Heavy Rainfall Forecast On Wednesday Into Thursday Across T&T

A mid to low-level trough is forecast to affect T&T over the next 48 hours, bringing periods of rainfall and isolated thunderstorms. Here is what you need to know:

— Through Thursday, a mid to low-level trough, in the presence of a favorable low to mid-level atmospheric environment, is forecast to produce scattered showers, periods of rain, and isolated thunderstorms. However, overall heavier rainfall is forecast to remain mainly across southern and eastern areas of T&T due to persisting strong westerly wind shear.
— The main hazards include heavy rainfall resulting in flooding, mainly across the eastern half of Trinidad, and gusty winds. Generally, the street and flash flooding threat will be medium to high. The riverine flood threat remains low at this time.
– Additionally, sustained winds up to 30 KM/H and wind gusts in excess of 45 KM/H are likely in heavy rain or thunderstorms, with gusts in excess of 50 KM/H possible. Winds of this strength can cause power dips and outages, fallen trees, utility poles, and lines, as well as sporadic areas of wind damage.
Lightning will accompany thunderstorms. Funnel clouds and waterspouts are unlikely through the forecast period due to elevated low-level winds, and landslides are possible across elevated areas.
— At this time, an Adverse Weather Alert (Yellow Level) is in effect for Trinidad and Tobago from the Trinidad and Tobago Meteorological Service from 11 PM Tuesday through 4 PM Thursday.

The Forecast

Moisture associated with a trough system east of Trinidad and Tobago. (SSEC/CIMSS/UW-Madison)

Heavier showers, thunderstorms, and overall higher rainfall accumulations are forecast to remain south and east of the county through the forecast period.

Tuesday night: Variably cloudy skies with intervals of showers to continue, with thunderstorms possible nearing midnight favoring southern and eastern Trinidad. Gusty winds are likely to accompany rainfall.

Wednesday: Scattered showers and thunderstorms are forecast to continue from the overnight hours through the day, with breezy and cloudy conditions. Gusty winds are likely to accompany rainfall activity. By the afternoon, rainfall is forecast to begin subsiding, leading to a mostly settled evening, barring the brisk shower.

Thursday: Partly to mostly cloudy skies, interrupted by intervals of showers favoring Trinidad through the day. One or two of these showers may become heavy during the late morning through the afternoon. A mostly settled, partly cloudy night is expected.

Scattered showers east of Trinidad and Tobago on Tuesday evening. (Weathernerds.org)
Scattered showers east of Trinidad and Tobago on Tuesday evening. (Weathernerds.org)

Marine: Seas are forecast to be agitated through the forecast period due to increased low-level winds. In open waters, waves are forecast to be between 2.0 meters to 2.5 meters, higher near eastern Tobago. Seas are forecast to be near 1.0 meter and choppy in sheltered areas due to swell periods up to 13 seconds arriving from the north. Small craft operators are advised to exercise caution.

Temperatures

Through the forecast period, the minimum low in Trinidad and Tobago is forecast to be between 22.0°C and 24.0°C.

Maximum highs across the country are forecast to be generally up to 30.0°C in Trinidad and Tobago. However, with increased cloud cover, maximum highs are forecast to be relatively cooler.

In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 31.0°C.

By Thursday into Friday, with the return of sunny skies, temperatures are forecast to trend warmer.

Forecast Impacts

Forecast impacts and confidence for the approaching trough system.
Forecast impacts and confidence for the approaching trough system.

As mentioned above, the main hazards include heavy rainfall, flooding, and gusty winds, though lightning will accompany thunderstorms.

Rainfall & Flooding

Through the forecast period, wind shear is forecast to remain unfavorable for persisting heavy rainfall. Still, favorable lower-level conditions are forecast to produce shallow pockets of deep convection that may produce heavy to violent rainfall rates and thunderstorm activity mainly on Wednesday.

Over the next 48 hours across Trinidad, widespread totals up to 35 millimeters are forecast, with high totals up to 75 millimeters across southern and eastern areas of the island, with generally 10-25 millimeters across Tobago. In highly isolated areas, mainly across eastern and southern coastal Trinidad, totals in excess of 75 millimeters are likely.

  • Tuesday: Generally between 5-20 millimeters of rainfall across the country, trending wetter across southern and eastern areas in both islands. Isolated rainfall totals up to 25 millimeters are possible, and higher in isolated or persisting heavy showers or thunderstorms.
  • Wednesday: Generally between 5-30 millimeters of rainfall across the country, trending wetter across southern and eastern areas in both islands. Isolated rainfall totals up to 75 millimeters are possible, and higher in isolated or persisting rainfall, heavy showers or thunderstorms.
  • Thursday: Overall totals up to 15 millimeters in showery activity, with isolated amounts across eastern and southern areas up to 25 millimeters.

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.

Street flooding and flash flooding are forecast on Wednesday, with low chances (at this time) for riverine flooding, mainly across eastern river basins like the Ortoire, Cunapo, and North Oropuche Rivers.

Gusty Winds

Low-level winds are forecast to remain elevated. Peak sustained surface winds are forecast after midday on Tuesday through Thursday, between 25 to 35 KM/H. In heavy to violent shower and thunderstorm activity, gusts up to and in excess of 45 KM/H are likely. Gusts in excess of 55 KM/H are possible, particularly across northern and eastern areas of the country.

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.

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, Southern Trinidad, and Tobago. The threat of landslides will remain elevated on Wednesday into Thursday.

Severe Thunderstorms: Severe thunderstorms are not expected.

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.

Scattered to widespread rainfall is forecast through Wednesday, becoming more isolated to scattered on Thursday.

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

Forecast Discussion

Scattered showers and cloudiness approaching T&T as a trough east of the Lesser Antilles near (NASA/RAMMB/CIRA)

Tropical Waves: There are no tropical waves in the Atlantic Basin.

A mid to low-level trough, accompanied by favorable surface to low-level confluence is forecast to bring a surge in deep tropical moisture and instability with peak favorable conditions forecast on Wednesday.

Mainly, the surface to mid-levels of the atmosphere will remain favorable for showers and thunderstorms to develop but strong wind shear remains present across the country. As showers and thunderstorms move across T&T, convection may weaken, with cloudiness associated with showers and thunderstorms that develop across western Trinidad or Tobago moving eastward. This wind shear will also keep heavier rainfall south and east of the country.

A surface to mid-level ridge will return through Thursday with a decrease in deep moisture, mainly bringing settled and breezy conditions into Thursday evening.

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