ITCZ, Tropical Wave 21 Forecast To Bring Scattered Showers, Thunderstorms to T&T Over Next 72 Hours.

As of the 2:00 PM Tropical Weather Outlook from the National Hurricane Center, a tropical wave we’ve been keeping an eye on is not expected to develop. This was expected due to generally unfavorable conditions for development. While the axis of this wave is over 600 kilometers east of Trinidad and Tobago, convergence ahead of the wave, aided by the ITCZ is already beginning to affect T&T and the Southern Windwards. Tropical Wave 21 forecast to bring inclement weather to T&T and the Southern Windwards over the next 48-72 hours.

4:45 PM Update: An adverse weather alert has been issued for Northern Trinidad and Tobago.

What We Know

Tropical Wave 21 (previously Invest 93L) clearly visible on the above satellite imagery with a concentrated area of showers and thunderstorms. Tropical Wave 21 forecast to move across T&T by Monday into Tuesday. Heavy rainfall is forecast. Credit: Weathernerds.org

Invest 93L was dropped as of 8:00 PM Saturday due to the low-pressure center given no chances of development. However, Tropical Wave 21, located at along 23N to 3N, between 55W and 53W continues to move westward with scattered showers and thunderstorms.

The development of this system was never quite expected, but locally heavy rainfall was. This heavy rainfall threat is materializing as an east-west oriented area of showers and thunderstorms are moving westward at this time.

These showers and thunderstorms are located in a favorable low-level convergent and upper-level divergent area with relatively low wind shear between 10-20 knots. There is also high total precipitable moisture in the region, fuelling these heavy showers and thunderstorms.

An ASCAT-C pass at 13:30 Z on Sunday showed winds up to 45 KM/H, mainly east of the wave axis, north of 10N and east of 55W. This area of higher sustained winds are forecast to move across Tobago, Barbados, Grenada and St. Vincent, and the Grenadines on Monday.

Tropical Wave 21 has been moving fairly briskly westward at 30-40 KM/H westward into an area of warmer sea surface temperatures of 28 – 29°C. Thankfully, this brisk movement means that heavy showers will be moving quickly across the region, but our next concern is “training” activity, where showers and thunderstorms fall across the same areas continuously.

Vorticity associated with this system, though still present, has reduced and become more elongated. Generally, Tropical Wave 21 is located in a generally unfavorable environment for development.

What We Expect

Impacts

The Takeaway: As we have been repeatedly forecasting, increased cloudiness and showers during the second half of Sunday persisting into Monday, with thunderstorms possible Monday into Tuesday, generally interrupting overcast to mostly cloudy skies.

Street/Flash flooding, gusty winds up to 60 KM/H, landslides and downed trees/utility poles possible.

The heaviest activity is forecast to remain across the Eastern and Northern halves of Trinidad, Tobago, and north of T&T. Showery spells will interrupt partly cloudy skies throughout the next 72 hours.

GOES-East IR Satellite Imagery showing scattered convection (showers and thunderstorms) north and east of Trinidad. Tropical Wave 21, ITCZ approaching the Lesser Antilles, forecast to bring possible inclement weather to the islands beginning this evening, through Monday lingering into Wednesday. Street/Flash Flooding, Gusty Winds possible. The heaviest rainfall and thunderstorms likely across Northern Trinidad, Tobago and north of T&T.
GOES-East IR Satellite Imagery showing scattered convection (showers and thunderstorms) north and east of Trinidad. Tropical Wave 21, ITCZ approaching the Lesser Antilles, forecast to bring possible inclement weather to the islands beginning this evening, through Monday lingering into Wednesday. Street/Flash Flooding, Gusty Winds possible. The heaviest rainfall and thunderstorms likely across Northern Trinidad, Tobago and north of T&T.

Generally, increasing cloudiness is forecast to begin late Sunday, with isolated showers occurring through Sunday night into Monday morning. A few isolated thunderstorms and locally heavy showers are possible during the latter half of Sunday. Street/Flash Flooding and gusty winds possible.

On Monday, during the late morning through the afternoon, scattered showers and isolated thunderstorms are possible across parts of Trinidad and northeastern Venezuela. The latter activity will likely cause mostly cloudy skies across the southern half of Trinidad. This activity will be directly associated with Tropical Wave 21 and is forecast to subside by the late afternoon into the evening.

1:30 PM Sunday 14th July 2019 Radar Imagery from the Trinidad and Tobago Meteorological Service showing a line of showers moving west away from Trinidad after producing brief light to moderate rainfall across T&T. Across Tobago, isolated to scattered showers with cloudy skies are ongoing.
1:30 PM Sunday 14th July 2019 Radar Imagery from the Trinidad and Tobago Meteorological Service showing a line of showers moving west away from Trinidad after producing brief light to moderate rainfall across T&T. Across Tobago, isolated to scattered showers with cloudy skies are ongoing.

