Forecast: Tropical Wave 21 On The Approach, Isolated Thunderstorms Likely

While Tropical Wave 21 is forecast to remain fairly weak, light winds and high moisture levels are forecast to fuel isolated heavy showers and thunderstorms across Trinidad on Wednesday. Then, a surge of Saharan Dust will arrive on Thursday, leading to a mostly sunny and hazy weekend for the country.

What you need to know

Saharan Dust: A significant dust surge is forecast to arrive by early Thursday, lingering through the upcoming weekend.
Rainfall: Isolated to scattered showers and isolated thunderstorms are forecast from early Wednesday, with less impactful rainfall forecast on Thursday. Over the next five days, between 25 and 50 millimeters of rainfall is forecast across most of the country. Across the southern and eastern halves of Trinidad, as well as western coastal Trinidad, isolated totals of up to 100 millimeters of rainfall are possible over the next five days, with the wettest conditions expected on Wednesday.
Hazards: The main hazards, mainly on Wednesday, when most of the forecast impactful weather is forecast for T&T, are gusty winds up to and in excess of 45 KM/H accompanying heavy showers or thunderstorms, as well as localized street/flash flooding favoring eastern and southern halves of Trinidad, and along western coastal areas. The chances for riverine flooding remain low at this time. Lightning will accompany thunderstorms.
Alerts/Watches/Warnings: There are no alerts, watches, or warnings in effect for T&T from the Trinidad and Tobago Meteorological Service at this time.

The Forecast

Tuesday Night

Tuesday Night
3 10 0 1
Partly cloudy skies with brisk showers favoring eastern and southern areas. There is a low chance of an isolated thunderstorm favoring southern Trinidad, nearing midnight.
Partly cloudy skies with brisk showers favoring eastern and southern areas. There is a low chance of an isolated thunderstorm favoring southern Trinidad, nearing midnight.
3/10
Thunderstorm Chances
Low

Wednesday

Wednesday
6 10 0 1
Early morning showers and isolated thunderstorms are forecast to interrupt partly cloudy skies, gradually settling by mid-morning. Isolated showers and thunderstorms are forecast to interrupt mostly sunny to partly cloudy skies by the late morning through the mid-afternoon. With near calm to light winds from the south/southeast, heavier showers and stronger thunderstorms are forecast to favor southern-facing slopes of the Northern Range, the Main Ridge, Tobago, as well as western coastal Trinidad. Conditions to settle by the evening, with partly cloudy and increasingly hazy skies.
Early morning showers and isolated thunderstorms are forecast to interrupt partly cloudy skies, gradually settling by mid-morning. Isolated showers and thunderstorms are forecast to interrupt mostly sunny to partly cloudy skies by the late morning through the mid-afternoon. With near calm to light winds from the south/southeast, heavier showers and stronger thunderstorms are forecast to favor southern-facing slopes of the Northern Range, the Main Ridge, Tobago, as well as western coastal Trinidad. Conditions to settle by the evening, with partly cloudy and increasingly hazy skies.
6/10
Thunderstorm Chances
Medium – High

Thursday

Thursday
4 10 0 1
Mostly hazy and breezy, with brief passing showers across both islands. There is a low to medium chance of isolated thunderstorms favoring Trinidad during the pre-dawn and late morning through afternoon hours. A partly cloudy night with a few isolated showers.
Mostly hazy and breezy, with brief passing showers across both islands. There is a low to medium chance of isolated thunderstorms favoring Trinidad during the pre-dawn and late morning through afternoon hours. A partly cloudy night with a few isolated showers.
4/10
Thunderstorm Chances
Low – Medium

Friday

Friday
1 10 0 1
Mostly sunny/partly cloudy and hazy day barring the isolated and brisk showers, favoring southern Trinidad.
Mostly sunny/partly cloudy and hazy day barring the isolated and brisk showers, favoring southern Trinidad.
1/10
Thunderstorm Chances
Very Low

