Heavy Rainfall Forecast From Friday Into Next Week Due To Back-To-Back Tropical Waves

Tropical Update Overview:
There are no systems in the Atlantic, as of 8:00 PM Wednesday, being officially monitored for tropical cyclone development.
Tropical Wave 3 (Now Tropical Wave 5) – This tropical wave’s wind signature was absorbed by the dominating trade wind flow of the Atlantic high-pressure system. Tropical Wave 3 is forecast to merge with a stationary low-level trough off the coast of South America, now analyzed as the 5th tropical wave for the 2021 Atlantic Hurricane Season. This large envelope of moisture and instability is forecast to move across T&T on Friday, with cloudiness and showers beginning on Thursday.
Tropical Wave 4 – A large tropical wave is located in the far eastern Atlantic Ocean, forecast to move across the region late Monday into Tuesday of the upcoming week. This will bring additional rainfall across the country.

Total Precipitable Water Imagery showing the Tropical Waves (analyzed by the National Hurricane Center, Weather Prediction Center, and TTWC) that exist east of Trinidad and Tobago.

Impacts to T&TThere are no direct tropical threats facing T&T over the next 5 days. Tropical-wave-enhanced rainfall is forecast from late Thursday through the weekend. From Friday, the main hazards include heavy rainfall and gusty winds, particularly in moderate to heavy showers or thunderstorms, bringing the risk of street flooding or flash flooding. In thunderstorm activity, frequent lightning is possible. Gusty winds up to and in excess of 55 KM/H may cause power dips/outages, downed trees, and localized wind damage. Landslides and minor to moderate riverine flooding may be possible by the start of the upcoming week due to saturated soils and additional forecast rainfall.

Before we dive into the forecast and 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.
https://ttweathercenter.com/tropical-waves/

Tropical Wave 3 (Now Tropical Wave 5)

Tropical Wave 5 analyzed from 12N southward, along 55W, with scattered moderate convection and widespread cloudiness. (Weathernerds)
Tropical Wave 5 analyzed from 12N southward, along 55W, with scattered moderate convection and widespread cloudiness. (Weathernerds)

As of the 2:00 AM Tropical Update, the 5th tropical wave for the 2021 Atlantic Hurricane Season is located from 12N, along 56/57W, based on our analysis. The wave axis is moving westward around 10 to 15 knots (18 to 27KM/H). This tropical wave, introduced by the National Hurricane Center in their 18Z analysis, was a stationary low-level trough near the north coast of French Guiana. Presently, no significant convection is noted with this tropical wave.

Typical wind profile and weather associated with a tropical wave
Typical wind profile and weather associated with a tropical wave

Tropical Wave 3, which was located slightly further east, along 41/42W, has lost its characteristic wave signature and was absorbed by the dominating trade wind flow of the Atlantic high-pressure system. However, the moisture and instability are forecast to move westward and gradually move across T&T following the passage of Tropical Wave 5 this weekend.

Tropcial Wave 5 is forecast to move across Trinidad and Tobago on Friday. Moisture ahead of the wave axis will begin to spread across the country near and after midnight Thursday into Friday, bringing increased cloudiness and isolated showers.

With high levels of atmospheric moisture and highly favorable mid- to upper-level conditions, scattered to widespread showers, scattered thunderstorms, and cloudy skies are forecast mainly from Friday through Sunday. The heaviest rainfall is forecast to occur on Friday into Saturday across T&T. Wind shear is also forecast to be light to moderate from the west to southwest, allowing for heavy shower or thunderstorm development but still keeping heavier overall rainfall offshore eastern Trinidad.

One hindering Notably, low to mid-level conditions may be unfavorable for much of Friday, which could delay activity from forming.

Tropical Wave 4

Tropical Wave 4 is analyzed further east from 12N southward along 30/31W. (Weathernerds)
Tropical Wave 4 is analyzed further east from 12N southward along 30/31W. (Weathernerds)

As of the 2:00 AM Tropical Update, the 4th tropical wave for the 2021 Atlantic Hurricane Season is located from 12N southward, along 30/31W, based on our analysis. The wave axis is moving westward around 10 to 15 knots (18 to 27 KM/H). Scattered moderate and isolated strong convection is noted from 01N to 10N between 24W and 27W.

This wave is forecast to move across the region on Monday into Tuesday of the upcoming week.

Overall instability and moisture will be favorable for the development of cloudy weather, scattered showers, and isolated thunderstorms. Strong to very strong wind shear from the west will limit the strength of showers and thunderstorms, as well as keeping overall heavier rainfall totals east of T&T.

The Forecast

At this time, there are no alerts, watches, or warnings issued for T&T from the Trinidad and Tobago Meteorological Service. Due to the potential for heavy rainfall on Friday or Saturday, an Adverse Weather Alert may be possible.

Thursday: Initially, a mostly hot, breezy, and sunny day is forecast. By the late morning through the afternoon, a gradual increase in cloud cover is forecast through the night due to an approaching tropical wave. Though heavier rainfall will stay offshore, showers and the odd thunderstorm will favor Trinidad’s eastern and southern halves during the afternoon through midnight, gradually spreading across the country.

