Tropical Waves Forecast To Bring Scattered Showers, Isolated Thunderstorms This Week

Tropical Update Overview:
Area of Interest: The National Hurricane Center is forecasting an area of low pressure to develop by the end of this week, giving the area a low chance of tropical cyclone formation over the next 5 days. The system poses no threat to T&T.
Tropical Wave 4 – A weak tropical wave, east of T&T, is forecast to move across the country on Monday into early Tuesday.
Tropical Wave 6 – A low-latitude, weak tropical wave in the Central Tropical Atlantic, is forecast to move across T&T on Wednesday into Thursday, bringing scattered showers and isolated thunderstorms.
Tropical Wave 7 – A tropical wave has just emerged off West Africa. Significant Saharan Dust will hinder this wave as it moves across the Atlantic, forecast to move across T&T early next week.

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.
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 mainly on Monday, Wednesday into Thursday, and possibly from late Sunday into next week. 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 riverine flooding may be possible mainly on Monday to saturated soils and elevated river levels.

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.

Area of Interest In The Southwest Caribbean

The National Hurricane Center Tropical Weather Outlook as of 8:00 PM Sunday 6th June 2021
The National Hurricane Center Tropical Weather Outlook as of 8:00 PM Sunday 6th June 2021

As of the 8:00 PM Tropical Update, the National Hurricane Center is forecasting an area of low pressure developing in the southwestern Caribbean Sea by Thursday or Friday. Some gradual development will be possible thereafter as the system moves slowly northwestward toward Central America over the weekend.

Between 80% and 85% of the European ensemble models show this system producing winds of tropical depressions strength over the next 5 days, with up to 30% of the EPS runs showing tropical-storm-force winds in the Gulf of Mexico. The operational UK model is also hinting at development later this weekend.

The European EMCWF ensemble models (EPS) showing the probability of tropical-storm-force winds over the next 5 days across the Atlantic Basin. (weathermodels.com)
The European EMCWF ensemble models (EPS) showing the probability of tropical-storm-force winds over the next 5 days across the Atlantic Basin. (weathermodels.com)

This system, whether it forms or not, poses no threat to T&T and the Lesser Antilles. The next name on the list of names for the 2021 Atlantic Hurricane Season is Bill.

List of names for the 2021 Atlantic Hurricane Season, including the supplemental list of names.
List of names for the 2021 Atlantic Hurricane Season, including the supplemental list of names.

Tropical Wave 4

Tropical Wave 4 analyzed from 124 southward, along 56W, with scattered moderate convection and widespread cloudiness. (Weathernerds)
Tropical Wave 4 analyzed from 124 southward, along 56W, with scattered moderate convection and widespread cloudiness. (Weathernerds)

As of 11:00 PM, the 4th tropical wave for the 2021 Atlantic Hurricane Season is located from 14N, along 56W, based on our analysis. The wave axis is moving westward around 15 knots (18 to 27KM/H). The wave is at the edge of a Saharan
Dust surge. The wave is also embedded in a strong wind shear environment, which is limiting the convection, resulting in scattered moderate showers 5N to 10N between 46W and 58W.

This tropical wave is forecast to move across T&T on Monday, with moisture moving across the island as early as the early-morning hours. However, there is dry air at the low- to mid-level of the atmosphere, and with strong wind shear, heavy showers and thunderstorms will struggle to develop. Still, cloudiness will increase from mid-morning through the afternoon, and with increased moisture, few showers and thunderstorms are possible from the late morning through the afternoon.

Heavier rainfall, though sparse, is forecast to favor southern and eastern areas of Trinidad, with rainfall spreading across the country as the day progresses into early Tuesday. Conditions will dry out as a surge of Saharan Dust arrives across the region.

Tropical Wave 6

Tropical Wave 6 is analyzed further east from 11N southward along 35W. (Weathernerds)
Tropical Wave 6 is analyzed further east from 11N southward along 35W. (Weathernerds)

As of 11:00 PM, the 6th tropical wave for the 2021 Atlantic Hurricane Season is located from 11N southward, along 35W, based on our analysis. The wave is embedded in a dry, Saharan Dust surge in which dry and dusty air is hindering the development of strong convection. Isolated showers are noted in the vicinity of the ITCZ from 02N to 05N between 37W and 34W.

