Forecast: Heavy Rainfall, Thunderstorms Over Next 48 Hours

Heavy rainfall in Port of Spain on May 5th, 2021 (Azlan Mohammed)

A low-level trough, favorable low-level convergence, and an overall supportive atmosphere with abundant moisture are forecast to trigger and fuel showers and thunderstorms for T&T over the next 48 hours.

Overall, heavier rainfall will favor the southern and eastern areas of Trinidad and Tobago. In areas where isolated heavy showers or thunderstorms occur, isolated higher rainfall totals are expected, particularly along Western Coastal Trinidad.

Based on model guidance, through the next 48 hours, high accumulated precipitation is forecast for T&T across all locations of the country. Chances of flooding will be medium to high as soils remain saturated from rainfall on Tuesday.

The Trinidad and Tobago Meteorological Service has discontinued the Adverse Weather Alert for Trinidad and Tobago at 6:06 PM Saturday 8th May 2021.

The Trinidad and Tobago Meteorological Service has also issued a Riverine Flood Alert for Northern and Central Trinidad, in effect from 3:00 PM Saturday 8th May 2021 through 3:00 PM Sunday 9th May 2021.

The Forecast

Showers and light to moderate rainfall have already begun across Trinidad and Tobago. On Friday, light rain with isolated to scattered showers are forecast through the mid-morning, accompanied by mostly cloudy to overcast skies. From the mid-morning through the afternoon, light to moderate rain and scattered to widespread showers are forecast across Trinidad and Tobago. During the late morning through the afternoon, isolated thunderstorm activity and heavy to violent showers are forecast to initially favor southern and eastern areas of Trinidad and western coastal Trinidad during the afternoon. Intermittent rain and showers will persist through the evening, gradually settling.

On Saturday, inclement weather is likely. From midnight, periods of rain, scattered to widespread showers, and isolated thunderstorms are forecast, favoring Tobago, northern and eastern Trinidad. These conditions will persist through the morning with mostly cloudy to overcast skies. By the late morning through the afternoon, isolated to scattered showers at heavy to violent intensities are forecast across both islands with isolated thunderstorm activity. Conditions are forecast to gradually settle into the evening with intermittent light rain and shower activity. Chances of gusty winds up to and in excess of 75 KM/H and street/flash flooding highest on this day!

On Sunday, conditions are forecast to gradually settle, with isolated to scattered showers throughout the day, favoring Trinidad, interrupting partly to mostly cloudy and breezy conditions.

Note that Saharan Dust levels are forecast to remain low through the weekend, with no major surges forecast over the next 5 days.

Marine: Seas are forecast to be moderate through the weekend, with waves up to 2.0 meters in Trinidad and Tobago’s open waters. Across sheltered coastlines, waves up to 1.0 meter are likely. In heavy showers or thunderstorms, sheltered seas may become choppy. Note that long period swells are forecast to affect T&T’s northern and eastern coastlines from Friday afternoon into the weekend, with swell periods up to 14 seconds from the north.


Overview: A low-level trough (not a tropical wave or the ITCZ – more on this later) is forecast to move across T&T on Friday. The passage of this trough will trigger an increased southeasterly flow of deep, tropical moisture.

An upper-level and mid-level low are northeast of the Lesser Antilles, moving northeastward. A trough extending from both of these lows places T&T on the divergent side of the low-pressure systems, with an upper-level jet north of T&T. This optimal atmospheric setup is forecast to allow for strong upper-level divergence and favorable low-level convergence.

Combined with the increased atmospheric moisture (up to 65mm), conditions will be ripe for heavy to violent showers and thunderstorm activity – capable of producing flooding rainfall. Moisture is forecast to decrease on Sunday as a high-pressure system regains dominance markedly. However, lingering moisture and instability will favor Trinidad, leading to a few light to moderate showers. The central Atlantic high-pressure system will remain dominant across T&T into Monday, where a dry and subsident environment will allow for fair conditions.

Through the forecast period, wind shear will be weak to moderate up to 30 knots from the west to southwest, bringing upper-level clouds from South America. This will limit the strength of thunderstorms mainly on Friday, keeping heavy showers and thunderstorms relatively brief and most of the heavy rainfall east of T&T. Wind shear is forecast to abate somewhat on Saturday, strengthening again on Sunday into Monday.

Temperatures

Across both islands, overnight lows are forecast to be mild, with daytime highs cool due to cloud cover throughout the week. As skies clear somewhat at times through the week, temperatures may trend warmer during the day.

The minimum low for Trinidad is forecast to be near 23.0°C, and as low as 22.0°C in valleys and forested areas. In Tobago, a minimum low near 24.0°C is forecast.

