Tropical Wave 53, ITCZ Forecast To Bring Showers, Thunderstorms To T&T

Moisture associated with Tropical Wave 53, the ITCZ, and a favorable mid to upper-level atmosphere will fuel isolated to scattered showers and thunderstorms beginning Wednesday night through Friday.

A low-level jet (narrow band of fast-moving winds) and a surge of low-level winds are forecast to also move across the region, mainly north of T&T. In areas where strong showers and thunderstorms occur, gusty winds may mix down to the surface, producing downburst-like activity with damaging wind gusts, particularly late Wednesday through Friday.

Showers and thunderstorms can be intense, producing localized street/flash flooding and gusty winds. Showers and thunderstorms will be scattered across Trinidad and Tobago over the next 72 hours, with heavier and more widespread rainfall forecast on Thursday and Friday. It should be noted that higher rainfall totals are forecast to remain north of T&T, mainly affecting Tobago as well as the northern and eastern halves of Trinidad.

Tropical Wave 51 (far west), Tropical Wave 52 (across the Lesser Antilles), and Tropical Wave 53 (east of the Lesser Antilles) seen on GOES-EAST IR Satellite Imagery (Weathernerds.org)
Tropical Wave 51 (far west), Tropical Wave 52 (across the Lesser Antilles), and Tropical Wave 53 (east of the Lesser Antilles) seen on GOES-EAST IR Satellite Imagery (Weathernerds.org)

The Trinidad and Tobago Meteorological Service has issued an Adverse Weather Alert Yellow Level on Wednesday afternoon at 4:38 PM, which will go into effect at 5:00 AM Thursday 1st October 2020, and will remain in effect through 8:00 AM Friday 2nd October 2020, though this may be extended as severe weather is possible into the weekend.

The Forecast (Through Friday)


Wednesday: Beginning after midnight Tuesday into Wednesday, showers and thunderstorms are forecast to increase across T&T. This activity will be scattered across both islands, interrupting partly to mostly cloudy skies allowing for some sunshine during the day. Heavier showers and thunderstorms are forecast to initially affect the eastern halves of both islands, gradually moving west to west-northwest. Conditions are forecast to initially settle into the evening, with a resurgence in showers and thunderstorms nearing midnight into Thursday.

Thursday: The axis of Tropical Wave 53 and the ITCZ will move across the island today. Showers and thunderstorms are forecast to increase again after midnight, with periodic rainfall throughout the day, into Friday, interrupting cloudy skies. Similarly to Wednesday, heavier rainfall will favor Northern and Eastern Trinidad and Tobago, with lesser rainfall accumulations across Southwestern Trinidad. Thursday is forecast to be the wettest of the three days.

Friday: The midnight through early morning increase in shower and thunderstorm activity across Eastern and Northern Trinidad, as well as Tobago, is expected yet again, gradually moving across the remainder of the country. Conditions are forecast to settle by daybreak, with a partly cloudy day. Still, showers and thunderstorms will return by the afternoon across Trinidad and both islands by nightfall, with mostly cloudy to overcast skies.

Maximum wind gusts across the Lesser Antilles due to the presence of a low-level jet and a surge in trade winds later this week. (Windy.com)
Maximum wind gusts across the Lesser Antilles due to the presence of a low-level jet and a surge in trade winds later this week. (Windy.com)

Forecast Discussion: Increasing low-level moisture is forecast as Tropical Wave 53 nears the islands. Mid to upper-level instability will trigger shower and thunderstorm development on Wednesday, with this enhancement remaining in place through Thursday. Particularly on Thursday into Friday as both the ITCZ and Tropical Wave 53 move across the islands, a low-level jet will be in place across the region, causing an overall increase in wind speeds. This will enhance convergence due to favorable confluence (winds slowing down and piling up) through Saturday.

Generally, wind shear is forecast to be low to moderate, from the northwest to the west, keeping heavier activity east of Trinidad and Tobago but bringing mid- to upper-level clouds across the country through the weekend.

