 18th, 2017. This system formed into Tropical Storm Bret on Monday 19th June 2017, making landfall across Southeastern Trinidad early June 20th, 2017. Image: National Hurricane Center.
The first Potential Tropical Cyclone ever, PTC-2, southeast of Trinidad and Tobago on June 18th, 2017. This system formed into Tropical Storm Bret on Monday 19th June 2017, making landfall across Southeastern Trinidad early June 20th, 2017. Image: National Hurricane Center.

The National Hurricane Center began using this term in 2017, referring to a disturbance that has not yet become a tropical cyclone, but which poses the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours.

This allows the National Hurricane Center, in conjunction with local governments and meteorological services to issue advisories, watches, and warnings in anticipation of tropical cyclone impacts.

It will be classified as a potential tropical cyclone (PTC) with a two-digit PTC number (for example, PTC-09 or PTC-15E) that otherwise looks identical to a Tropical Cyclone (TC) number.

For example, the first instance of a potential tropical cyclone was Potential Tropical Cyclone Two of 2017, which eventually formed into Tropical Storm Bret – making landfall across Southeastern Trinidad in late June.

Tropical Depressions

Tropical Depression Seven degenerated into a Tropical Wave on August 11th, 2012, shortly before moving across the Lesser Antilles. This tropical system eventually developed into Tropical Storm Helene. Image: NASA Worldview/MODIS
Tropical Depression Seven degenerated into a Tropical Wave on August 11th, 2012, shortly before moving across the Lesser Antilles. This tropical system eventually developed into Tropical Storm Helene. Image: NASA Worldview/MODIS

A tropical depression is a low-pressure system with a closed surface circulation that has maximum sustained surface wind speed (1-minute mean) of 62 KM/H or less. In addition to a closed, and well-defined circulation, meeting the wind threshold, a tropical depression needs organized, persisting, deep convection near the center.

This last criterion is usually determined using the Dvorak technique, to estimate intensity, with a tropical depression usually classified when estimates reach T2.0. It is a widely used system to estimate tropical cyclone intensity (which includes tropical depression, tropical storm, and hurricane intensities) based solely on visible and infrared satellite images, as seen below.

This chart shows common developmental patterns for tropical cyclones, and their intensities per the Dvorak technique (NOAA/Vernon F. Dvorak)
This chart shows common developmental patterns for tropical cyclones, and their intensities per the Dvorak technique (NOAA/Vernon F. Dvorak)

However, these weaker tropical cyclones can still pack a punch. In Trinidad and Tobago, though gusty winds pose hazards for our trees, poorly constructed roofing and overhead lines – our main hazards with these systems tend to be locally heavy rainfall.

Tropical Depression Seven, which degenerated into a tropical wave in 2012, triggered severe weather causing flooding and landslides throughout the country, particularly along the northwestern peninsula. The damaging effects of the severe weather considered a 1 in 50-year event were concentrated in the Diego Martin Corporation. Two deaths and heavy infrastructure damage occurred.

Tropical Cyclone History: Tropical Depressions within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.
Tropical Cyclone History: Tropical Depressions within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.

To date, eight tropical depressions made landfall or passed within 20 nautical miles of Trinidad and Tobago’s coasts (what we consider to be near misses).

Tropical Storms

Tropical Storm Don on its approach to the Windward Islands. This tropical storm degenerated northeast of Tobago. However, the remnants of this system produced severe weather and flash flooding across parts of Northern Trinidad on July 18th, 2017. Image: NASA Worldview/VIIRS
Tropical Storm Don on its approach to the Windward Islands. This tropical storm degenerated northeast of Tobago. However, the remnants of this system produced severe weather and flash flooding across parts of Northern Trinidad on July 18th, 2017. Image: NASA Worldview/VIIRS

A tropical storm is another low-pressure system with a closed surface circulation that has a maximum sustained surface wind speed (1-minute mean) ranging from 63 KM/H to 118 KM/H.

Similar to tropical depressions, tropical storms across Trinidad and Tobago’s main hazards tend to be locally heavy rainfall, even with the sustained and gusty winds posing a threat to power, roofs, and trees.

Tropical Cyclone History: Tropical Storms within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.
Tropical Cyclone History: Tropical Storms within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.

To date, 18 tropical storms made landfall or passed within 20 nautical miles of Trinidad and Tobago’s coasts (what we consider to be near misses).

Subtropical Depressions & Subtropical Storms

Subtropical Storm Andrea in the Western Atlantic on May 20, 2019, starting off the 2019 Atlantic Hurricane Season early for the 5th year in a row (at that date). Image: NASA Worldview/VIIRS
Subtropical Storm Andrea in the Western Atlantic on May 20, 2019, starting off the 2019 Atlantic Hurricane Season early for the 5th year in a row (at that date). Image: NASA Worldview/VIIRS

A subtropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Subtropical cyclones originate over tropical or subtropical waters and have a closed circulation about a well-defined center. Across the North Atlantic, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere.

