Invest 96L: Medium Chance of Development. Heavy Rainfall Threat To Lesser Antilles

There are no tropical storm or hurricane watches or warnings in effect for Trinidad and Tobago and the Lesser Antilles concerning Tropical Wave 32/Invest 96L at this time.

On Friday, models have continued its southward trend bringing the area of lowest pressure across the French Antilles, with few developing this system into an organized tropical cyclone. This system is now a heavy rainfall threat, mainly for the Windward Islands.

What We Know

Tropical Wave 32/Invest 96L remains well east of the Lesser Antilles with scattered convection around the low-pressure center. This area of low pressure and the associated tropical wave is forecast to move across the region Tuesday into Wednesday of the upcoming week, though showers and thunderstorms are likely to begin on Monday associated with TW32. Credit: Weathernerds.org
Tropical Wave 32/Invest 96L remains well east of the Lesser Antilles with scattered convection around the low-pressure center. This area of low pressure and the associated tropical wave is forecast to move across the region Tuesday into Wednesday of the upcoming week, though showers and thunderstorms are likely to begin on Monday associated with TW32. Credit: Weathernerds.org

The axis of Tropical Wave 32 is along 45-47W from 03N- 14N, moving west at 20 KM/H to 30 KM/H. Scattered showers are occurring within the vicinity of the wave axis, west of the low-pressure center. As of the 18Z surface analysis Friday evening, a 1010 millibar low pressure is present at approximately 10.6N, 45.7W, moving west to west-northwestward at 14 knots.

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.

Favorable upper-level divergence (yellow lines) and low-level convergence (light blue lines) are mainly across the western areas of Invest 96L.
Favorable upper-level divergence (yellow lines) and low-level convergence (light blue lines) are mainly across the western areas of Invest 96L.

Wind shear across much of the wave axis and the low-pressure center have waned to favorable levels. However, the vorticity (spin) associated with this wave remains very elongated.

Sea surface temperatures are favorable for convection to develop, between 29 and 30 degrees Celsius ahead of the wave.

As seen above, the National Hurricane Center, in their 8:00 PM Tropical Weather Outlook on Friday 2nd August 2019, there is a medium chance for development over the next 5 days at 40% and a low chance of development over the next 48 hours, at 20%.

What We Expect

Track

The Takeaway: Based on the 12Z and 18Z model runs on Friday evening from the top global models for tropical cyclone development, the low-pressure center of Invest 96L is forecast to move across the Leewards/French Antilles on Tuesday into Wednesday, slower than initally forecast.

The notable southern shift in model guidance conitnued on Friday due to a stronger subtropical high pressure system north of Invest 96L.

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

On Friday, models and their ensembles have made a notable southward shift for the eventual path of Invest 96L, the low-pressure center associated with Tropical Wave 32.

A 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. Models have trended to a stronger subtropical high-pressure system over the next coming days, anchored north and just east of the Lesser Antilles.

GFS (GEFS) Ensemble Model Runs for Tropical Wave 32/Invest 96L in the Atlantic Ocean. Credit: Alan Barammer

In addition, models suggest a slower speed compared to yesterday, now bringing Tropical Wave 32/Invest 96L across the Lesser Antilles Tuesday into Wednesday. Note that due to its large moisture envelope and circulation, showers and thunderstorms are still forecast to begin affecting the Lesser Antilles beginning Monday.

By Tuesday, nearly all operational runs of top global models for tropical development bring the center of circulation (or remnants of it) of this system either across or near Guadeloupe and Dominica. The GFS has been trending further south, across the French Antilles while the EMCWF keeps the center of the circulation across the northernmost Leewards.

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 Friday evening from the top global models for tropical cyclone development, models briefly develop this wave over the weekend east of the Lesser Antilles as a weak tropical depression, but quickly degenerates into a tropical wave as it moves across the region on Tuesday.

Because of the broad circulation, large moisture field and generally favorable atmosphere, scattered showers and thunderstorms are possible across the entire Lesser Antilles, beginning on Monday across the Southern Windwards. Generally, below 1 inch (25 mm) of rainfall is forecast across the Lesser Antilles with isolated areas seeing between 2-3 inches (50-75 mm).

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 4 days away, with very little organization at this time, exact impacts are difficult to pin down.

