Forecast: Hot Friday, Then Potentially Flooding Rainfall Into Next Week

Key Messages:
— One more hot day is forecast for T&T, then a surge in trade winds and Tropical Wave 51 are forecast to arrive overnight Friday into Saturday, bringing the first round of gusty winds and rainfall.
— By Monday, a trough is forecast to bring heavy rainfall and thunderstorms to the country.
Street and flash flooding are possible, particularly from Saturday. Winds are forecast to be light and variable through Friday, with sustained winds and gusts increasing from early Saturday through Sunday, with wind gusts in excess of 55 KM/H possible. Maximum temperatures are still forecast to be hot on Friday.
Lightning may accompany thunderstorms. Funnel clouds and waterspouts are possible mainly through Friday.
— Weatherwise, there are no alerts, watches, or warnings in effect from the Trinidad and Tobago Meteorological Service at this time.

The Forecast

Moisture associated with a trough south of Hurricane Sam and a tropical wave approaching the region. (SSEC/CIMSS/UW-Madison)

Friday (Approaching Tropical Wave 51/Trade Wind Surge): Overnight showers and thunderstorms, favoring offshore areas and Tobago, are forecast to give way to a mostly hot and partly cloudy day across the country. By the late morning, isolated showers and thunderstorms are forecast across western and hilly areas of Trinidad. A mostly settled evening and, initially, night are expected. Winds are forecast to increase across the region overnight.

Saturday (Passing of Tropical Wave 51 & Trade Wind Surge): After midnight, showers and isolated thunderstorms are forecast to move in from the northeast spreading across both islands. Rainfall is forecast to initially subside by daybreak, giving way to a variably cloudy day. By the late morning through the afternoon, isolated to scattered showers and thunderstorms are forecast to develop across much of Trinidad. Intervals of rainfall with isolated thunderstorms are forecast to persist into the night. Gusty winds are likely. Street/flash flooding possible!

Sunday (Lingering instability from Tropical Wave 51, approaching trough): Overnight showers and the isolated thunderstorm are forecast to give way to a breezy and variably cloudy day across the country, with brisk scattered showers and isolated thunderstorms favoring southern and western halves of Trinidad. Conditions to settle by the evening, with brisk showers overnight. Gusty winds are likely, street/flash flooding likely!

Monday (Ex-Invest 91L, now trough system, affecting T&T): Breezy and mostly cloudy conditions across the country, with scattered showers and isolated thunderstorms across Trinidad through the afternoon, favoring southern and western Trinidad. Lingering isolated rainfall is possible overnight. Street/flash flooding likely!

Tuesday (Ex-Invest 91L, now trough system, affecting T&T): Broad areas of rain with isolated heavy showers and thunderstorms are forecast, interrupting breezy and cloudy skies throughout the day. Conditions to gradually settle into the evening, with a resurgence of rainfall overnight. Street/flash flooding likely!

Marine: Through the first half of Saturday, seas in open waters are forecast to be moderate, with waves between 1.5 to 2.0 meters in open waters and up to 1.0 meters but occasionally choppy in sheltered areas. As winds increase over the region, from mid-Saturday through Wednesday, seas in open waters are forecast to be moderate to occasionally rough in open waters with waves up to 2.5 meters, occasionally up to 3.0 meters north and east of the country. In sheltered areas, seas will be slight with waves up to 1.0 meters and choppy. Larger waves are possible in nearshore areas due to long-period swells between 11 to 15 seconds from the north, peaking on Sunday into Monday.

Temperatures

Through the forecast period, the minimum low in Trinidad is forecast to be between 23.0°C and 25.0°C. In Tobago, the minimum low is forecast to be between 25.0°C and 27.0°C.

Maximum highs across the country are forecast to be generally up to 34.0°C in Trinidad and 32.0°C in Tobago on Friday. However, with rainfall and cloudy skies expected from Saturday, maximum high temperatures may remain relatively cooler compared to the last two weeks, with maximum highs up to 32.0°C in Trinidad and 31.0°C in Tobago.

In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 34.0°C. The feels-like temperature, or heat index, is forecast to range between 35.0°C and 48.0°C, mainly on Friday.

This level of heat puts everyone at risk for heat illnesses; however, the risks are greatest for the elderly, young children, people with chronic illnesses such as breathing difficulties, heart conditions, or psychiatric illnesses, people who work or who exercise in the heat, and the socially displaced.

Forecast Impacts

Forecast impacts due to Tropical Wave 51, a trough, and a surge in trade winds over the next 5 days.
Forecast impacts due to Tropical Wave 51, a trough, and a surge in trade winds over the next 5 days.

Light winds south of Hurricane Sam are forecast to continue across Trinidad and Tobago, producing localized showers and thunderstorms.

However, the combination of a trade wind surge and Tropical Wave 51 are forecast to bring higher rainfall accumulations and potentially damaging, gusty winds after midnight Friday into Saturday.

