Forecast: ITCZ To Affect T&T Through Weekend

What you need to know:
— Through the next 48 hours, the Intertropical Convergence Zone will linger across Trinidad and Tobago, with Tropical Depression 10’s influence on winds from Sunday onward, allowing for the development of heavy showers and isolated strong thunderstorms.
— Main impacts include locally heavy rainfall that may trigger street or flash flooding. Riverine flooding remains unlikely at this time. Localized gusty winds in excess of 45 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, mainly along the Northern Range and Tobago on Sunday and Monday. With light southeasterly to southerly winds and the potential for strong thunderstorms mainly on Sunday through Tuesday, funnel clouds, and waterspouts are possible.
— An Adverse Weather Alert will be in effect for T&T and surrounding marine areas from 2:00 AM Sunday through 6:00 PM Sunday from the Trinidad and Tobago Meteorological Service at this time. Trinidad and Tobago is not under a tropical storm watch or warning. Tropical Depression 10 poses no direct threat to T&T.

The Forecast

Moisture associated with Tropical Wave 39 (Invest 98L, now Tropical Depression 10), as well as the Intertropical Convergence Zone moving across and approaching T&T (SSEC/CIMSS/UW-Madison)

Saturday: Variably cloudy with brief showers and isolated thunderstorms mainly favoring Trinidad initially, then drifting across the country through the day. Isolated to scattered, brief, heavy showers and thunderstorms are also possible across both islands from the late morning through the afternoon. Conditions to settle into the evening, with lingering showers. Isolated heavy showers and thunderstorms are possible near and after midnight.

Sunday: Scattered showers and isolated thunderstorms are possible after midnight through the early morning, with heavier activity south and east of Trinidad initially, drifting across the island, with isolated showers across Tobago. By daybreak through the late morning, mostly hot and sunny conditions are forecast with increasing cloudiness across western and northern areas of Trinidad by midday. Isolated thunderstorms and heavy showers are forecast, mainly across northern and western areas of Trinidad and northern Tobago through the mid-afternoon, settling into the night.

Monday: Early morning showers, with the odd thunderstorm favoring eastern offshore areas, are forecast to give way to initially partly cloudy and hot conditions with light winds. By the late morning through the afternoon, isolated heavy showers and thunderstorms are forecast to develop across the western half of Trinidad with mostly settled conditions elsewhere. A mostly settled night is forecast.

Marine: Seas are forecast to be moderate on Saturday into Sunday but improving to slight on Monday into Tuesday. In open waters, waves are forecast to be up to 2.0 meters and below 1.0 meter to near calm in sheltered areas through Sunday, with waves in open waters up to 1.25 meters Monday onward. Note seas may become locally choppy or rough 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 26.0°C.

On Saturday, the maximum high for Trinidad and Tobago is forecast to be between 30.0°C and 32.0°C. From Sunday onward, the maximum high for Trinidad and Tobago is forecast to be between 32.0°C and 35.0°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 and Tropical Wave 39
Forecast impacts due to the Intertropical Convergence Zone and Tropical Wave 39

Through the next 72 hours, the main hazards include locally heavy rainfall that may trigger street or flash flooding. Riverine flooding remains unlikely at this time. Localized gusty winds in excess of 45 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, mainly along the Northern Range and Tobago on Sunday and Monday. With light southeasterly to southerly winds and the potential for strong thunderstorms mainly on Sunday through Tuesday, funnel clouds, and waterspouts are possible.

Winds

Sustained surface winds up to 35 KM/H are forecast on Saturday, diminishing to 15 KM/H on Sunday onward. 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

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

  • Saturday: Between 10-25 millimeters of rainfall across Trinidad and Tobago, with isolated areas receiving up to 40 millimeters mainly across southern and eastern areas of Trinidad.
  • Sunday: Between 5-20 millimeters of rainfall across Trinidad and Tobago, with isolated areas receiving up to 30 millimeters mainly across southern and western areas of Trinidad and Tobago.
  • Tuesday: Between 5-15 millimeters across the country. In isolated heavy showers or thunderstorms across western and hilly areas of Trinidad, totals may reach and exceed 25 millimeters.

As mentioned earlier, street flooding and flash flooding are likely with medium to high chances and a very low chance of riverine flooding.

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 easterly to southeasterly 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 mainly on Sunday onward.

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 Saturday, with isolated to scattered rain on Sunday and Monday.

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

Forecast Discussion

Tropical Weather Outlook from the National Hurricane Center, tracking Invest 97L, Invest 98L, Hurricane Ida, and a new area off the coast of Africa for tropical cyclone formation. (NHC)
Tropical Weather Outlook from the National Hurricane Center, tracking Invest 97L, Invest 98L, Hurricane Ida, and a new area off the coast of Africa for tropical cyclone formation. (NHC)

Tropical Waves: At 06Z Saturday 28th August 2021, the axis of Tropical Wave 39 is along 50W, south of 18N, moving westward at 15 knots (28 KM/H). The axis of Tropical Wave 40 is along 31W, south of 18N, moving westward at 15-20 knots (28-37 KM/H).

Over the next 48 hours, the Intertropical Convergence Zone, coupled with moisture ahead of Tropical Wave 39/Invest 98L – which poses no direct threat to T&T – is forecast to affect the country.

Easterly winds, which remain moderate through Saturday, are forecast to diminish on Sunday through the upcoming week as Invest 98L (now Tropical Depression 10), east of the Lesser Antilles, begin to pull north and northeastward, slackening the pressure gradient.

These light winds and high moisture will allow for the development of slow-moving, heavy afternoon showers triggered by daytime heating, sea breeze convergence, and orographic effects as southerly to southeasterly winds move across the islands.

Though atmospheric moisture will be abundant, providing the fuel for heavy to violent showers, T&T’s saving grace on Saturday will be unfavorable wind shear from the south to southwest, tearing apart showers and thunderstorms, possibly limiting the strength and duration of rainfall across the country.

However, from Sunday through Tuesday, light to no wind shear is forecast across T&T. This will result in any shower or thunderstorm that forms will have ample moisture, little to no formation hindrances as well as slow movement due to light winds, potentially settling up conditions ripe for localized but severe street/flash flooding.

A typical wet season pattern is forecast through much of next week with afternoon heavy showers and isolated thunderstorms favoring western and hilly areas of Trinidad following a mostly hot and sunny morning. The exception to this may occur on Thursday due to the passage of a Tropical Wave.

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