Tropical Update Overview:
— Tropical Storm Humberto – Tropical Storm Humberto continues to move north of the Northwestern Bahamas, bringing locally heavy rainfall to parts of the region. This system is forecast to move north and east towards Bermuda over the next several days, strengthening into our next hurricane.
— Tropical Wave 46 (Ex-Invest 96L) – Showers and thunderstorms associated with this tropical wave have mostly remained along the north of the wave axis, with very little activity. This wave is forecast to move across the Lesser Antilles on Sunday, bringing an increase in moisture across the region, but showers and thunderstorms will mainly impact the Leewards. No threat of development nor any impacts to T&T expected.
— Tropical Wave 47 – Another tropical wave located in the Eastern Atlantic has a similar structure to Tropical Wave 46, with much of its showers and thunderstorms along the north of the wave axis. This wave is forecast to move across the Lesser Antilles Monday into Tuesday, bringing increased moisture and showers to much of the islands. Street flooding and locally gusty winds possible in heavier showers or thunderstorms, but no development or widespread severe weather forecast.
— Tropical Wave 48 – This tropical wave has gained model support from most models on Saturday, with medium chances of development, though it is yet to be tagged as an “Invest” by the National Hurricane Center. Because of Humberto’s weakening effect on the dominant Subtropical high-pressure system, if this system develops, it is likely to move well north and east of the Lesser Antilles, with no direct impacts to and landmasses.
— Gulf of Mexico System – An upper level low in the Gulf of Mexico is moving westward and is producing an area of showers and thunderstorms throughout Friday and Saturday. This area of low pressure may work its way to the surface over the next 3-4 days as it approaches the Texas Gulf Coast, increasing rain chances along the Texas Gulf Coast. This area has low chances of tropical cyclone formation over the next five days.
– Impacts to T&T – Increasing moisture from the three tropical waves east of Trinidad and Tobago, in combination with the surface trough east of T&T and the ITCZ, periods of isolated to scattered showers are forecast next week, a welcome interruption from the oppressive heat. No direct tropical threats to Trinidad and Tobago are forecast over the next week. We may experience street flooding and gusty winds in those heavier showers and thunderstorms.
Before we dive into the Tropical Update, a few notes:
- Tropical waves are a normal part of the rainy season.
- Not every tropical wave will form into a tropical cyclone.
- Weaker tropical waves produce more rainfall across Eastern parts of the islands with mostly cloudy conditions and a few showers across western parts of the islands.
- Rainfall will be more isolated and intermittent with weaker tropical waves that do not have ITCZ or upper-level support.
- Saharan Dust may weaken tropical waves.
You can read more about the weather associated with Tropical Waves, as well as what to expect as these waves move through the region below.
Tropical Storm Humberto
At 5:00 PM Saturday AST, the National Hurricane Center continues to issue advisories on Tropical Storm Humberto, 115 kilometers north of Great Abaco Island.
The center of Tropical Storm Humberto was located near latitude 27.4 North, longitude 77.3 West. Humberto is moving toward the north-northwest near 11 KM/H, and this general motion with a gradual turn to the north is expected during the next day or so. A sharp turn to the northeast is expected on Monday. On the forecast track, the center of Humberto should continue to move away from the northwestern Bahamas tonight, and then move well offshore of the east coast of Florida this weekend and early next week.
Maximum sustained winds are near 85 KM/H with higher gusts. Humberto is forecast to become a hurricane by Sunday night or early Monday well east of the east coast of Florida.
Tropical-storm-force winds extend outward up to 220 kilometers to the north and east of the center. The estimated minimum central pressure is 1005 millibars.
Tropical Wave 46 (Ex-Invest 96L)
Ex-Invest 96L has lost much of its showers and thunderstorms as it quickly moved westward over the last 36 hours, having grown disorganized and lost much of its heavy thunderstorm activity since Friday, as seen on satellite imagery. 96L was encountering dry air from the Saharan Air Later (SAL), and the combination of this dry air and the fast forward speed of the system put an end to its development.