In comparison to tropical cyclones, the maximum winds occur relatively far from the center (greater than 60 nautical miles) and have a less symmetric wind field and distribution of convection.

Under certain conditions, subtropical cyclones can evolve into tropical cyclones. For the low-pressure system.

Those with sustained winds below 62 KM/H (33 knots or 38 MPH) are called subtropical depressions, while those at or above this speed are referred to as subtropical storms.

Subtropical cyclones with hurricane-force winds of 119 KM/H (64 knots, or 74 MPH) or greater are not officially recognized by the National Hurricane Center. Once a subtropical storm intensifies enough to have hurricane-force winds, it is then automatically assumed to have become a fully tropical hurricane.

Generally, these systems form well north of our area. No subtropical cyclones have directly affected Trinidad and Tobago, or are considered near misses based on our definition (within 20-nautical miles) in recorded history.

Category 1 Hurricane

Category 1 Hurricane Joyce on September 29th, 2000 east of Trinidad and Tobago (far left). Joyce weakened into a tropical storm as it made landfall across Tobago shortly before 8:00 AM on October 1st, 2000. No serious damage was reported across T&T. Image: NASA Worldview/VIIRS
Category 1 Hurricane Joyce on September 29th, 2000 east of Trinidad and Tobago (far left). Joyce weakened into a tropical storm as it made landfall across Tobago shortly before 8:00 AM on October 1st, 2000. No serious damage was reported across T&T. Image: NASA Worldview/VIIRS

Category 1: Very dangerous winds will produce some damage.

Category 1 storms usually cause no significant structural damage to most well-constructed permanent structures; however, they can topple unanchored mobile homes, as well as uproot or snap weak trees. Poorly attached roofs, galvanize or tiles can blow off.

Coastal flooding and pier damage are often associated with Category 1 storms. Power outages are typically widespread to extensive, sometimes lasting several days. Even though it is the least intense type of hurricane, they can still produce widespread damage and can be life-threatening storms.

Tropical Cyclone History: Category 1 hurricanes within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.
Tropical Cyclone History: Category 1 hurricanes within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.

To date, 6 Category 1 hurricanes made landfall or passed within 20 nautical miles of Trinidad and Tobago’s coasts (what we consider to be near misses).

Category 2 Hurricane

Damage in Barbados following a direct hit from Category 2 Hurricane Janet. A tropical storm warning was issued for Tobago when this cyclone traversed the region in 1955 and a storm shelter collapsed in Port of Spain, killing ten. Image courtesy of The National Archives.
Damage in Barbados following a direct hit from Category 2 Hurricane Janet. A tropical storm warning was issued for Tobago when this cyclone traversed the region in 1955 and a storm shelter collapsed in Port of Spain, killing ten. Image courtesy of The National Archives.

Category 2: Extremely dangerous winds will cause extensive damage

Storms of Category 2 intensity often damage roofing material (sometimes exposing the roof) and inflict damage upon poorly constructed doors and windows. Poorly constructed signs and piers can receive considerable damage and many trees are uprooted or snapped.

Poorly constructed homes, whether anchored or not, are typically damaged and sometimes destroyed, and many manufactured homes also suffer structural damage.

Small craft in unprotected anchorages may break their moorings. Extensive to near-total power outages and scattered loss of potable water are likely, possibly lasting many days.

No Category 2 hurricanes have directly affected Trinidad and Tobago, or are considered near misses based on our definition (within 20-nautical miles) in recorded history.

Major Category 3 Hurricane

Hurricane Ivan just west of Grenada in the Caribbean Sea on September 7, 2004 at 19:45 UTC (15:45 AST). At the time, Ivan had maximum sustained winds of 120 mi/h (195 km/h), placing it at Category 3 on the Saffir-Simpson Hurricane Scale. At the time, Ivan was located at about 11.9° N, 61.2° W. Photo as taken from a satellite with the GOES-12 1 km visible imagery.
Hurricane Ivan just west of Grenada. north of Trinidad and Tobago, in the Caribbean Sea on September 7, 2004, at 19:45 UTC (15:45 AST). At the time, Ivan had maximum sustained winds of 120 mi/h (195 km/h), placing it at Category 3 on the Saffir-Simpson Hurricane Scale. At the time, Ivan was located at about 11.9° N, 61.2° W. Photo as taken from a satellite with the GOES-12 1 km visible imagery.

Category 3: Devastating damage will occur

Tropical cyclones of Category 3 and higher are described as major hurricanes in the Atlantic or Eastern Pacific basins.

These storms can cause some structural damage to small residences and utility buildings, particularly those of wood frame or manufactured materials with minor curtain wall failures.

Buildings that lack a solid foundation are usually destroyed, and gable-end roofs are peeled off. Manufactured homes usually sustain severe and irreparable damage.

Flooding near the coast destroys smaller structures, while larger structures are struck by floating debris. A large number of trees are uprooted or snapped, isolating many areas. Additionally, terrain may be flooded well inland.

