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Forecast: Locally Heavy Rainfall Across T&T This Week

July 22, 2024
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Variably cloudy skies with isolated to scattered showers and thunderstorms are forecast daily this week as a tropical wave and a low-level trough interact with the Intertropical Convergence Zone (ITCZ).

What you need to know

— Rainfall: Over the next five days, through Saturday night, overall rainfall accumulations across the country are forecast to range between 25 and 75 millimeters, with five-day totals between 75 and 150 millimeters favoring southern and eastern halves of Trinidad. In highly isolated areas, daily rainfall totals exceeding 25 to 50 millimeters are likely, mainly Tuesday through Thursday.
— Saharan Dust: Through Wednesday afternoon, mild to moderate dust levels are forecast, with moderate to high dust levels through the remainder of the forecast period.
— Hazards: Daily through Friday, locally heavy rainfall is likely in heavy showers/thunderstorms, producing localized street/flash flooding and accompanied by gusty winds. From Wednesday, gusty winds are likely to exceed 55 KM/H. Frequent lightning is possible in thunderstorm activity. As the week progresses, with increasing soil saturation, the risk of landslides is likely to increase and particularly in southern and eastern areas, river levels will need to be closely monitored.
— Marine: Generally moderate seas are forecast over the next five days.

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The Forecast

Monday Night

Monday Night
5 10 0 1
A variably cloudy night with isolated showers and thunderstorms, mainly near and after midnight, favoring eastern and southern areas.
A variably cloudy night with isolated showers and thunderstorms, mainly near and after midnight, favoring eastern and southern areas.
5/10
Thunderstorm Chances
Medium

Tuesday

Tuesday
6 10 0 1
Early morning showers and thunderstorms, favoring Trinidad initially, forecast to spread across the country through mid-morning, with variably cloudy skies. Mostly cloudy day, with isolated showers and thunderstorms developing by the late morning, favoring western coastal areas. An initially settled evening, with a return of rain, showers and thunderstorms nearing midnight across both islands.
Early morning showers and thunderstorms, favoring Trinidad initially, forecast to spread across the country through mid-morning, with variably cloudy skies. Mostly cloudy day, with isolated showers and thunderstorms developing by the late morning, favoring western coastal areas. An initially settled evening, with a return of rain, showers and thunderstorms nearing midnight across both islands.
6/10
Thunderstorm Chances
Medium – High

Wednesday

Wednesday
7 10 0 1
Scattered showers and thunderstorms from the early morning through dawn across both islands, with heavier showers/thunderstorms favoring Trinidad. By the mid-morning, partly to mostly cloudy skies are forecast. A return of isolated to scattered showers and thunderstorms is forecast mainly after midday favoring Trinidad. Increasingly settled and hazy conditions by the late afternoon and continue into the night. A few showers are possible after nightfall favoring eastern and southern areas.
Scattered showers and thunderstorms from the early morning through dawn across both islands, with heavier showers/thunderstorms favoring Trinidad. By the mid-morning, partly to mostly cloudy skies are forecast. A return of isolated to scattered showers and thunderstorms is forecast mainly after midday favoring Trinidad. Increasingly settled and hazy conditions by the late afternoon and continue into the night. A few showers are possible after nightfall favoring eastern and southern areas.
7/10
Thunderstorm Chances
High

Thursday

Thursday
7 10 0 1
Early morning showers and isolated thunderstorms to favor Tobago initially, then spreading south and west through dawn. A partly to mostly cloudy day, with showers and isolated thunderstorms initially favoring eastern and southern halves of Trinidad during the late morning, spreading west and north through the afternoon. Isolated strong thunderstorms/heavy showers are likely along western coastal Trinidad, as well as northern areas. Conditions to become gradually settled by the late afternoon. An initially settled night, with showers and isolated thunderstorms returning nearing midnight.
Early morning showers and isolated thunderstorms to favor Tobago initially, then spreading south and west through dawn. A partly to mostly cloudy day, with showers and isolated thunderstorms initially favoring eastern and southern halves of Trinidad during the late morning, spreading west and north through the afternoon. Isolated strong thunderstorms/heavy showers are likely along western coastal Trinidad, as well as northern areas. Conditions to become gradually settled by the late afternoon. An initially settled night, with showers and isolated thunderstorms returning nearing midnight.
7/10
Thunderstorm Chances
High

