Forecast: Two Tropical Waves, ITCZ To Bring Heavy Rainfall To T&T Into Weekend

What you need to know:
— Through the next 5 days, the Intertropical Convergence Zone will linger across Trinidad and at times Tobago, with adverse weather likely during the passages of Tropical Wave 34 on Wednesday but lesser so with Tropical Wave 35 on Thursday.
— Main impacts include locally heavy rainfall that may trigger street or flash flooding, and possible riverine flooding, mainly across the South Oropouche, Caroni, North Oropouche and Ortoire River Basins. Localized gusty winds in excess of 55 KM/H and lightning may accompany thunderstorms and heavy to violent showers causing power dips/outages, downed trees, and localized wind damage. In elevated areas, there is also the potential for landslides. With light southerly winds and the potential for strong thunderstorms, funnel clouds, and waterspouts are possible.
— The Adverse Weather Alert (Yellow Level) has been discontinued for Trinidad while the Riverine Flood Alert (Orange Level) is in effect for Trinidad until 6:00 PM Thursday from the Trinidad and Tobago Meteorological Service. Trinidad and Tobago is not under a tropical storm watch or warning.

The Forecast

Moisture associated with Tropical Waves 34 and 35, as well as the Intertropical Convergence Zone moving across and approaching T&T (SSEC/CIMSS/UW-Madison)
Moisture associated with Tropical Waves 34 and 35, as well as the Intertropical Convergence Zone moving across and approaching T&T (SSEC/CIMSS/UW-Madison)

Tuesday: Hot and sunny conditions initially, with showers and thunderstorms developing across southern Trinidad by the late morning. Through the afternoon, mostly cloudy with periods of rain, scattered showers, and isolated thunderstorms mainly favoring the southern half of Trinidad initially, gradually drifting northward and continuing into the night.

Wednesday: Mostly cloudy skies with periods of rain, scattered showers, and isolated thunderstorms favoring Trinidad, with heavier rainfall favoring southern and western areas. Though conditions are expected to settle through the evening, intermittent showers and variably cloudy skies are forecast to continue into the night.

Thursday: Mostly hot and sunny skies. Isolated thunderstorms and heavy showers are possible across western and hilly areas of Trinidad during the afternoon, diminishing by the evening. A mostly fair night, barring the odd shower.

Friday: Mostly hot and sunny skies. Isolated thunderstorms and heavy showers are possible across western and hilly areas of Trinidad during the afternoon, diminishing by the evening. A mostly fair night, barring the odd shower.

Saturday: Mostly hot and sunny skies until the late morning, where cloudiness is forecast to build across Trinidad. Isolated thunderstorms and heavy showers are possible mainly across Trinidad, diminishing by the evening.

Marine: Through Thursday, seas are forecast to be moderate in open waters with waves up to 2.0 meters. By late Thursday through Saturday, seas are forecast to be slight in open waters with waves up to 1.25 meters. Through the entire forecast period, seas are forecast to be near calm but choppy seas in the vicinity and during heavy showers or thunderstorms.

Temperatures

Through the forecast period, the minimum low for Trinidad and Tobago is forecast to be between 23.0°C and 25.0°C. The maximum high for Trinidad and Tobago is forecast to be between 29.0°C and 33.0°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 Intertropical Convergence Zone, Tropical Wave 34, and Tropical Wave 35

Through the next five days, the main hazards include heavy rainfall, particularly across Trinidad, with street or flash flooding likely, and a low to medium chance of riverine flooding, mainly across the South Oropouche and Ortoire River Basins. Localized gusty winds and lightning may accompany thunderstorms and heavy to violent showers causing power dips/outages, downed trees, and localized wind damage. In elevated areas, there is also the potential for landslides. With light southerly winds and the potential for strong thunderstorms, funnel clouds, and waterspouts are possible. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes are possible.

Winds

Sustained surface winds up to 35 KM/H are forecast. In shower activity, gusts up to and in excess of 55 KM/H are possible. With wind gusts in excess of 55 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

Through Saturday, overall higher rainfall is forecast to favor much of Trinidad, with accumulations between 25 and 75 millimeters. However, localized totals up to and in excess of 125 millimeters are possible in prolonged heavy showers or isolated thunderstorm activity, particularly across the southern half of Trinidad.

  • Tuesday: Between 5-25 millimeters of rainfall across Trinidad and isolated areas receiving up to 50 millimeters, with highly isolated higher totals mainly across South Trinidad. Across Tobago, less than 10 millimeters.
  • Wednesday: Across Trinidad, between 15-40 millimeters of rainfall and isolated areas receiving near 50 millimeters, with isolated higher totals. Across Tobago, less than 5 millimeters.
  • Thursday: Less than 5 millimeters across the country. In isolated heavy showers or thunderstorms across western, southwestern and hilly areas of Trinidad, totals may reach and exceed 25 millimeters.
  • Friday: Less than 5 millimeters across the country. In isolated heavy showers or thunderstorms, rainfall totals may reach and exceed 25 millimeters.
  • Saturday: Less than 5 millimeters across the country. In isolated heavy showers or thunderstorms, rainfall totals may reach and exceed 25 millimeters.

As mentioned earlier, all types of flooding are on the table through the next five days, with high to very high chances for street flooding and flash flooding, and a low to medium chance of riverine flooding, mainly across the South Oropouche and Ortoire River Basins.

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 or persisting rains occur.

Tornadic Activity: With light southerly winds and the potential for strong thunderstorms, funnel clouds, and waterspouts are possible. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes are possible.

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.

The most widespread showers and thunderstorms are forecast to occur on Tuesday into Wednesday, with isolated rain chances Thursday onwards.

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

Forecast Discussion

Infrared satellite imagery showing scattered showers and thunderstorms near Trinidad and Tobago associated with the Intertropical Convergence Zone. (Weathernerds)
Infrared satellite imagery showing scattered showers and thunderstorms near Trinidad and Tobago associated with the Intertropical Convergence Zone. (Weathernerds)

Tropical Waves: At 12Z Tuesday 17th August 2021, the axis of Tropical Wave 34 is along 58W, south of 20N, moving westward at 10-15 knots (18-28 KM/H), forecast to move across the region on Wednesday. A second wave, Tropical Wave 35, is along 42W from 23N southward and moving west at 15 knots (28 KM/H), forecast to move across the region on Thursday. A third wave, Tropical Wave 36, is along 22W from 23N southward, moving westward at 10-15 knots (18-28 KM/H).

Over the next 36 hours, the Intertropical Convergence Zone, coupled with the passage of Tropical Wave 34 is forecast to produce scattered to widespread showers and thunderstorms across Trinidad. Favorable atmospheric conditions are forecast with high moisture and low wind shear through Wednesday morning, with shear increasing through late Wednesday.

Atmospheric moisture is forecast to decrease late Wednesday through Thursday as most of the moisture and instability associated with Tropical Wave 35 moves north of the country.

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