Tropical Depression 7 Key Messages:
– Track: Tropical Depression 7 is forecast to move across the region on Saturday, with the center of circulation remaining just north of Tobago, likely making a direct impact on Grenada.
– Intensity: This tropical depression is expected to strengthen into a tropical storm by Wednesday, moving north of T&T with sustained winds of 60 MPH (93 KM/H) and gusts to 70 MPH (111 KM/H). Across T&T, wind gusts in excess of 65 KM/H are likely in heavier showers or thunderstorms.
– Rainfall: As of Tuesday night, much of the active weather 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. 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. On Saturday, seas are forecast to be moderate to rough, with waves up to 2.5 meters, occasionally up to 3.0 meters in open waters. In sheltered areas, up to 1.5 meters and choppy.
NHC: Tropical Depression Seven To Strengthen Over Central Atlantic
The National Hurricane Center, as of 11:00 PM, continues to issue advisories on Tropical Depression Seven, 2,185 KM east of the Windward Islands.
The center of Tropical Depression Seven was located near latitude 10.0 North, longitude 41.3 West. The depression is moving toward the west-northwest near 9 MPH (15 KM/H). A faster westward motion is expected during the next few days.
Maximum sustained winds are near 35 MPH (55 KM/H) with higher gusts. Strengthening is forecast during the next couple of days, and the depression is expected to become a tropical storm on Wednesday. The estimated minimum central pressure is 1008 millibars.
What We Know
Tropical Depression 7 continues to produce a compact area of strong convection in the Central Atlantic, well east of Trinidad and Tobago. It is located in an area with little to no vertical wind shear, favorable upper-level divergence and low-level convergence and a well defined circulation.
This low-pressure system, previously 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 this tropical cyclone to take advantage of.
Tropical Wave 26, which Invest 99L and now Tropical Depression 7 formed from, is now well west of the low-pressure center as it continues to ward off dry Saharan air to the north and west of the system.
Tropical Depression 7 will be traversing warmer waters, with sea surface temperatures between 29°C and 30°C over the next 48 hours, allowing for strengthening. If this system gets named on Wednesday, it will be named Gonzalo. It will also break the current record for the earliest 7th named storm formation in the Atlantic, edging out Gert on July 24th, 2005.
This eventual tropical storm is forecast to bring adverse weather to the Southern Windwards, including T&T on Saturday.
What We Forecast
North of Tobago, south of Barbados on Saturday 25th July 2020.
Based on present model guidance, the core (i.e. where the heaviest showers and thunderstorms may occur) is forecast to track north of Tobago and south of Barbados on Saturday.
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. There has been little to no changes in the track over the last 2 advisories.
However, a strengthening deep-layer subtropical ridge to the north of the depression should cause the system to accelerate some toward the west or west-northwest during the next several days. This steering pattern should take the cyclone across the eastern Caribbean Islands and into the Caribbean Sea this weekend. The models are in fairly good agreement and there is high confidence in the track forecast.
Scenario 1: A weaker tropical storm north of Tobago – the current NHC forecast
As of Tuesday night, models continue to grapple with the intensity of Tropical Depression 7, with a notable split in the intensity guidance.
In this scenario, which is in line with most global models, there is little change in strength as TD7 moves westward, with some even dissipating it as it nears the Lesser Antilles.
A combination of the cyclone’s fast forward speed and associated shear and dry air entrainment should prevent strengthening or lead to weakening. However, global models have underperformed on compact systems like TD7 as these systems do have the ability to fluctuate in strength much more rapidly than larger tropical cyclones.
Hence, a modest tropical storm moving north of Tobago, with sustained winds of 60 MPH (93 KM/H) and gusts to 70 MPH (111 KM/H) is the the likely scenario. There may be slight adjustments to this intensity forecast over the next 3-4 days, as small systems are difficult to forecast.
In this scenario, Tobago may experience tropical storm conditions, while across Trinidad, wind gusts in excess of 65 KM/H become likely, particularly near showers and thunderstorms.
Scenario 2 (unlikely): TD7 rapidly intensifies into a hurricane, moving slower and across the French Antilles.
Statistical-dynamical models and some of the hurricane regional models showing the system becoming a hurricane within the next few days. With compact systems like TD7, these outcomes are possible.
Generally, the stronger (intensity) a tropical cyclone is east of the Lesser Antilles, the further north it recurves. This intensity depends on how much organization the depression can attain over the next 24 hours. The SHIPS model does show above average indicators for rapid intensification.
However, just north of TD7, a surge of trade winds and dry Saharan air will serve as a detriment to the cyclone,.
In this scenario, little to no impacts are expected across T&T. In fact, winds may calm across the islands, allowing for near-record high temperatures with locally heavy showers and thunderstorms possible – but due to localized climatic features.
Impacts From Tropical Depression Seven
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 night, This can change over the coming week.
Tobago may experience tropical storm conditions, while across Trinidad, wind gusts in excess of 65 KM/H become likely, particularly near showers and thunderstorms.
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. 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.
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
On Saturday, seas are forecast to be moderate to rough, with waves up to 2.5 meters, occasionally up to 3.0 meters in open waters. In sheltered areas, up to 1.5 meters 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 night) are in effect by the Trinidad and Tobago Meteorological Service.
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.
This Tropical Depression 7 is still four days away, so a lot can change. Tropical storm watches may be issued 48 hours in advance if tropical-storm-force winds are possible, while tropical storm warnings would be issued 36 hours ahead of expected tropical-storm-force winds. Given that strongest winds are forecast to remain north of T&T, do not wait until a watch or warning is issued. You should continue to monitor updates from official and trusted sources.
If you live in a flood-prone area, it is always recommended to prepare sandbags and ensure watercourses and guttering on your property are cleared. Secure loose objects outdoors and trim larger trees. You can request assistance from your local city, borough, or regional corporation. If your roof is not secured, consider securing your roof with hurricane straps. A hurricane strap costs between $8 to $10. It takes about 50 straps to secure the roof of an average three-bedroom house. Most straps are available in your neighbourhood hardware and are made with galvanized steel.
The Office of Disaster Preparedness and Management has put together a comprehensive guide for preparing for the Wet and Hurricane Season.