Tropical Wave 21 – Low Chance of Tropical Cyclone Formation

Tropical Wave 21 Key Messages:
[WINDS] For Trinidad and Tobago: Most of the activity associated with this tropical wave, as well as its strongest winds, are forecast to move north of both islands. Peak wind gusts in excess of 55 KM/H are possible.
– [RAIN] For Trinidad and Tobago: Most of the rainfall activity across T&T will be due to the ITCZ, interacting with the passages of Tropical Wave 20 and Tropical Wave 21. Generally, less than 25 millimeters of rainfall forecast across T&T over the next 3 days, with isolated overall 3-day totals in excess of 75 millimeters in areas of persisting heavy rainfall and thunderstorm activity.
– [WINDS] For islands mainly north of Barbados: Wind gusts up to 75 KM/H possible with showers and thunderstorms on Tuesday, with locally heavy rainfall producing isolated accumulations up to and in excess of 50 millimeters on Windward facing slopes.
For Trinidad and Tobago: There is an elevated threat of street flooding and flash flooding, particularly on Monday and Tuesday across Trinidad, favoring Southern, Central, and Western areas. As of Sunday night, there is a negligible threat of riverine flooding.

– Seas are forecast to remain moderate into Wednesday with waves up to 2.0 meters in open waters east of T&T and near 1.0 meter in sheltered areas. Note that in heavy showers or thunderstorms, seas may become locally rough and choppy.
— Gusty winds in excess of 55 KM/H are possible. Winds of this strength will produce wind damage including downed trees, utility poles, and roof damage. Localized power outages possible.
— Development chances for Tropical Wave 21 are low over the next 48 hours and the next 5 days as of 8:00 PM AST Sunday, at 10% respectively. While the wave is well defined and strong, significant development is not expected due to hostile environmental conditions later tomorrow (Monday).

Chances for Tropical Cyclone Development: Low

Tropical Weather Outlook as of 8:00 PM AST Sunday 5th July 2020
Tropical Weather Outlook as of 8:00 PM AST Sunday 5th July 2020

We’ve been monitoring this tropical wave over the last few days as it became increasingly organized, producing sustained winds up to 30 knots (50 KM/H).

The National Hurricane Center, in their 8 PM ASTTropical Weather Outlook, has now tagged Tropical Wave 21 for tropical cyclone development. There is no model support for the development of this system into a tropical cyclone.

Regardless of development, it is forecast to bring locally heavy rainfall and gusty winds to islands north of T&T mainly on Tuesday.

From the National Hurricane Center’s Tropical Weather Outlook, as of 8:00 PM EST, “A strong tropical wave, centered about 700 miles east of the Windward Islands, is producing disorganized showers and thunderstorms. Some slight development of this system is possible on Monday before environmental conditions become hostile for development on Tuesday. The wave is forecast to move through the Lesser Antilles on Tuesday and could produce locally heavy rainfall as well as gusty winds on some of those islands.”

What We Know

An active tropical wave, Tropical Wave 21 with a center of circulation, located at 50W, 10N producing scattered showers, isolated thunderstorms and has low chances of development over the next 5 days. (RAMMB/CIRA)
An active tropical wave, Tropical Wave 21 with a center of circulation, located at 50W, 10N producing scattered showers, isolated thunderstorms and has low chances of development over the next 5 days. (RAMMB/CIRA)

A tropical wave is along 45/50W from 20N southward moving west at (15-20 knots) 27 KM/H to 38 KM/H. The approximate center of circulation for this tropical disturbance is at 10°N and 49.5°W. This is approximately 1,250 kilometers east of Trinidad and Tobago.

The wave is forecast to cross the Windward Islands on Tuesday. Regardless of development, locally heavy rainfall and gusty winds are possible, mainly north of Trinidad and Tobago

Based on our counts, this is the 21st tropical wave of the 2020 Atlantic Hurricane Season.

As of the 8:00 PM EST Tropical Weather Outlook, this system has a low chance, 10%, of tropical cyclone formation over the next 48 hours and a low chance, 10%, of tropical cyclone formation over the next 5 days.

What We Forecast

Track

Based on present model guidance, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to track north of Trinidad and Tobago on Tuesday based on the EMCWF (European) and GFS (US) models

The UKMET (UK) and other top models, like the ICON (German) and ARPEGE (French), have similar outputs to the EMCWF.

This system is then forecast to move into the Caribbean Sea by late Tuesday into Wednesday.

Intensity

Regardless of any development, the primary threat from this system will be locally heavy rainfall and gusty winds on Tuesday, north of T&T and Barbados.

12Z Model Run of the EMCWF on July 5th 2020, showing peak winds associated with Tropical Wave 21 remaining well north of T&T. (Windy)
12Z Model Run of the EMCWF on July 5th, 2020, showing peak winds associated with Tropical Wave 21 remaining well north of T&T. (Windy)

There are no top models that support tropical cyclone development (GFS, EMCWF, and UKMET), as of Sunday night.