The axis of the wave is along 58/59W, from 21N southward, moving west around 15 knots. Scattered moderate convection is within 100 nautical miles of the wave axis from 13N-17N.
It is forecast to traverse the Lesser Antilles overnight through Sunday, bringing a surge of moisture across the region. Over the next 12 hours, much of the showers and thunderstorms associated with this wave is forecast to move across islands as south as Dominica to as north as Antigua and Barbuda. Between 1-2 inches (25-50 millimeters) of rainfall is forecast but no significant impacts are expected beyond isolated street/flash flooding and gusty winds in heavier showers and thunderstorms.
For Trinidad and Tobago, by late Sunday, moisture from this wave will begin to affect the islands, bringing welcome showers and thunderstorms to the region. See below for details.
Tropical Wave 47
Similarly to Tropical Wave 46, Tropical Wave 47 has much of its activity and moisture surge along the northern axis of the wave.
This tropical wave is along 50W from 20N southward, moving W at 15-20 knots. Scattered showers are within 180 nautical miles west of the wave axis from 12N-17N. This wave is well depicted in model guidance, and satellite imagery shows turning. Conditions could become a little more conducive for development in a few days when the wave moves over the eastern Caribbean Sea.
No development of this wave is forecast, but welcome rainfall is expected for much of the Central and Northern Windwards, as well as the Leewards on Monday into Tuesday as this wave traverses the region. Generally, between 1-2 inches (25-50 millimeters) of rainfall is forecast across islands as far south as St. Vincent to as north as Antigua and Barbuda. No significant impacts are expected beyond isolated street/flash flooding and gusty winds in heavier showers and thunderstorms.
For Trinidad and Tobago, moisture from this wave, as well as a surface trough preceding this wave will begin to affect the islands, bringing isolated showers and thunderstorms to the region. See below for details.
Tropical Wave 48
As of the 2:00 PM Tropical Weather Outlook and the 4:00 PM Tropical Update, Tropical Wave 48 is located along 34W from 05N-15N, moving W at 15-20 knots. The wave is well depicted in satellite imagery and TPW imagery. Scattered moderate convection is from 06N-12N between 32W-36W. Gradual development of this system is possible during the next several days, and a tropical depression could form early next week while it moves westward across the tropical Atlantic.
Presently, it has a low chance, 20%, of development over the next 48 hours and a medium chance, 60%, of development over the next 5 days.
This wave had not yet been assigned an “Invest” designation by NHC. The system had a modest amount of spin and strong but disorganized convection on Saturday, as seen on satellite imagery. Conditions appeared favorable for slow development, with light wind shear of 5 – 10 knots and warm SSTs near 28°C.
As of the latest Tropical Update, The wave was located far to the south in the Intertropical Convergence Zone (ITCZ), away from the dry air from the Saharan Air Later (SAL), so dry air should be less of an impediment than we’ve seen for other tropical waves this year.
The tropical wave is predicted to take a track to the west, then west-northwest, over the coming week, passing several hundred miles to the northeast of the Lesser Antilles Islands on Friday.
The 12Z Saturday runs of our three top models for predicting tropical cyclone genesis—the GFS, European and UKMET—all predicted that development into a tropical depression would occur by Wednesday.
The movement well north and east of the Lesser Antilles is fairly confident due to a weakness in the subtropical high-pressure system induced by Tropical Storm, and soon to be Hurricane Humberto. Because of this weakness, instead of the usual westward movement by the subtropical high, this eventual system will begin to move towards the northwest.
Models are in good agreement that this system will develop into our next named tropical cyclone by mid-week this upcoming week.
This system will be of no threat to Trinidad and Tobago.
Gulf of Mexico Area of Interest
NHC is now flagging a large area of convection in the eastern Gulf of Mexico, just east of the same upper low that’s been shearing Humberto. Much like Humberto, this convection is strung out along a northwest-to-southeast band. Because of the shear, this area is unlikely to develop into a tropical cyclone right away.
However, as the upper low retreats toward the western Gulf, conditions may become more favorable. Any such development would likely move westward on the heels of the upper low, and the resulting system would have little time to develop before coming onshore. The 0Z UKMET model run takes this system into the Texas coast early next week, but the GFS and European runs did not develop it.
