Tropical Wave 51 & The ITCZ Forecast To Bring Showers, Thunderstorms

A weak tropical wave (Tropical Wave 51) is forecast to traverse the Windward Island on Sunday. Moisture ahead of the wave, coupled with an upper-level trough favorably positioned across the region will fuel and enhance showers and thunderstorms on Saturday through Monday across T&T.

Winds are forecast to follow the typical profile of a tropical wave, generally northeasterly through Sunday, becoming easterly and then southeasterly Sunday through Monday. Wind speeds generally to remain below 25 KM/H, gusting to 50 KM/H through Monday.

Showers and thunderstorms can be intense, producing localized street/flash flooding and gusty winds in downburst-like activity. Showers and thunderstorms will be scattered across Trinidad and Tobago over the next 72 hours. With light winds, funnel clouds are also possible.

Moisture moving across the Lesser Antilles associated with the ITCZ and tropical waves 51 and 52 over the next week, as shown by the EMCWF model for total precipitable water as of 12Z Friday 25th September 2020. (weathermodels.com)
Moisture moving across the Lesser Antilles associated with the ITCZ and tropical waves 51 and 52 over the next week, as shown by the EMCWF model for total precipitable water as of 12Z Friday 25th September 2020. (weathermodels.com)

While scattered activity is forecast through the next 72 hours, with isolated heavy activity, Tropical Wave 52 and the ITCZ will begin to affect T&T on Wednesday 30th September 2020 through the end of the upcoming week. This poses a greater heavy rainfall threat and gusty winds as this tropical wave coincides with a surge in trade winds.

The Trinidad and Tobago Meteorological Service issued an Adverse Weather Alert (Yellow Level) for Trinidad and Tobago, extended until 2:00 PM Monday 28th September 2020.

The Forecast (Through Monday)


Overview: Increasing low-level moisture is forecast as Tropical Wave 51 nears the islands. An upper-level trough will provide enhancement for isolated strong thunderstorms and heavy showers to develop, particularly on Saturday and Sunday. Wind shear is forecast to be low to moderate, from the west, keeping heavier activity east of Trinidad and Tobago.

As the ITCZ will also be present, 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 in activity between 5:00 PM and 8:00 PM. These are the general time frames for inclement weather, favoring Northern and Eastern areas of T&T through Monday, with isolated inclement weather possible along Western Coastal Trinidad due to localized climatic effects.

Light winds and partly cloudy skies will allow for daytime heating to trigger shower and thunderstorm development. Strong daytime heating, orographic lift (along the Northern and Central Range), and sea breeze convergence are forecast to as triggers for shower and thunderstorm development.

Sea breeze convergence mechanisms for the Gulf of Paria and the Atlantic Ocean across Trinidad which can produce thunderstorm activity. On Thursday, sea breeze convergence will be possible across all coasts of Trinidad and Tobago due to light to no winds.
Sea breeze convergence mechanisms for the Gulf of Paria and the Atlantic Ocean across Trinidad which can produce thunderstorm activity. On Saturday, sea breeze convergence will be possible across all coasts of Trinidad and Tobago due to light to no winds.

Forecast For Saturday through Monday: A variably cloudy day is forecast, with periods of showers and isolated showers from midnight through the midmorning, and again through the afternoon. Activity will be isolated to scattered across both islands, with heavier showers and thunderstorms Eastern and Northern areas initially, then across Western areas during the afternoon, gradually moving westward to southwestward on Saturday. A resurgence of activity is forecast after nightfall across both islands.

Near identical conditions are forecast on Sunday and Monday, with Monday being the wettest of the three days. Due to the wind shifts associated with the tropical wave, on Monday, activity will be moving from the southeast to the northwest, allowing heavier rainfall to focus along the south-facing slopes of the Northern Range.


Marine: Seas are forecast to be slight across T&T. In open waters of both islands, waves up to 1.25 meters are forecast. In sheltered areas, near calm conditions are forecast with the exception of during heavy showers or thunderstorms, where waves will remain generally below 1 meter and choppy seas. Seas by late Monday in open waters will become moderate, with waves up to 1.5 meters.

Winds are forecast to be generally calm, with light winds up to 15 knots from the northeast to east on Saturday into Sunday, and by late Sunday into Monday, east to southeast.

Hazards: The main hazards include heavy rainfall, which may reduce visibility and gusty winds in downburst-like fashion, up to and in excess of 55 KM/H. Street or flash flooding will be possible in slow-moving 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 light winds, funnel clouds are also possible.

Temperatures

Through the weekend, the ongoing hot spell will be broken. Temperatures are forecast to be notably cooler than the past week due to increased cloud cover from Sunday.

Across both islands on Saturday, with a mostly sunny to partly cloudy morning, still hot and humid conditions are forecast. The maximum high for Trinidad is forecast to be near 34.0°C, possibly higher in urban and built-up areas, and areas that experience partly cloudy skies with little rainfall. In Tobago, the maximum high is forecast to be near 32.0°C or slightly above.

On Sunday and Monday, still warm and humid conditions are forecast. The maximum high for Trinidad is forecast to be near 31.0°C, possibly higher in urban and built-up areas, and areas that experience partly cloudy skies with little rainfall. In Tobago, the maximum high is forecast to be near 30.0°C.

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

Tropical Wave 51 and the ITCZ Impacts and Forecast Confidence.
Tropical Wave 51 and the ITCZ Impacts and Forecast Confidence.

Winds

Sustained surface winds will be light to nonexistent. However, during heavy showers and thunderstorms, sustained winds between 25 KM/H and 35 KM/H with gusts up to and in excess of 55 KM/H are possible.

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

Across most areas of Trinidad and Tobago, daily maximum accumulation totals are forecast to be below 15 millimeters. However, in areas where heavy thunderstorms and showers occur, up to 25 millimeters are possible daily, with isolated daily totals up to and in excess of 50 millimeters, particularly in slow-moving downpours or thunderstorms.

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.

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 as Tropical Wave 51 moves across the islands.

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

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