ITCZ Forecast To Affect T&T Through Monday

The ITCZ, as anticipated, will be enhanced by a low-level trough and remain present across T&T over the next three days, possibly into next week bringing periods of showers and thunderstorms, but heavy rainfall will not be widespread.

Higher rainfall totals are forecast to remain north of T&T, mainly affecting Tobago as well as the northern and eastern halves of Trinidad on Saturday. This heavier rainfall threat expands to southern coastal areas on Sunday into Monday.

Tropical Wave 53 west of the Lesser Antilles seen on GOES-EAST IR Satellite Imagery with the ITCZ affecting the Southern Windwards (Weathernerds.org)
Tropical Wave 53 west of the Lesser Antilles seen on GOES-EAST IR Satellite Imagery with the ITCZ affecting the Southern Windwards (Weathernerds.org)

There are no alerts, watches, or warnings from the Trinidad and Tobago Meteorological Service in effect for T&T as of 3:00 AM Saturday 3rd October 2020.

The Forecast (Through Monday)

Saturday: Partly cloudy skies are forecast to become increasingly cloudy closer to daybreak, with scattered showers and isolated thunderstorms moving initially across Tobago, then Trinidad by sunrise.

Mostly cloudy skies will be interrupted by isolated showers through the morning across both islands. By the late morning, isolated thunderstorms and heavier showers are possible, favoring Trinidad and persisting through the afternoon, becoming initially settled by the evening. Partly to mostly cloudy skies persist across Tobago into the evening. Rainfall will be brisk and highly isolated during the day, but these brief showers can be intense with very gusty winds.

By nightfall, showers and thunderstorms are forecast to increase across both islands, with heavier activity favoring Tobago initially, gradually spreading across Trinidad. Activity will be scattered, interrupting mostly cloudy skies and persisting into Sunday.


Sunday: Overnight showers and thunderstorms persist across both islands, with scattered activity and cloudy periods. Though models are indicating thunderstorm activity generally remaining offshore closer to daybreak, isolated showers will still be possible across T&T through the morning with partly to mostly cloudy skies.

By the late morning, scattered showers and thunderstorms are forecast to return, with heavier activity favoring southern and eastern Trinidad, as well as Tobago initially. These conditions will spread across both islands through the early afternoon with variably cloudy skies, settling by the late afternoon into the evening.

Another resurgence in shower and thunderstorm activity is forecast closer to midnight. Heavier showers and isolated thunderstorms will generally remain east, affecting parts of Tobago, eastern and southern areas of Trinidad initially.


Monday: Isolated overnight showers and thunderstorms persist mainly offshore with cloudy periods across T&T. By 3:00 AM, showers and thunderstorms move across both islands, settling by daybreak.

A variably cloudy start to Monday is forecast, with sunny periods. By the afternoon, isolated showers and the odd thunderstorm are possible, favoring Southern and Central Trinidad, settling into the evening with partly to mostly cloudy skies. A mostly settled night is forecast, with the odd shower affecting coastal areas of both islands, interrupting partly cloudy to clear skies. Showers may return early Tuesday morning, favoring Trinidad.


Forecast Discussion: The ITCZ will be present across T&T, at times just north of our area, with gradually decreasing moisture levels into next week. Activity with this feature has its most prominent peak between 12:00 AM and 8:00 AM. A secondary peak is also observed between 12:00 PM and 3:00 PM, with minimum activity between 5:00 PM and 8:00 PM. These are the general time frames for possible inclement weather.

Surface to mid-level convergence and upper-level divergence, with favorable upper-level support, remains in place through Saturday, with a low-level trough enhancing shower and thunderstorm activity Saturday into early Monday, with peak rainfall possible between Saturday afternoon and Sunday afternoon.

Generally, across Trinidad and Tobago, wind shear is forecast to be near zero at times. Hence, the limiting factors to shower and thunderstorm development would be a dry mid-level environment due to Saharan Dust and a low-shear environment resulting in the downdraft of a shower/thunderstorm undercutting the updraft, causing dissipation.

