Activity Associated With Tropical Wave 53, ITCZ Still Forecast To Affect T&T

Even with abundant moisture and favorable atmospheric conditions, shower and thunderstorm activity completely stayed to our north and east of Thursday. The ITCZ still remains north of T&T, with the axis of Tropical Wave 53 now moving across the Lesser Antilles as of Thursday afternoon.

Winds have picked up across the region, with sustained winds across Trinidad and Tobago up to 30 KM/H. Further north, in shower and thunderstorm activity, winds have been stronger. At Grantley Adams International Airport in Barbados, sustained winds have varied between 30 KM/H and 45 KM/H, with a wind gust up to 85 KM/H reported earlier on Thursday.

Models have significantly backed off heavy rainfall across T&T over the next 24 hours, but as the tropical wave axis moves across T&T overnight, favorable atmospheric conditions could still support isolated to scattered showers and isolated thunderstorms. By Friday night, as well as Saturday night, as the ITCZ drifts southward, our rainfall chances increase. 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 53 moving across the Lesser Antilles seen on GOES-EAST IR Satellite Imagery (Weathernerds.org)
Tropical Wave 53 moving across the Lesser Antilles seen on GOES-EAST IR Satellite Imagery (Weathernerds.org)

The Trinidad and Tobago Meteorological Service has maintained the Adverse Weather Alert (Yellow Level), which began at 5:00 AM Thursday 1st October 2020 through 8:00 AM Friday 2nd October 2020. Note that on Saturday into Sunday, even though heavier activity is forecast to generally remain north and east of T&T, inclement weather is possible.

The National Hurricane Center, in their 2:00 PM 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)

In their Tropical Weather Outlook, “Another tropical wave located just east of the Lesser Antilles is also producing a large area of disorganized showers and thunderstorms. This wave is forecast to move westward at 15 to 20 mph during the next several days, and environmental conditions could become a little more conducive for development when the system is over the central or western Caribbean Sea early next week.” This tropical wave has little operational model support, with no major models showing this wave developing at this time.

The Forecast (Through Sunday)


Friday: During the early morning, brisk, isolated showers and thunderstorms are possible across both islands, with heavier activity favoring Northern and Eastern Trinidad, as well as Tobago. By daybreak, an increase in shower and thunderstorm activity is forecast, mainly across Trinidad, subsiding by the late morning. Throughout the remainder of the day, partly to mostly cloudy skies are anticipated across the island, with brief intervals of showers and isolated thunderstorms. Activity is forecast to increase into the night with periods of persistent rainfall, mainly favoring Northern and Eastern Trinidad and Tobago, with lower rainfall accumulations across Southwestern and Western Trinidad. Note that rainfall will be brisk and highly isolated, but these brief showers can be intense with very gusty winds.

Saturday: After midnight, rainfall activity will subside with a few isolated but brisk showers possible across both islands. A partly cloudy and warm day is anticipated across T&T with isolated heavy showers and thunderstorms possible by the late morning through the afternoon across both Trinidad and Tobago. The ITCZ is forecast to drift southward through the evening, with scattered to widespread showers and thunderstorms overnight into Sunday. Note that from 12:00 PM Saturday into 12:00 PM Sunday is forecast to be the wettest 24-hour period during the forecast.

Sunday: As the ITCZ drifts southward, scattered to widespread showers and thunderstorms are possible, still with heavier activity favoring Northern and Eastern Trinidad and Tobago. These conditions will gradually subside into daybreak with a partly cloudy day forecast. By the late morning through the afternoon, usual wet season shower and thunderstorms are possible across Northern and Western areas of Trinidad, with isolated showers elsewhere. Conditions to settle into the night with a possible resurgence in activity after midnight.


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 Friday. In addition, a low-level jet is 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 Sunday., in addition to the ITCZ driven activity.

Generally, across Trinidad and Tobago, 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 across T&T, at times north of our area. 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 night through Friday, seas will still be moderate, with waves between 2.0 meters and 2.5 meters in open waters, returning to up to 2.0 meters by Saturday into Sunday, 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. With heavy rainfall across elevated areas, landslides and fallen trees will also be possible through the weekend.

Temperatures

With the increased cloud cover and rainfall, temperatures are forecast to be noticeably cooler on Friday through the weekend, 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.

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. 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.

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

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