Overcast Skies, Showers & Thunderstorms Due to ITCZ, Tropical Wave 53

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

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 late Thursday through Saturday. 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 52 (west of the Lesser Antilles), and Tropical Wave 53 (east of the Lesser Antilles) seen on GOES-EAST IR Satellite Imagery (Weathernerds.org)
Tropical Wave 52 (west of 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 National Hurricane Center, in their 8:00 AM Tropical Weather Outlook, the NHC has given this system a low chance (20%) of tropical cyclone formation when it moves into the far northwestern Caribbean Sea in 5 days.

Tropical Weather Outlook for Tropical Wave 53, showing a low chance of Tropical Cyclone Formation over the next 5 days as it moves across the Caribbean Sea. (National Hurricane Center)
Tropical Weather Outlook for Tropical Wave 53, showing a low chance of Tropical Cyclone Formation over the next 5 days as it moves across the Caribbean Sea. (National Hurricane Center)

In their Tropical Weather Outlook, “Another tropical wave located a few hundred miles east of the Lesser Antilles is producing widespread cloudiness and disorganized shower activity. This disturbance is forecast to move westward during the next several days where environmental conditions could become a little more conducive for development over the central or western Caribbean Sea by early next week.” This tropical wave has little operational model support, with no major models showing this wave developing at this time. 35% of the European ensemble models (EPS) bring this system into a tropical depression early next week.

The Forecast (Through Saturday)


Thursday: The axis of Tropical Wave 53 and the ITCZ will move across the island today. Showers and thunderstorms are forecast to increase by the early morning (after 2 AM) with periodic rainfall throughout the day, into Friday, interrupting cloudy skies. Heavier rainfall will favor Northern and Eastern Trinidad and Tobago, with lower rainfall accumulations across Southwestern Trinidad.

Friday: After a decrease in activity overnight, showers and thunderstorms will return by the late morning through the afternoon, interrupting partly to mostly cloudy skies. Strong and severe thunderstorms are possible across Western Coastal Trinidad during the afternoon. Showers will persist through the afternoon, with a brief settled period during the evening. A resurgence of showers and thunderstorms is forecast after nightfall, closer to 9 PM.

Saturday: Showers and thunderstorms are forecast to continue moving across T&T, with heavier activity across Eastern and Northern Trinidad, as well as Tobago. Mostly settled conditions are forecast into the day but late morning through afternoon thunderstorms will return across Trinidad and isolated showers across Tobago. These showers and thunderstorms will interrupt a partly cloudy day. Another resurgence of showers and thunderstorms is forecast by nightfall, closer to midnight.

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: Tropical Wave 53 will move across the Lesser Antilles on Thursday night into Friday. Mid to upper-level instability and upper-level divergence will enhance shower and thunderstorm development on Thursday into Friday. In addition, a low-level jet is in place across the region on Thursday through Friday, causing an overall increase in wind speeds. This will enhance convergence due to favorable confluence (winds slowing down and piling up) through Sunday.

Generally, wind shear is forecast to be moderate, from the northwest to the north, keeping heavier activity north and 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 Thursday through Saturday, seas will still be moderate, with waves below 2.5 meters in open waters, returning to up to 2.0 meters by Saturday, particularly north and east of Trinidad. Long period swells are forecast to affect our area 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 Friday and Saturday, 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 30.0°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 29.0°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.
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. 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 into Friday.

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

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