As the ITCZ will also be present, activity with this feature has its most prominent peak between 12:00 AM and 8:00 AM. A secondary peak is also observed between 12:00 PM and 3:00 PM, with minimum in activity between 5:00 PM and 8:00 PM. These are the general time frames for inclement weather, favoring Northern and Eastern areas of T&T into the weekend, with isolated enhancement (stronger showers/thunderstorms) are possible along Western Coastal Trinidad due to localized climatic effects.

On days where sunshine is dominant during the morning daytime heating, orographic lift (along the Northern and Central Range), and sea breeze convergence may also act as triggers for shower and thunderstorm development.


Marine: On Wednesday, seas are forecast to be moderate across T&T with waves up to 2.0 meters in open waters. In sheltered areas, waves are forecast to remain generally below 1 meter. During heavy showers or thunderstorms, seas can become choppy. On Thursday and Friday, seas will still be moderate, with waves below 2.5 meters in open waters, particularly north and east of Trinidad. Long period swells are forecast to affect our area Wednesday night through Friday morning. In sheltered areas, seas are still forecast to be below 1 meter and choppy, with large breaking waves in nearshore areas of our northern and eastern coastlines.

Winds are forecast to be fresh to strong, up to 20 knots (37 KM/H) from the southeast to east, peaking at 25 knots (46 KM/H) on Thursday night.

Hazards: The main hazards include heavy rainfall, which may reduce visibility and gusty winds in downburst-like fashion, up to and in excess of 55 KM/H. Street or flash flooding will be possible in slow-moving showers and thunderstorms. In thunderstorm activity, frequent lightning is also possible. Gusty winds may cause power dips/outages, downed trees, and localized wind damage. Funnel clouds and waterspouts are also possible, particularly on Thursday into Friday. With heavy rainfall across elevated areas, landslides and fallen trees will also be possible on Thursday into the weekend.

Temperatures

With the increased cloud cover and rainfall, temperatures are forecast to be noticeably cooler, with maximum highs in the low thirties, even in the upper twenties on days with scattered to widespread showers.

On Wednesday and Friday, with a partly cloudy morning, the maximum high for Trinidad is forecast to be near 32.0°C, possibly higher in urban and built-up areas. In Tobago, the maximum high is forecast to be near 31.0°C or slightly above.

On Thursday, cooler conditions are anticipated due to rainfall activity. The maximum high for Trinidad is forecast to be near 29.0°C, possibly higher in urban and built-up areas, and areas that experience partly cloudy skies with little rainfall. In Tobago, the maximum high is forecast to be near 28.0°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

Tropical Wave 53 and the ITCZ Impacts and Forecast Confidence.

Winds

Sustained surface winds will increase through the forecast period, between 25 KM/H and 35 KM/H with gusts up to and in excess of 55 KM/H are possible, particularly by late Wednesday through Friday. Stronger winds at higher elevations could mix down towards the surface and cause downburst-like winds, which can be damaging.

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

If you live across Southwestern and the Western half of Trinidad, rainfall accumulations will be low, and higher accumulations will be highly isolated due to brisk, isolated downpours and thunderstorms. In these areas, daily accumulations are forecast to be less than 10 millimeters, and isolated daily totals up to and in excess of 25 millimeters, particularly if violent (>50 mm/hr) rainfall rates occur.

Across Eastern and Northern halves of Trinidad, as well as Tobago, heavier rainfall is anticipated with daily maximum accumulation totals are forecast to be up to 25 millimeters. However, in areas where persisting or frequent heavy thunderstorms and showers occur, up to and in excess of 50 millimeters are possible daily.

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. There is no threat of riverine flooding at this time.

Frequent Lightning: Frequent 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 over the next three days, meaning with partly cloudy skies, some sunshine will also be mixed in as Tropical Wave 53 moves across the islands. Scattered to widespread activity is forecast on Thursday.

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

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