T&T’s Adverse Weather Event: How Bad Is It Forecast To Be?

It’s a question everyone is asking – will the winds over the next three days be as bad as earlier (in reference to early September)? That severe weather event was due to a large cluster of thunderstorms associated with Tropical Wave 41, in the presence of a low-level jet (an area of fast-moving winds at low levels of the atmosphere). Thunderstorms then organized into what is known as a squall line and produced wind gusts as high as 95 KM/H across Trinidad.

There is a similar setup over the next five days, but models aren’t showing the development of a squall line like what we experienced a few months ago. Instead, T&T is expected to experience several days of elevated winds and strong gusts accompanying brief showers and thunderstorms.

Here is what you need to know:

— Tropical Wave 58, Tropical Wave 59, an area of low pressure, and the Intertropical Convergence Zone are forecast to bring rainfall to Trinidad and Tobago over the next five days, with heavier rain remaining south and east of T&T due to strong westerly wind shear.
— The main hazard through Sunday will be elevated sustained winds up to 40 KM/H and wind gusts in excess of 55 KM/H, with peak winds forecast on Saturday. Winds of this strength can cause power dips and outages, fallen trees, utility poles, and lines, as well as sporadic areas of wind damage.
— On Saturday and generally from Monday through the first half of next week, the hazards shift to mainly heavy rainfall and flooding. Generally, the street and flash flooding threat will increase by the weekend, with the potential for riverine flooding by Monday.
Lightning will accompany thunderstorms. Funnel clouds and waterspouts are unlikely through the forecast period due to elevated low-level winds. Stronger thunderstorms are possible from Monday.
— An Adverse Weather Alert (Yellow Level) is in effect from the Trinidad and Tobago Meteorological Service at this time until 6:00 PM Saturday.

The Forecast

Moisture associated with Tropical Wave 58, Tropical Wave 59, the ITCZ, and a surface trough east of T&T. (SSEC/CIMSS/UW-Madison)
Moisture associated with Tropical Wave 58, Tropical Wave 59, the ITCZ, and a surface trough east of T&T. (SSEC/CIMSS/UW-Madison)

Thursday: Early morning showers and thunderstorms are forecast to give way to a partly to mostly cloudy, hazy, and breezy day. By the late morning through the afternoon, brief showers and gusty winds are forecast to develop across both islands, with heavier rainfall favoring Trinidad, interrupting hazy skies. Isolated to scattered showers and thunderstorms are forecast to develop by the evening and continue through the night, favoring Trinidad initially, gradually spreading across Tobago.

Friday: After midnight, showers and isolated thunderstorms are forecast to move from the southeast spreading across both islands. Rainfall is forecast to initially subside by daybreak, giving way to a variably cloudy, breezy, and hazy day. By mid-morning through the afternoon, isolated to scattered showers and thunderstorms are forecast to develop across much of Trinidad, with heavier rainfall and thunderstorms favoring southern areas. Lingering showers are forecast into the evening, with isolated thunderstorms.

Saturday: Mostly cloudy to overcast skies with scattered showers and thunderstorms are forecast, favoring Trinidad. Activity is forecast to persist through the day and night, with heavy to violent rainfall and gusty winds likely across Trinidad. Damaging, gusty winds are likely.

Sunday: Partly cloudy, breezy, and somewhat hazy, interrupted by brisk isolated showers through the day, favoring Trinidad. By the evening, increasing cloudiness, scattered showers, and isolated thunderstorms are forecast to affect both islands. Heavier rainfall is forecast to favor southern Trinidad generally.

Monday: Scattered showers, thunderstorms, and periods of rain are forecast to affect the country after midnight, interrupting cloudy and breezy conditions. Rainfall is forecast to decrease in intensity and coverage from late afternoon into the evening, but isolated showers and thunderstorms are possible overnight. Similar conditions to Monday are forecast on Tuesday and Wednesday.

Scattered showers and thunderstorms east of the Lesser Antilles on Wednesday night. (Weathernerds.org)

Marine: Hazardous seas are forecast through the weekend due to long period swells and increased winds. Seas in open waters are forecast between 2.0 and 3.0 meters, occasionally above 3.0 meters on Saturday. In sheltered areas, between 1.0 and 1.5 meters, higher along northern and eastern coastlines.

Temperatures

Through the forecast period, the minimum low in Trinidad and Tobago is forecast to be between 23.0°C and 26.0°C.

Maximum highs across the country are forecast to be generally up to 32.0°C in Trinidad and Tobago. However, with increased cloud cover, maximum highs are forecast to be relatively cooler.

In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 33.0°C on days with sunny mornings.

Forecast Impacts

As mentioned above, through Sunday, elevated sustained winds up to 40 KM/H and wind gusts in excess of 55 KM/H are likely, with peak winds forecast on Saturday. On Saturday and generally from Monday through the first half of next week, the hazards shift to mainly heavy rainfall and flooding. Generally, the street and flash flooding threat will increase by the weekend, with the potential for riverine flooding by Monday. Lightning will accompany thunderstorms. Funnel clouds and waterspouts are unlikely through the forecast period due to elevated low-level winds. Stronger thunderstorms are possible from Monday.

Rainfall & Flooding

Forecast chances of heavy to violent showers and thunderstorms associated with tropical waves and the ITCZ over the next five days.
Forecast chances of heavy to violent showers and thunderstorms associated with tropical waves and the ITCZ over the next five days.

Through the forecast period, wind shear is forecast to remain unfavorable for persisting heavy rainfall, with shear lessening by next week. Still, favorable upper-level conditions are forecast to produce pockets of deep convection that may produce heavy to violent rainfall rates and thunderstorm activity.

