Tropical Storm Dorian – Tropical Threat to the Windward Islands. Tropical Storm Watches, Warnings in Effect.

As of 11:00 AM Sunday 25th August 2019, Tropical Storm Dorian is approximately 850 kilometers east of Tobago and is forecast to move near Barbados, and across St. Vincent and the Grenadines, and St. Lucia over the next 48 hours.

What We Know

Tropical Storm Dorian watches and warnings in effect for Barbados, St. Vincent and the Grenadines and St. Lucia as of 11:00 AM Sunday 25th August 2019.
Tropical Storm Dorian watches and warnings in effect for Barbados, St. Vincent and the Grenadines and St. Lucia as of 11:00 AM Sunday 25th August 2019.

At 11:00 AM Sunday, the center of Tropical Storm Dorian was located near latitude 11.2 North, longitude 52.9 West. Dorian is moving toward the west near 14 mph (22 km/h), and this motion should continue through tonight.

Maximum sustained winds are near 65 KM/H with higher gusts to 85 KM/H. Presently, this system has a minimum central pressure of 1008 millibars.

The convective organization of Dorian has improved a little since the last advisory, as a more persistent area of central convection has formed along with increased, but ragged, outer banding. However, the various satellite intensity estimates remain clustered around 65 KM/H (35 knots), and that remains the initial intensity.

Dorian is located in a very warm area of the Eastern Caribbean, with ocean temperatures near 29 degrees Celsius. However, significant dry air is present around the entirety of the system, and it is moving into an area of stronger wind shear, which will hinder rapid intensification. Dry air has also infiltrated the circulation of the storm.

The Government of Barbados has issued a tropical storm watch for St. Vincent and the Grenadines, as well as St. Lucia. It has also upgraded the tropical storm watch to a tropical storm warning for Barbados.

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.

Additional watches could be issued later today for portions of the Windward and Leeward Islands. Elsewhere, interests in Puerto Rico, the Virgin Islands, and Hispaniola should monitor the progress of Dorian.

What We Expect

The Track

Dorian is moving toward the west near 22 KM/H, and this motion should continue through tonight. A turn toward the west-northwest is expected on Monday, with this motion continuing through Tuesday night. On the forecast track, the center of Dorian is expected to be near the Windward Islands late Monday or early Tuesday and move into the eastern Caribbean Sea on Tuesday.

Dorian continues to remain south of where official forecasts from the National Hurricane Center, as well as model guidance, is forecasting. Hence, over the last day, we’ve continually seen the track of Dorian shift further south.

From the National Hurricane Center:

A combination of 1-minute GOES-16 visible imagery and microwave satellite data have helped pinpoint the center, and the initial motion is 280/12. The subtropical ridge to the north of the cyclone should steer it generally westward to west-northwestward for the next 3-4 days, with a more northwestward motion possible thereafter as Dorian approaches an upper-level low pressure area forecast to be over the north-central Caribbean. The model guidance continues the previous trend of being south of and a bit faster than the previous runs, and as a result the new forecast track is again shifted a little to the south. The new forecast lies between the previous forecast and the various consensus models. However, it is south of the forecasts of the HWRF, the UKMET, and the UKMET Ensemble mean. Additional adjustments to the track may be required on the next advisory if the current model trends continue.

National Hurricane Center, Forecast Discussion, Advisory 5

This is significant because confidence is increasing that Trinidad, Tobago, and Grenada *may* be affected by outer bands of this system, but not sustained tropical-storm-force winds. Here’s why.

Much of the “active” weather associated with Dorian is concentrated near the center of the system. Presently, Dorian’s circulation extends outward from the center of the system at 100 nautical miles or approximately 185 kilometers (light green in the above image).

Based on the present track, this 100 nautical mile circulation radius covers islands as far south as Grenada, to as far north as the southern tip of Dominica. But, as mentioned above, Dorian is trending further south than forecast. This means that the area of circulation – the area that is directly influenced by Dorian’s low-pressure center – may include Trinidad and Tobago over the next track adjustments.

Reiterating, this is just periphery effects of the circulation and NOT tropical storm conditions, hence a tropical storm watch or warning is not necessary. Tropical Storm Watches/Warnings are only issued for areas that sustained tropical-storm-force winds are possible or expected. It does not take into account gusts up to tropical-storm-force strength, locally heavy rainfall and thunderstorms which may trigger street, flash, and riverine flooding; downed trees, utility poles, and lines; landslides/landslips; power outages; hazardous seas, and coastal flooding in association with storm surge.

Therefore, persons residing in the Southern Windwards need to pay close attention to the upcoming forecasts issued by respective authorities in the region. From the 12Z model runs, nearly all models are moving this system between Tobago and Barbados, then across St. Vincent and the Grenadines.