About 60-70% of European and GFS model ensemble members from Friday morning generate a depression from 96L east of the Leewards. Only about 20% of the ensemble members maintain 96L as a tropical cyclone beyond that point, with great uncertainty over where such a system might move. 

A tropical depression is possible to form well east of the Lesser Antilles this weekend, as conditions are favorable for development. However, due to several mitigating factors, this area of low pressure is likely to degenerate into a tropical wave as it moves across the Lesser Antilles early next week. See below for a brief overview of why this is expected.

Generally, winds are not forecast to be strong. Across the Lesser Antilles, winds are generally forecast to remain between 20-40 KM/H, normal for the region. Wind gusts may occasionally occur up to and in excess of 60 KM/H in mountainous areas, as well as in the vicinity of heavy showers or thunderstorms. The highest winds are forecast to move across the Leewards, with peak sustained winds of 45 KM/H possible.

Widespread, persistent rains are not forecast as a result of Invest 96L. Instead, due to this influence on the ITCZ, this feature is forecast to move across the Southern Windwards, including Trinidad and Tobago. As the ITCZ interacts with the axis of Tropical Wave 32, and in combination with a favorably located upper-level trough (TUTT), scattered showers and thunderstorms are likely across the Southern Windwards beginning on Monday, lingering through Wednesday.

Tropical Wave 33 is also forecast to bring another round of showers and thunderstorms to the region on Wednesday into Thursday. This is another wave we’re monitoring, though models have backed off development.

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.

Why is Invest 96L likely to degenerate as it moves across the Lesser Antilles?

Invest 96L is forecast to interact with a feature called a Tropical Upper Tropospheric Trough (TUTT). This upper-level trough is forecast to rapidly increase wind shear across the forecast weak tropical depression this weekend.

This is a tricky forecast. If Invest 96L’s low-pressure center moves closer to the TUTT, the wind shear will increase and vice versa. If Invest 96L manages to strengthen beyond a tropical depression (which very few models are indicating), then the interaction between potential tropical storm (Chantal) and the TUTT will result in lower wind shear. Another complicating factor will be the tilt and amplitude of this TUTT. A more negatively tilted (SW to NE) trough at a higher amplitude (sharper) will result in favorable conditions for convection.

Beyond this TUTT, the modeled, strengthening subtropical ridge to the north is forecast to increase the forward speed of this system early next week, which is detrimental to the organization of a tropical cyclone.

Thankfully, we have several days to monitor this before inclement weather begins early next week.

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. Based on latest modeling, this system is forecast to move across the Lesser Antilles as an open wave (tropical wave or trough).

The interaction with Tropical Wave 32, the broad area of circulation associated with Invest 96L, its influence on the ITCZ, a favorably located upper-level trough (TUTT) and high total atmospheric moisture, the atmosphere is primed for showers and thunderstorms, beginning Monday through Wednesday of the upcoming week.

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 Invest 96L but influenced by its passage.

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.

Other Concerns on Invest 96L
(A Tropical Wave)

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 weak 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 and associated features, mostly scattered showers and thunderstorms are forecast. Hence, we are likely to see intermittent showers and thunderstorms, interrupting mostly cloudy to overcast skies between beginning on Monday.

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, especially since this tropical wave is more than 5 days away from moving across the Windwards.

Always check the National Hurricane Center for the latest information for tropical cyclones and your local meteorological offices for country-specific advisories concerning Invest 96L.

What is an Invest?

It sounds ominous, but from the outset, it really isn’t. Invest is short for investigation, followed by the numbers 90 through 99 and either the letter “L” for the Atlantic basin systems or “E” for the Eastern Pacific Systems.

This naming convention is used by the National Hurricane Center to identify features they are monitoring for potential future development into a tropical depression or a tropical storm.

According to the National Hurricane Center, by designating a tropical weather system as an “Invest”, the collection of specialized data sets and computer model guidance on the area of interest can begin. This collection and processing of data are shown on a number of government and academic websites for analyzing.

That said, the “Invest” assignment does not correspond to how likely a system may develop into a tropical depression or storm.

What should I do?

Firstly, don’t panic. There is a high likelihood of Invest 96L remaining a tropical wave moving across the Lesser Antilles by the beginning of next week.

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.

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