Thunderstorms

Thunderstorms are no stranger to Trinidad and Tobago, occurring year-round. These storms produce some of our highest localized rainfall with accompanying lightning and gusty winds, particularly during the Wet Season, during the latter half of the year.

Content
The Life Cycle of a Thunderstorm

Stage 1: Developing
—— Triggering Mechanisms
—— Sea Breeze Convergence
—— Orographic Precipitation

Stage 2: Mature
Stage 3: Dissipation
Thunderstorm Classifications
Single-cell Thunderstorms
Multi-cell Thunderstorms
—— Squall Lines
—— Bow Echos

Supercell Thunderstorms
Thunderstorm Frequency in T&T
Thunderstorm Forecasting
Thunderstorm Detection
Hazards
Frequently Asked Questions

The Life Cycle of a Thunderstorm

Stage 1: Developing

Thunderstorms are developed through convection (warm air rising, cool air sinking) and this is how that process works.

When conditions are favorable and there is a triggering mechanism (more on this later) for warm, moist air to rise, this starts the process of developing a cumulus cloud.

Cumulus clouds, the beginnings of possible thunderstorms across Trinidad. Photo: Trinidad and Tobago Meteorological Service
Cumulus clouds, the beginnings of possible thunderstorms across Trinidad. Photo: Trinidad and Tobago Meteorological Service

As the moisture cools at a higher altitude, the water vapor condenses into liquid, and heat is released. This is where the cumulus cloud is visible. However, in favorable conditions at the low, mid and upper levels of the atmosphere, the heat then warms the air, causing it to become less dense than the surrounding drier air and continues to rise in what will now be called an updraft. This is the beginning of the convective process.

As the air continues to rise, a low pressure aloft (above the ground, in the atmosphere) develops beneath the cumulus cloud or the forming thunderstorm.

In a typical thunderstorm, approximately 500 million kilograms of water vapor are lifted into the Earth’s atmosphere.

Triggering Mechanisms

One of the simplest triggering mechanisms is the sun heats the surface of land, and a runway convective process causes a warm updraft and a cool downdraft to occur. This is more typical of showers across Trinidad and Tobago. In thunderstorms, we usually have a bit more going on at the surface and aloft to trigger and sustain these heat engines.

Sea Breeze Convergence

This is the most common trigger during the wet season, as well as days where a light wind regime is in place across Trinidad.

Sea breeze convergence mechanisms for the Gulf of Paria and the Atlantic Ocean across Trinidad which can produce thunderstorm activity.. Note that sea breeze convergence is possible across all coasts of both islands. Diagram: Trinidad and Tobago Weather Center
Sea breeze convergence mechanisms for the Gulf of Paria and the Atlantic Ocean across Trinidad which can produce thunderstorm activity.. Note that sea breeze convergence is possible across all coasts of both islands. Diagram: Trinidad and Tobago Weather Center

The entire process begins as soon as the sun rises.  Very slowly, the sun begins to heat up the land surface.  The existing air over the land surface warms, and the air rises.  As the air over coastal areas rises, cooler air from over the surrounding water bodies moves in to replace the rising pocket of air near the surface.

This generally occurs across Western Trinidad with winds from the Gulf of Paria year-round when moisture is present, Southern Trinidad with winds from the Columbus Channel during the wet season and across Eastern Trinidad with winds from the Atlantic Ocean when the overall wind regime is light.

For the Gulf of Paria and Western Trinidad, during the afternoon hours, the sea breeze from the Gulf of Paria, and prevailing easterly winds or meet aloft, right over parts of Western Trinidad. When two air masses from opposite directions collide, it forces air upwards. This is called a convergence zone. 

The converging air masses contain water vapor collected from the Atlantic and the Gulf.  As the air rises, it cools, and water vapor condenses into water droplets, forming clouds, and eventually showers and thunderstorms.

Orographic Precipitation

Orographic precipitation occurs when warm, moist air is forced upward by hilly or mountainous topography. This can trigger shower development, or intensify shower and/or thunderstorms. Diagram: Trinidad and Tobago Weather Center
Orographic precipitation occurs when warm, moist air is forced upward by hilly or mountainous topography. This can trigger shower development, or intensify shower and/or thunderstorms. Diagram: Trinidad and Tobago Weather Center

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down, which can create showers and thunderstorms, or in some cases, enhance already existing convection.

In Trinidad, this tends to occur when winds are from the south. Winds from the surface, heated as it moves across the terrain of Trinidad, are forced upwards by the Southern, Central and predominantly Northern Range. In Tobago, this occurs when moisture moves off the Atlantic Ocean and forced upwards by the Main Ridge of the island.

