Invest 93L To Bring Gusty Winds, Heavy Rainfall To The Lesser Antilles

Invest 93L Key Messages:
– Invest 93L (Tropical Wave 55) has low chances of development over the next 5 days as it moves west to west-northwest across the northern half of the Lesser Antilles.
For T&T: Active weather associated with this tropical disturbance will remain well north of the country, with light winds and mostly dry air moving across the region on Thursday into Friday. This will lead to very hot and sunny conditions across T&T, though afternoon showers and thunderstorms are possible across Trinidad.
For the Central and Northern Windwards, Leewards: Periods of rain, showers, and thunderstorms with gusty winds are possible from Wednesday night through Friday as this disturbance and the associated tropical wave tracks across the region.

Invest 93L: Low Chances for Tropical Cyclone Formation

Tropical Weather Outlook as of 2:00 AM Wednesday 14th October 2020

Over the last several days, we’ve been monitoring a tropical wave in the Atlantic, producing scattered showers and thunderstorms.

The National Hurricane Center, in their 2 AM AST Tropical Weather Outlook, continue to monitor Tropical Wave 55, now Invest 93L, for tropical cyclone development. However, there is little to no model support for tropical cyclone formation at this time.

From the National Hurricane Center’s Tropical Weather Outlook, as of 2:00 AM EST, “A broad area of low pressure located a few hundred miles east of the Windward Islands continues to produce a large area of disorganized showers and thunderstorms, mainly to the east of its center. Strong upper-level winds are expected to inhibit significant development while the system moves west-northwestward over the next couple of days. Regardless of development, the system could produce locally heavy rainfall across portions of the central and northern Lesser Antilles today, the Virgin Islands and Puerto Rico on Thursday, and Hispaniola on Friday.”

As of the 2:00 AM 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 Know

Tropical Wave 55/Invest 93L in the Atlantic, producing an area of scattered showers and thunderstorms. (Weathernerds)
Tropical Wave 55/Invest 93L in the Atlantic, producing an area of scattered showers and thunderstorms. (Weathernerds)

A tropical wave is along 55W from 20N southward moving west at 10 knots (18 KM/H), with a 1009 millibar low-pressure center near 14N, 56.4W. This wave is producing widely scattered moderate to isolated strong convection within 500 nautical miles to the east of the tropical wave from 10N to 18N, and within 600 nautical miles to the west of the tropical wave from 12N to
20N.

Over the last several days, a well-defined circulation did form. However, due to strong westerly wind shear, the low-level center moved ahead of the stronger showers and thunderstorms, weakening the disturbance. Strong wind shear continues to affect this tropical disturbance, keeping shower and thunderstorm activity east of the low-pressure center and the wave axis.

Strong (unfavorable) wind shear affecting Invest 93L on Wednesday morning as it nears the Lesser Antilles (UW-CIMMS)
Strong (unfavorable) wind shear affecting Invest 93L on Wednesday morning as it nears the Lesser Antilles (UW-CIMMS)

Invest 93L is located in an area of somewhat favorable upper-level divergence and low-level convergence. Moisture is limited to the northern half of the wave axis, with notably drier air to the south of the center of circulation. This dry air will move across T&T through the end of the week.

What We Forecast

Invest 93L: Track

The tropical disturbance will generally progress west-northwestward with active weather remaining to T&T’s north and east, moving across the Central and Northern Lesser Antilles.

Forecast models as of 0Z October 14th, 2020 for Invest 93L, showing the system generally moving across the Leeward Islands towards the Greater Antilles over the next 3-5 days. (NCAR)
Forecast models as of 0Z October 14th, 2020 for Invest 93L, showing the system generally moving across the Leeward Islands towards the Greater Antilles over the next 3-5 days. (NCAR)

A surface to low-level ridge (high-pressure system), anchored just northeast of the Lesser Antilles, is dominating the steering flow of Invest 93L, moving this disturbance to the west-northwest. This movement is forecast over the next several days, taking the system over the Central and Northern Lesser Antilles into the Greater Antilles this weekend.

On this track, little to no impacts are anticipated for Trinidad and Tobago.

Invest 93L: Intensity

There is little model support, mainly from ensemble runs, for this system. It is likely to remain a tropical wave, though gusty winds up to 74 KM/H and locally heavy rainfall are possible across the Central and Northern Lesser Antilles.

Forecast models as of 0Z October 14th, 2020 for Invest 93L, showing the system remaining below tropical-storm-strength over the next 3-5 days. (NCAR)
Forecast models as of 0Z October 14th, 2020 for Invest 93L, showing the system remaining below tropical-storm-strength over the next 3-5 days. (NCAR)

Dynamical model guidance on systems that have not yet formed has a difficult time showing the eventual track or intensity of undeveloped systems.

Still, none of the operational models from top global models (EMCWF, GFS, UKMET) shows this system developing this week. Less than 40% of ensemble runs from the EPS (European EMCWF ensembles) bring winds of tropical depression strength (20+ knots) east of the Lesser Antilles, with similar odds from the UKMET and GFS ensembles.

The latest model runs of the EMCWF EPS runs showing the percent probability of tropical depression winds as of 0Z Sunday 11th October 2020. (weathermodels.com)

However, what models continue to show is an area of 50 millimeters to 100 millimeters (2-4 inches) of rainfall across the Central and Northern Lesser Antilles through Saturday. Note that across Trinidad and Tobago, little to no widespread rainfall is forecast.

The latest model runs of the EMCWF and GFS runs showing total rainfall accumulations over the next 72 hours as of 0Z Wednesday 14th October 2020. (weathermodels.com)

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?

For those in Trinidad and Tobago, nothing. This system will move well north and east of the country resulting in mostly hot and sunny skies through the end of the week.

For those north of Barbados and St. Vincent and the Grenadines, follow the advice of your local meteorological offices as flood alerts, watches and warnings may be issued for your areas over the next 72 hours.

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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