This wave is forecast to move across the region on Wednesday into Thursday. Overall instability and moisture will be favorable for the development of cloudy weather, scattered showers, and isolated thunderstorms.

Atmospheric conditions are forecast to be marginally favorable through the 48 hour period, particularly on Thursday, for heavy showers and thunderstorms. However, Saharan Dust and 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.

Tropical Wave 7

Tropical Wave 7 is analyzed further east from 10N southward along 19W. (Weathernerds)
Tropical Wave 7 is analyzed further east from 10N southward along 19W. (Weathernerds)

As of 11:00 PM, the 7th tropical wave for the 2021 Atlantic Hurricane Season is located from 10N southward, along 19W, based on our analysis. The wave axis is moving westward around 10 to 15 knots (18 to 27 KM/H) with numerous strong showers and isolated thunderstorms.

This wave recently emerged in the Atlantic Ocean, and model guidance shows the wave making its way across the Lesser Antilles early next week. However, it is too early to tell what impacts, if any, this wave may have on T&T. Notably, dense Saharan Dust will be present across the Atlantic as this wave moves westward.

The Forecast

At this time, there are no alerts, watches, or warnings issued for T&T from the Trinidad and Tobago Meteorological Service.

Monday: Initially, partly to mostly cloudy and windy conditions are forecast through the early morning, with brisk showers. By mid-morning, cloudiness is forecast to build across Trinidad and later Tobago, with few showers and isolated thunderstorms favoring Trinidad through the afternoon. Periodic showers with brisk rainfall and isolated thunderstorms are forecast throughout the remainder of Monday into the overnight hours with partly to mostly cloudy skies.

Tuesday: Early morning showers and the isolated thunderstorm are forecast to give way to a mostly hazy, windy, and sunny day with few brisk isolated showers. However, cloudiness is forecast to increase near midnight, with showers returning to T&T.

Wednesday: Mostly cloudy to overcast conditions with scattered showers and thunderstorms and periods of rain through the day, with thunderstorm activity favoring Trinidad. Barring the lingering shower or two, conditions will briefly settle into the evening, with a resurgence of rainfall possible near midnight.

Thursday: From midnight, periods of rain with scattered showers and isolated thunderstorms are forecast, mainly favoring Trinidad, and continue through the late afternoon, with cloudy, windy, and hazy periods. Conditions will settle into the evening, becoming increasingly hazy due to a surge in Saharan Dust

Friday: Windy and hazy conditions are forecast throughout the day, with brisk, isolated showers across the country, interrupting mostly sunny to partly cloudy skies. Similar conditions are forecast into the night and through the weekend.

Through the forecast period, Saharan Dust concentrations will be high to significant, particularly from Thursday onward. Seas are forecast to remain moderate in open waters and less than 1.0 meters by choppy 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 Tuesday and Friday.

On days with increased cloud cover, such as Monday, Wednesday and Thursday, 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

Tropical-wave-enhanced rainfall is forecast mainly on Monday, Wednesday into Thursday, and possibly from late Sunday into next week. 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 the very low risk of riverine flooding may be possible mainly on Monday to saturated soils and elevated river levels.

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 25 to 50 millimeters. However, localized totals in excess of 50 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.

  • Monday: Between 5 to 15 millimeters of rainfall across the country, with isolated totals up to 25 millimeters, favoring southern and eastern Trinidad. In highly isolated areas such as across western coastal Trinidad, isolated totals could exceed 25 millimeters.
  • Tuesday: Less than 5 millimeters across the country, with highly isolated totals exceeding that amount.
  • Wednesday: Between 5-20 millimeters across the country, with higher rainfall totals favoring eastern and southern halves of Trinidad in excess of 25 millimeters.
  • Thursday: Less than 10 millimeters across the country, with higher rainfall totals favoring southern and southwestern Trinidad, up to 20 millimeters.
  • Friday: Across the country, less than 5 millimeters. Little to no rainfall is expected.

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. On Monday, there is a low risk of riverine flooding due to elevated river levels coming out of this past weekend.

Other Weather Impacts

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.

Landslides: With saturated soils, landslides are possible in elevated areas, particularly hilly areas in southern and northeastern Trinidad, as well as across Tobago.

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 Monday, Wednesday, and the first half of Thursday, isolated to scattered rainfall is forecast, with highly isolated to no rainfall through the remainder of the forecast period.

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

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