While this forecast is through Monday, models show rainfall totals between Monday and Wednesday to be reasonably high, but this time frame is too far out to forecast reliably.

Over the next five days across Trinidad, isolated totals up to 125 millimeters are forecast across southern and eastern areas of the island, with generally 25-75 millimeters across the country. Higher rainfall accumulations are forecast mainly across southern and eastern regions of Trinidad, and isolated high totals are also possible along the western coastal areas of Trinidad.

  • Thursday: Less than 10 millimeters across the country with isolated totals up to 25 millimeters favoring highly localized areas of Trinidad, particuarly across southern and eastern areas.
  • Friday: Less than 10 millimeters across the country with totals up to 25 millimeters favoring the eastern half of Trinidad and highly localized areas across southern Trinidad.
  • Saturday: Up to 15 millimeters across the country, trending wetter across the eastern and southern halves of Trinidad. Isolated totals in excess of 30 millimeters are possible mainly across southern Trinidad. Note that the GFS is trending significantly wetter, with totals across Trinidad up to 75 millimeters.
  • Sunday: Up to 15 millimeters across the country, trending wetter across the eastern and southern halves of both islands. The heaviest rainfall is expected across and offshore southern Trinidad, with totals between 25 and 50 millimeters. Note that the GFS is showing rainfall totals up to 100 millimeters within a 24 hour period across much of Southern Trinidad on Sunday.
  • Monday: Up to 25 millimeters across the country with isolated totals up to 50 millimeters favoring southern and eastern areas of both islands. Note that the GFS is showing rainfall totals up to 100 millimeters within a 24 hour period across parts of Northern Trinidad and north of Tobago.
Forecast chances of street or flash flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.
Forecast chances of street or flash flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.

Street flooding or flash flooding is forecast over the next five days, with medium to high chances becoming very high on Monday. Chances for riverine flooding remain very low at this time, increasing by Monday, as model guidance suggests heavier rainfall remains east of the country.

Putting the rainfall forecast into context, rainfall rates in excess of 50 millimeters per hour or areas that receive in excess of 25 millimeters within an hour tend to trigger street flooding across the country or flash flooding in northern Trinidad. For riverine flooding to occur, a large area of the country (not just in highly localized areas of western coastal Trinidad) would have to record upwards of 75 millimeters within 24 hours, and rainfall would have to fall across major rivers’ catchment areas.

Forecast chances of riverine flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.
Forecast chances of riverine flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.

Gusty Winds

Forecast chances for gusty winds associated with tropical waves and the ITCZ over the next five days.

Initially, this adverse weather event is forecast to mainly consist of gusty winds at low levels of the atmosphere, working their way to the surface in showers and thunderstorms.

Through the forecast period, sustained surface winds are forecast to be between 25 to 40 KM/H. In heavy to violent shower and thunderstorm 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.

Elevated winds will be present across the region due to a persisting low-level jet across the Windward Islands. A low-level jet is an area of strong winds at low levels of the atmosphere. These stronger winds can make it down to the surface in heavy showers and thunderstorms.

When these strong winds make it to the surface, the winds can hit the ground in a concentrated area and radiate outward like a ripple in a pond. This activity is known as a downburst and causes substantial wind damage across a fairly large area.

Diagram of convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively “raining itself out.”
Convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively “raining itself out.” 

Other Adverse 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. Dangerous, frequent lightning is expected on Friday and Saturday.

Landslides: Landslides and fallen trees are possible, particularly along the Northern Range, Southern Trinidad, and Tobago.

Severe Thunderstorms: The potential for severe thunderstorms, funnel clouds, and waterspouts remain low. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes can form.

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 forecast period, isolated to scattered rainfall is forecast, with widespread rains possible from Monday.

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

Forecast Discussion

Satellite update as of 11:00 PM Wednesday 20th October 2021 (NASA/CIRA/RAMMB)
Satellite update as of 11:00 PM Wednesday 20th October 2021 (NASA/CIRA/RAMMB)

Tropical Waves: At 00Z Thursday 21st October 2021, the axis of Tropical Wave 58 is along 61°W moving westward at 15 knots (27 KM/H) with scattered showers and thunderstorms mainly east of the Lesser Antilles. Tropical Wave 59 is along 38°W from 14°N southward, moving west at 15 knots (27 KM/H), and is forecast to move across T&T on Sunday into Monday.

Tropical Wave 58 is forecast to continue moving westward into Thursday. Following the passage of this wave, a surge in low-level winds and Saharan Dust is forecast to move across the region. This wave is also forecast to bring the Intertropical Convergence Zone northward and linger near T&T into the weekend.

By Saturday, a surface trough embedded within the ITCZ is forecast to move across Trinidad and Tobago, bringing a surge of moisture, instability, and gusty winds.

The ITCZ will remain near T&T on Sunday, but Tropical Wave 59 is forecast to near and move across the country on Monday by nightfall. This wave will be interacting with the ITCZ, and wind speeds are forecast to decrease, allowing for slower-moving and more widespread rainfall through the first half of next week.

Through the forecast period, upper-level conditions are forecast to remain favorable for shower and thunderstorm development. However, strong wind shear from the west is forecast to remain across Trinidad and Tobago, keeping shower and thunderstorm activity short-lived, with cloudiness associated with showers and thunderstorms that develop across western Trinidad or Tobago moving eastward. This wind shear will also keep heavier rainfall south and east of the country. Then, from Monday onwards, wind shear is forecast to subside, varying between light to moderate levels, allowing for longer-lived rainfall.

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