TTMS: Adverse Weather Alert Goes Into Effect for T&T At 6:00 AM Sunday As Invest 99L Nears the Islands

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 6 inches (150 millimeters) are possible across the Southern Windwards. There is an elevated threat of street flooding, flash flooding and riverine flooding next week. Review your flood plan now!
— Rough seas with waves between 2.5 meters and 4.5 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.

The Adverse Weather Alert

The Trinidad and Tobago Meteorological Service has issued the Adverse Weather Alert (Yellow Level) as an active tropical wave with medium chances of development nears Trinidad, Tobago and the Southern Windwards. The alert goes into effect from 6:00 AM Sunday 22nd September 2019 through 8:00 PM Monday 23rd September 2019.

This is forecast to be a medium to high impact event, with 2 to 4 inches (50 to 100 millimeters) with isolated totals up to 6 inches (150 millimeters) are possible across the Southern Windwards. There is an elevated threat of street flooding, flash flooding and riverine flooding next week. Rough seas with waves between 2.5 meters and 4.5 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 is expected across both islands

Trinidad and Tobago is NOT under any tropical storm watch or warning AT THIS TIME.

Based on the latest model guidance and analysis, these showers, with scattered heavy showers and thunderstorms, are forecast to produce severe weather across Trinidad. This activity is forecast to begin by the late Saturday night and continue into early Tuesday, though the Adverse Weather Alert begins at 6:00 AM Sunday and ends at 8:00 PM Monday.

Street floodingflash flooding and riverine flooding by late Sunday are possible. Gusty winds are likely. These gusty winds and heavy rainfall will be capable of triggering landslides in landslide-prone areas, as well as downing trees and utility poles. See below for more details on what impacts to expect.

Information from the Trinidad and Tobago Meteorological Service concerning the Adverse Weather Alert for September 22nd-23rd, 2019.

There is medium (40-60%) chance of heavy to intense showers and/or thunderstorms developing from Sunday morning. Significant rainfall will continue into Monday. Heavy/intense downpours will lead to street/flash flooding. Gusty winds in excess of 65 km/hr can be expected. The risk of landslides/landslips is very high in areas so prone.” according to the Trinidad and Tobago Meteorological Service. This “alert” status takes into account the possibility of the event occurring. This adverse weather event is possible.

The color of the watch indicates the severity of the event and the probability of the event occurring. Currently, the alert level is at Yellow. This means that the hazard is possible to be aware of the impacts of Street/Flash Flooding and Gusty Winds in your area. Areas prone to landslides should also be aware of the hazard and impacts.

Image Credit: Trinidad and Tobago Meteorological Service
Image Credit: Trinidad and Tobago Meteorological Service

What We Know

An active tropical wave, Invest 99L, is located along 46W producing scattered showers and thunderstorms and has low chances of development over the next 5 days. Credit: Weathernerds

An Atlantic tropical wave with axis along 46W S of 15N with a low centered near 09N46W, moving W at 10 kt. Model guidance depicts this wave well. Scattered moderate to strong convection is from 04N-13N between 45W-50W.

However, the approximate center of Invest 99L is located in the North Atlantic basin at 10.2°N and 49.5°W. The current intensity was 25 knots and the center was moving at 20 knots at a bearing of 275 degrees (westward). The minimum central pressure was 1011 mb. This places the center of Invest 99L approximately 1200 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 2:00 PM Tropical Weather Outlook, this system has a low chance, 30%, of tropical cyclone formation over the next 48 hours and a medium chance, 40%, 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 Friday morning, as seen on satellite imagery.

Conditions appeared favorable for development, with the SHIPS model diagnosing light wind shear of 5 – 10 knots after the next 12 hours and 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 early next week. Credit: National Hurricane Center

Invest 99L is forecast to move mostly westward over the next 3 days, 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 on Saturday 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. 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. Islands as far north as Guadeloupe to as south as Trinidad should remain aware of this system over the next several days.

The GEFS (GFS) ensemble models for Invest 99L as of 12Z Friday 20th 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 Hurricane Jerry is forecast to move Invest 99L west to the west-southwest, 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 northwestward 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, as of Friday morning, to tropical storm strength, or even 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 Friday 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

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.

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.

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.

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 6 inches (150 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 55 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 4.5 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 high likelihood of this area of disturbed weather, Invest 99L, remaining a strong tropical wave moving across our region in 3 days. 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 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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