Saturday

Saturday
1 10 0 1
Mostly sunny/partly cloudy and hazy day barring the isolated and brisk showers, favoring southern and western Trinidad during the afternoon.
Mostly sunny/partly cloudy and hazy day barring the isolated and brisk showers, favoring southern and western Trinidad during the afternoon.
1/10
Thunderstorm Chances
Very Low

Sunday

Sunday
1 10 0 1
Mostly sunny/partly cloudy and hazy day barring the odd brisk shower.
Mostly sunny/partly cloudy and hazy day barring the odd brisk shower.
1/10
Thunderstorm Chances
Very Low

Marine: Through Sunday, seas are forecast to be slight to mostly moderate, with waves in open waters up between 1.25 and 2.0 meters. In sheltered areas, waves are forecast to be below 1.0 meter and occasionally choppy on Tuesday and Wednesday in the vicinity of heavy showers/thunderstorms.

Temperatures

Through the forecast period, the minimum low in Trinidad and Tobago is forecast to be between 24.0°C and 27.0°C.

On Wednesday, maximum highs across the country are forecast to be below 31.0°C in Trinidad and Tobago due to increased cloud cover. In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 31.0°C.

From Thursday, maximum highs across the country are forecast to be generally up to 32.0°C in Trinidad and Tobago. In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 33.0°C.

Forecast Impacts

Flooding

Flooding
7 10 0 1
Over the next five days, between 25 and 50 millimeters of rainfall is forecast across most of the country. Across the southern and eastern halves of Trinidad, as well as western coastal Trinidad, isolated totals of up to 100 millimeters of rainfall are possible over the next five days, with the wettest conditions expected on Wednesday.
Over the next five days, between 25 and 50 millimeters of rainfall is forecast across most of the country. Across the southern and eastern halves of Trinidad, as well as western coastal Trinidad, isolated totals of up to 100 millimeters of rainfall are possible over the next five days, with the wettest conditions expected on Wednesday.
7/10
Likelihood
High

There is a high chance of street and flash flooding, mainly on Wednesday, particularly across the southern and eastern halves of Trinidad. Chances for riverine flooding remain low as major rivers across the country are well within their banks.

Forecast Rainfall Totals

  • Wednesday: Between 15 and 35 millimeters across the country, with rainfall totals trending higher across western coastal Trinidad and central and southern areas of Trinidad. Isolated rainfall totals, particularly along western and hilly areas, could exceed 50 millimeters.
  • Thursday: Less than 15 millimeters across the country, with rainfall totals trending higher across eastern and southern areas of Trinidad.
  • Friday: Less than 5 millimeters of rainfall accumulating in isolated areas across the country.
  • Saturday: Less than 5 millimeters of rainfall accumulating in isolated areas across the country.
  • Sunday: Less than 5 millimeters of rainfall accumulating across the country. In isolated areas, up to 10 millimeters are possible.

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
2 10 0 1
Conditions do not appear favorable for the development of deep convection – leading to stronger thunderstorms and heavy, persisting showers.
Conditions do not appear favorable for the development of deep convection – leading to stronger thunderstorms and heavy, persisting showers.
2/10
Likelihood
Very Low – Low

Though the chances of strong thunderstorms remain very low to low, funnel cloud development is possible mainly on Wednesday due to near calm to light and variable winds. The highest chances for funnel cloud/waterspout development exist along western coastal areas of both Trinidad and Tobago, as well as along southern and eastern coastal areas of Trinidad. If a funnel cloud touches down on a body of water, it becomes a waterspout; if it touches down on land, it becomes a tornado.

While abundant frequent cloud-to-ground lightning isn’t likely, lightning is expected in thunderstorm activity.

