Forecast: ITCZ To Bring Periods of Heavy Rainfall To T&T

The Intertropical Convergence Zone (ITCZ) is forecast to affect T&T on Wednesday and Thursday. Enhancing rainfall will be favorable mid and upper-level conditions, creating conditions for potentially heavy rainfall across much of Trinidad. Strong wind shear is forecast to keep the heaviest rains east and south, mainly affecting eastern and southern halves of Trinidad initially.

October has historically produced significant flood events and based on the current model guidance; the Intertropical Convergence Zone may trigger high 3-day rainfall accumulations across the country. Severe flooding has already affected parts of the South Oropouche River Basin. If you live in a flood-prone area, it may be pertinent to review your flood safety plan and preposition resources if needed.

Here is what you need to know:

— Through Thursday, the ITCZ, in the presence of a highly favorable upper-level environment, is forecast to produce scattered showers and isolated thunderstorms, potentially producing flooding rainfall across both islands. However, overall heavier rainfall is forecast to remain south and east of T&T due to persisting strong westerly wind shear. In addition, peak activity is generally forecast during the overnight hours into the early morning and again during the early afternoon.
— The main hazard through Thursday will mainly be heavy rainfall and flooding. Generally, the street and flash flooding threat will be high, with the potential for riverine flooding mainly across the South Oropouche River and Ortoire River Basins.
– Additionally, sustained winds up to 30 KM/H and wind gusts in excess of 45 KM/H are likely, with gusts in excess of 55 KM/H less likely. 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. Landslides are possible across elevated areas.
— An Adverse Weather Alert (Yellow Level) has been issued for Trinidad, Tobago, and surrounding marine areas from the Trinidad and Tobago Meteorological Service from 11:00 PM Tuesday through 4:00 PM Thursday.

The Forecast

Moisture associated with the ITCZ east of T&T with Tropical Wave 61 just off the African Coast. (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.

Wednesday: Scattered showers and thunderstorms are forecast to develop by 3:00 AM, drifting northwest, initially affecting eastern Trinidad and Tobago. Near dawn, additional showers and thunderstorms are forecast to move across Trinidad from the southeast, with heavier activity favoring southern and eastern areas. Though the day, periodic showers, at times heavy to violent, thunderstorms and rain, with cloudy skies, are forecast to affect the country. Coverage and the intensity of rainfall are expected to decrease by nightfall, with a few lingering showers.

Thursday: After midnight, showers and thunderstorms are forecast mainly across Tobago, eastern and southern coastal Trinidad, giving way to variably cloudy skies across the country, with additional brisk isolated showers and thunderstorms from the late morning through the afternoon. Conditions are forecast to settle by the evening, with isolated showers and thunderstorms generally remaining offshore T&T.

Friday: Partly cloudy and hot conditions forecast through the day, interrupted by slow-moving, isolated showers and thunderstorms across the country by the late morning through the afternoon, favoring southern, western, and hilly areas of Trinidad, lesser so Tobago. A mostly settled and partly cloudy night is forecast.

Saturday: Partly cloudy and hot conditions forecast through the day, interrupted by isolated showers and low chance of an isolated thunderstorm across the country by the late morning through the afternoon, favoring southern, western, and hilly areas of Trinidad, lesser so Tobago. A mostly settled and partly cloudy night is forecast initially, with showers developing near and after midnight.

Sunday: Partly cloudy and hot conditions forecast through the day, interrupted by isolated showers and thunderstorms across the country by the late morning, becoming settled by the late afternoon. Isolated showers and thunderstorms are forecast to develop near and after midnight.

Scattered showers and thunderstorms east of the Lesser Antilles on Tuesday night. (Weathernerds.org)

Marine: Seas are forecast to return to normal levels through the remainder of the week. Seas in open waters are forecast between 1.5 and 2.0 meters. In sheltered areas, near 1.0 meters and occasionally choppy, higher along northern and eastern coastlines.

Temperatures

Through the forecast period, the minimum low in Trinidad and Tobago is forecast to be between 23.0°C and 26.0°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.

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.

On Friday into the weekend, maximum high temperatures are forecast to be slightly warmer.

Forecast Impacts

As mentioned above, street and flash flooding will be likely through Thursday, with elevated chances for riverine flooding mainly across the South Oropouche River and Ortoire River Basins. Additionally, sustained winds up to 30 KM/H and wind gusts in excess of 45 KM/H are likely, with gusts in excess of 55 KM/H less likely. 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. Landslides are possible across elevated areas.