The maximum highs for Trinidad and Tobago are forecast to be near or below 30.0°C, up to 31.0°C in urban and built-up areas, particularly between 9:00 AM and 3:00 PM.

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

The main hazards are heavy rainfall and gusty winds, particularly in heavy to violent showers or thunderstorms, bringing the risk of street flooding or flash flooding. By mid-Saturday, due to heavy rainfall earlier this week, there will be the potential for riverine flooding, particularly in eastern and southeastern areas. With gusty winds and saturated soils, downed trees and landslides will also become a concern, particularly on Saturday.

Winds

Sustained surface winds moderate to fresh (20-38 KM/H). In shower activity, gusts up to and in excess of 55 KM/H are possible. Wind gusts up to 75 KM/H are possible on Saturday.

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

Over the next 48 hours, high rainfall accumulations are forecast. The most likely totals are between 25 millimeters and 50 millimeters for Trinidad and Tobago, with totals between 50 and 75 millimeters across Eastern and Southern Trinidad, as well as Eastern Tobago. In highly isolated areas, overall rainfall totals could reach and exceed 100 millimeters.

  • Friday: 10-20 millimeters are forecast, with isolated totals up to (and in excess of) 35 millimeters across Trinidad and Tobago. Higher totals are possible across eastern and southern Trinidad and areas of isolated heavy showers or thunderstorms.
  • Saturday: 15-30 millimeters are forecast, with isolated totals up to (and in excess of) 50 millimeters across Trinidad. Higher totals are possible across eastern and southern Trinidad and areas of isolated heavy showers or thunderstorms.
  • Sunday: Between 5-15 millimeters are forecast, with isolated totals in excess of 15 millimeters across southern and eastern Trinidad.

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. By mid-Saturday, riverine flooding will be possible, particularly in southeastern and eastern areas due to heavy rainfall earlier this week.

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.

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 next 48 hours, scattered rainfall is forecast interrupting mostly cloudy to overcast skies.

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

This is likely not a tropical wave or the ITCZ

There has been some discussion on whether the approaching trough was actually a tropical wave. Based on our analysis, it isn’t. In fact, there is another trough system out in the Atlantic, also being called a tropical wave at approximately 32W. A trough system is an area in the atmosphere with lower than usual pressure, generally expressed with a cyclonic curvature of winds. Both surface troughs became embedded within the flow of the Sub-tropical High-Pressure system and progressed eastward.

Diagram showing troughs and ridges.
Diagram showing troughs and ridges.

Interestingly, the Barbados Meteorological Service designated the Eastern Atlantic trough system as the first tropical wave for 2021 since the evening of May 4th, 2021. The Service analyzed the tropical wave initially as a wave aloft (higher in the atmosphere) but worked its way down to the surface on May 5th. Given that there is no clear indication this feature originated from Africa – the key characteristic of designating a Tropical Wave – we’d lean against calling it a tropical wave. For standardization (and counting tropical waves annually), we utilize the National Hurricane Center’s Tropical Analysis and Forecast Branch’s Unified Surface Analysis, in addition to our own analyses.

Tropical Wave Formation
Tropical Wave Formation

Another question – as the U.S. National Weather Service’s Weather Prediction Center‘s International Desk shows the Intertropical Convergence Zone (ITCZ) affecting us this weekend – but is it actually the ITCZ? Based on our analyses, which is also in line with the National Hurricane Center’s Tropical Analysis and Forecast Branch, the ITCZ is still on the northern coast of Brazil, forecast to remain there through the weekend.

VIIRS/MODIS Satellite Imagery showing the Intertropical Convergence Zone across the Southern Caribbean, Atlantic, and West Africa. The diagram also illustrates the northeasterly and southeasterly trade winds that form the ITCZ. Image: Trinidad and Tobago Weather Center.

If this was later in May, it would be more believable to think this was our first tropical wave. In fact, models indicate our first weak tropical wave possibly leaving the African coast on May 15th, with a more robust wave entering the Atlantic Ocean on May 25th. Generally, it takes 5-7 days for these waves to move across the Atlantic and then across T&T.

Meteorologically, this would be significant, as the passage of the first true tropical wave or the ITCZ across T&T would begin the 2021 Wet Season. What you would expect weather-wise, there is little difference between a trough with abundant moisture and a tropical wave – reduced wind speeds, cloudiness, showers, and thunderstorms.

Tropical Wave Structure
Tropical Wave Structure

However, this is likely not a tropical wave. What these trough passages do show is that the wetter than average 2021 Dry Season has come to fruition.

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