Friday

Friday
4 10 0 1
Early morning showers and isolated thunderstorms are forecast across southern and eastern areas of Trinidad, leading to a partly cloudy day. Isolated showers and thunderstorms are possible by the late morning through the afternoon, favoring western Trinidad. A mostly settled, hazy and breezy late afternoon, evening and night, barring the odd nighttime shower.
Early morning showers and isolated thunderstorms are forecast across southern and eastern areas of Trinidad, leading to a partly cloudy day. Isolated showers and thunderstorms are possible by the late morning through the afternoon, favoring western Trinidad. A mostly settled, hazy and breezy late afternoon, evening and night, barring the odd nighttime shower.
4/10
Thunderstorm Chances
Low – Medium

Saturday

Saturday
3 10 0 1
A few early morning showers and isolated thunderstorms to give way to a partly cloudy day. Isolated late morning through afternoon showers and thunderstorms are possible, favoring western and northern areas of Trinidad, and to a lesser extent, Tobago. A mostly breezy, hazy and settled night.
A few early morning showers and isolated thunderstorms to give way to a partly cloudy day. Isolated late morning through afternoon showers and thunderstorms are possible, favoring western and northern areas of Trinidad, and to a lesser extent, Tobago. A mostly breezy, hazy and settled night.
3/10
Thunderstorm Chances
Low

Marine Forecast

Temperatures

Tuesday

Low: 23-26°C

High: 31-33°C

Wednesday

Low: 23-25°C

High: 29-31°C

Thursday

Low: 23-25°C

High: 31-33°C

Friday

Low: 24-26°C

High: 31-33°C

Saturday

Low: 25-27°C

High: 32-34°C

Forecast Impacts

Flooding

Over the next five days, through Saturday night, overall rainfall accumulations across the country are forecast to range between 25 and 75 millimeters, with five-day totals between 75 and 150 millimeters favoring southern and eastern halves of Trinidad. In highly isolated areas, daily rainfall totals exceeding 25 to 50 millimeters are likely.

Street & Flash Flooding

Street & Flash Flooding
7 10 0 1
The chances of street/flash flooding are high, particularly across the East-West Corridor, as well as the southern and eastern halves of Trinidad.
The chances of street/flash flooding are high, particularly across the East-West Corridor, as well as the southern and eastern halves of Trinidad.
7/10
Likelihood
High

Riverine Flooding

Riverine Flooding
3 10 0 1
With saturated soils across the country, particularly across the South Oropouche River Basin, the chances of riverine flooding are low at this time. However, these chances are likely to increase as the week progresses.
With saturated soils across the country, particularly across the South Oropouche River Basin, the chances of riverine flooding are low at this time. However, these chances are likely to increase as the week progresses.
3/10
Likelihood
Low

Forecast Rainfall Totals

  • Tuesday: Between 10 and 25 millimeters of rainfall across both islands, with isolated totals between 25 and 50 millimeters, particularly in heavy thunderstorm/shower activity.
  • Wednesday: Between 5 and 15 millimeters of rainfall across both islands, with isolated totals across up to 25 millimeters favoring coastal areas of southern, western, and eastern Trinidad.
  • Thursday: Between 5 and 15 millimeters of rainfall across both islands. Across eastern and southern halves of Trinidad, as well as western coastal Trinidad, between 25 and 50 millimeters are possible, with higher totals in heavy showers/thunderstorms.
  • Friday: Less than 10 millimeters of rainfall across both islands, with isolated totals up to 25 millimeters favoring southwestern coastal areas of Trinidad, as well as eastern Trinidad.
  • Saturday: Less than 5 millimeters of rainfall across the country, with isolated higher totals favoring eastern and northern Trinidad, and Tobago.

Understanding Rainfall Accumulations

Putting the rainfall forecast into context, rainfall rates in excess of 50 millimeters per hour or areas that receive in excess of 25 millimeters within an hour tend to trigger street flooding across the country or flash flooding in northern Trinidad. For riverine flooding to occur, a large area of the country (not just in highly localized areas of western coastal Trinidad) would have to record upwards of 75 millimeters within 24 hours, and rainfall would have to fall across major rivers’ catchment areas.

Strong Thunderstorms

Strong Thunderstorms
5 10 0 1
From Wednesday, the threat of damaging wind gusts exists as winds at the low levels of the atmosphere range between 25 and 35 knots (46-65 KM/H) which can move to the surface in heavy showers/thunderstorms. The atmosphere this week will also be conducive for locally intense rainfall, mainly Tuesday through Thursday. Frequent lightning is also possible in isolated strong thunderstorm activity.
From Wednesday, the threat of damaging wind gusts exists as winds at the low levels of the atmosphere range between 25 and 35 knots (46-65 KM/H) which can move to the surface in heavy showers/thunderstorms. The atmosphere this week will also be conducive for locally intense rainfall, mainly Tuesday through Thursday. Frequent lightning is also possible in isolated strong thunderstorm activity.
5/10
Likelihood
Medium
What is a strong or severe thunderstorm?