Friday: Isolated to scattered showers and periods of rain with mostly cloudy to overcast skies are forecast across Trinidad and Tobago. Isolated to scattered thunderstorm activity is also expected throughout the day, with heavier rainfall favoring eastern and northern areas of both islands. However, overall higher totals are forecast to remain offshore. Lingering showers and isolated thunderstorms are forecast through the overnight hours.

Saturday: Partly cloudy conditions, becoming increasingly cloudy after daybreak with occasional showers and isolated heavy showers/thunderstorms from the last morning through the afternoon. Heavy showers and thunderstorm activity is forecast to favor southern and western areas, though overall higher rainfall totals are forecast offshore eastern and northern Trinidad. Conditions to gradually settle into the evening, with a few lingering showers.

Sunday: Partly cloudy skies are forecast across Trinidad and Tobago, with isolated showers throughout the day. During the late morning through the afternoon, isolated heavy showers and the odd thunderstorm are possible across western areas of Trinidad. A mostly settled and slightly hazy night is expected, barring the odd shower.

Monday: Partly cloudy skies are forecast across Trinidad and Tobago, with isolated showers throughout the day. During the late morning through the afternoon, isolated heavy showers and the odd thunderstorm are possible across western areas of Trinidad. Increasing cloudy weather is forecast through the afternoon with isolated showers.

Tuesday: Isolated to scattered showers with cloudy periods are forecast across T&T, gradually settling into the evening. Heavier rainfall to favor southern and eastern areas, with isolated thunderstorms possible during the late morning through the afternoon across western Trinidad.

Wednesday: Partly cloudy skies are forecast across Trinidad and Tobago, with isolated showers throughout the day. During the late morning through the afternoon, isolated heavy showers and the odd thunderstorm are possible across western areas of Trinidad.

Through the forecast period, Saharan Dust concentrations will be low, though a slight increase is expected Sunday into Monday, with seas remaining moderate in open waters and less than 1.0 meters to near calm in sheltered areas. In heavy showers or thunderstorms, sheltered seas may become choppy.

Temperatures

Across both islands, overnight lows are forecast to be mild, with daytime highs warm.

The minimum low for Trinidad is forecast to be 23.0°C to 25.0°C. In Tobago, a minimum low between 24.0°C to 26.0°C is forecast. The maximum high for Trinidad and Tobago is forecast to be between 30.0°C and 33.0°C, higher across Trinidad.

Temperatures may record higher in urban and built-up areas, particularly between 9:00 AM and 3:00 PM. The maximum heat index in Trinidad is forecast to be between 35.0°C and 40.0°C while across Tobago between 33.0°C and 36.0°C. The warmest temperatures are likely on Thursday, Sunday and Monday.

On days with increased cloud cover, such as Friday, Saturday and Tuesday, maximum high temperatures may be lower.

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, absorbing solar radiation more than surrounding areas.

Forecast Impacts

From Friday, the main hazards include heavy rainfall and gusty winds, particularly in moderate to heavy showers or thunderstorms, bringing the risk of street flooding or flash flooding. In thunderstorm activity, frequent lightning is possible. Gusty winds up to and in excess of 55 KM/H may cause power dips/outages, downed trees, and localized wind damage. Landslides and minor to moderate riverine flooding may be possible by the start of the upcoming week due to saturated soils and additional forecast rainfall.

Winds

Sustained surface winds up to 35 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

Over the next seven days, overall higher rainfall is forecast to favor eastern and southern Trinidad and Tobago with accumulations between 75 to 100 millimeters. However, localized totals up to 150 millimeters are possible in prolonged heavy showers or isolated thunderstorm activity. Note that highly isolated areas across Western areas of Trinidad may exceed daily forecast totals.

  • Thursday: Less than 5 millimeters across Trinidad, with less than 15 millimeters across Tobago. Isolated totals favoring eastern and southern Trinidad up to 25 millimeters.
  • Friday: Between 10-25 millimeters across the country, with higher rainfall totals favoring northern and eastern areas up to 50 millimeters. Highly isolated areas may exceed 50 millimeters.
  • Saturday: Between 10-20 millimeters across the country, with higher rainfall totals favoring eastern and northern areas in excess of 25 millimeters. Note that the GFS is trending higher, up to 50 millimeters across the country.
  • Sunday: Less than 5 millimeters across the country, with higher rainfall totals favoring eastern and southern areas up to 15 millimeters. Note that the GFS is trending higher, up to 25 millimeters across the country.
  • Monday: Across the country, less than 5 millimeters. In isolated areas, up to 20 millimeters favoring southern and eastern areas.
  • Tuesday: Less than 10 millimeters across the country, with areas across eastern Trinidad up to 15 millimeters. The GFS remains the outlier with highly isolated areas up to 40 millimeters.
  • Wednesday: Less than 10 millimeters across the country, with isolated totals up to 20 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. By the end of the weekend, there is a minor to moderate risk of riverine flooding.

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.

On Friday into Saturday, scattered to widespread rainfall is forecast, with isolated to scattered rainfall through the remainder of the forecast period.

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

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