On days where sunshine is dominant during the morning daytime heating, orographic lift (along the Northern and Central Range), and sea breeze convergence may also act as triggers for shower and thunderstorm development.


Marine: On Saturday and Sunday, seas will still be moderate, with waves up to 2.0 meters in open waters, particularly north and east of Trinidad. In sheltered areas, seas are still forecast to be below 1 meter but choppy when showers and thunderstorms are present. On Monday, seas are forecast to be slight to moderate in open waters, with waves between 1.0 meter and 1.5 meters and near calm in sheltered areas.

Winds are forecast decrease through the period, between 15 and 20 knots through Sunday, decreasing to 10 to 15 knots on Monday, generally from the southeast to east.

Hazards: The main hazards include isolated, heavy rainfall, which may reduce visibility and gusty winds up to and in excess of 55 KM/H. Street or flash flooding will be possible in violent showers and thunderstorms. In thunderstorm activity, frequent lightning is also possible. Gusty winds may cause power dips/outages, downed trees, and localized wind damage. With heavy rainfall across elevated areas, landslides and fallen trees will also be possible through the weekend, particularly across Northeastern Trinidad and Tobago.

Temperatures

With the increased cloud cover and rainfall, temperatures are forecast to be slightly cooler through the weekend, with maximum highs in the low thirties.

On Saturday and Sunday, with a partly to mostly cloudy days, the maximum high for Trinidad is forecast to be near 32.0°C, possibly higher in urban and built-up areas. In Tobago, the maximum high is forecast to be near 31.0°C or slightly above. On Monday, slightly hotter conditions are forecast, with the maximum highs closer to the mid-thirties.

Temperatures in cities, such as Port of Spain, tend to be much higher than surrounding locations due to a phenomenon called the Urban Heat Island Effect. There are several causes, but the main instigator for this phenomenon tends to be increased dark surfaces such as roads and pavement in cities, which absorb solar radiation more than surrounding areas.

Forecast Impacts

ITCZ Impacts and Forecast Confidence for T&T through Monday 3rd October 2020.
ITCZ Impacts and Forecast Confidence for T&T through Monday 3rd October 2020.

Winds

Sustained surface winds will increase through the forecast period, between 25 KM/H and 35 KM/H with gusts up to and in excess of 55 KM/H are possible. Stronger winds at higher elevations could mix down towards the surface and cause downburst-like winds, which can be damaging.

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

Higher rainfall totals are forecast to remain north of T&T, mainly affecting Tobago as well as the northern and eastern halves of Trinidad on Saturday. This heavier rainfall threat expands to southern coastal areas on Sunday into Monday.

If you live across Southwestern and the Western half of Trinidad, rainfall accumulations will be low, and higher accumulations will be highly isolated due to brisk, isolated downpours and thunderstorms. In these areas, daily accumulations are forecast to be less than 5 millimeters, and isolated daily totals up to and in excess of 15-20 millimeters, particularly if violent (>50 mm/hr) rainfall rates occur.

Across Eastern and Northern halves of Trinidad, as well as Tobago, heavier rainfall is anticipated with daily maximum accumulation totals are forecast to be up to 25 millimeters.

From midnight through Sunday, thunderstorms may produce isolated rainfall totals up to and in excess of 50 millimeters, particularly across northeastern areas of Trinidad. Across the remainder of T&T, up to 10 millimeters is forecast, with isolated totals up to 25 millimeters.

Generally, with heavy showers and thunderstorm activity, street flooding, particularly in flood-prone areas or areas with poor drainage, is possible as well as flash flooding in areas where more prolonged heavy rainfall may occur. There is no threat of riverine flooding at this time.

Frequent Lightning: Frequent 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.

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.

Isolated to scattered rainfall is forecast over the next three days, meaning with partly cloudy skies, some sunshine will also be mixed in.

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

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