Invest 99L Forecast to Being Heavy Rainfall to T&T, Southern Windwards Regardless of Development

Key Messages:
— Regardless of tropical cyclone development, locally heavy rainfall and gusty winds are likely across Trinidad, Tobago, Barbados and the remainder of the Windwards between Sunday and Monday.
— Between 2 to 4 inches (50 to 100 millimeters) with isolated totals up to 8 inches (200 millimeters) are possible across the Southern Windwards. There is an elevated threat of street floodingflash flooding and riverine flooding next week. Review your flood plan now!
— Rough seas with waves between 2.5 meters and 4.0 meters are possible in open waters east of T&T on Sunday into Monday, with battering swells along the Atlantic Coasts.
— Gusty winds in excess of 65 KM/H expected. Winds of this strength will produce wind damage including downed trees, utility poles and roof damage. Localized power outages possible.
— Although chances of development are medium as of 8:00 AM Saturday, if the development of this system occurs, short-notice tropical storm watches and warnings may be issues. This should not take away from the main threat of this system – heavy rainfall.

Key messages for Invest 99L as of 11:00 AM Saturday 21st September 2019
Key messages for Invest 99L as of 11:00 AM Saturday 21st September 2019

Since 2:00 AM Wednesday, the National Hurricane Center has been monitoring a Tropical Wave, now designated Invest 99L, for tropical cyclone development. Model support for the development of this system into a tropical cyclone over the next several days have remained minimal as it traverses the islands on Sunday into Monday.

From the National Hurricane Center’s Tropical Weather Outlook, as of 8:00 AM, for Invest 99L, “A tropical wave located several hundred miles east of the Windward Islands is producing a large area of showers and thunderstorms. The wave is forecast to move quickly westward to west-northwestward at 15 to 20 mph for the next day or two, crossing the Windward Islands on Sunday. Although the system is currently disorganized, environmental conditions appear to be conducive for some development and a tropical depression could form later this weekend or early next week. An NOAA Hurricane Hunter aircraft is scheduled to investigate this system this afternoon. Regardless of development, heavy rainfall is possible over much of the Lesser Antilles over the weekend, and interests on those islands and Puerto Rico should monitor the progress of this disturbance.

What We Know

An active tropical wave, Invest 99L, is located along 52W producing scattered showers and thunderstorms and has medium chances of development over the next 5 days. Credit: Weathernerds
An active tropical wave, Invest 99L, is located along 52W producing scattered showers and thunderstorms and has medium chances of development over the next 5 days. Credit: Weathernerds

An Atlantic tropical wave with axis along 52W S of 15N is moving W at around 15 kt. Model guidance and satellite imagery depict this wave well. Clusters of scattered moderate convection are from 05N-15N between 44W-57W.

Environmental conditions are currently conducive for some development, and a tropical depression could form on Sunday or early next week while the system moves quickly westward to west-northwestward, crossing the Windward Islands on Sunday. Upper-level winds are forecast to become less conducive for development by the middle of next week once the wave moves out of the northeastern Caribbean Sea.

An NOAA Hurricane Hunter aircraft is en route to investigate this system this afternoon. Regardless of development, heavy rainfall is possible over much of the Lesser Antilles over the weekend, and interests on those islands and Puerto Rico should monitor the progress of this disturbance.

The approximate center of Invest 99L is at 10.4°N and 56.5°W, moving west at 18 knots, or 33 kilometers per hour. This places the center of Invest 99L approximately 415 kilometers east of Trinidad and Tobago.

Based on our counts, this is the 49th tropical wave of the 2019 Atlantic Hurricane Season.

As of the 8:00 AM Tropical Weather Outlook, this system has a medium chance, 50%, of tropical cyclone formation over the next 48 hours and a medium chance, 60%, of tropical cyclone formation over the next 5 days.

This wave has been designated Invest 99L by NHC as of Friday morning. The system is located in an area of favorable upper-level divergence and low-level convergence. It also had a good deal of low-level and mid-level vorticity (spin). There is a good amount of heavy thunderstorm and shower activity, which was slowing increasing in intensity and areal coverage on Saturday morning, as seen on satellite imagery.

However, there is unfavorable wind shear across much of the northern and eastern halves of the system, up to 40 knots. This has limited persisting convection needed to lower pressures in the area and favor development. In addition, the fast forward movement have been detrimental in the consolidation of showers and thunderstorms and will continue to be the case over the next 24-48 hours.

Conditions appeared neutral for development, with the SHIPS model diagnosing moderate to strong shear of 20 – 30 knots over the next several days, but warm sea surface temperatures between 28°C and 29°C and a fairly supportive moisture envelope.

What We Forecast For Invest 99L

Track

Invest 99L’s path and potential area for development across the Southern Windwards over the next 5 days. Credit: National Hurricane Center Note that the area that is hatched orange is the area in which tropical cyclone development may occur and is not where impacts are only expected. Impacts will extend far away from the low-pressure center due to its disorganized nature.

Invest 99L is forecast to move mostly westward to slightly west-northwest over the next 48 hours, forecast to move directly across the Southern Windward Islands, including Trinidad and Tobago on Sunday into Monday. However, the weather ahead of this system is forecast to begin affecting Trinidad and Tobago beginning today and linger through Tuesday.

Based on present model guidance, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to track across the Southern Windwards, just north of Tobago, with the strongest sustained winds remaining north of Tobago and South of Barbados. The possible center of circulation (if any forms) is forecast to move across anywhere between Northern Trinidad and South of Barbados.