This can also cause the mixing of stronger winds at height down towards the surface and aid in the development of higher rainfall-producing showers and thunderstorms.

Stage 2: Mature

A mature thunderstorm across Southern Trinidad. Photo: Trinidad and Tobago Weather Center
A mature thunderstorm across Southern Trinidad. Photo: Trinidad and Tobago Weather Center

Thunderstorms at this point can produce heavy rain, possible hail, strong winds, severe lightning, and even tornadoes.

At this stage, the warmed air continues to rise until it reaches an area of warmer air and can rise no farther. This is considered a “cap” and usually occurs in the troposphere.

The air (and cloud mass) is then forced to spread out, giving the storm a characteristic anvil shape which we see in stronger thunderstorms. The resulting cloud is called cumulonimbus incus.

In typical convective showers and normal thunderstorms, moisture (water droplets) freeze as they rise, become ice particles and coalesce, which makes, the now-suspended chunks of ice, to fall. As these ice particles fall, they melt and become rain. However, when the updrafts (rising air) are particularly strong, the ice particles become so large that they do not melt completely when falling and fall as hail.

While updrafts are still present, the falling rain (or hail) drags the surrounding air with it, creating downdrafts as well. The simultaneous presence of both an updraft and a downdraft marks the mature stage of the storm and produces cumulonimbus clouds.

A mature thunderstorm moving into the Gulf of Paria during the 2019 Wet Season in Trinidad. Image: Karen Johnstone/Trinidad and Tobago Weather Center.
A mature thunderstorm moving into the Gulf of Paria during the 2019 Wet Season in Trinidad. Image: Karen Johnstone/Trinidad and Tobago Weather Center.

Stage 3: Dissipation

At this stage, the downdraft dominates the thunderstorm, weakening it as time progresses. In Trinidad and Tobago, particularly during the rainy season and in the afternoon, if there is little wind shear, the storm will rapidly enter the dissipating stage and ‘rain itself out’, in 20-30 minutes.

When a thunderstorm reaches its dissipation stage, the downdraft will push down out of the thunderstorm, hit the ground and spread out. This phenomenon is known as a downburst. The cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate.

Diagram of convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively "raining itself out." Diagram: Trinidad and Tobago Weather Center.
Diagram of convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively “raining itself out.” Diagram: Trinidad and Tobago Weather Center.

Thunderstorms in an atmosphere with virtually no vertical wind shear (where the wind changes speed or direction with height) weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air, and kills the thunderstorm’s further growth.

However, if there is sufficient change in wind speed or direction, the downdraft will be separated from the updraft, and the storm may become a supercell, where the mature stage can sustain itself for several hours.

Thunderstorm Classifications

In Trinidad and Tobago, we experience three types of thunderstorms (in order of most common to rarest): single-cell, multi-cell (squall lines such as those in feeder bands from tropical cyclones) and supercell thunderstorms.

Single-cell Thunderstorms

A weakening thunderstorm moving into the Gulf of Paria as a weak tropical wave traversed the region on September 9th, 2019. Photo: Karen Johnstone.
A weakening thunderstorm moving into the Gulf of Paria as a weak tropical wave traversed the region on September 9th, 2019. Photo: Karen Johnstone.

These are our most common thunderstorms in Trinidad and Tobago. They typically occur during the wet season and can be called “popcorn” convection.

They are small, brief and mostly weak storms that grow and die within 60-90 minutes. These types of storms are typically driven by heating on a hot afternoon, and tend to favor Northern and Western halves of Trinidad.

These thunderstorms typically produce locally heavy rainfall that triggers street or flash flooding, gusty winds, frequent thunder and lightning, and reduced visibility.

Multi-cell Thunderstorms

A cluster of thunderstorms affecting the southern half of Trinidad the night of September 13th into 14th 2018. Photo: Anthony Martin Ganpat
A cluster of thunderstorms affecting the southern half of Trinidad the night of September 13th into 14th 2018. Photo: Anthony Martin Ganpat

This is another common thunderstorm in which new updrafts form along the leading edge of rain-cooled air (the gust front). Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours.

Multicell storms may produce heavy rainfall that triggers street or flash flooding, gusty winds, frequent thunder and lightning, and reduced visibility. These cells can also support hail and brief tornadoes.

Squall Lines

These are a type of multi-cell thunderstorms, or can also be a multi-cell line of showers that are often accompanied by “squalls” or brief periods of high wind and heavy rain. These types of showers and thunderstorms tend to pass quickly and less prone to producing tornadoes – but not impossible.

We frequently see this type of activity in and around tropical storms and hurricanes.

Bow Echos

The "bow echo" of August 15th, 2019, which produced significant wind damage across parts of Southwestern Trinidad.
The “bow echo” of August 15th, 2019, which produced significant wind damage across parts of southwestern Trinidad.