However, as Tropical Wave 21 progresses westward, the ITCZ is forecast to remain across the Eastern and Northern halves of Trinidad and Tobago through Tuesday. This will facilitate scattered to possibly widespread showers across the aforementioned areas. The heaviest rainfall is likely across Eastern parts of Trinidad and Tobago.

Based on the latest model guidance, this activity is likely to linger into Wednesday with intermittent showers across both islands. An adverse weather alert may be issued for Trinidad and Tobago from the Trinidad and Tobago Meteorological Service, particularly for Monday into Tuesday.

Note: Street/Flash flooding, frequent lightning, gusty winds, landslides/mudslides are all possible beginning Sunday evening through Tuesday evening. In addition, locally heavy showers and thunderstorms may produce isolated areas, in excess of 100 millimeters of rainfall, across the Windward Islands, particularly Eastern areas of Trinidad and Tobago and north of T&T.

Locally Heavy Rainfall & Flooding: Based on the latest model runs on Sunday, from Sunday (2:00 PM) through Wednesday (2:00 PM), generally, over the 72 hour period, models are in agreement of fewer than 30 millimeters across Western halves of Trinidad and Tobago and between 30-50 millimeters across the Eastern halves of both islands. Isolated rainfall totals up to 75 millimeters are possible across Eastern coastal Trinidad and Tobago. As mentioned above, much of the heaviest showers and thunderstorms are forecast to move north of Trinidad and Tobago, particularly between Tobago and Barbados.

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, based on the above-mentioned forecast rainfall totals.

Frequent Lightning: In addition, with thunderstorms, frequent lightning is likely. Thunderstorms are likely to begin on Monday through Tuesday.

Gusty Winds: Gusty winds, generally up to 60 KM/H are possible Gusty winds are most likely prior to, in the vicinity of, or occur during heavy showers or thunderstorms. With wind gusts up to 60 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.

Across parts of Tobago, Grenada, St. Vincent and the Grenadines and Barbados, winds may gust up to 65 KM/H at times, particularly in the vicinity of heavy showers and thunderstorms.

High Wind Event Precautions
High Wind Event Precautions

Landslides: In landslide-prone areas, particularly in areas that receive heavy rainfall, landslides and/or mudflows may occur. These landslides, in addition to gusty winds, may down trees, utility poles and impede traffic on roadways.

Other Concerns on Tropical Wave 21 (Previously Invest 93L)

Why I May Not/Will Not See Constant Rainfall?

Tropical waves typically have fair weather ahead of the wave axis, though other features in the area such as surface troughs, increased atmospheric moisture, and local climatic effects, to name a few, may trigger showers and the odd thunderstorm. Following the passage of the wave, much of the active weather typically associated with a tropical wave occurs.
Tropical waves typically have fair weather ahead of the wave axis, though other features in the area such as surface troughs, increased atmospheric moisture, and local climatic effects, to name a few, may trigger showers and the odd thunderstorm. Following the passage of the wave, much of the active weather typically associated with a tropical wave occurs.

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.

Generally, because of winds associated with tropical waves, showers and thunderstorms tend to follow a west-northwest track, generally missing areas across Southwestern Trinidad and even Northwestern Trinidad – though activity due to local climatic effects such as daytime heating may trigger shower and thunderstorm development across these areas. See the below graphic for a simple explanation.

With this tropical wave, mostly scattered showers are forecast. Hence, we are likely to see intermittent showers and thunderstorms, interrupting partly cloudy to overcast skies between late Sunday and Tuesday. In addition, most of the heavier showers, thunderstorms, much like Tropical Wave 19 that moved across the region on Friday through Saturday, will occur between Tobago and Barbados.

But this model shows…

Individual model runs are just one possible outcome from a myriad of outcomes. Weather does not always follow what is modeled, and even what may be forecast. Beware of individual model runs being posted on social media, as it is not exactly what will occur.

Always check the National Hurricane Center for the latest information for tropical cyclones and your local meteorological offices for country-specific advisories concerning the impending inclement weather.

What should I do?

Firstly, don’t panic. As was expected, Invest 93L remained a tropical wave moving across Trinidad, Tobago and the Southern Windwards over the next 48 hours.

Secondly, if you are a risk-averse person, now is a good time to check your inclement weather or hurricane season plan, ensuring your preparedness supplies are not expired, stocked and in a safe location.

The Office of Disaster Preparedness and Management has put together a comprehensive guide for preparing for the 2019 Wet and Hurricane Season.

This tropical wave is forecast to produce locally heavy rainfall, but not enough to trigger riverine flooding, the more prolonged and potentially life-threatening flooding. This tropical wave is part of the regular wet-season or rainy season activity and should be treated as such.

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