Rainfall & Flooding

Forecast chances of heavy to violent showers and thunderstorms associated with the ITCZ over the next five days.
Forecast chances of heavy to violent showers and thunderstorms associated with the ITCZ over the next five days.

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

Over the next five days across Trinidad, isolated totals up to 125 millimeters are forecast across southern and eastern areas of the island, with generally 30-75 millimeters across the country. Higher rainfall accumulations are forecast mainly across southern and eastern regions of Trinidad and offshore eastern Trinidad, and isolated high totals are also possible along the western coastal areas of Trinidad.

  • Wednesday: Up to 35 millimeters across the country, trending wetter across the eastern and southern halves of both islands. Isolated totals up to 50 millimeters are possible. The heaviest rainfall is expected across and offshore southern and eastern Trinidad, with totals between 25 and 50 millimeters. This is forecast to be the wettest day of the 5-day period.
  • Thursday: Up to 25 millimeters across southern and eastern Trinidad. Generally, less than 15 millimeters elsewhere.
  • Friday: Up to 20 millimeters of rain across the country, with heavier rains favoring eastern areas of both islands.
  • Saturday: Across Trinidad, less than 10 millimeters, with higher rainfall accumulations favoring southern and eastern coastal Trinidad.
  • Sunday: Less than 10 millimeters across the country with totals up to 25 millimeters favoring localized areas of southern, western and hilly areas of Trinidad and Tobago.

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.

Forecast chances of street or flash flooding resulting from heavy rainfall associated with the ITCZ over the next five days.

Street flooding or flash flooding is forecast over the next five days, with very high chances on Wednesday, lowering as we progress through the week. Chances for riverine flooding remain elevated through the forecast period, gradually lowering as we head into the weekend.

Forecast chances of riverine flooding resulting from heavy rainfall associated with the ITCZ over the next five days.
Forecast chances of riverine flooding resulting from heavy rainfall associated with the ITCZ over the next five days.

Gusty Winds

Forecast chances for gusty winds associated with tropical waves and the ITCZ over the next five days.
Forecast chances for gusty winds associated with tropical waves and the ITCZ over the next five days.

Low-level winds are on the decline through the next five days. On Wednesday and Thursday, sustained surface winds are forecast to be 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 possible.

Through the week and into the weekend, the chances for gusty winds will gradually lower as winds at low levels of the atmosphere decrease. However, with possible stronger thunderstorms on Wednesday and Thursday and then again on Sunday, the chance for gusty winds remains elevated.

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 through Thursday.

Severe Thunderstorms: The potential for severe thunderstorms, funnel clouds, and waterspouts remain low. 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.

Scattered rainfall is forecast through Thursday, becoming isolated into and through the weekend.

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

Forecast Discussion

Satellite update as of 11:40 PM Tuesday 26th October 2021 (NASA/CIRA/RAMMB)
Satellite update as of 11:40 PM Tuesday 26th October 2021 (NASA/CIRA/RAMMB)

Tropical Waves: At 00Z Wednesday 27th October 2021, the axis of Tropical Wave 61 is along 28°W moving westward at 10-15 knots (18-27 KM/H).

The ITCZ is forecast to modulate northward across Trinidad, Tobago, and the Southern Windward Islands on Wednesday through Thursday. In addition, a highly favorable mid to upper-level environment associated with an upper-level low-pressure system will be in place across the region, allowing for deep convection to develop.

However, strong wind shear from the west is forecast to remain across Trinidad and Tobago. While showers and thunderstorms will move across T&T, as activity moves west, 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 weak surface to mid-level ridge will be dominant by Friday into the weekend, mainly bringing settled conditions. However, with the ITCZ nearby and ample moisture, daytime heating, and sea breeze convergence will act as triggers for isolated but slow-moving showers and thunderstorms favoring western and hilly areas of the country.

An induced surface to low-level trough is forecast to bring more rainfall to the country by Sunday night into Monday, but westerly wind shear will remain strong.

Through the forecast period, fluctuating concentrations of Saharan Dust and volcanic smoke (or Vog) will remain present across the region, acting as an inhibitor for showers and thunderstorms and keeping skies hazy.

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