The Intensity

Maximum sustained winds are near 65 KM/H with higher gusts to 85 KM/H. Some strengthening is forecast during the next 48 hours, and Dorian could be near hurricane strength by Tuesday over the Eastern Caribbean Sea.

Tropical-storm-force winds extend outward up to 35 kilometers from the center.

Probabilities of an area experiencing Tropical-Storm-Force and Hurricane-Force Winds. It is important for users to realize that wind speed probabilities that might seem relatively small at their location might still be quite significant since they indicate that there is a chance that a damaging or even extreme event could occur that warrants preparations to protect lives and property.

While vertical wind shear is decreasing over Dorian, the cyclone still appears to be ingesting dry air based on the ragged convective pattern.

Some dry air entrainment is expected to continue for the next few days, and based on this the new intensity forecast again calls for gradual strengthening through 72 hours. This part of the intensity forecast is in best agreement with the SHIPS model, and it lies near the upper edge of the intensity guidance.

The intensity forecast becomes low confidence after 72 hours due to uncertainties in the amount of shear and land that Dorian will encounter. The new forecast from the National Hurricane Center calls for some weakening due to shear before Dorian moves over Hispaniola, followed by weakening to a depression due to passage over the island.

However, the large range of possibilities includes both Dorian going north of Hispaniola and remaining a hurricane and the small cyclone dissipating completely over Hispaniola.

Based on the latest modeling as of 12Z runs, this system is likely to remain a compact and weak tropical storm as it moves through the Windward Islands.

The Impacts

For Barbados

Winds: Sustained surface winds between 64 KM/H and 84 KM/H with higher gusts are expected to spread across Barbados late Monday afternoon and persist into early Tuesday morning.

Rainfall: In addition, pockets of moderate to heavy showers, periods of rain and scattered thunderstorms are expected. Rainfall accumulations of at least 2 to 4 inches (50 to 100 mm) are possible with isolated higher amounts. As a result, some flash-flooding is likely in low-lying areas thus, residents are urged to be on the alert and take all necessary precautions.

Seas: Large easterly to south-easterly swells of 2.5 to 3.5m (8 to 12ft) are also forecast to accompany the system. Low-lying coastlines around the island will be particularly vulnerable at times of high tide. Large waves and dangerous rip-tides can be expected. These will create unsafe conditions for small-craft operators and fishermen. Sea-bathers and other users of the sea are also advised to stay out of the water. Thus, a High-Surf Advisory and Small-Craft Warning will be in effect from 12 noon Monday 26th August 2019 until 6:00 a.m. Tuesday, 27th August 2019.

A small-craft Warning means in this case that mean wind-speeds of 25 to 33 knots (47 to 62 KM/H) and/or seas equal to or greater than 3m (10ft) will be affecting the marine area. A High-Surf Advisory is issued when breaking wave action poses a threat to life and property within the surf zone.

For St. Vincent and the Grenadines (And St. Lucia)

Similar conditions are forecast for St. Lucia.

For Grenada

As of 1:00 PM Sunday 25th August 2019, there are no alerts, watches or warnings in effect for Grenada.

Winds: Sustained surface winds between 20 KM/H and 40 KM/H with higher gusts up to 70 KM/H are possible across Grenada late Monday evening and persist into early Wednesday morning as Tropical Storm Dorian traverses the region.

Rainfall: In addition, pockets of moderate to heavy showers, periods of rain and scattered thunderstorms are expected. Rainfall accumulations of at least 1 to 2 inches (25 to 50 mm) are possible with isolated higher amounts. As a result, some street and flash flooding are possible in low-lying areas thus, residents are urged to be on the alert and take all necessary precautions. Landslides are also possible in landslide-prone areas.

Seas: Large easterly to south-easterly swells of 2.0 to 3.0 meters are also forecast to accompany the system. Low-lying coastlines around the island will be particularly vulnerable at times of high tide. Large waves and dangerous rip-tides can be expected. These will create unsafe conditions for small-craft operators and fishermen. Note that king tides are forecast to begin on Wednesday, which means higher than normal high tides are expected. Coastal flooding possible, particularly along the northern coastlines of Grenada.

For Trinidad and Tobago

As of 1:00 PM Sunday 25th August 2019, there are no alerts, watches or warnings in effect for Trinidad and Tobago.

Dorian, though some periphery effects are possible, is not forecast to have major impacts across Trinidad and Tobago, regardless of the minor southward track adjustments.

However, because of Trinidad and Tobago’s high vulnerability to flooding, landslides and wind damage, all it takes is one severe thunderstorm to wreak havoc across the islands.

