[UPDATED] Feeder Band Activity From Tropical Storm Elsa Forecast To Affect T&T Through Saturday

What you need to know:
Tropical Storm Elsa is forecast to track north of T&T on Friday. Periphery impacts are forecast for T&T. Direct, tropical-storm-force winds are not expected.
— From late-night Thursday through early Saturday, scattered showers and thunderstorms with variably cloudy skies are forecast across T&T. Showers will be accompanied by gusty winds and some may become heavy. Seas are forecast to be agitated, particularly near Tobago.
— An Adverse Weather Alert (Yellow Level) is in effect for T&T from the Trinidad and Tobago Meteorological Service from 8:00 PM Friday through 12:00 PM Saturday.

The Forecast

Friday: Mostly cloudy to overcast skies with scattered showers through the morning. By the late morning through the afternoon, scattered heavy showers and thunderstorms, are forecast to favor Trinidad, gradually settling by the evening. However, a resurgence of scattered showers thunderstorms is forecast after nightfall, some of which becoming strong.

Saturday: Showers and thunderstorms continue from midnight with few showers through midday, diminishing to partly/mostly cloudy skies. Increasingly hazy skies with few brisk showers are forecast for the remainder of the day.

Sunday: Mostly hazy with partly cloudy skies interrupted by a few isolated showers.

Marine: Seas are forecast to be moderate over the next 3 days, with waves between 1.5 to 2.0 meters in Trinidad and Tobago’s open waters. On Friday into Saturday, seas may become occasionally rough with waves in open waters between 2.5 to 3.5 meters, mainly northeast of Tobago. Across sheltered coastlines, waves between 1.0 and 1.5 meters are expected, with choppy seas in the vicinity and during heavy showers or thunderstorms.

Forecast Discussion

Through Sunday, the main influencer for T&T’s weather will be the passage of Tropical Storm Elsa north of Tobago on Friday, with lingering influence through the weekend.

Tropical Storm Elsa is approaching the region as of early Friday morning, with increased moisture, instability, and convergence ahead of the system, supporting showers and thunderstorms with winds shifting from the northeast to the northwest into mid-Friday.

As Elsa moves north of T&T on Friday afternoon, winds are forecast to markedly shift again, now coming from the southwest to the southeast into Saturday, with feeder band activity likely impacting Trinidad and Tobago. Forecast models indicate a highly favorable atmosphere overnight Friday into Saturday, with strong showers and thunderstorms possible. Wind shear will be from the northeast, spreading cloudiness from Elsa southwestward.

Elsa will be moving into the Caribbean Sea on Saturday, briskly moving to the west-northwest. Following Elsa’s passage, a high-pressure system is forecast to regain dominance by the afternoon on Saturday, accompanied by a surge in Saharan Dust. Another tropical wave will be approaching and moving across the region Monday into Tuesday.

Infrared satellite imagery showing Tropical Storm Elsa northeast of Trinidad and Tobago. (Weathernerds.org)

Temperatures

Across both islands, overnight lows are forecast to be mild, with daytime highs relatively cool due to increased cloud cover.

Over the next 72 hours, the minimum low for Trinidad and Tobago is forecast to be between 23.0°C and 26.0°C. The maximum high for Trinidad and Tobago is forecast to be between 29.0°C and 32.0°C.

Forecast Impacts

Forecast impacts due to Tropical Storm Elsa

From Thursday through Saturday afternoon, the main hazards include heavy to violent rainfall triggering localized street flooding and flash flooding. Lightning and gusty winds up to and in excess of 55 KM/H may accompany thunderstorms or heavy showers and cause power dips/outages, downed trees, and localized wind damage. Landslides are also possible across elevated areas, particularly in Tobago.

If a feeder band develops and persists across Trinidad on Friday into Saturday, high rainfall accumulations could produce significant enough rainfall to trigger riverine flooding,

Winds

Sustained surface winds up to 40 KM/H are forecast. In shower activity, gusts up to and in excess of 55 KM/H are possible. With wind gusts in excess of 55 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

Over the next 3 days, in addition to Thursday’s rainfall, additional rainfall accumulations of 50 to 100 millimeters are possible, with isolated totals of up to 125 millimeters. If intense feeder band activity develops, with strong showers and thunderstorms persisting across a particular area of the country, isolated totals may reach up to 150 millimeters.

  • Friday: Between 10 and 25 millimeters of rainfall across the country. In isolated areas, particularly where heavy to violent showers and thunderstorms occur, localized totals may exceed 25 millimeters. Again, the GFS remains the outlier, as it shows a stationary feeder band developing across Trinidad, with isolated totals up to 100 millimeters.
  • Saturday: Less than 20 millimeters of rainfall across the country with isolated totals up to 30 millimeters.
  • Sunday: Between 0-10 millimeters of rainfall across the country. Isolated areas may exceed 10 millimeters but little to no rainfall is forecast after daybreak on Sunday.

As mentioned earlier, heavy rainfall may trigger street flooding and flash flooding. If a stationary feeder band develops, dumping copious rainfall amounts across Trinidad, there would be the potential for riverine flooding.

Other Adverse Weather Impacts

Lightning: Lightning is possible in thunderstorm activity. Lightning can cause power outages, voltage dips, damage to life and property, particularly during cloud-to-ground lightning strikes.

Landslides: With potentially heavy rainfall and gusty winds, landslides are possible in elevated areas, particularly along the Northern Range and across Tobago on Friday into Saturday.

Waterspouts: Though conditions are not highly favorable, the varying wind profiles on Friday into Saturday, combined with the presence of shower and thunderstorms offshore, waterspouts are possible, particularly in the Gulf of Paria, off Trinidad’s eastern coast, and in Tobago’s coastal waters.

Periphery vs. Direct Impacts

Too often, when a tropical system is moving near or north of Trinidad and Tobago, you hear the country may experience periphery impacts but no further explanation is added.

Direct Impacts: The core of the tropical cyclone (i.e. the strongest winds and rain near the center of circulation) moves across the country. This can also be considered a direct hit, when the cyclone passes to within a distance equal to the cyclone’s radius of maximum wind.

Indirect or Periphery Impacts: The core of the tropical cyclone is far away from the country. However, the strong low-pressure can alert the typical wind regime of the area, causing winds to become light or non-existent or even reversing the typical direction of winds for that area. Strong winds associated with a tropical cyclone agitate seas or cause long-period swells, though the tropical cyclone is far away from where is being affected. Periphery impacts also mean feeder band activity. A feeder band is a spiral-shaped band of convection (showers and thunderstorms) surrounding and moving toward the center of a tropical cyclone.

Why is T&T not under a tropical storm watch or warning?

Tropical Storm Elsa’s forecast probabilities for tropical-storm-force winds as of 8:00 PM Thursday 1st July 2021. (National Hurricane Center)

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.

These specific watches and warnings are only issued for areas that may or will experience sustained tropical storm winds (above 63 KM/H).

For this particular tropical storm, the pressure gradient between the tropical cyclone and the steering ridge suggests stronger winds on the north side of the system than would be typical for a storm in a normal environment. In fact, based on forecast wind-radii from the National Hurricane Center, tropical-storm-force winds are only expected on the northern half of the circulation.

Why I May Not/Will Not See 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. Widespread showers mean that nearly all persons and areas may experience rainfall.

Through the forecast period, scattered to widespread rainfall is anticipated.

Forecast - Isolated, Scattered, Widespread - What do they mean?

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