Invest 99L Producing Near Tropical-Storm-Force Gusts, Heavy Showers & Thunderstorms Approaching T&T.

Invest 99L 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. Adverse Weather Alert (Orange Level) In Effect.
– Between 2 to 4 inches (50 to 100 mm) with isolated totals up to 8 inches (200 mm) are possible across the Southern Windwards. There is an elevated threat of street floodingflash flooding and riverine flooding Sunday into Monday.
– 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.
Development chances are high over the next 48 hours. This system is already producing near tropical-force-force gusts up to 74 KM/H, but lacks a closed low-level center. This should not take away from the main threat of this system – heavy rainfall.

What to Expect Over the Next 24-48 Hours

Key messages for Invest 99L as of  2:00 AM Sunday 22nd September 2019
Key messages for Invest 99L as of 2:00 AM Sunday 22nd 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 2:00 AM, for Invest 99L, “Satellite, radar and surface data indicate that the area of low pressure located just north of Tobago in the Windward Islands is becoming better organized and is producing winds to near tropical storm force in the eastern portion of the system. Further development of this disturbance is expected over the next couple of days, and a tropical depression or tropical storm will likely form while it moves west-northwestward and then northwestward at 10 to 15 mph across the Windward Islands and over the eastern Caribbean Sea. The system is then expected to turn northward, moving near Puerto Rico and the Virgin Islands on Tuesday. Heavy rainfall and strong gusty winds are likely over much of the Leeward and Windward Islands during the next couple of days and will spread across Puerto Rico and the Virgin Islands by Monday night or Tuesday. Interests across the eastern Caribbean should monitor the progress of this low, and tropical storm watches and warnings will likely be required for portions of the Leeward Islands and Puerto Rico later today. An Air Force Hurricane Hunter is scheduled to investigate the system this afternoon.

What We Know

An active tropical wave, Invest 99L, is located along 60W producing scattered showers and thunderstorms and has high chances of development over the next 5 days. Credit: Weathernerds
An active tropical wave, Invest 99L, is located along 60W producing scattered showers and thunderstorms and has high chances of development over the next 5 days. Credit: Weathernerds

An Atlantic tropical wave with axis along 60W and S of 16N is moving W at 15-20 kt. Model guidance and satellite imagery depict this wave well. Scattered moderate to strong convection is from 08N-14N between 55W-64W.

Sustained winds and gusts at Grantley Adams International Airport, Barbados resulting from Invest 99L
Sustained winds and gusts at Grantley Adams International Airport, Barbados resulting from Invest 99L

Recent satellite-derived surface wind data and observations from Barbados indicate that the disturbance is producing winds to near tropical storm force in the eastern portion of the system. Further development of this disturbance is expected over the next couple of days, and a tropical depression or tropical storm will likely form while it moves westward and then northwestward at 10 to 15 mph across the Windward Islands and over the eastern Caribbean Sea.

The system is then expected to turn northward, moving near Puerto Rico and the Virgin Islands on Tuesday. Regardless of development, heavy rainfall and strong gusty winds are likely over much of the Lesser Antilles during the next couple of days and will likely spread across Puerto Rico and the Virgin Islands by Monday night or Tuesday.

Interests across the eastern Caribbean should monitor the progress of this disturbance since tropical storm watches and warnings could be required for portions of the Lesser Antilles and Puerto Rico and the Virgin Islands on Sunday. This system has a high chance for tropical development during the next 48 hours.

The approximate center of Invest 99L is at 11.9°N and 59.5°W, moving west-northwest 24 kilometers per hour. This places the center of Invest 99L approximately 130 kilometers east-northeast of Charlottesville, Tobago.

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

As of the 2:00 AM Tropical Weather Outlook, this system has a high chance, 80%, of tropical cyclone formation over the next 48 hours and a high chance, 80%, 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 scattered in intensity and areal coverage on Saturday night, as seen on satellite imagery.

However, unfavorable wind shear continues 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.

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 red 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’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 red 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 west-northwest to northwest over the next 48 hours, forecast to move directly across the Southern Windward Islands with its showers and thunderstorms moving across Trinidad and Tobago on today into Monday. However, the weather ahead of this system is forecast to continue affecting Trinidad and Tobago tonight 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, as none have been discovered by the NOAA plane investigating the system) is forecast to move north of Tobago.

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 across the Southern Windwards, and as it enters the Caribbean Sea, it is forecast to make a turn to the north towards the Leewards and the Greater Antilles later this upcoming week.

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.

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 afternoon, to tropical storm strength, or with few ensembles bringing this system to a tropical depression with a closed low-level low-pressure center.

The GFS ensembles (GEFS) have 73% of its members bringing this system to tropical depression strength while 86% of its members bring it to tropical storm strength over the next 5 days as it moves across the Eastern Caribbean.

The EMCWF ensembles (ECMF/EPS) have 80% of its members bring Invest 99L to tropical depression strength but only 20% of its members bring it to tropical storm strength in the Eastern Caribbean.

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 Sunday morning, we continue to expect 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.

It should be noted that this system is already producing tropical-storm-force gusts up to 74 KM/H, but lacks a closed low-level center. This should not take away from the main threat of this system – heavy rainfall.

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.

As mentioned, regardless of development, this system will be a heavy rain threat above all else.

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

The rainfall forecast has decreased across T&T, but still remains a flood threat. 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. This is near one month’s worth of the average rainfall for the month of September inside 48 hours.

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

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.

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. 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 Orange Level is in effect.

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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