Trough, ITCZ Forecast To Bring Showers, Thunderstorms Through Thursday

What you need to know:
— A trough system is forecast to pull moisture and the Intertropical Convergence Zone northward, affecting T&T through Thursday.
— Rainfall is forecast to begin after midnight tonight (Tuesday night into Wednesday) with scattered showers, isolated thunderstorms, and variably cloudy skies continuing through Thursday. Note that the heaviest rainfall is forecast to affect Trinidad. Drier conditions are briefly expected on Friday, ahead of Tropical Wave 26.
— A Riverine Flood Alert is in effect for Trinidad by the Trinidad and Tobago Meteorological Service from 8:25 AM Thursday through 12:00 AM Friday.

The Forecast

Moisture associated with the ITCZ approaching T&T  (SSEC/CIMSS/UW-Madison)
Moisture associated with the ITCZ approaching T&T (SSEC/CIMSS/UW-Madison)

The remainder of Tuesday: Partly to mostly cloudy and hazy skies across the country, with isolated showers favoring eastern Trinidad. Increasingly cloudy skies are forecast near midnight.

Wednesday: Scattered showers and isolated thunderstorms are forecast to spread across Trinidad and Tobago between 12:00 AM and 3:00 AM and continuing through the day with mostly cloudy to overcast skies. Showers and thunderstorms are likely through the evening, briefly subsiding after nightfall. Gusty winds, street/flash flooding are possible.

Thursday: Scattered showers and isolated thunderstorms are forecast across both islands with heavier activity favoring offshore areas. Activity is forecast to subside by the afternoon with variably cloudy skies. A generally settled night is forecast, barring the odd shower. Gusty winds, street/flash flooding are possible.

Friday: A partly cloudy, warm, and hazy day is forecast across Trinidad with brisk, isolated showers. Across Tobago, variably cloudy skies are forecast from the late morning and persist into the night with periodic showers and the odd thunderstorm, mainly staying north of the country.

Marine: Seas are forecast to be slight over the three days, with waves between 1.0 and 1.5 meters in Trinidad and Tobago’s open waters. Across sheltered coastlines, near calm seas are expected, with choppy seas in the vicinity and during heavy showers or thunderstorms.

Forecast Discussion

Infrared satellite imagery showing scattered showers east of Trinidad and Tobago associated with an induced trough and the ITCZ. (Weathernerds)

Tropical Waves: At 12Z Tuesday 27th July 2021, the axis of Tropical Wave 26 is along 33W, south of 20N, moving westward at 15 knots (27 KM/H). Saharan Dust is limiting showers and thunderstorms at this time north of 10N. This wave is forecast to move across the Lesser Antilles on Friday into Saturday. Another wave, Tropical Wave 27, is along 17W, south of 18N. This wave is forecast to move across the region next week Tuesday.

An upper-level low-pressure system has worked its way to the mid and low levels of the atmosphere, producing a surface trough east of the Lesser Antilles. This trough system is forecast to move westward on Wednesday, pulling moisture from the Intertropical Convergence Zone (ITCZ) and the ITCZ itself northward toward T&T. Moisture, instability, and favorable mid- to upper-level conditions are forecast on Wednesday through Thursday afternoon, diminishing late Thursday into Friday, when the Atlantic high-pressure system regains dominance briefly.

Tropical Wave 26 is forecast to move across the region late Friday into Saturday, with most of the moisture remaining north of Trinidad, mainly affecting Tobago northward. Moisture and instability associated with the wave are forecast to affect the entire country by late Saturday through Monday.

Wind shear and low-level winds are forecast to be light through the forecast period, allowing for heavier showers or thunderstorms to develop and persist.

Infrared satellite imagery showing the ITCZ and Tropical Wave 26 at 33W (Weathernerds)
Infrared satellite imagery showing the ITCZ and Tropical Wave 26 at 33W (Weathernerds)

Temperatures

Across both islands, overnight lows are forecast to be mild, with daytime highs warm.

From Wednesday through Friday, the minimum low for Trinidad and Tobago is forecast to be between 24.0°C and 26.0°C. The maximum high for Trinidad and Tobago is forecast to be between 29.0°C and 31.0°C. Higher temperatures are possible in urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas. Lower daytime temperatures are possible on days with increased cloud cover.

Forecast Impacts

Forecast impacts due to the trough system and the ITCZ

The main hazards include heavy rainfall triggering street flooding and flash flooding mainly on Wednesday and Thursday. Lightning and gusty winds up to and in excess of 45 KM/H may accompany thunderstorms or heavy showers and cause power dips/outages, downed trees, and localized wind damage.

Winds

Sustained surface winds up to 25 KM/H are forecast. In shower activity, gusts up to and in excess of 45 KM/H are possible. With wind gusts 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.

Rainfall

Over the next 3 days, overall higher rainfall is forecast to favor western coastal Trinidad and eastern areas of Trinidad and Tobago, with accumulations between 20 and 50 millimeters. However, localized totals up to and in excess of 75 millimeters are possible in prolonged heavy showers or isolated thunderstorm activity.

  • Wednesday: Between 10-25 millimeters of rainfall across the country with isolated areas recieving between up to 50 millimeters. Heavier rainfall to favor coastal eastern and western Trinidad, as well as southern areas of Trinidad and Tobago.
  • Thursday: Between 10-25 millimeters of rainfall across the country with isolated areas recieving between up to 50 millimeters. Heavier rainfall to favor coastal eastern and western Trinidad, as well as southern areas of Trinidad and Tobago.
  • Friday: Less than 5 millimeters of rainfall across Trinidad. Between 5-15 millimeters across Tobago with localized higher accumulations.

As mentioned earlier, mainly on Wednesday and Thursday, heavy rainfall may trigger street flooding and flash flooding. There is a very low to a low chance of short-lived riverine flooding by late Thursday.

Other Adverse Weather Impacts

Lightning: Lightning is possible in thunderstorm activity. Lightning can cause power outages, voltage dips, damage to life and property, particularly during cloud-to-ground lightning strikes.

Landslides: Landslides and fallen trees are possible through the forecast period, particularly in areas where heavy thunderstorms occur or persisting rains over the next 2 days.

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 Wednesday and Thursday, scattered to widespread rainfall is forecast with isolated showers possible on Friday.

Forecast - Isolated, Scattered, Widespread - What do they mean?

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