Invest 96L Forecast to Bring Locally Heavy Rainfall To Northern Half of Lesser Antilles, Regardless of Development

Since Monday, the National Hurricane Center has been monitoring a Tropical Wave, now designated Invest 96L, for tropical cyclone development. Though there was some model support for development of this wave on Thursday, over the last 24 hours, these models have not shown any development of this system as it traverses the islands on Monday.

From the National Hurricane Center’s Tropical Weather Outlook, as of 2:00 PM, for Invest 96L, “A tropical wave located about midway between the Cabo Verde Islands and the Lesser Antilles is producing a large but disorganized area of cloudiness and showers. Conditions are expected to become more conducive for development in a couple of days, and a tropical depression could form early next week while the system moves westward across the tropical Atlantic and approaches the eastern Caribbean Sea.

What We Know

The tropical wave associated with this area of disturbed weather is located along 42/43W from 06N to 20N, moving west at 10 to 15 knots. However, the approximate center of Invest 96L is moving slightly faster, at 18 knots, west to west-southwestward at 13.5°N and 40.9°W.

Based on our counts, this is the 46th tropical wave of the 2019 Atlantic Hurricane Season, as waves were added onto the Tropical Analysis & Forecast Branch Surface Analysis.

As of the 2:00 PM Tropical Weather Outlook, this system has a low chance, 20%, of tropical cyclone formation over the next 48 hours and a medium chance, 50%, of tropical cyclone formation over the next 5 days.

This wave was designated Invest 96L by NHC on Friday morning. The system had a good deal of spin and a modest amount of heavy thunderstorm activity, which was slowing increasing in intensity and areal coverage on Friday morning, as seen on satellite imagery.

Conditions appeared favorable for development, with the SHIPS model diagnosing light wind shear 5 – 10 knots and warm SSTs near 28°C. However, 96L was encountering dry air from the Saharan Air Later (SAL), and this dry air was likely slowing development.

The National Hurricane Center plans on sending an aircraft reconnaissance flight into Invest 96L on Sunday morning to investigate the area and collect crucial data, as this area of disturbed weather is forecast to move across the region on Monday.

What We Forecast

Track

Invest 96L in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Friday 13th September 2019.
Invest 96L in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Friday 13th September 2019.

Invest 96L, is forecast to move mostly westward over the next 4 days, forecast to move across the Lesser Antilles sometime on Monday. Tropical Wave 45, ahead of Invest 96L, will bring a plume of moisture, beginning to affect the islands by late Sunday through early Tuesday.

Based on present model guidance, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to remain well north of Trinidad and Tobago.

Models are indicating a weaker system may take a southerly but faster path, across the Northern Windwards and French Antilles on Monday. However, a slightly stronger system (i.e. an organized tropical storm) may move across the Leewards, but slightly slower, on Monday into Tuesday.

Of the big three – EMCWF (European), GFS (US) and UKMET (UK), the UKMET and EMCWF bring this system across the Southern and Central Leewards while the GFS takes it across the Northern and Central Windwards. Other top models, like the ICON (German) and ARPEGE (French) have similar outputs to the EMCWF and UKMET.

There is still great uncertainty on any potential track and intensity of a system that has not even formed. Islands as far south as the St. Vincent and the Grenadines to as north as the northernmost Leewards should closely monitor this system.

If this system moves into the Caribbean Sea, based on model runs Friday afternoon, most dissipate the system over Hispanola as it moves along the southern coasts of the Greater Antilles islands. If this system moves across or north of the Leewards, most models recurve it away from the Bahamas – a welcome relief. However, this is where an already uncertain track becomes even more uncertain, as we venture beyond the 5-7 day range.

Intensity

Invest 96L in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Friday 13th September 2019.
Invest 96L in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Friday 13th September 2019.

Reiterating: there is great uncertainty on any potential track and intensity of a system that has not even formed.

Based on the latest modeling, Invest 96L is forecast to move across the Lesser Antilles anywhere between a strong tropical wave to a weak tropical storm. If this system moves north and east of the Lesser Antilles, or across the Northernmost Leewards, then a stronger tropical storm is likely.

Models that take this system across the Leewards develop this into a modest tropical storm as it traverses the region while models that move this system across the Northern Windwards into the Caribbean Sea keep this system as a strong tropical wave or tropical depression. Amongst other things, its eventual forward speed may be its major hindrance for development.

Impacts

Central, Northern Windwards & Leewards

This area encompasses islands from as south as St. Vincent to as north as St. Kitts and Nevis.

Of the big three – EMCWF (European), GFS (US) and UKMET (UK), the UKMET and EMCWF bring much of the heavy rainfall and potentially gusty winds across the Southern and Central Leewards while the GFS takes this high rainfall accumulation across the Northern and Central Windwards. Other top models, like the ICON (German) and ARPEGE (French) have similar outputs to the EMCWF and UKMET.