Monday: A mostly sunny start to Monday is forecast. However, by the late morning through the afternoon, daytime heating and sea breeze convergence will result in isolated showers, mainly across Western and Northern Trinidad. A few of these showers, particularly along Western Coastal Trinidad and the north-facing slopes of the Northern Range may develop into heavy showers or thunderstorms.

Showers and thunderstorms from the periphery of Tropical Storm Dorian to our north may begin to affect parts of Tobago and the Northern half of Trinidad by the late afternoon through the night, interrupting mostly settled skies.

Street and flash flooding, gusty winds and landslides are possible in the vicinity of, or accompanying heavy showers and thunderstorms.

Winds are forecast to be light, mainly from the east initially, becoming more from the northeast as the afternoon and evening progresses as Tropical Storm Dorian nears the Windwards. What this means weather-wise, is that after an initially sunny morning, any showers that develop due to daytime heating and sea breeze convergence will move southwesterly, particularly in the late evening into the night.

By nightfall, increasing showers are likely, mainly across the Northern half of Trinidad and Tobago as Tropical Storm Dorian moves south of Barbados and north of Tobago. The Southern half of Trinidad is forecast to remain mostly settled, with the odd isolated shower moving southwest across the island.

Note that King Tides are forecast to begin on Wednesday. This means higher than usual high tides and lower than usual low tides. Seas are forecast to become moderate with waves up to 2.5 meters in open waters. Large waves and dangerous rip-tides are possible. These will create unsafe conditions for small-craft operators and fishermen and persons with marine interests. Extreme caution is advised.

Tuesday: Winds are forecast to be from the north as Tropical Storm Dorian is north of T&T. This means that sea breeze convergence is forecast to initiate showers across the Northern Range and the northern coast of Tobago and the prevailing winds will cause orographic lift (air being forced upwards by the mountainous terrain). These factors, combined with our usual local climate regime and increased moisture may result in locally heavy showers and thunderstorms across the northern half of Trinidad, as well as parts of Tobago.

Isolated showers, particularly by the late morning through the afternoon, are forecast across Southern Trinidad. Parts of Southwestern Trinidad may experience thunderstorm activity.

Street and flash flooding, gusty winds and landslides are possible in the vicinity of or accompanying heavy showers and thunderstorms.

Note that King Tides are forecast to begin on Wednesday. This means higher than usual high tides and lower than usual low tides. Seas are forecast to become moderate with waves up to 2.5 meters in open waters. Large waves and dangerous rip-tides are possible. These will create unsafe conditions for small-craft operators and fishermen and persons with marine interests. Extreme caution is advised.

Wednesday: Winds are forecast to be from the Southeast as Tropical Storm Dorian moves into the Caribbean Sea. This means that sea breeze convergence and daytime heating are forecast to initiate showers across Trinidad, particularly along the south-facing slopes of Northern Range and the southern coast of Tobago. The prevailing winds will cause orographic lift (air being forced upwards by the mountainous terrain). These factors, combined with our usual local climate regime and increased moisture may result in locally heavy showers and thunderstorms across the northern half of Trinidad, as well as parts of Tobago.

Isolated showers, particularly by the late morning through the afternoon, are forecast across Southern Trinidad. Parts of Southwestern Trinidad may experience thunderstorm activity.

Street and flash flooding, gusty winds and landslides are possible in the vicinity of or accompanying heavy showers and thunderstorms.

Note that King Tides are forecast to begin on Wednesday. This means higher than usual high tides and lower than usual low tides. Seas are forecast to become moderate with waves up to 2.5 meters in open waters. Large waves and dangerous rip-tides are possible. These will create unsafe conditions for small-craft operators and fishermen and persons with marine interests. Extreme caution is advised.

Over the 3-day period for T&T

Winds: Sustained surface winds between 20 KM/H and 40 KM/H with higher gusts up to 65 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago late Monday evening through Wednesday as Tropical Storm Dorian traverses the region.

Rainfall: In addition, pockets of moderate to heavy showers, periods of rain and scattered thunderstorms are expected. Rainfall accumulations of at least 1 to 2 inches (25 to 50 mm) are possible with isolated higher amounts. As a result, some street and flash flooding are possible in low-lying areas thus, residents are urged to be on the alert and take all necessary precautions. Landslides are also possible in landslide-prone areas.

Seas: Waves and swells of 2.0 to 2.5 meters, occasionally up to 3.0 meters are also forecast to accompany the system, particularly on Tuesday. Low-lying coastlines around the island will be particularly vulnerable at times of high tide. Large waves and dangerous rip-tides can be expected. These will create unsafe conditions for small-craft operators and fishermen. Note that king tides are forecast to begin on Wednesday, which means higher than normal high tides are expected. Coastal flooding possible, particularly along the northern and eastern coastlines of Trinidad and Tobago.

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