A large thunderstorm can form into a Mesoscale Convective System, known as a bow echo, because of its characteristic bow shape on radar displays.

On radar, a bow-echo looks like a comma, with a round head on one end and a tail on the other. Because in Trinidad, our weather moves from the east to west (generally), this signature looks like a vertically and horizontally flipped comma.

The leading edge has a sharp reflectivity gradient (oranges, reds and black on the above radar image), and there are notches (of dry air) dug into the weak reflectivity gradient on the trailing edge.

A bow echo is a radar signature of a squall line or dissipating thunderstorm that “bows out” as winds fall behind the line of showers/thunderstorms or squall line. A strongly bowed echo may indicate high winds particularly in the middle of the line, where storms are moving forward the fastest.

Sometimes, the periphery of the bow echo, to the north and south, becomes dominant over time, gradually evolving into a comma-shaped storm complex.

Supercell Thunderstorms

A supercell thunderstorm across west-central Trinidad on the 30th March, 2018 which produced a funnel cloud across the Waterloo area. Image: Trinidad and Tobago Meteorological Service Radar
A supercell thunderstorm across west-central Trinidad on the 30th March 2018 which produced a funnel cloud across the Waterloo area. Image: Trinidad and Tobago Meteorological Service Radar

Supercell thunderstorms are long-lived, highly organized and powerful systems lasting longer than 1 hour with a tilted and rotating updraft. These types of thunderstorms are uncommon in Trinidad and Tobago, generally smaller than supercells that frequent North America.

The rotating updraft can be as large as 10 miles in diameter and up to 50,000 feet (15 kilometers) tall. This type of activity can be present between 20 to 60 minutes before a tornado forms. A tornado is essentially an extension of this larger rotation. Most large and violent tornadoes originate from supercells.

There are other types of convective systems such as a mesoscale convective complex, a mesoscale convective vortex, and a derecho. These systems typically occur in the mid-latitudes such as within the United States and Europe but with few records in Trinidad and Tobago.

Thunderstorm Frequency in T&T

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. At any given time approximately 2,000 thunderstorms are occurring on Earth. Any area in Trinidad can experience, on average, 30 to 40 thunderstorms annually. On rare occasions, as many as two to three a day in a given area.

During the wet season, thunderstorms generally occur during the late morning through the afternoon across the western half of Trinidad predominantly.

When particular weather features traverse the region – tropical waves, tropical cyclones, surface troughs, mid- and upper-level troughs, and the Inter-Tropical Convergence Zone, thunderstorms can be abundant.

Isolated thunderstorm affecting parts of West-Central Trinidad, taken from Mt. St. Benedict. Photo: Jason Kanhai.
Isolated thunderstorm affecting parts of West-Central Trinidad, taken from Mt. St. Benedict. Photo: Jason Kanhai.

Thunderstorms bring most of our rainfall and nearly all of our street/flash flooding events. Torrential downpours, such as the one today, can produce upwards of 20-30 millimeters of rainfall within an hour.

In addition, these thunderstorms can produce severe winds up to and in excess of 65 KM/H across localized areas of the country.

Stationary thunderstorms, such as those that caused the Divali 2017 floods, can produce upwards of 100 millimeters in one to two hours.

Thunderstorms can last from 30 minutes to as long as two hours, depending on the speed of low to mid-level winds. More prolonged thunderstorms begin to trigger different types of flooding – street, then flash and lastly riverine flooding.

During the wet season and the Atlantic Hurricane Season, tropical waves gradually become stronger and the ITCZ may linger across the region for possible prolonged periods. Showers resulting from daytime heating and sea breeze convergence will become more common.

Thunderstorm Forecasting

Forecasters rely on data and numerical weather prediction models that are run by supercomputers globally to determine whether conditions will be right to thunderstorm development.

Models are designed to calculate what the atmosphere may do at certain points over different spatial scales from the surface of the earth to the top of the atmosphere.

One integral data collection points are weather balloons (atmospheric soundings) launched around the globe two times daily. In Trinidad and Tobago, these soundings are done at 7:00 AM and 7:00 PM by the Trinidad and Tobago Meteorological Service at Piarco. Special soundings can also occur due to requests by the National Hurricane Center ahead of or during a tropical cyclone or in the United States ahead or during a severe weather event.

Special Rawinsonde Sounding (Weather Balloon Release) for Tropical Storm Kirk for NOAA National Weather Service at our…

Posted by Trinidad and Tobago Meteorological Service on Thursday, September 27, 2018

This is in addition to data gathered from satellites, aircraft, ships, temperature profilers, and surface weather stations.