There is still great uncertainty on any potential track and intensity of a system that has not even formed. All of the Windward and Leeward Islands should remain aware of this system.

The GEFS (GFS) ensemble models for Invest 99L as of 06Z Saturday 21st September 2019. Credit: Alan Brammer
The GEFS (GFS) ensemble models for Invest 99L as of 06Z Saturday 21st September 2019. Credit: Alan Brammer

There is remarkable consistency in the forecast track of this system. Based on model guidance, an expanding high-pressure, building behind Tropical Storm Jerry is forecast to move Invest 99L west directly across Trinidad, Tobago, and the Southern Windwards.

Of the EMCWF (European), GFS (US) and UKMET (UK) all bring this system across the Southern Windwards. Other top models, like the ICON (German) and ARPEGE (French), have similar outputs to the EMCWF and UKMET.

This system is then forecast to move into the Caribbean Sea on Tuesday, with showers tapering off across the Windwards, but may then move toward the Greater Antilles later next week.

Intensity

Regardless of any development, the primary threat from this system will be heavy rainfall between Sunday and Monday across T&T.

There are few models that bring this system, none of them the operational top three models for tropical cyclone development (GFS, EMCWF and UKMET), as of Saturday morning, to tropical storm strength, or with few ensembles bringing this system to a tropical depression with a closed low-level low-pressure center.

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

However, nearly all models (EMCWF (European), GFS (US), UKMET (UK), ICON (German), ARPEGE (French) and CMC (Canadian)) bring heavy rainfall and winds between 40-55 KM/H and gusts in excess of 65 KM/H to Barbados, Trinidad, Tobago and the remainder of the Southern Windwards, particularly Sunday into Monday.

Hence, as of Saturday morning, we are expecting a very strong tropical wave to traverse the region on Sunday into Monday, bringing locally heavy rainfall, gusty winds and agitated seas to all Southern Windward Islands.

Impacts From Invest 99L

Impacts for T&T resulting from Invest 99L
Impacts for T&T resulting from Invest 99L

The Southern and Central Windwards are forecast to be affected by this strong tropical wave, with most impacts across the Southern and Central Windwards. This area includes Dominica, Martinique, St. Lucia, St. Vincent and the Grenadines, Barbados, Grenada, and its dependencies, and Trinidad and Tobago.

As mentioned, regardless of development, this system will be a heavy rain threat above all else. An NOAA Hurricane Hunter aircraft is en route to investigate this system this afternoon.

All models, the EMCWF (European), GFS (US), UKMET (UK), ICON (German), ARPEGE (French) and CMC (Canadian) bring heavy rainfall and winds between 40-55 KM/H and gusts in excess of 65 KM/H to Trinidad, Tobago and the remainder of the Southern Windwards, particularly Sunday into Monday.

Wind

Peak sustained surface winds of 25 KM/H to 55 KM/H with gusts in excess of 65 KM/H are possible in heavy showers or thunderstorms across the entirety of the Southern and Central Windwards, including Trinidad and Tobago. These peak sustained winds are forecast to occur Sunday into Monday, particularly Sunday night.

With wind gusts in excess of 65 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. Localized power outages expected.

High Wind Event Precautions
High Wind Event Precautions

Rainfall

Across much of the Southern Windwards, including Trinidad and Tobago, we expect rainfall totals between 2 to 4 inches (50 to 100 millimeters) with isolated totals up to 8 inches (200 millimeters). Across Trinidad and Tobago, the heaviest rainfall is forecast between Sunday and Monday.

Because of the particularly wet period we’ve had this week, there is an elevated threat of street flooding, flash flooding and riverine flooding next week.

Thunderstorms and Lightning

With thunderstorms, locally severe wind (in excess of 65 KM/H), violent rainfall rates (in excess of 50 millimeters per hour) and lightning are all possible.

Gusty winds and lightning pose a threat to our power grid, causing localized power outages. Lightning may also strike trees, homes or utility poles causing damage to property. Lightning can also be deadly to persons outside during a thunderstorm.

Seas and Surf

By Saturday night, seas are forecast to be moderate to rough with long-period swells between 2.0 to 3.0 meters in open waters, battering the Atlantic Coasts of the Southern Windwards.

On Sunday, seas are forecast to be rough in open waters with long-period swells between 2.5 and 4.0 meters in open waters, continuing to batter the Altlantic Coasts of the Southern Windwards.

On Monday, seas are forecast to be moderate to rough with long-period swells between 2.0 to 3.0 meters in open waters, with the third day of battering the Atlantic Coasts of the Southern Windwards.

Seas will return to a moderate state by Tuesday as whatever system comes out of Invest 99L. However, between late Saturday and early Tuesday, large waves and dangerous near-shore conditions will make marine conditions unsafe for mariners and sea bathers. Large, battering waves may pose a threat to life and property within the surf zone.

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 still 3 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 99L.

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 medium likelihood of this area of disturbed weather, Invest 99L, remaining a strong tropical wave moving across our region in 36 hours. However, as repeatedly mentioned, the main threat is flooding. Adequate preparations should be made to safeguard life and property. Create a safety plan in case of emergency as an Adverse Weather Alert is in effect.

An important note: if this system organizes further, a short-notice tropical storm watch or warning may be issued for parts of the Southern Windwards, including Trinidad and Tobago. However, regardless of if a warning is issued or not, be prepared.

Secondly, 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 2019 Wet and Hurricane Season.

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