2:00 PM Update: The National Hurricane Center has raised the chances of formation for INVEST 99L to 90% (high) over the next 48 hours through 5 days.
From the National Hurricane Center’s Tropical Weather Outlook, as of 2:00 PM EST, “ Showers and thunderstorms associated with the low-pressure area located about midway between the west coast of Africa and the Lesser Antilles continue to get better organized, and a tropical depression appears to be forming. If current trends continue, advisories could be initiated on this system this afternoon. Regardless of development during the next couple of days, less favorable conditions should limit additional development of the system by the weekend.”
11:05 AM Update: The National Hurricane Center has raised the chances of formation for INVEST 99L to 80% (high) over the next 48 hours through 5 days.
From the National Hurricane Center’s Special Tropical Weather Outlook, as of 11:05 AM EST, “ Satellite imagery and recent satellite wind data indicate that the low-pressure system located about midway between the west coast of Africa and the Lesser Antilles continues to get better organized. If current trends continue, advisories could be initiated on this system as a tropical depression later today. Regardless of development during the next couple of days, less favorable conditions should limit additional development by the weekend.”
While conditions remain favorable for development over the next 48 hours, its survival after Wednesday is largely dependent on its strength. A stronger tropical cyclone will be able to mix out the dry air and resist deformation due to the Saharan Dust surge to the north and west.
Invest 99L Key Messages:
– Timing: The low pressure is forecast to move across the Windwards Saturday into Sunday, through Tropical Wave 26, where this low pressure originated, will move across the islands on Friday.
– Track: Anywhere between Trinidad and Barbados (as of 8:00 AM Tuesday 21st July 2020). There is still some uncertainty in this.
– Rainfall (For T&T): As of Tuesday morning, much of the active weather, regardless of development, is forecast to remain north of Trinidad and Tobago. Slight adjustments in the track may have significant changes in the rainfall forecast. Isolated heavy rainfall will still be possible across T&T. Generally, less than 25 millimeters of rainfall forecast Friday through Sunday, with isolated totals in excess of 75 millimeters in areas of persisting heavy rainfall and thunderstorm activity, mainly across Northern, Eastern Trinidad and Tobago.
– Rainfall (For the Lesser Antilles): Locally heavy rainfall producing isolated accumulations up to and in excess of 75 millimeters on Windward facing slopes of the Windwards and the French Antilles.
– Winds: Stronger winds are forecast to remain offshore and north of T&T. Again, slight adjustments in the track may have significant changes in the wind forecast. Wind gusts in excess 65 KM/H possible with showers and thunderstorms late Friday due to Tropical Wave 26 and again Saturday through Sunday due to Invest 99L or its eventual tropical system.
– For Trinidad and Tobago: There is an elevated threat of street flooding and flash flooding as showers and thunderstorms associated with tropical systems can produce violent rainfall rates. As of Tuesday morning, there is a negligible threat of riverine flooding.
– Between late Friday and Sunday, seas are forecast to remain moderate with waves up to 2.0 meters in open waters east of T&T, occasionally up to 2.5 meters, and near 1.0 meter in sheltered areas. If this system organizes further, or track adjustments closer to T&T, further agitation of seas are likely. Note that in heavy showers or thunderstorms, seas may become locally rough and choppy.
– Development chances for INVEST 99L are medium over the next 48 hours and the next 5 days as of 8:00 AM AST Tuesday, at 60% respectively.
INVEST 99L Chances for Tropical Cyclone Development: Medium
We’ve been monitoring this tropical wave over the two days as convection persisted around an area of weak low pressure, producing sustained winds up to 30 knots (56 KM/H) and gusts to 35 knots (65 KM/H) as of Tuesday morning.
The National Hurricane Center, in their 8 AM AST Tropical Weather Outlook, tagged (previously Tropical Wave 26) INVEST 99L, for tropical cyclone development. There is increasing support for short-lived development before reaching the Lesser Antilles.
Regardless of development, it is forecast to bring locally heavy rainfall and gusty winds to islands north of T&T late Friday through Sunday.
From the National Hurricane Center’s Tropical Weather Outlook, as of 8:00 AM EST, “Showers and thunderstorms have become better organized in association with a low-pressure system located about midway between the west coast of Africa and the Lesser Antilles. Environmental conditions are expected to be somewhat conducive for development, and a tropical depression could form during the next day or two while the low moves westward at around 10 mph over the tropical Atlantic. By the weekend, however, less favorable conditions should limit additional development.”
As of the 8:00 AM EST Tropical Weather Outlook, this system has a medium chance, 60%, of tropical cyclone formation over the next 48 hours and a medium, 60%, of tropical cyclone formation over the next 5 days.
What We Know
A tropical wave is along 42W from 19N southward moving west at (15 knots) 27 KM/H. The tropical wave has started to move faster than the low and has moved west of the surface low (INVEST 99L). The wave will continue to move further away from the surface low over the next few days. With high pressure strengthening over the subtropics, the steering flow will increase, and help the wave to increase its westward motion to around 20 to 25 knots within the next 24 to 48 hours.