The models start with these current weather observations and attempt to predict future weather using physics and dynamics to mathematically describe the atmosphere’s behavior. The predictions are usually output in text and graphics.

Thunderstorm Detection

Satellite

Weather satellites are some forecaster’s best friends. Most areas of Earth are covered by a number of geostationary satellites, taking images of Earth from thousands of kilometers away at regular intervals.

Due to advancements in technology, in Trinidad and Tobago, we can get new satellite images as frequently as every 5 minutes from the GOES-East (GOES-16) and GOES-West (GOES-17) satellites. More incredibly, we can also get up to the minute lightning data from the Global Lightning Mapper instrument onboard these satellites. In special circumstances, we can also get up to the minute satellite imagery for our region, usually when a tropical cyclone is nearby.

Lightning data from the Global Lightning Mapper (GLM) overlaid on the GOES-EAST (GOES-16) Infrared Satellite Image as thunderstorms affect T&T on October 28th 2019.

Generally, satellites take pictures of Earth at regular intervals from space, telling us where clouds are located. Meteorologists and forecasters watch these pictures over time to watch for rapidly growing clouds, a clue to a possible thunderstorm. Satellites also can tell us the temperature of the clouds. Clouds with cold tops are usually very high up in the atmosphere could mean the cloud is tall enough to be a thunderstorm. Lightning data from newer satellites support the “thunderstorm” designation.

Radar & Lightning Detectors

Automated Lightning Detector detecting lightning strikes from a thunderstorm on October 21st, 2019.
Automated Lightning Detector detecting lightning strikes from a thunderstorm on October 21st, 2019.

Doppler radar sends out electromagnetic wave fields that can be reflected back to the radar by things in the air like precipitation. The amount of energy that is reflected back can tell us how heavy the rain might be or tell us there is hail.

Advanced Doppler radars can also show us how the wind is blowing near and inside the storm. This is helpful in understanding what kinds of hazards the thunderstorm might have (tornado, microburst, gust fronts, etc.) associated with it. It also helps us understand how the thunderstorm is feeding itself.

Automated weather stations with lightning detectors or stand-alone lightning detectors can also aid in the detection of thunderstorms.

Hazards

Thunderstorms bring a number of hazards to Trinidad and Tobago. The three most common are lightning, gusty winds and street/flash flooding resulting from heavy rainfall.

Less commonly, we experience damaging downbursts associated with dissipating thunderstorms. Even less common – tornadoes, waterspouts, and funnel clouds. The rarest hazard for Trinidad and Tobago has been hail which is typically associated with the strongest of thunderstorms in the tropics.

Frequently Asked Questions

It’s the dry season, why are we having thunderstorms?

Once conditions are optimal (moisture, unstable air mass, and heat or a lifting mechanism, see Stages of a Thunderstorm above), a thunderstorm can form. Since these conditions, particularly ample atmospheric moisture, are predominantly present during the Wet Season, thunderstorm activity becomes more common in May through December.

What is a severe thunderstorm?

A thunderstorm is classified as “severe” when it contains one or more of the following: hail one inch (2.54 cm) or greater, winds gusting in excess of 50 knots (92.6 mph), or a tornado, based on the United States definition.

How many thunderstorms are there?

Worldwide, there are an estimated 16 million thunderstorms each year, and at any given moment, there are roughly 2,000 thunderstorms in progress. Any area in Trinidad can experience, on average, 30 to 40 thunderstorms annually. On rare occasions, as many as two to three a day in a given area.

Where and when are thunderstorms most likely?

Thunderstorms are most likely in the Wet Season across Trinidad and Tobago, mainly during the afternoon and evening hours, but they can occur year-round and at all hours.

Thunderstorms tend to favor the Western halves of Trinidad and Tobago during the late mornings through the late afternoons due to the abundant heat present, in addition to sea breeze convergence (as the lifting mechanisms).

A thunderstorm is ongoing, producing gusty winds, street/flash flooding and frequent lightning. Why aren’t we under an alert/watch/warning?

Advisories (in this case, an Adverse Weather Alert/Watch/Warning) would originate from the Trinidad and Tobago Meteorological Service. These advisories are issued based on the risk to public safety, livelihoods, and property for a given geographical area.

However, these advisories are only issued when the threats are widespread, instead of the typical isolated activity we experience during the Wet Season. If an advisory was issued for each thunderstorm, not only is this difficult to do with any appreciable lead time, but we would potentially be under an alert throughout the entire Wet Season, which reduces the likelihood of people heeding the warnings.

In the same way not every tropical wave brings inclement weather or requires a name to bring impactful weather to T&T, not every thunderstorm requires an alert for us to be prepared.

You can read more about The Early Warning System here.

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