Tropical Update – Tropical Waves, ITCZ To Bring Heavy Rainfall To T&T This Week

Tropical Update Overview:
Tropical Wave 59 – This tropical wave moved across the region on Thursday 31st October 2019. This wave brought heavy rainfall across parts of the French Antilles and interacted with our local climatic features in Trinidad, triggering intense thunderstorms across Northwestern Trinidad causing widespread street flooding.
Tropical Wave 60 – This tropical wave is forecast to move across the Lesser Antilles Sunday into Monday, bringing elevated moisture that may fuel showers and thunderstorms across Trinidad and Tobago, as well as increased cloudiness. This wave is forecast to bring the ITCZ across our region this week, potentially creating a multi-day rainfall event. A surge of low-level winds are forecast to follow this wave, peaking during the passage of Tropical Wave 61.
Tropical Wave 60 – This tropical wave is forecast to move across Trinidad and Tobago beginning late Tuesday through Thursday, interacting with the ITCZ. Periods of heavy showers and thunderstorms are possible/likely during this time frame, with severe weather likely. Gusty winds are also likely to accompany shower and thunderstorm activity.
The ITCZ – Following the passage of Tropical Wave 60, the ITCZ is forecast to linger across Trinidad and Tobago while a Saharan Dust plume moves across the remainder of the Lesser Antilles. A multi-day rainfall event is forecast through much of the upcoming week, with rainfall totals between 2-4 inches (50-100 millimeters) and isolated totals, through 2 AM Saturday, up to 8 inches (200 millimeters). No direct tropical threats to Trinidad and Tobago are forecast over the next week but heavy rainfall is forecast, particularly on Tuesday and Wednesday. Street flooding, flash flooding, gusty winds in excess of 65 KM/H, landslides and frequent lightning possible. There is the very low chance of riverine flooding, by the end of the week.

Before we dive into the Tropical Update, a few notes:

  • Tropical waves are a normal part of the rainy season.
  • Not every tropical wave will form into a tropical cyclone.
  • Weaker tropical waves produce more rainfall across Eastern parts of the islands with mostly cloudy conditions and a few showers across western parts of the islands.
  • Rainfall will be more isolated and intermittent with weaker tropical waves that do not have ITCZ or upper-level support.
  • Saharan Dust may weaken tropical waves.

You can read more about the weather associated with Tropical Waves, as well as what to expect as these waves move through the region below.

Tropical Wave 59

This tropical wave broke off from Tropical Wave 60. Though the wave didn’t bring prolonged active weather to Trinidad and Tobago, periods of showers and thunderstorms affected the Northern Windwards, including the French Antilles, dumping up to 4 inches of rainfall inside 24 hours on Thursday.

As of the 5:00 PM Tropical Update, the axis of Tropical Wave 59 analyzed with axis along 71W from 06N-18N. After carefully revising the latest scatterometer data, model guidance and satellite signature, this wave’s axis will likely be repositioned for the 18 UTC map to 65W.

Scattered moderate convection is noted east of the wave’s axis affecting the Lesser Antilles and adjacent waters.

This wave continues to influence our wind regime across Trinidad and Tobago, which created ideal conditions for heavy showers and thunderstorms to develop across Western and Northern Trinidad.

For Trinidad and Tobago, moisture from this wave will linger across the region through the weekend, ahead of Tropical Wave 60, fuelling heavier showers and thunderstorms resulting from daytime heating, sea breeze convergence, and orographic precipitation. See below for details.

Tropical Wave 60

Activity associated with Tropical Wave 60 along 55W nearing the Lesser Antilles with minimal convective activity. This wave is forecast to traverse the region Sunday into Monday, bringing with it a surge of Saharan Dust and low-level winds, mainly North of Trinidad and Tobago. Credit: Weathernerds
Activity associated with Tropical Wave 60 along 55W nearing the Lesser Antilles with minimal convective activity. This wave is forecast to traverse the region Sunday into Monday, bringing with it a surge of Saharan Dust and low-level winds, mainly North of Trinidad and Tobago. Credit: Weathernerds

As of the 2:00 PM Tropical Update, Tropical Wave 60 is along 55W S of 18N, moving west at 10-15 knots. This wave is in the leading edge of a dry/dusty Saharan air mass and is well depicted in satellite imagery. Little convective activity is noted with this wave at this time.

The tropical wave is forecast to move across the Lesser Antilles Sunday into Monday bringing another round of showers and thunderstorms to Trinidad and Tobago.

No development is expected of this wave but a surge of trade winds is forecast to accompany this wave next week. Wind gusts in excess of 65 KM/H may accompany shower and thunderstorm activity, lingering through much of this upcoming week.

Generally, less than 1 inch (25 millimeters) is expected, with up to 2 inches possible in isolated areas, mainly across Trinidad. No significant impacts are expected beyond isolated street/flash flooding and gusty winds in heavier showers and thunderstorms.

This wave is forecast to drag the ITCZ across the region, lingering through much of this week, triggering a multi-day rainfall event.

Tropical Wave 61

Activity associated with Tropical Wave 61 along 42W nearing the Lesser Antilles. This wave is embedded within the ITCZ and is forecast to bring a multi-day rainfall event to T&T this upcoming week as of this tropical update. Credit: Weathernerds
Activity associated with Tropical Wave 61 along 42W nearing the Lesser Antilles. This wave is embedded within the ITCZ and is forecast to bring a multi-day rainfall event to T&T this upcoming week as of this tropical update. Credit: Weathernerds

As of the 2:00 PM Tropical Update, Tropical Wave 61 is a low amplitude central Atlantic tropical wave extends its axis along 42W from 00N-12N, moving west at 10-15 knots. This wave is well depicted in model guidance. Scattered moderate convection is noted in the wave’s environment from 05N10N between 38W-50W. This convection is also related to the ITCZ.