By late Sunday into Monday, moisture and instability associated with surface to mid-level troug, are forecast to move across Trinidad and Tobago. This trough system is forecast to produce potentially flooding rainfall across T&T through the first half of next week. This trough was ex-Invest 91L, a tropical disturbance in the Atlantic’s Main Development region.

Over the next five days, the main hazards are forecast to be isolated heavy to violent rainfall triggering street or flash flooding and gusty winds in excess of 55 KM/H. Higher wind gusts are forecast across Tobago and Northern Trinidad and accompanying heavy showers and thunderstorm activity. In thunderstorms, cloud-to-ground lightning is possible. Funnel clouds and waterspouts are possible mainly on Friday due to weak winds, with landslides also possible as heavy rainfall falls over elevated terrain through the weekend.

Winds

Sustained surface winds less than 25 KM/H are forecast through Friday, with sustained winds increasing through the weekend to 35 KM/H during the afternoons. In heavy to violent shower and thunderstorm 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

Scattered to widespread heavy rainfall is possible from Saturday, but mainly Sunday through Tuesday.

Across Tobago, up to 50 millimeters of rainfall is forecast through the next five days. Across Trinidad, between 50-75 millimeters are forecast across the country, with totals in excess of 125 millimeters, up to 200 millimeters, possible across the southern half of the island. Isolated high totals are also possible along the western coastal areas of Trinidad.

  • Friday: This is forecast to be the driest day of the next five days. Less than 10 millimeters across the country with isolated totals up to 25 millimeters favoring western coastal Trinidad resulting from localized heavy showers or thunderstorms.
  • Saturday*: Up to 25 millimeters across the country with totals up to 75 millimeters favoring much of Trinidad.
  • Sunday*: Up to 25 millimeters across the country with totals up to 75 millimeters favoring southern and western Trinidad.
  • Monday*: Up to 25 millimeters across the country with totals up to 50 millimeters favoring much of Trinidad.
  • Tuesday: Less than 10 millimeters across the country with isolated totals up to 25 millimeters favoring eastern and northern Trinidad.

*On these days, the latest runs of the American GFS model are producing significant rainfall totals across Trinidad. Year to date, the GFS model has proved more reliable on overall precipitation forecasts. If these higher rainfall totals materialize, a riverine flood event is possible across southern Trinidad.

Street flooding or flash flooding is forecast over the next five days with high chances. Chances for riverine flooding remain low at this time, but as indicated above, this could change. The riverine flood threat at this time is mainly for the South Oropouche River Basin. Based on current model outputs, riverine flooding is not expected for other river basins at this time, including the Caroni River Basin.

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.

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. Dangerous, frequent lightning is expected on Friday and Saturday.

Landslides: Landslides and fallen trees are possible, particularly along the Northern Range.

Tornadic Activity: The potential for thunderstorms, funnel clouds, and waterspouts remain low to medium, mainly through Friday, as winds remain fairly light and temperatures remain high. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes can form.

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 Friday, isolated showers and thunderstorms are forecast. From Saturday, scattered to widespread rain, isolated to scattered heavy showers, and isolated thunderstorms are forecast.

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

Forecast Discussion

Main weather features across the Atlantic Basin as of 7:00 PM Thursday 30th September 2021 (NASA/CIRA/RAMMB)
Main weather features across the Atlantic Basin as of 7:00 PM Thursday 30th September 2021 (NASA/CIRA/RAMMB)

Tropical Waves: At 18Z Thursday 30th September 2021, the axis of Tropical Wave 51 is along 50°W moving westward at 10 knots (18 KM/H) with scattered shower and thunderstorm activity. Tropical Wave 49 formed into Hurricane Sam, Tropical Wave 50 dissipated, and Tropical Wave 52 formed into Tropical Storm Victor.

Over the next 24 hours, the unstable surface to mid-level conditions south of Hurricane Sam and an upper-level diffluent pattern is forecast to enhance convection across the Windward Islands, including Trinidad and Tobago.

Then, a pocket of dry and stable air is forecast to move in through Friday, resulting in mostly hot and sunny conditions across the region, ahead of an approaching surge in trade winds and Tropical Wave 51.

Tropical Wave 51 and the accompanying surge in trade winds are forecast to bring variably cloudy skies, scattered showers, and isolated thunderstorms, with sustained winds up to 35 KM/H and wind gusts in excess of 55 KM/H across the region, with higher gusts north of Trinidad from Saturday through Sunday.

As moisture and instability linger from the passage of this tropical wave, additional instability and deep tropical moisture associated with a surface to mid-level trough, an ex-tropical disturbance in the central Main Development Region, is forecast to approach the Windwards.

This area of disturbed weather is forecast to bring periods of heavy rainfall and gusty winds to Trinidad and Tobago from late Sunday through the middle of the upcoming week.

However, through the weekend, moderate to strong wind shear from the northwest to the west is forecast to remain across Trinidad and Tobago, keeping shower and thunderstorm activity short-lived, with cloudiness associated with showers and thunderstorms that develop across western Trinidad or Tobago moving eastward. Then, from Monday onwards, wind shear is forecast to subside, varying between light to moderate levels, allowing for longer-lived rainfall.

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