Tropical Wave 32: Potential Tropical Cyclone Threat to the Leeward Islands

There are no alerts, watches or warnings in effect for Trinidad and Tobago and the Lesser Antilles concerning Tropical Wave 32 at this time.

What We Know

Tropical Wave 32, well defined on satellite imagery, remains well east of the Lesser Antilles. This wave is forecast to move across the region early next week (Tuesday into Wednesday). An area of low pressure is forecast to develop by the late weekend east of the Lesser Antilles and track near or across the Leeward Islands by early next week as a tropical depression or weak tropical storm. Credit: Weathernerds.org
Tropical Wave 32, well defined on satellite imagery, remains well east of the Lesser Antilles. This wave is forecast to move across the region early next week (Tuesday into Wednesday). An area of low pressure is forecast to develop by the late weekend east of the Lesser Antilles and track near or across the Leeward Islands by early next week as a tropical depression or weak tropical storm. Credit: Weathernerds.org

The axis of Tropical Wave 32 is along 32/33W from 04N- 18N, moving west at 15 knots. Scattered moderate convection is noted close to where the wave intersects the monsoon trough, from 06N-10N between 33W-36W, in an area where a recent scatterometer pass indicated a small scale low-pressure area may be forming along with the monsoon trough, part of a much broader gyre along the monsoon trough between 25W and 40W.

As of the 18Z surface analysis Wednesday evening, a 1011 millibar low pressure has formed at approximately 10N, 32W.

Presently, this wave is located in an area of favorable upper-level divergence and low-level convergence, as well as a large envelope of high atmospheric moisture, protecting the convection from a dense plume of Saharan Dust to the north of the wave axis.

Wind shear across the southern portion of the wave axis is neutral, bordering on unfavorable while across the northern portion of the wave axis, wind shear is favorable. Interestingly, much of the convection is occurring across the southern areas of this wave, in conjunction with the ITCZ. Wind shear has been on the decrease in the area of this wave over the last 24 hours.

Sea surface temperatures are favorable for convection to develop, between 27 and 28 degrees Celsius ahead of the wave.

As seen above, the National Hurricane Center, in their 2:00 PM Tropical Weather Outlook on Wednesday 31st July 2019, a medium chance for development over the next 5 days at 60%. Development is not expected over the next 48 hours, as those chances are near 0%.

What We Expect

Track

The Takeaway: Based on the 12Z and 18Z model runs on Wednesday evening from the top global models for tropical cyclone development, the low pressure center slated to develop over the next 5 days is forecast to track just north and east of the northern-most Leewards, or directly over the area.

Note that for large waves such as this, impacts may extend far away from the center of circulation.

Surprisingly, nearly all top global models for tropical cyclone formation are in agreement for the eventual track of what may become Tropical Depression Four or Tropical Storm Chantal by early next week.

A weak subtropical high-pressure system will keep this wave and its associated low pressure on a mostly west to west-northwesterly path over the next 5 days.

As conditions become more conducive for development, the low pressure will begin to strengthen, taking a more west-northwesterly to northwesterly path by next Monday.

By late Monday into Tuesday, nearly all operational runs of top global models for tropical development bring the center of circulation of this system either across or near Antigua and Barbuda.

It then continues west-northwest over and just north of the Greater Antilles into the Turks and Caicos and Bahamas by the end of next week.

Intensity & Impacts

The Takeaway: Based on the 12Z and 18Z model runs on Wednesday evening from the top global models for tropical cyclone development, much of the heaviest showers and thunderstorms are forecast to affect the Northern Leewards into the Greater Antilles.

Note that for large waves such as this, impacts may extend far away from the center of circulation.

Taking into account that this potential tropical system is over 5 days away, exact impacts are difficult to pin down.

The European model (EMCWF) is showing a compact tropical storm with maximum sustained winds up to 80 KM/H and gusts to 100 KM/H, with these higher winds remaining east and north of Antigua and Barbuda. However, it is also showing a wide swath of rainfall across the Leewards, with 25 mm to 75 mm from as far south as Dominica to as north as Anguilla between Monday and Wednesday of next week. Isolated totals between 75 mm to 125 mm possible.

EMCWF (EPS) Ensemble Model Runs for Tropical Wave 32 in the Atlantic Ocean. Credit: Alan Barammer

The United States model (GFS) has a slightly more south solution, with a larger but generally weaker tropical storm, with winds up to 70 KM/H and gusts to 90 KM/H, with the higher winds moving across Antigua, Barbuda and surrounding islets. This model produces much more widespread heavier precipitation, with a wide swath of 25 mm to 75 mm as far south as Barbados to as north and west as the Greater Antilles. Isolated totals up to 150 millimeters possible across islands as far south as Dominica to as north and west as the Virgin Islands.

GFS (GEFS) Ensemble Model Runs for Tropical Wave 32 in the Atlantic Ocean. Credit: Alan Barammer

Regardless, persons with interests or residing in the Leeward Islands need to review their 2019 Atlantic Hurricane Season Preparedness plan, as well as any necessary flood and landslide preparedness plans. Follow guidance from your local meteorological and disaster management offices such as the Antigua and Barbuda Meteorological Service.

Now the big question on everyone’s mind, predominantly those who read Trinidad and Tobago Weather Center’s posts, is “What about Trinidad and Tobago”?

This system will not directly impact Trinidad and Tobago. However, similar to Invest 95L’s influence on our wind field, this potential tropical system will bring very light winds from the south and southeast across the island and high atmospheric moisture.

Particularly Monday through Wednesday, this is forecast to be the case across T&T, with showers and thunderstorms possible.

Tropical Wave 33, following the axis of this wave, is forecast to move across T&T during the second half of next week, bringing additional showers and thunderstorms to T&T.

Based on the latest model guidance, between Monday and Friday of next week, rainfall accumulation totals between 50-75 millimeters are possible across both Trinidad and Tobago, with isolated totals up to 100 millimeters.

Reiterating, this rainfall will not be directly associated with the low-pressure system affecting the Leewards but influenced by its passage. The main trigger will be local climatic features such as daytime heating and sea breeze convergence, in addition to the ITCZ being dragged across the region. This activity is normal during the wet season across Trinidad and Tobago.

Street/flash flooding, locally gusty winds, and landslides would be possible if these rainfall totals and thunderstorm activity come to fruition across Trinidad and Tobago.

This whole event is several days away, and we expect to have more clarity as we near the passage of what may become Tropical Depression Four or Tropical Storm Chantal early next week.

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.

Through next week, isolated to scattered showers are forecast, not widespread activity, across Trinidad and Tobago. Hence, we are likely to see intermittent showers and thunderstorms, interrupting partly cloudy to overcast skies throughout the upcoming week.

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