Forecast: Tropical Wave 41 To Bring Gusty Winds, Rainfall To T&T

What you need to know:
— An active and fast-moving tropical wave (Tropical Wave 41) is approaching the region, forecast to move across the area on Friday into Saturday, with a surge of Saharan Dust following on Sunday.
— Periods of heavy rainfall may trigger street or flash flooding across both islands over the next 48 hours. The main impacts include increased sustained winds, with wind gusts in excess of 55 KM/H possible, causing power dips/outages, downed trees, and localized wind damage. Frequent lightning may accompany thunderstorms. Landslides are possible across the Northern Range and Tobago. Funnel clouds and waterspouts are possible, though chances remain low. Seas are forecast to be agitated.
— An Adverse Weather Alert (Yellow Level) is in effect for T&T from the Trinidad and Tobago Meteorological Service from 12:00 AM Friday through 8:00 PM Saturday. Trinidad and Tobago is not under a tropical storm watch or warning.

The Forecast

Moisture associated with Tropical Wave 41 approaching T&T (SSEC/CIMSS/UW-Madison)

Friday: Windy and mostly cloudy to overcast skies are forecast to be interrupted by periods of showers and thunderstorms throughout the day. Heavier rainfall is forecast to initially favor Tobago, eastern and northern Trinidad, gradually spreading across the island. Rainfall is forecast to briefly subside by the evening, with showers and thunderstorms returning before midnight.

Saturday: From midnight, scattered showers and thunderstorms are forecast across both islands, gradually subsiding by daybreak. Isolated showers and thunderstorms are possible again from the late morning through the afternoon, favoring Trinidad and initially subsiding by the evening. Then, additional showers and thunderstorms are forecast to return by the late night, favoring eastern Trinidad and offshore areas.

Sunday: Mostly hot, sunny, and hazy conditions, interrupted by brisk isolated showers. Air quality may be reduced to moderate levels.

Marine: Over the next 48 hours, seas are forecast to become agitated with waves in open waters up to 2.5 meters and less than 1.5 meters in sheltered areas.

Temperatures

Through the forecast period, the minimum low for Trinidad and Tobago is forecast to be between 23.0°C and 25.0°C. The maximum high for Trinidad and Tobago is forecast to be between 28.0°C and 31.0°C, with temperatures near that higher end of the range favoring Trinidad.

Higher temperatures are possible in urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas.

Forecast Impacts

Forecast impacts due to localized climatic effects and three tropical waves over the next 5 days.

Over the next 48 hours, the main hazards include increased sustained winds, with wind gusts in excess of 55 KM/H likely causing power dips/outages, downed trees, and widespread wind damage. Periods of heavy rainfall may trigger street or flash flooding across both islands. Frequent lightning may accompany thunderstorms. Landslides are possible across the Northern Range and Tobago. With strong surface winds, funnel clouds and waterspouts are unlikely. Seas may become agitated.

Winds

Sustained surface winds up to 45 KM/H are forecast. 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

Overall, higher rainfall is forecast to favor northern and easter Trinidad and Tobago, with accumulations between 25-75 millimeters. However, localized totals up to and in excess of 75 millimeters are possible in prolonged heavy showers or isolated thunderstorm activity.

  • Friday: Between 10-25 millimeters across the country, with totals up to 45 millimeters across the eastern and northern halves of Trinidad and Tobago. In isolated areas, totals in excess of 50 millimeters possible.
  • Saturday: Between 5-15 millimeters across the country. In heavy showers or thunderstorms across the country, totals may reach and exceed 25 millimeters favoring eastern Trinidad.
  • Sunday: Less than 10 millimeters across the country. Note that the GFS model, which has been the more reliable model regarding forecast precipitation totals for 2021 continues to show heavy rainfall with accumulations in excess of 25 mm across the country on Sunday.

As mentioned earlier, street flooding and flash flooding are likely with medium to high chances in heavy showers or thunderstorms and a very low chance of riverine flooding.

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.

Landslides: Landslides and fallen trees are possible through the forecast period, particularly in areas where heavy thunderstorms or persisting rains occur.

Tornadic Activity: With the potential for strong thunderstorms, funnel clouds, and waterspouts are possible though chances are low as winds at the surface are forecast to be relatively strong. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes are possible.

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.

The scattered showers and thunderstorms are forecast to occur on Friday and Saturday, with isolated activity on Sunday.

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

Forecast Discussion

Tropical Weather Outlook from the National Hurricane Center, tracking Hurricane Larry and two tropical disturbances, Invest 91L and a new area off the coast of Africa for tropical cyclone formation. (NHC)
Tropical Weather Outlook from the National Hurricane Center, tracking Hurricane Larry and two tropical disturbances, Invest 91L and a new area off the coast of Africa for tropical cyclone formation. (NHC)

Tropical Waves: At 18Z Thursday 2nd September 2021, the axis of Tropical Wave 41 is along 57W moving across Trinidad and Tobago, south of 20N, moving westward at 10-15 knots (18-27 KM/H) with significant shower and thunderstorm activity following the wave axis. Tropical Wave 40 was dropped from the analysis on September 1st.

On Thursday, a high-pressure system regained dominance, with moisture at the low- to mid-levels of the atmosphere reducing and wind speeds increasing. Tropical Wave 41 is just east of the region, accompanied by a surge in low-level winds. Moisture ahead of the wave is forecast to move across the region overnight tonight, fuelling showers and thunderstorms and contributing to cloudy skies.

Marginally favorable upper-level conditions and favorable low- to mid-level support, and moderate to light wind shear will allow for deeper convection to develop, potentially producing heavy to violent rainfall rates and gusty winds working their way to the surface. Stronger winds are forecast to favor Tobago.

This pattern will hold through mid-Saturday when the Atlantic high-pressure system regains dominance briefly and remain in place through Sunday.

This pattern will be favorable for hot conditions during the day, localized heavy showers, and thunderstorms moving from south to north, mainly from late Sunday and heading into the upcoming week. However, Hurricane Larry will be moving northeast of the region, slackening the pressure gradient and causing light, northerly winds. Swells from Larry will begin to affect T&T’s northern and eastern coasts from Sunday evening, with abnormal waves affecting western coasts.

Infrared satellite imagery showing scattered showers and thunderstorms Tropical Wave 41, forecast to move across the region, and Hurricane Lenny. (Weathernerds)
Infrared satellite imagery showing scattered showers and thunderstorms Tropical Wave 41, forecast to move across the region, and Hurricane Larry. (Weathernerds)
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