Soon to be Designated Invest 96L, A Tropical Wave Potential Threat to the Lesser Antilles

Since Monday, the National Hurricane Center has been monitoring a Tropical Wave, likely to be designated Invest 96L, for tropical cyclone development. Because 1) this wave was so far away, 2) there have been lots of chances on whether this wave would actually develop or not and 3) it’s slow movement, we’ve been fairly mum on the subject – but we have been keeping a close eye.

Today, Thursday, the top three models for tropical development are now all on board for a strong tropical wave to a disorganized tropical cyclone to traverse the region (Lesser Antilles) sometime between late Sunday and early Tuesday.

What We Know

As of 11:00 AM, a cluster of moderate to heavy showers and thunderstorms associated with this tropical wave being monitored was located at approximately 32W and 14N.

This wave, which had not yet been given an “Invest” assignment by NHC, had a good deal of spin and a modest amount of heavy thunderstorm activity that was slowing increasing in intensity and areal coverage on Thursday morning, as seen on satellite imagery. The next “Invest” would be designated Invest 96L Conditions appeared favorable for slow development, with moderate wind shear of 10 – 15 knots, marginally warm SSTs near 27°C, and a moist atmosphere.

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, near 0%, of tropical cyclone formation over the next 48 hours and a medium chance, 40%, of tropical cyclone formation over the next 5 days.

From the NHC’s 2:00 PM update, “A tropical wave located several hundred miles west of the Cabo Verde Islands is producing disorganized showers and thunderstorms. Conditions appear conducive for development, and a tropical depression could form early next week while the system moves westward over the tropical Atlantic.”

What We Forecast

Track

This wave, future Invest 96L, is forecast to move mostly westward over the next 4 days, forecast to move across the Lesser Antilles sometime on Monday. Its plume of moisture may begin to affect the islands by late Sunday through early Tuesday.

Based on present model guidance, of the 00Z, 06Z and 12Z runs, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to remain north of Trinidad and Tobago.

However, there is 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.

As it moves into the Caribbean Sea early next week, conditions do appear to become more favorable for development. Models indicate that it may move along, just south of the Greater Antilles, then make a northward turn past Hispanola, moving across the Bahamas. This is where an already uncertain track becomes even more uncertain, as we venture beyond the 5-7 day range.

Intensity

Tropical Wave 46 in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Thursday 12th September 2019.
Tropical Wave 46 in the East Atlantic, with a cluster of disorganized showers and thunderstorms as of 2:00 PM Thursday 12th 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, this system, future Invest 96L, is forecast to move across the Lesser Antilles anywhere between a strong tropical wave to a weak tropical storm or tropical depression.

As it moves into the Caribbean Sea, this system may strengthen into a tropical storm, paralleling the southern coasts of the Greater Antilles, making a northward turn in the vicinity of Hispanola. It is too far out to tell a definitive intensity beyond this period. However, those in the Greater Antilles and the Bahamas should keep a close eye on this system later next week.

Impacts

Central, Northern Windwards & Leewards

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

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.

The UKMET, GFS, and EMCWF – in addition to the ICON – all have the heaviest rainfall (i.e. the core of this system), moving 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 this area of disturbed weather, future Invest 96L, is compact, meaning direct impacts from the passage of this area of low pressure will remain near the center of circulation (heaviest showers, thunderstorms, gustiest winds). This means that, based on the latest analysis, as of Thursday afternoon, no direct impacts to Trinidad and Tobago are forecast.

However, this system will influence our wind field. On Sunday, 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, 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.

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, across Trinidad, Tobago and Grenada, 10-30 millimeters of rainfall are possible with isolated to scattered showers interrupting partly cloudy skies. Similar conditions are forecast on Tuesday, with hot conditions persisting.

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 this system, future Invest 96L, traverses the region, isolated to scattered showers are forecast for Trinidad and Tobago. 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 future 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 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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