Given how rare these types of thunderstorms are in our region – we classify a severe or strong thunderstorm as one that produces any of the following:

  • Damaging wind gusts exceeding 55 KM/H;
  • Frequent lightning (more than 30 cloud-to-ground strikes within a 10-minute period);
  • Hail (of any size);
  • Rainfall of more than 50 millimeters or more within an hour or exceeding 75 millimeters or more within three hours;
  • The sighting of a funnel cloud or touchdown of a waterspout/tornado associated with the thunderstorm.

Gusty Winds

Gusty Winds
5 10 0 1
As mentioned earlier, mainly from Wednesday and Thursday, gusts are likely to exceed 45 KM/H, with sustained winds reaching as high as 35 KM/H. In isolated strong thunderstorms or intense showers, gusts between 55 and 65 KM/H are possible.
As mentioned earlier, mainly from Wednesday and Thursday, gusts are likely to exceed 45 KM/H, with sustained winds reaching as high as 35 KM/H. In isolated strong thunderstorms or intense showers, gusts between 55 and 65 KM/H are possible.
5/10
Likelihood
Medium

With winds gusting above 50 KM/H, whole trees can be in motion, with larger trees and weaker branches falling. Light outdoor objects can topple or become airborne, such as garbage cans, loose galvanize, construction material, and outdoor furniture. Tents may also jump. Due to saturated soils, with strong winds, trees may fall easier, particularly along the Northern Range.

Other Hazards

Saharan Dust Forecast

Why I May Not/Will Not See Rainfall?

A frequent complaint is the forecast is wrong because I didn’t experience any rainfall. Scattered showers mean that you, individually, may experience some showers intermittently throughout the day, and there is a higher chance for this activity than isolated activity. Widespread showers mean that nearly all persons and areas may experience rainfall.

On Tuesday through Thursday, isolated to scattered rainfall is likely, with isolated rainfall possible on Friday and Saturday.

Forecast Discussion

A weak low-level trough embedded within the Intertropical Convergence Zone (ITCZ) is forecast to move across T&T and the Windward Islands tonight (Monday night) through Tuesday, bringing sufficient instability for scattered showers and isolated thunderstorms through Tuesday.

Then, on Wednesday, low-level winds are forecast to increase, with enhanced convergence trailing the low-level trough, as well as the ITCZ remaining across T&T. All of these features will lead to scattered showers and thunderstorms across both islands but favoring Trinidad.

Generally, between Tuesday and Thursday (outside of Wednesday afternoon through the pre-dawn hours of Thursday morning), relative humidities are forecast to remain above 70% with low-, mid-, and upper-levels becoming increasingly moist; wind shear is forecast to be light to moderate, up to 20 knots from the west to northwest, and increasing instability will lead to periods of heavy rainfall during this three-day period.

Conditions are set to settle briefly into Wednesday evening as a dry pocket of air and a short-lived ridge pattern ahead of Tropical Wave 24 move across the region. By early Thursday morning, the weak tropical wave moves across the Lesser Antilles but interacts with the ITCZ still across Trinidad and Tobago, leading to scattered showers and thunderstorms once more due to a surface-to-mid-level confluent pattern and deep-layered moisture.

While dust models also show a moderate to high concentration surge of Saharan Dust moving in by Wednesday evening and Thursday morning, forecast models continue to show an amply moist atmosphere from late Thursday morning through the afternoon.

MLCAPE values are forecast to be between 1000 J/KG and 1500 J/KG, with relative humidity above 70% and high precipitable water above 2 inches—all ingredients for locally heavy rainfall during Thursday’s late morning into the afternoon. A low-level jet is also forecast to develop, with southeasterly to easterly winds between 25 and 35 knots, advecting moisture across the region and feeding into shower and thunderstorm activity.

With the ITCZ around, it is important to remember that peak activity associated with this area of convergence typically happens during the early morning and early afternoon hours. Heavy overnight rainfall is possible, and those living in flood-prone areas should be ready to act in the event persistent heavy rainfall materializes, leading to street, flash, and even riverine flooding.

With soils already relatively saturated, the risk of landslides in elevated areas and trees falling in gusty winds will increase throughout the week. If you live in a flood-prone area, particularly with the threat of heavy overnight rainfall, be prepared to act quickly if flooding materializes—with or without an official alert/watch/warning in place.

By Friday, the ITCZ is forecast to shift southward as a ridge rebuilds by the afternoon. As a result, rainfall is expected during the first half of the day through the early afternoon, but conditions become increasingly dry and hazy.

By Saturday, the atmosphere is forecast to continue becoming increasingly dry as a high-pressure ridge rebuilds and winds at the low levels remain elevated.

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