Forecast: As Gusty Winds Subside, Flooding Rainfall Is Possible Next Week

The forecast gusty wind event materialized across the region, but thankfully, it was not as severe as anticipated. As winds gradually subside through the weekend, a new system will be taking shape next week. October has historically produced a significant flood event, and based on the current model guidance; the Intertropical Convergence Zone may trigger high 3-day rainfall accumulations across the country.

While there are no alerts, watches, or warnings in effect for next week’s anticipated rainfall, if you live in a flood-prone area, it may be pertinent to review your flood safety plan and preposition resources if needed.

Here is what you need to know:

— Through the weekend, following the passage of Tropical Wave 59 on Saturday, a weak ridge pattern is forecast to move in, with the Intertropical Convergence Zone (ITCZ) lingering nearby. Heavier rain during the weekend is forecast to remain south and east of T&T due to strong westerly wind shear.
— From Monday through Wednesday, Tropical Wave 60 is forecast to move across the region, interacting with the ITCZ and in the presence of a highly favorable upper-level environment. Scattered to widespread showers and isolated thunderstorms are forecast, potentially producing flooding rainfall across both islands. However, overall heavier rainfall is forecast to remain south and east of T&T due to persisting strong westerly wind shear.
— The main hazard through Sunday will be elevated sustained winds up to 40 KM/H and wind gusts in excess of 55 KM/H, with peak winds forecast on Saturday. Winds of this strength can cause power dips and outages, fallen trees, utility poles, and lines, as well as sporadic areas of wind damage.
— From Monday through the first half of next week, the hazards shift to mainly heavy rainfall and flooding. Generally, the street and flash flooding threat will increase Sunday night, with the potential for riverine flooding by Tuesday.
Lightning will accompany thunderstorms. Funnel clouds and waterspouts are unlikely through the forecast period due to elevated low-level winds. Stronger thunderstorms are possible from Monday.
— An Adverse Weather Alert (Yellow Level) is in effect from the Trinidad and Tobago Meteorological Service at this time until 6:00 PM Saturday.

The Forecast

Moisture associated with Tropical Wave 59, Tropical Wave 60, and the ITCZ east of T&T. (SSEC/CIMSS/UW-Madison)
Moisture associated with Tropical Wave 59, Tropical Wave 60, and the ITCZ east of T&T. (SSEC/CIMSS/UW-Madison)

Saturday: Early morning showers and thunderstorms are forecast to give way to variably cloudy skies across the country, with additional brisk isolated showers and thunderstorms favoring southern and eastern areas of Trinidad and Tobago, gradually moving westward through the day. Conditions are forecast to settle by the evening, with isolated showers and thunderstorms generally remaining offshore T&T.

Sunday: Partly cloudy, breezy, and somewhat hazy, interrupted by brisk isolated showers through the day, favoring Trinidad. By the evening, increasing cloudiness, scattered showers, and isolated thunderstorms are forecast to affect both islands. Heavier rainfall is forecast to favor southern Trinidad generally.

Through the next three days, heavier showers, thunderstorms, and overall higher rainfall accumulations are forecast to remain south and east of the county.

Monday: Scattered showers, thunderstorms, and periods of rain are forecast to affect the country after midnight, interrupting cloudy and somewhat breezy conditions. Rainfall is forecast to decrease in intensity and coverage from late afternoon into the evening, but isolated to scattered showers and thunderstorms are forecast to return overnight.

Tuesday: Scattered showers, thunderstorms, and periods of rain are forecast to affect the country after midnight with cloudy and breezy conditions. Intermittent heavy showers and thunderstorms are forecast within periods of rain and cloudy skies through the evening and night.

Wednesday: Widespread showers, thunderstorms, and rain, with cloudy skies, are forecast to affect the country after midnight and persist through the day, with coverage and the intensity of rainfall decreasing by the evening leading to a mostly cloudy night.

Scattered showers and thunderstorms east of the Lesser Antilles on Wednesday night. (Weathernerds.org)

Marine: Hazardous seas are forecast through the weekend due to long period swells and increased winds. Seas in open waters are forecast between 2.0 and 3.0 meters, occasionally above 3.0 meters on Saturday. In sheltered areas, between 1.0 and 1.5 meters, higher along northern and eastern coastlines.

Temperatures

Through the forecast period, the minimum low in Trinidad and Tobago is forecast to be between 23.0°C and 26.0°C.

Maximum highs across the country are forecast to be generally up to 32.0°C in Trinidad and Tobago. However, with increased cloud cover, maximum highs are forecast to be relatively cooler.

In urbanized areas like Port of Spain, Scarborough, San Fernando, and Chaguanas, the maximum high temperatures could exceed 33.0°C on days with sunny mornings.

Forecast Impacts

As mentioned above, through Sunday, elevated sustained winds up to 40 KM/H and wind gusts in excess of 55 KM/H are likely, with peak winds forecast on Saturday. From Monday through the first half of next week, the hazards shift to mainly heavy rainfall and flooding. Generally, the street and flash flooding threat will increase from Monday, with the potential for riverine flooding by Tuesday. Lightning will accompany thunderstorms. Funnel clouds and waterspouts are unlikely through the forecast period due to elevated low-level winds. Stronger thunderstorms are possible from Monday.

Rainfall & Flooding

Forecast chances of heavy to violent showers and thunderstorms associated with tropical waves and the ITCZ over the next five days.
Forecast chances of heavy to violent showers and thunderstorms associated with tropical waves and the ITCZ over the next five days.

