Forecast: Cloudy Skies, Showers & Thunderstorms To Move Across The Region

Key Messages:
— Tropical Wave 53, a series of troughs, and the Intertropical Convergence Zone are forecast to bring rainfall to the Windward Islands over the next five days, with heavier rain remaining north and east of T&T.
— Generally, street and flash flooding will favor Northern Trinidad and across Tobago. By Monday, the flooding threat will be elevated. Sustained winds and gusts will be elevated, mainly from Sunday, with wind gusts in excess of 45 KM/H possible.
Lightning will accompany thunderstorms. Funnel clouds and waterspouts are possible mainly Friday through Saturday, with severe thunderstorms possible on Friday.
— An Adverse Weather Alert (Yellow Level) is in effect from the Trinidad and Tobago Meteorological Service at this time until 6:00 PM Friday.

The Forecast

Moisture trailing Tropical Wave 53, associated with the ITCZ, moving across the region. (SSEC/CIMSS/UW-Madison)
Moisture trailing Tropical Wave 53, associated with the ITCZ, moving across the region. (SSEC/CIMSS/UW-Madison)

Thursday night: Mostly cloudy skies, mainly across Trinidad with isolated showers and the odd thunderstorm, generally remaining offshore.

Friday (Lingering instability, moisture from Tropical Wave 53): After midnight, showers and isolated thunderstorms are forecast to move in from the northeast spreading across both islands. Persisting heavy rainfall is likely across Tobago. 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. Isolated, strong to severe thunderstorms are possible across the southern half of Trinidad during this period. Conditions are forecast to gradually settle into the evening, with isolated showers, thunderstorms, and variably cloudy skies returning after nightfall. Gusty winds, street/flash flooding likely!

Saturday (Lingering instability from Tropical Wave 53, approaching trough): Overnight showers and isolated thunderstorms are forecast to continue through the day, with variably cloudy skies. Conditions to settle by the evening, barring the odd shower. Gusty winds, street/flash flooding possible!

Sunday (Trough system affecting T&T): Brisk isolated showers are possible overnight, favoring Tobago, giving way to an initially partly to a mostly cloudy day. By the late morning through the afternoon, isolated heavy showers and isolated thunderstorms are forecast to move across the country. Similar conditions are forecast through the evening and night with cloudy periods. Gusty winds likely, street/flash flooding possible!

Monday (Trough system drifts northwest, lingering moisture and instability): Isolated early morning showers and thunderstorms are forecast to move across the country, giving way to initially settled by cloudy conditions by daybreak. By the late morning through the afternoon, scattered showers and thunderstorms are forecast across the country, settling by nightfall. Showers and thunderstorms are forecast to return near and after midnight.

Tuesday (Elevated moisture, instability present): Partly cloudy conditions across the country, with brief isolated showers by the late morning through the afternoon, with the odd thunderstorm favoring western and hilly areas of Trinidad. Mostly settled conditions by the evening, with isolated showers returning overnight.

Scattered showers and thunderstorms across the Lesser Antilles on Thursday evening (Weathernerds.org)
Scattered showers and thunderstorms across the Lesser Antilles on Thursday evening (Weathernerds.org)

Marine: Seas are forecast to be moderate in open waters with waves up to 2.0 meters. Seas are forecast to be near or below 1.0 meter in sheltered areas, becoming choppy during heavier showers or thunderstorms. 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 24.0°C and 27.0°C.

Maximum highs across the country are forecast to be generally up to 32.0°C in Trinidad and Tobago. However, with increased cloud cover, maximum highs are forecast to be relatively cooler than the last three weeks.

In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 33.0°C on days with sunny mornings.

Forecast Impacts

Forecast impacts due to Tropical Wave 53, a trough, and the ITCZ over the next five days.
Forecast impacts due to Tropical Wave 53, a trough, and the ITCZ over the next five days.