In its 2:00 PM EDT Tropical Update, the NHC gave this system a 10% chance of developing into at least a tropical depression by Monday and a 30% chance by Thursday.
What does all of this mean for Trinidad and Tobago?
With the passages of these three tropical waves, the Intertropical Convergence Zone and a surface trough throughout the upcoming week (Sunday through Friday), showers and thunderstorms are in the forecast as of this Tropical Update.
However, these showers and thunderstorms are going to follow the typical wet season pattern.
An initially clear morning, at times with a few isolated showers during the pre-dawn through dawn will lead to a hot and sunny late morning. By midday, isolated showers and thunderstorms may develop across the western halves of mainly Trinidad, but also Tobago as well as hilly areas. Thunderstorms may favor western coastal Trinidad during the afternoon hours.
This week, Tuesday 17th September is forecast to be our wettest day, with scattered showers and isolated thunderstorms possible as Tropical Wave 47 traverses the region.
This activity will be isolated to scattered, with no widespread severe weather forecast. However, with all thunderstorms and heavy showers, there is the possibility of street flooding and gusty winds.
An unwanted by-product of this increased moisture will be particularly humid conditions across Trinidad and Tobago. This means that while the actual recorded temperature on some days may not top 34°C, that feels like temperature may easily top 40°C due to that high humidity. However, those showers will bring some brief heat relief.
The Heat Index
The heat index is a combination of air temperature and relative humidity determining what the air feels like to a person i.e. how hot it actually feels.
The heat index is important because of sweating. Your body sweats in order to cool the skin and maintain a constant, healthy body temperature. This cooling process means that the sweat has to evaporate off the skin to remove heat. However, if the sweat is unable to evaporate, the body isn’t able to regulate temperature.
With high volumes of moisture in the air, also known as high relative humidity, which T&T regularly experience due to its tropical climate, the rate of sweat evaporation decreased. This is because the atmosphere is unable, or has limited potential to hold additional moisture in the atmosphere.
This results in you feeling warmer in humid conditions and cooler in less humid conditions i.e. when relative humidity decreases. As temperature increases, the heat index increases. As relative humidity increases, so does the heat index.
The heat index is generally classified into four categories: caution, extreme caution, dangerous and extremely dangerous. Generally across Trinidad and Tobago, we experience heat indices of caution to extreme caution in times of hot days, with isolated areas experiencing dangerous heat indices such as urban areas.
This upcoming week, heat index levels are likely to remain at extreme cautious to dangerous levels. Heat cramps and heat exhaustion are likely with heatstroke probable with continued activity.
Impacts This Upcoming Week
Winds: Sustained surface winds between 15 KM/H and 30 KM/H with gusts in excess of 50 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago throughout the week, particularly on Tuesday.
With wind gusts in excess of 50 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.
Rainfall: Throughout the week, daily rainfall accumulations are forecast to be isolated totals of 5-10 millimeters, with up to 25 millimeters in isolated thunderstorms or heavy downpours.
On Tuesday 17th September 2019, 15-25 millimeters of rainfall is possible across mainly Trinidad, with isolated totals up to 50 millimeters in those highly localized thunderstorms and downpours.
Generally, with heavy showers and thunderstorm activity, street flooding, particularly in flood-prone areas or areas with poor drainage, is possible as well as flash flooding in areas where more prolonged heavy rainfall may occur.
Frequent Lightning: In addition, with forecast thunderstorms, frequent lightning is likely.
Why I May Not/Will Not See Constant Rainfall?
A frequent complaint is the forecast is wrong because I didn’t experience any rainfall. Scattered showers mean that you, individually, may experience some showers intermittently throughout the day and there is a higher chance for this activity than isolated activity.
Throughout the week, mainly isolated activity is forecast, with the exception of Tuesday, where scattered activity is forecast.
Tropical Cyclone Climatology
Tropical Cyclone Points of Origin during the month of September. This month is the peak of the Atlantic Hurricane Season, hence all low-pressure systems in the Atlantic should be closely monitored, as we do in the Tropical Update