Forecast: Showers, Chance of Thunderstorms For T&T Through Friday

Increased cloudy periods with showers are forecast over the next 24-48 hours as a low-level trough moves across T&T. Conditions may support brief thunderstorm activity during Thursday and Friday afternoons, favoring Trinidad. Overall heavier rainfall will favor the eastern and southern areas of Trinidad.

There are no alerts, watches, or warnings in effect from T&T from the Trinidad and Tobago Meteorological Service at this time.

The Forecast

For tonight, increasingly cloudy skies are forecast with brisk showers moving across both islands. Heavier activity to favor eastern areas of both islands mainly nearing midnight.

On Thursday, isolated to scattered showers, with periods of light to moderate rain, are forecast across both islands. Showers may become moderate to heavy at times, with brief and isolated thunderstorm activity favoring Trinidad. This could bring street flooding, gusty winds, and lightning.

Conditions will briefly settle into the evening, with a resurgence of showers possible nearing midnight.

On Friday, another day of isolated to scattered showers, periods of rain, and isolated but brief thunderstorms are possible interrupting mostly cloudy skies. Thunderstorm activity to favor Trinidad during the late morning through the afternoon, with conditions settling into the evening. Brisk, isolated showers are possible overnight.

Even with the anticipated rainfall, moderate levels of Saharan Dust will remain present, with brief air quality improvements anticipated during rainfall. Sensitive groups are still advised to take the necessary precautions.

Marine: Seas are forecast to be moderate over the next 48 hours with waves between 1.5 to 2.0 meters in open waters. Along northern, eastern, and sheltered coastlines, waves near 1.0 meter are likely, with higher waves possible due to long-period northerly swells, particularly on Friday. In heavy showers or thunderstorms, sheltered seas may become choppy.


Overview: On Wednesday night into Thursday, a surface to low-level trough is forecast to move across T&T. This feature will bring moisture and instability at both low- and mid-levels of the atmosphere from equatorial regions, triggering showers and cloudiness.

As the trough moves westward, moisture and instability will linger into Friday, with daytime heating and possible sea breeze convergence allowing for heavier showers and brief thunderstorm activity to develop across Trinidad during the late morning through the afternoon.

Dominating the overall wind flow, a high-pressure system will remain in place into the weekend. However, the passage of this trough will weaken the pressure gradient with wind speeds gradually weakening. The high-pressure system will regain its strength into the weekend with mostly sunny and hazy skies but still, brisk, isolated showers are possible.

Wind shear will be strong, up to 50 knots from the southwest on Thursday and Friday. This will limit the strength of thunderstorms, keeping heavy showers and thunderstorms brief.

Temperatures

Across both islands, overnight lows are forecast to be mild to cool, with daytime highs cool due to cloud cover on Thursday. With partly cloudy to mostly cloudy skies on Friday, temperatures may trend warmer during the day.

The minimum low for Trinidad is forecast to be near 22.0°C, and as low as 20.0°C in valleys and forested areas. In Tobago, a minimum low near 24.0°C is forecast.

For Thursday, the maximum high for Trinidad is forecast to be near 30.0°C, up to 33.0°C in urban and built-up areas, particularly between 9:00 AM and 3:00 PM. In Tobago, the maximum high is forecast to be near 31.0°C.

On Friday, the maximum high for Trinidad is forecast to be near 31.0°C, up to 36.0°C in urban and built-up areas, particularly between 9:00 AM and 3:00 PM. In Tobago, the maximum high is forecast to be near 31.0°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

The main hazards over the next 48 hours would be locally heavy rainfall and gusty winds, particularly in moderate to heavy showers or thunderstorms, bringing the risk of street flooding or flash flooding.

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.

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 Trinidad, between 25 millimeters to 50 millimeters are possible, with overall higher rainfall totals favoring southern and eastern areas of the island through Saturday morning.

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.

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.

Isolated to scattered rainfall is forecast on Thursday and Friday.

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

This is likely not a tropical wave

GOES-EAST Imagery showing an approaching trough with a large upper-level trough spanning the Atlantic Ocean. (NOAA)
GOES-EAST Imagery showing an approaching trough with a large upper-level trough spanning the Atlantic Ocean. (NOAA)

There has been some discussion on whether the approaching trough was actually a tropical wave. Based on our analysis, it isn’t. This trough originated from a persisting upper-level trough (TUTT) across the Central Atlantic, which deepened in the atmosphere and produced a surface to low-level trough. A trough system is an area in the atmosphere with lower than usual pressure, expressed generally with a cyclonic curvature of winds. This surface trough 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 U.S. National Weather Service’s Weather Prediction Center‘s International Desk designated this trough system as the first tropical wave for 2021.

The Chief of the International Desk, Michel Davison, explained given the weather feature moved independently of the TUTT, he designated it as a tropical wave. Looking at a montage of visible satellite imagery, called a Hovmöller Diagram, he also indicated a feature emerging from Africa last weekend – the key characteristic of designating a Tropical Wave.

Tropical Wave Formation
Tropical Wave Formation

If this was May, it would be more believable to think this was our first tropical wave. It is still March – one of T&T’s driest months – with at least 2 more months of relatively dry weather ahead.

Meteorologically, this would be significant as the passage of the first true tropical wave 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 the possibility of 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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