Forecast: Two Tropical Waves, Another Trough To Bring Rainfall To T&T Over Next 5 Days

What you need to know:
— Isolated to scattered showers and isolated thunderstorms are forecast through the weekend associated with the passage of Tropical Wave 27.
— From late Sunday, a trough system is forecast to pull moisture and the Intertropical Convergence Zone northward, affecting T&T through Wednesday, with another tropical wave moving through the region on Tuesday.
— A Riverine Flood Alert has been discontinued for Trinidad by the Trinidad and Tobago Meteorological Service from as of 11:37 AM Saturday. Note that flooding is still ongoing across the island.

The 5-Day Forecast

Moisture associated with Tropical Wave 27 approaching T&T  (SSEC/CIMSS/UW-Madison)
Moisture associated with Tropical Wave 27 approaching T&T (SSEC/CIMSS/UW-Madison)

Saturday: Partly cloudy skies, with occasional cloudy periods associated with brisk shower and thunderstorm activity throughout the day. By the late morning through the afternoon, thunderstorms and heavier showers are forecast across much Trinidad. Conditions are forecast to initially settle into the evening, with showers returning near midnight, favoring Tobago and Eastern Trinidad.

Sunday: After midnight and through the morning, scattered showers and isolated thunderstorms are forecast across both islands, with heavier activity favoring Trinidad. Rainfall is forecast to subside by the afternoon, with variably cloudy skies continuing into the night. Isolated showers and thunderstorms are possible overnight.

Monday: Mostly cloudy with scattered showers and thunderstorms across both islands, with heavier activity favoring western and northern halves of Trinidad. Rainfall is forecast to gradually diminish through the afternoon with variably cloudy skies.

Tuesday: Scattered showers and isolated thunderstorms are forecast to spread across Trinidad and Tobago and continue through the day with mostly cloudy to overcast skies. Conditions are forecast to settle into the evening with variably cloudy skies. Thunderstorms are possible in the Gulf of Paria overnight.

Wednesday: Partly to mostly cloudy skies with scattered showers and thunderstorms developing mainly across the Trinidad’s western and northern halves during the late morning through the afternoon.

Marine: Through Sunday, seas in open waters are forecast to be between 1.5 meters and 2.0 meters. From Monday through Wednesday, with light winds, seas are forecast to be near 1.0 meter in open waters. Through the forecast period, across sheltered coastlines, near calm seas are expected, with 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 24.0°C and 26.0°C. The maximum high for Trinidad and Tobago is forecast to be between 29.0°C and 32.0°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 Tropical Waves 27, 28 and a trough system

Over the next five days, the main hazards include heavy rainfall that may trigger street or flash flooding and exacerbate riverine flooding. 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.

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 five days, 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 100 millimeters are possible in prolonged heavy showers or isolated thunderstorm activity.

  • Saturday: Between 5-20 millimeters of rainfall across the country, with isolated areas receiving up to 25 millimeters. Heavier rainfall to favor southern and western areas of Trinidad, as well as eastern offshore Tobago.
  • Sunday: Between 10-25 millimeters of rainfall across the country with isolated areas in excess of 25 millimeters, up to 35 millimeters. Heavier rainfall to favor western and northern Trinidad.
  • Monday: Between 15-25 millimeters of rainfall across the country with isolated areas in excess of 40 millimeters. Heavier rainfall to western and northern Trinidad, particularly along south-facing slopes of the Northern Range
  • Tuesday: Overall accumulations between 15-25 millimeters with isolated totals in excess of 25 millimeters favoring along south-facing slopes of the Northern Range, western and southwestern Trinidad.
  • Wednesday: Between 10-15 millimeters of rainfall across the country with isolated areas favoring western Trinidad receiving in excess of 25 millimeters.

As mentioned earlier, all types of flooding are on the table through the next five days, including street flooding, flash flooding, and 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.

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.

Through the next five days, isolated to scattered showers and thunderstorms are forecast.

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

Forecast Discussion

Infrared satellite imagery showing scattered showers east of Trinidad and Tobago associated with Tropical Wave 27. (Weathernerds)
Infrared satellite imagery showing scattered showers east of Trinidad and Tobago associated with Tropical Wave 27. (Weathernerds)

Tropical Waves: At 00Z Saturday 31st July 2021, the axis of Tropical Wave 27 is along 60W, south of 20N, moving westward at 15 knots (27 KM/H). Saharan Dust and moderate to strong wind shear are limiting showers and thunderstorms at this time. This wave is forecast to move across the Lesser Antilles tonight, with moisture and instability trailing the wave through tomorrow. Another wave, Tropical Wave 28, is along 35W, south of 14N. This wave is forecast to move across the region on Tuesday.

From Saturday into Sunday, the forecast is relatively straightforward. Moisture and instability associated with Tropical Wave 27 are forecast to move across the Lesser Antilles, including Trinidad and Tobago, on Saturday and linger into Sunday. Marginally favorable upper-level and low-level conditions are forecast, allowing for showers and thunderstorms to develop, mainly after midnight and again during the afternoon on both days. Moderate wind shear from the northwest is forecast to be the limiting factor, keeping overall heavier rainfall to the country’s east.

From Sunday, the forecast becomes tricky as it entirely depends on the strength of an upper-level low-pressure system (tropical upper tropospheric trough or TUTT) as we progress through Wednesday. The general expectation is that this TUTT is forecast to develop on Sunday and gradually move westward, staying north of the Lesser Antilles. This TUTT is forecast then to work its way to the surface, creating a deep-layered trough system across the Lesser Antilles.

The strength of this eventual trough system is key in determining T&T’s weather from late Sunday through Wednesday. The GFS and several other global models show a stronger TUTT, thus a stronger trough system, which pulls abundant moisture from the equatorial regions and the ITCZ towards Trinidad and Tobago. This is the same setup that occurred last Wednesday and Thursday, which produced widespread flooding rainfall. The European (EMCWF) model has a weaker TUTT and eventual trough system, limiting moisture and leaving T&T to the typical local climate during the Wet Season of afternoon showers and thunderstorms across the western half of Trinidad. A complicating factor is the passage of Tropical Wave 28 on Tuesday, which is forecast to bring its moisture and instability.

As it stands now, the forecast favors the GFS and other models’ solution with a wet start to next week. This forecast will be updated as we head through the weekend and models gain more clarity on what we can expect next week, particularly as soils are very saturated across Trinidad.

Infrared satellite imagery showing Tropical Wave 27 at 60W and Tropical Wave 27 at 35W (Weathernerds)
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