The approximate center of circulation for this tropical disturbance is now at 9.20°N and 38.7°W. This is approximately 2,450 kilometers east of Trinidad and Tobago.
As of Tuesday morning, the closed wind circulation associated with INVEST 99L has tightened up some from this morning with 25-30 knot winds on the west side in convection which also cooled over the last 3-6 hours (down to less than -70°C) – indicating strengthening showers and thunderstorms.
This low pressure, INVEST 99L, was spawned by the interaction of the ITCZ, Tropical Wave 26, and the African Monsoon Trough, which has extended into the Atlantic, producing a natural area of vorticity (spin) for INVEST 99L to take advantage of.
INVEST 99L is located in an area of favorable upper-level divergence, low-level convergence and abundant moisture, with mostly favorable wind shear ahead of it. However, it’s biggest obstacle is abundant Saharan Dust ahead of the low pressure system. Because INVEST 99L is a smaller circulation, moist air can quickly be recirculated into the core, so this may be a negligible effect.
Tropical Wave 26 is forecast to move across the Lesser Antilles on Friday, ahead of the low-pressure system bringing the first round of showers and isolated thunderstorms. INVEST 99L, and its eventual tropical cyclone is forecast to cross the Windward Islands late Saturday into Sunday. Regardless of development, locally heavy rainfall and gusty winds are possible, mainly north of Trinidad and Tobago.
What We Forecast
Between Trinidad and Barbados, but it is too soon to tell, as there is still a wide model spread. Given that this system has a compact circulation with slight northward or southward adjustments meaning no or significant impacts to T&T respectively, it is too soon to tell.
Based on present model guidance, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to track anywhere between Trinidad and Tobago and Barbados on Saturday into Sunday based on model consensus.
Given the small radii of circulation, slight track adjustments northward would result in minimal impacts to T&T, while track adjustments southward would mean widespread heavy showers, thunderstorms and gusty winds to the islands.
With this uncertainty, we continue to closely monitor model trends, not individual model runs, to give us an idea of where this system may eventually move, but the center of circulation is trending presently to remain north of T&T, with peripheral effects possible.
One of the top global dynamical models, the EMCWF and its ensembles (EPS) show INVEST 99L remaining north of T&T, passing between Tobago and Barbados. This solution would keep much of the active weather offshore, mainly affecting Northern Trinidad and Tobago as it heads towards St. Vincent and the Grenadines, as well as St. Lucia.
Another top global model, the GFS, and its ensembles, also have a tightly clustered track between Tobago and Barbados, slightly south from the EMCWF/EPS solutions, meaning more impacts for T&T.
Regardless of any development, the primary threat from this system will be locally heavy rainfall and gusty winds on Saturday through Sunday, north of T&T and Barbados.
As of Tuesday morning, INVEST 99L is basically at tropical depression strength, having a well defined, closed, and robust low-level circulation with sustained winds of 30 knots (56 KM/H).
Dynamical model guidance on small circulations tend to under-perform, due to compact systems tenacity to rapidly organize and degenerate. The 06Z runs show a split in intensity, with some models keeping this system at below tropical storm strength, while others bring it to a tropical storm.
The top performing intensity models (particularly the HWRF and HMON) bring INVEST 99L near hurricane strength over the next 48 hours as conditions remain favorable. However, as INVEST 99L approaches the Lesser Antilles, weakening is forecast (as of Tuesday morning).
However, these models do bring winds between 30-45 KM/H and gusts in excess of 65 KM/H to Trinidad and Tobago, particularly during those heavier showers and thunderstorms.
Hence, as of Tuesday morning, we expect a tropical storm, albeit weakening, to traverse the region on Saturday into Sunday, bringing locally heavy rainfall, gusty winds, and agitated seas, mainly north of T&T, with more adverse effects affecting Grenada through St. Lucia.
Impacts From INVEST 99L
As we keep reiterating, much of the worst weather associated with this system will remain offshore and north of Trinidad and Tobago, as of Tuesday morning, This can change over the coming week.
Peak sustained surface winds of 30 KM/H to 45 KM/H with gusts in excess of 65 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago.
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.
Overall rainfall accumulations remain relatively low during the three-day period between Friday through Sunday across T&T, but isolated totals may be high. Overall, less than 1 inch (25 mm) are forecast across Trinidad and Tobago. Isolated totals up to 2.5-3.0 inches (75 mm) are possible across Trinidad and Tobago, with heavier rainfall favoring Northern, Eastern Trinidad and Tobago
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
Waves up to 2.0 meters in open waters east of T&T and near 1.0 meter in sheltered areas. Note that in heavy showers or thunderstorms, seas may become locally rough and choppy.
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.
What should I do?
Firstly, don’t panic. No alerts (as of Tuesday morning) are in effect by the Trinidad and Tobago Meteorological Service, so the overall threat to T&T this disturbance poses is low at this time.
An important note: if this system organizes further, tropical storm watches or warnings may be issued for parts of the Southern Windwards later this week. 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 Wet and Hurricane Season.