As with most of these weaker systems, the main threat is rainfall triggering street and flash flooding which may trigger landslides. Locally gusty winds in thunderstorms may down trees, utility poles and lines, and localized power outages.

Generally, across the Leewards, with rainfall totals of 3-5 inches, or 75-125 millimeters. Isolated totals may reach up to 8 inches, or 200 millimeters, mainly on the windward slopes of the Leewards, and the French Antilles.

Grenada, Trinidad, and Tobago

For Trinidad and Tobago, there are really three scenarios that typically play out when a tropical cyclone moves through the Lesser Antilles.

  1. The Tropical Cyclone moves well north of Trinidad and Tobago, through the northernmost Leewards. In this scenario, in most instances, hot and sunny conditions prevail across T&T, with a few showers during the afternoon, not atypical of the Wet Season. With larger systems (i.e. a larger circulation), a “feeder band” or “squall line” may move across the Southern Windwards, bringing heavy showers, thunderstorms, and gusty winds.
  2. The Tropical Cyclone moves north of T&T, through the Central Lesser Antilles. In this scenario, the size of the circulation matters. A larger circulation may affect Trinidad and Tobago with potential feeder bands or squall lines moving across the area. In addition, moisture from the system may move across T&T and interact with local climatic effects (daytime heating, sea breeze convergence, orographic precipitation) and enhance showers and thunderstorms. Smaller systems will keep effects localized nearer to the center of circulation and no impacts to T&T may occur.
  3. The Tropical Cyclone moves westward, South of Barbados or across T&T. The size of the circulation does matter, particularly if the center of circulation is nearer to Barbados, with a small circulation radius. However, this setup provides the highest chances for indirect and direct impacts to Trinidad and Tobago. This means periods of showers, thunderstorms, gusty winds and agitates seas are likely, triggering wind damage, possible storm surge, street/flash/riverine flooding, power outages, etc.

For this system, scenario 2 looks likely. The circulation size of Invest 96L, has broadened. This means that the core of the system will not directly impact T&T, but the further south the core travels, we may see some periphery rainfall. This means that, based on the latest analysis, as of Friday afternoon, no direct impacts to Trinidad and Tobago are forecast.

However, in addition to the periphery rainfall, this system will influence our wind field. On Sunday into Monday, winds will primarily be northeast to north, producing southward moving showers and thunderstorms across parts of Trinidad and Tobago.

The northern facing slopes of the Northern Range and Tobago will enhance low-level convergence and any showers moving onshore. These showers and possible thunderstorms, outside of any potential outer bands of the eventual system, will mainly occur during the late morning through the early evening.

On Monday and Tuesday, winds will be near calm, then from the south to southeast as the day progresses. With favorable low-level convergence and a moist atmosphere, day time heating will trigger showers and thunderstorms across both islands. These showers and thunderstorms may be stationary to slow-moving northwards and northwestward. This is when we expect to see rainfall from this system’s passage.

Southward facing slopes of the Northern Range and Tobago may enhance showers and thunderstorms along the East-West Corridor in Trinidad and along the Southern Coasts of Tobago.

Winds (For Trinidad and Tobago): Sustained surface winds between 15 KM/H and 30 KM/H with gusts in excess of 50 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago.

With wind gusts in excess of 50 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.

High Wind Event Precautions
High Wind Event Precautions

Rainfall: Generally, on Sunday, across Trinidad, less than 15 millimeters of rainfall is forecast with those isolated showers and thunderstorms. Across Grenada and Tobago, 15-25 millimeters are possible with isolated activity. Locally heavy downpours may produce totals in excess of 25 millimeters.

Generally, on Monday and Tuesday, across Trinidad, Tobago and Grenada, 10-30 millimeters of rainfall are possible with isolated to scattered showers interrupting partly cloudy skies.

Overall, isolated totals between 50-75 millimeters are possible, mainly across Eastern and Northern Trinidad between Sunday into Tuesday, with the highest threat of heavier rainfall to occur Monday through Tuesday.

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.

Frequent Lightning: In addition, with forecast thunderstorms, frequent lightning is likely.

Why I May Not/Will Not See Constant Rainfall?

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.

As Invest 96L, traverses the region, isolated to scattered showers are forecast for Trinidad and Tobago. For areas north of T&T and Grenada, scattered showers to, at times, widespread showers and thunderstorms are possible.

Forecast - Isolated, Scattered, Widespread - What do they mean?

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 this area of disturbed weather remaining a strong tropical wave moving across or even north of 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.

Facebook Comments