On Friday, as Tropical Wave 53 moves west, upper-level conditions are forecast to be favorable, capable of triggering severe thunderstorm development. Localized street/flash flooding, damaging gusty winds, and other severe thunderstorm impacts such as frequent lightning, funnel cloud or waterspout development, and even hail (though hail remains unlikely as surface temperatures are not forecast to get exceptionally hot.)

A trough is forecast to interact with the ITCZ with favorable upper-level conditions that will support cloudy skies, isolated thunderstorms, and scattered showers through the weekend. Street/flash flooding is possible across T&T, with gusty winds likely mainly on Sunday into Monday.

Winds

Sustained surface winds are forecast to be between 20 to 35 KM/H, becoming lighter on Friday through Saturday. In heavy to violent shower and thunderstorm 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

Scattered to widespread heavy rainfall is forecast to remain north of the country.

Higher rainfall totals are forecast to favor Tobago, with up to 75 millimeters across the eastern half of the island over the next five days, with up to 40 millimeters across the western half of the island during the same period.

Across Trinidad, isolated totals up to 75 millimeters are forecast across eastern areas of the island, with generally 25-50 millimeters across the country. Higher rainfall accumulations are forecast mainly across northern and eastern areas of Trinidad, and isolated high totals are also possible along the western coastal areas of Trinidad.

  • Friday: Less than 15 millimeters across the country with isolated totals up to 25 millimeters favoring Tobago and highly localized areas of Trinidad due to isolated heavy showers or thunderstorms.
  • Saturday: Less than 15 millimeters across the country with isolated totals up to 25 millimeters favoring Tobago and highly localized areas of western and southern Trinidad due to isolated heavy showers or thunderstorms.
  • Sunday: Up to 25 millimeters across the country, trending wetter across the western halves of both islands. Isolated totals in excess of 25 millimeters are possible mainly across Tobago.
  • Monday: Up to 25 millimeters across the country, trending wetter across the western halves of both islands. Isolated totals in excess of 25 millimeters are possible mainly across the eastern half of Trinidad and eastern Tobago.
  • Tuesday: Less than 10 millimeters across the country with isolated totals up to 25 millimeters favoring coastal areas of Trinidad due to localized heavy showers or thunderstorms.

Street flooding or flash flooding is forecast over the next five days with medium to high chances, mainly Sunday and Monday. Chances for riverine flooding remain very low at this time, as model guidance suggests heavier rainfall remains north of the country.

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.

Severe Thunderstorms: The potential for severe thunderstorms, funnel clouds, and waterspouts remain low to medium, mainly Friday and Saturday, as winds remain fairly light and temperatures remain high. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes can form. There is also the chance for hail, as CAPE levels are forecast to remain high on Friday. However, as mentioned above, hail chances remain low.

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 forecast period, isolated to scattered rainfall is forecast.

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

Forecast Discussion

Satellite update as of 7:00 PM Thursday 7th October 2021 (NASA/CIRA/RAMMB)

Tropical Waves: At 18Z Thursday 7th, October 2021, the axis of Tropical Wave 53 is along 64°W moving westward at 15 to 20 knots (27-38 KM/H) with scattered showers and thunderstorms and isolated strong thunderstorms. Tropical Wave 54 is along 33°W from 16°N southward.

Through Friday, moisture and instability associated with Tropical Wave 53 are forecast to affect the country and the Windward Islands. Favorable upper-level conditions are forecast to enhance showers and thunderstorms on Friday, potentially producing severe thunderstorms.

By Saturday, Tropical Wave 53 moves well west. A surface to mid-level trough interacting with the Intertropical Convergence Zone will approach and move across the country. Still, stronger convection will remain north and east of Trinidad and Tobago. Upper-level conditions are forecast to remain favorable for shower and thunderstorm development.

Deep-layered moisture and instability are forecast to remain across the region through Monday and into Tuesday.

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. This wind shear will also keep heavier rainfall north and east of the country. 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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