Near-total to total power loss is likely for up to several weeks and water will likely also be lost or contaminated.

Tropical Cyclone History: Category 3 hurricanes within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.
Tropical Cyclone History: Category 3 hurricanes within 20 nautical miles of Trinidad and Tobago since 1842. Data: NOAA National Hurricane Center HURDAT2 and NOAA National Centers for Environmental Information IBTrACS data sets.

To date, two Category 3 hurricanes made landfall or passed within 20 nautical miles of Trinidad and Tobago’s coasts (what we consider to be near misses).

Major Category 4 Hurricane

 Hurricane Allen approaching the Lesser Antilles on August 3, 1980, with Trinidad just visible center left, being covered by an upper level cirrus band. This system moved across Barbados as a Category Three, and between St. Lucia and St. Vincent as a Category 4 hurricane. Image: NASA
Hurricane Allen approaching the Lesser Antilles on August 3, 1980, with Trinidad just visible center left, being covered by an upper level cirrus band. This system moved across Barbados as a Category Three, and between St. Lucia and St. Vincent as a Category 4 hurricane. Image: NASA

Category 4: Catastrophic damage will occur

Category 4 hurricanes tend to produce more extensive curtainwall failures, with some complete structural failure on small residences. Heavy, irreparable damage and near complete destruction of gas station canopies and other wide span overhang type structures are common.

Poorly constructed and manufactured homes are often flattened. Most trees, except for the heartiest, are uprooted or snapped, isolating many areas.

These storms cause extensive beach erosion, while terrain may be flooded far inland. Total and long-lived electrical and water losses are to be expected, possibly for many weeks.

No Category 4 hurricanes have directly affected Trinidad and Tobago, or are considered near misses based on our definition (within 20-nautical miles) in recorded history.

Major Category 5 Hurricane

Category 5 Hurricane Maria devastated the island of Dominica on September 18th-19th 2017.
Category 5 Hurricane Maria devastated the island of Dominica on September 18th-19th 2017.

Category 5: Catastrophic damage will occur

Category 5 is the highest category of the Saffir–Simpson scale. These storms cause complete roof failure on many residences and industrial buildings and some complete building failures with small utility buildings blown over or away. The collapse of many wide-span roofs and walls, especially those with no interior supports, is common.

Very heavy and irreparable damage to many wood frame structures and total destruction to mobile/manufactured homes is prevalent. Only a few types of structures are capable of surviving intact, and only if located at least 3 to 5 miles (5 to 8 km) inland. They include office, condominium, and apartment buildings and hotels that are of solid concrete or steel frame construction, multi-story concrete parking garages, and residences that are made of either reinforced brick or concrete/cement block and have hipped roofs with slopes of no less than 35 degrees from horizontal and no overhangs of any kind, and if the windows are either made of hurricane-resistant safety glass or covered with shutters. Unless all of these requirements are met, the absolute destruction of a structure is certain.

Tree damage in Dominica following the passage of Category 5 Hurricane Maria across the island. Agriculture represents Dominica’s second industry (Tomás Ayuso/IRIN)
Tree damage in Dominica following the passage of Category 5 Hurricane Maria across the island. Agriculture represents Dominica’s second industry (Tomás Ayuso/IRIN)

The storm’s flooding causes major damage to the lower floors of all structures near the shoreline, and many coastal structures can be completely flattened or washed away by the storm surge. Virtually all trees are uprooted or snapped, and some may be debarked, isolating most affected communities. Massive evacuation of residential areas may be required if the hurricane threatens populated areas. Total and extremely long-lived power outages and water losses are to be expected, possibly for up to several months.

No Category 5 hurricanes have directly affected Trinidad and Tobago, or are considered near misses based on our definition (within 20-nautical miles) in recorded history.

Post Tropical Cyclones

A post-tropical cyclone is simply a former tropical cyclone. This generic term describes a cyclone that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds.

When a tropical cyclone reaches the end of its life cycle, depending on its location, one of three things may occur. If the tropical cyclone dissipates in the tropics, it may degenerate into a tropical wave. If a tropical cyclone degenerates – it can become a remnant low (more common overland) or an extratropical cyclone (in the northern latitudes).

Remnant Low

The remnant low of previously Hurricane Irma across the East-Central United States. Image: NASA
The remnant low of previously Hurricane Irma across the East-Central United States on September 13th, 2017. Image: NASA

This is a post-tropical cyclone that no longer possesses the convective organization required of a tropical cyclone and has maximum sustained winds of less than 34 knots. These shallow systems may meander for sometime before opening into a trough of low pressure, or being absorbed into an extratropical cyclone.

Extratropical Cyclone

Hurricane Dorian undergoing extratropical transition in the North Atlantic on September 7th, 2019. Image: RAMMB/CIRA
Hurricane Dorian undergoing extratropical transition in the North Atlantic on September 7th, 2019. Image: RAMMB/CIRA

An extratropical cyclone is a term used in advisories and tropical summaries to indicate that a cyclone has lost its “tropical” characteristics. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.

Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude

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