This tropical wave is forecast to gradually inch closer to Trinidad and Tobago, dragging the ITCZ further northward. Hence, periods of showers and thunderstorms are becoming increasingly likely this week, with the wettest day coinciding with the passage of this tropical wave on Wednesday, interacting with the ITCZ. See below for details on impacts to Trinidad and Tobago

What does all of this mean for Trinidad and Tobago?

Saturday: The axis of Tropical Wave 59 continues to move away from the Lesser Antilles but it continues to influence our wind regime. The axis of Tropical Wave 60 is approaching the Lesser Antilles, so we can expect the overall winds to chance from southeasterly to northeasterly through Saturday night into Sunday morning. With this, isolated showers and possible thunderstorms remain in the forecast for both islands, mainly for Northern and Eastern Trinidad, overnight tonight interrupting partly cloudy skies.

Sunday: The axis of Tropical Wave 60 will be moving across the Lesser Antilles. Though a significant surge in moisture is not expected, favorable upper-level conditions are forecast to support heavy shower and thunderstorm development. Nearing midnight into Sunday morning, showers are forecast to favor Northern and Eastern Trinidad, as well as Tobago, gradually moving across both islands. By daybreak Sunday morning, periods of showers and thunderstorms are forecast to subside, leading to a partly cloudy, and at times, sunny mid- to late morning, with showers and thunderstorms returning by midday through the afternoon across both islands. These showers and thunderstorms will interrupt partly cloudy to cloudy skies and persist into nightfall, gradually decreasing in coverage and intensity.

Monday: The axis of Tropical Wave 60 will continue to affect T&T, with showers and thunderstorms developing after midnight, affecting both islands. Periods of showers and thunderstorms are forecast to interrupt partly cloudy to overcast conditions. Note that activity will be intermittent throughout the day. Winds are forecast to increase on Monday, so particularly Monday through the remainder of the week, gusts in excess of 65 KM/H are likely in heavier showers and thunderstorms. Lingering showers, remaining isolated and interrupting partly cloudy skies, are forecast by the late evening through the night.

An adverse weather alert may be issued for Tuesday into Wednesday, due to the elevated threat of wind gusts in excess of 65 KM/H, landslides and heavy rainfall which may trigger flooding.

Tuesday: The ITCZ affects Trinidad and Tobago, with Tropical Wave 61 approaching, moving across the region by nightfall. Isolated showers are forecast to persist through the early morning into daybreak, becoming more scattered to widespread, and a few heavier showers and thunderstorms developing by mid-morning. Scattered showers, rain, and thunderstorms are forecast to persist throughout the day across both islands, with periods of mostly cloudy to overcast skies. Conditions are forecast to become temporarily settled by the evening, with a few lingering showers but activity is forecast to increase again after midnight. Increased wind speeds would result in higher gusts near thunderstorms.

Wednesday: Scattered to widespread rain, showers, and thunderstorms throughout the 24-hour period, with coverage reducing by the late evening.

Thursday: The ITCZ begins to drift southwards, as Tropical Wave 61 moves well west. Heavier showers and thunderstorms forecast to favor Western and Southern Trinidad, mainly during the late morning through the evening.

Activity will be isolated to scattered throughout the week, becoming widespread on Tuesday and Wednesday.

Impacts This Upcoming Week

Winds: Sustained surface winds between 15 KM/H and 30 KM/H with gusts in excess of 65 KM/H are likely in heavy showers or thunderstorms across Trinidad and Tobago throughout the week.

With wind gusts in excess of 65 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.

High Wind Event Precautions
High Wind Event Precautions

Rainfall: Throughout the week, daily rainfall accumulations are forecast to be isolated totals of 15-25 millimeters, with in excess of 50 millimeters in isolated thunderstorms or heavy downpours.

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 a very low chance of riverine flooding by the end of the week if overall rainfall totals materialize.

Frequent Lightning: In addition, with forecast thunderstorms, frequent lightning is likely. Lightning can cause power outages, voltage dips, damage to life and property, particularly during cloud-to-ground lightning strikes.

Landslides: Particularly across Northwestern Trinidad, soils are very saturated with persistent heavy afternoon thunderstorms since Thursday, with a number of landslides over the last 3 days. With additional rainfall in the forecast, landslides are possible through Tuesday and likely through Thursday in landslide-prone and elevated areas.

Hazardous Seas: A long period swell event is underway across Trinidad, Tobago, and the Eastern Caribbean. The peak of this swell event is forecast to subside overnight Saturday but swells will continue through Monday afternoon. As a result, a hazardous seas alert remains in effect for the Northern and Western coasts of both islands.

Though long period swells are forecast to subside by Monday, an increase in strong low-level winds will keep seas at moderate levels, with waves up to 2.5 meters in open waters and even at rough levels on Wednesday, with waves in excess of 2.5 meters, particularly along the Eastern coasts of both islands. Hence, it is within the realm of possibility another hazardous sea alert for the Eastern coasts of Trinidad and Tobago.

Why I May Not/Will Not See Constant 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.

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

Tropical Cyclone Climatology

Tropical Cyclone Points of Origin during the month of November. Though much of the points of origin are west or north of T&T as we move into the last month of the Hurricane Season, we still need to pay attention to some of these waves and the ITCZ, which have historically brought major flooding events to T&T, hence all low-pressure systems in the Atlantic should be closely monitored, as we do in the Tropical Update.

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