However, these models do bring winds between 30-45 KM/H and gusts in excess of 55 KM/H to Trinidad and Tobago, particularly during those heavier showers and thunderstorms.

Hence, as of Sunday night, we continue to expect an organized tropical wave to traverse the region on Tuesday, bringing locally heavy rainfall, gusty winds, and agitated seas, mainly north of T&T, with more adverse effects affecting islands north of Barbados. This will be due to favorable upper-level conditions are expected as the tropical wave passes across the area. As a result, this is likely to enhance scattered deep convective activity.

Weather Forecast For T&T


Monday: An active ITCZ interacting with Tropical Wave 20 is forecast to be present across Trinidad and Tobago, with scattered to widespread showers and isolated to scattered thunderstorms forecast across both islands. Severe weather is likely. Activity is forecast to begin after midnight, peaking during the morning, gradually settling by the late afternoon across both islands with isolated showers across both islands into the night.

Seas to remain moderate, with waves up to 2.0 meters in open waters. In sheltered areas, less than 1.0 meter. Note that in heavier showers or thunderstorms, seas may become locally choppy or rough. Air quality to be at good levels.

The main hazards would be locally heavy rainfall, reducing visibility, and localized wind damage from wind gusts associated with heavier showers or thunderstorms. 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.


Tuesday: The axis of Tropical Wave 21 is forecast to move across T&T, with much of the activity remaining north of the islands. Across Trinidad and Tobago, isolated showers are forecast to interrupt a mostly settled morning, isolated strong thunderstorms are possible by the late morning through the afternoon, favoring Western areas of Trinidad. This will likely be triggered due to a combination of localized climatic effects, TW21 and the ITCZ. Conditions are forecast to settle by the evening into the night, with brisk, isolated showers possible.

The main hazards would be locally heavy rainfall, reducing visibility, and localized wind damage from wind gusts associated with heavier showers or thunderstorms. 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. On Tuesday, given light winds are forecast across Western Trinidad, and severe thunderstorms are possible, localized severe street/flash flooding and funnel cloud activity are possible.

Seas to remain moderate, with waves up to 2.0 meters in open waters. In sheltered areas, less than 1.0 meter. Note that in heavier showers or thunderstorms, seas may become locally choppy or rough. Air quality to be at good levels.



Wednesday: As the ITCZ drifts southward, isolated to scattered showers are forecast after midnight through the early morning, with heavier activity remaining offshore and favoring Eastern Trinidad and Tobago. A few of these showers are forecast to move across both islands during the late morning through the afternoon. A surge of Saharan Dust is forecast to move across T&T by the evening, with air quality returning to moderate.

The main hazards continue to be gusty winds and localized street flooding in isolated heavy showers. There is a low to medium chance of a thunderstorm.

Seas to remain moderate, with waves increasing up to 2 meters in open waters, occasionally above 2.0 meters by nightfall. In sheltered areas, near 1.0 meter.


Impacts From Tropical Wave 21

As we keep reiterating, much of the worst weather associated with this system will remain offshore and north of Trinidad and Tobago.

Wind

Peak sustained surface winds of 30 KM/H to 45 KM/H with gusts in excess of 55 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago.

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. Localized power outages expected.

High Wind Event Precautions
High Wind Event Precautions

Rainfall

Overall rainfall accumulations remain relatively low during the three-day period, but isolated totals may be high. Overall, less than 1 inch (25 mm) are forecast across Trinidad and Tobago. Isolated totals up to 2.5-3.0 inches (75 mm) are possible across Trinidad and Tobago, with heavier rainfall favoring Southern, Central, and Eastern Trinidad.

There is an elevated threat of street flooding and flash flooding. As of Sunday night, there is a negligible threat of riverine flooding.

Thunderstorms and Lightning

With thunderstorms, locally severe wind (in excess of 55 KM/H), violent rainfall rates (in excess of 50 millimeters per hour) and lightning are all possible.

Gusty winds and lightning pose a threat to our power grid, causing localized power outages. Lightning may also strike trees, homes, or utility poles, causing damage to property. Lightning can also be deadly to persons outside during a thunderstorm.

Seas and Surf

Waves up to 2.0 meters in open waters east of T&T and near 1.0 meter in sheltered areas. Note that in heavy showers or thunderstorms, seas may become locally rough and choppy.

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.

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

What should I do?

Firstly, don’t panic. This is likely to be just another tropical wave, with isolated to scattered showers and isolated thunderstorms. No alerts (as of Sunday night) are in effect by the Trinidad and Tobago Meteorological Service, so the overall threat to T&T this wave poses is low at this time.

An important note: if this system organizes further, a short-notice tropical storm watch or warning may be issued for parts of the Southern Windwards. This is an unlikely scenario at this time. Regardless of if a warning is issued or not, be prepared.

Secondly, if you are a risk-averse person, now is a good time to check your inclement weather, flood 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 Wet and Hurricane Season.

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