Gusty Winds

Gusty Winds
5 10 0 1
Low-level winds across Trinidad and Tobago are forecast remain light to moderate but strong gusts could accompany heavier thunderstorms or shower activity. Sustained winds up to 30 KM/H are forecast, with gusts up to 45 KM/H accompanying heavy shower or thunderstorm activity on Tuesday into Wednesday.
Low-level winds across Trinidad and Tobago are forecast remain light to moderate but strong gusts could accompany heavier thunderstorms or shower activity. Sustained winds up to 30 KM/H are forecast, with gusts up to 45 KM/H accompanying heavy shower or thunderstorm activity on Tuesday into Wednesday.
5/10
Likelihood
Medium

With wind gusts up to and in excess of 45 KM/H, whole trees are expected to be in motion, and there may be some inconvenience when walking against the wind gusts. Light outdoor objects may topple or become airborne such as garbage cans, potted plants, loose galvanize or construction material, and other outdoor furniture. Tents may jump. Older/weaker trees may fall, bringing down utility poles and lines.

Other Hazards

Saharan Dust will increase from Thursday, reducing air quality and affecting visibility.

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.

Isolated to scattered rainfall is forecast on Wednesday, with highly isolated rainfall thereafter.

Forecast Discussion

8:00 PM Tuesday Tropical Weather Outlook from the National Hurricane Center
8:00 PM Tuesday Tropical Weather Outlook from the National Hurricane Center

Tropical Waves East of T&T: Based on the Tropical Analysis and Forecast Branch’s 18Z surface analysis on July 19th, 2022, as well as the latest satellite and model guidance:

  • The 21st tropical wave for 2022 (TW21) is along 51°W, south of 20°N, moving west at 20 knots (37 KM/H). A few weak showers have been noted near the wave as abundant Saharan Dust is present across the Atlantic Ocean, suppressing showers and thunderstorms. TW21 is forecast to move across the Lesser Antilles on Wednesday, with the Intertropical Convergence Zone near/across T&T on Wednesday into early Thursday.
  • The 22nd tropical wave for 2022 (TW22) is a large and broad tropical wave, along 25°W, south of 20°N, moving west at 10-15 knots (18-28 KM/H). Scattered moderate convection has been noted near the wave axis. Dry stable air from the mid-latitudes is moving southward, north of the wave axis, suppressing thunderstorm activity further west of the wave, north of 10°N, and west of 23°W. This tropical wave is forecast to move across the region on Sunday.

On Tuesday, a weak low-level trough moved across T&T, which brought the Intertropical Convergence Zone near to the country. However, with light winds and abundant moisture, local climatic features (sea breeze convergence and daytime heating) led to the development of persisting showers and isolated thunderstorms across the western half of Trinidad.

The ITCZ is forecast to be near Trinidad and Tobago again on Wednesday through early Thursday. Increased cloudy skies with isolated to scattered showers and isolated thunderstorms are forecast due to increasing atmospheric moisture and instability. Activity with the ITCZ has its strongest activity occurring between 12:00 AM and 6:00 AM local time, with another peak during the Northern Hemisphere summer months between 12:00 PM to 3:00 PM.

TW21 is forecast to move across the Lesser Antilles on Wednesday under a marginally favorable mid- to upper-level environment. Additionally, light and variable winds are forecast at low levels. With abundant moisture, localized climatic effects (sea breeze convergence along western Trinidad, orographic precipitation along hilly areas, and daytime heating) are forecast to aid in shower/thunderstorm development.

While abundant atmospheric moisture will be present into the night and early on Thursday, a significant surge of Saharan Dust will lead to the suppression of rainfall. Though models show that isolated to scattered showers and thunderstorms on Thursday, Saharan Dust and its drying out of the low to mid-levels are habitually understated in weather prediction models, leading to an erroneous abundance of rainfall activity. Still, ample instability and moisture will be present as the ITCZ will remain near/across T&T, leading to a low chance of heavy showers/thunderstorms Thursday.

On Thursday and into the weekend, a surface to mid-level ridge will dominate as an overall drier atmosphere remains in place. By Sunday, Tropical Wave 22 will be on the approach, but moderate to high concentrations are forecast to remain across the Lesser Antilles, suppressing rainfall.

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