Through the forecast period, wind shear is forecast to remain unfavorable for persisting heavy rainfall, with shear lessening by next week. Still, favorable upper-level conditions are forecast to produce pockets of deep convection that may produce heavy to violent rainfall rates and thunderstorm activity from Monday.

Over the next five days across Trinidad, isolated totals up to 200 millimeters are forecast across southern and eastern areas of the island, with generally 75-125 millimeters across the country. Higher rainfall accumulations are forecast mainly across southern and eastern regions of Trinidad, and isolated high totals are also possible along the western coastal areas of Trinidad. Again, overall higher rainfall accumulations are forecast to remain offshore eastern Trinidad.

  • Saturday: Less than 25 millimeters across the country with isolated totals up to 50 millimeters favoring highly localized areas of Trinidad, particuarly across southern and eastern areas.
  • Sunday: Less than 10 millimeters across the country with totals up to 25 millimeters favoring the eastern half of Trinidad and highly localized areas across southern Trinidad.
  • Monday: Up to 25 millimeters across the country, trending wetter across the eastern and southern halves of Trinidad. Isolated totals in excess of 50 millimeters are possible mainly across the eastern half of Trinidad.
  • Tuesday: Up to 25 millimeters across the country, trending wetter across the eastern and southern halves of both islands. The heaviest rainfall is expected across and offshore southern and eastern Trinidad, with totals between 25 and 50 millimeters.
  • Wednesday: Up to 50 millimeters across the country with isolated totals up to 100 millimeters favoring eastern areas of both islands. This is forecast to be the wettest day of the 5-day period.
Forecast chances of street or flash flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.
Forecast chances of street or flash flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.

Street flooding or flash flooding is forecast over the next five days, with low to medium chances through the weekend becoming high to very high by Monday. Chances for riverine flooding remain very low through the weekend, increasing through the upcoming week.

Putting the rainfall forecast into context, rainfall rates in excess of 50 millimeters per hour or areas that receive in excess of 25 millimeters within an hour tend to trigger street flooding across the country or flash flooding in northern Trinidad. For riverine flooding to occur, a large area of the country (not just in highly localized areas of western coastal Trinidad) would have to record upwards of 75 millimeters within 24 hours, and rainfall would have to fall across major rivers’ catchment areas.

Forecast chances of riverine flooding resulting from heavy rainfall associated with tropical waves and the ITCZ over the next five days.

Gusty Winds

Forecast chances for gusty winds associated with tropical waves and the ITCZ over the next five days.
Forecast chances for gusty winds associated with tropical waves and the ITCZ over the next five days.

Initially, this adverse weather event is forecast to mainly consist of gusty winds at low levels of the atmosphere, working their way to the surface in showers and thunderstorms. By Sunday, winds are forecast to gradually subside at low levels of the atmosphere.

Through the forecast period, sustained surface winds are forecast to be between 25 to 40 KM/H. In heavy to violent shower and thunderstorm 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.

Elevated winds will be present across the region due to a persisting low-level jet across the Windward Islands. A low-level jet is an area of strong winds at low levels of the atmosphere. These stronger winds can make it down to the surface in heavy showers and thunderstorms.

When these strong winds make it to the surface, the winds can hit the ground in a concentrated area and radiate outward like a ripple in a pond. This activity is known as a downburst and causes substantial wind damage across a fairly large area.

Diagram of convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively “raining itself out.”
Convective precipitation, the basis of shower and thunderstorm development. When a thunderstorm or shower begins to dissipate, the downdraft overpowers the updraft, effectively “raining itself out.” 

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. Dangerous, frequent lightning is expected on Friday and Saturday.

Landslides: Landslides and fallen trees are possible, particularly along the Northern Range, Southern Trinidad, and Tobago. The threat of landslides will increase during the upcoming week.

Severe Thunderstorms: The potential for severe thunderstorms, funnel clouds, and waterspouts remain low. If funnel clouds touchdown or waterspouts encounter land, weak tornadoes can form.

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, isolated to scattered rainfall is forecast, with widespread rains possible from Monday.

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

Forecast Discussion

Satellite update as of 4:00 PM Friday 22nd October 2021 (NASA/CIRA/RAMMB)
Satellite update as of 4:00 PM Friday 22nd October 2021 (NASA/CIRA/RAMMB)

Tropical Waves: At 18Z Friday 21st October 2021, the axis of Tropical Wave 59 is along 61°W moving westward at 10-15 knots (18-27 KM/H) with scattered showers and thunderstorms across T&T and along NE Venezuela and Guyana. Tropical Wave 60 is along 46°W from 16°N southward, moving west at 10-15 knots (18-27 KM/H), and is forecast to move across T&T from late Sunday into Monday.

On Saturday, Tropical Wave 59 will continue its westward path with trailing moisture and instability supporting isolated showers and thunderstorms across T&T, with the ITCZ lingering nearby. A weak ridge pattern is forecast to rebuild by the late afternoon, but moisture embedded within the ridge pattern will support brisk isolated showers on Sunday.

By late Sunday, Tropical Wave 60 and the ITCZ are forecast to move across Trinidad and Tobago bringing abundant moisture and instability. In addition, a highly favorable mid to upper-level environment will be in place across the region, allowing for deep convection to develop.

Following the passage of the tropical wave on Tuesday, a southeasterly low-level flow will bring abundant moisture across the country. Coupled with favorable upper-level conditions, widespread moderate to heavy rainfall is likely with cloudy skies through Wednesday.

However, strong wind shear from the west is forecast to remain across Trinidad and Tobago. While showers and thunderstorms will move across T&T, as activity moves west, convection may weaken, with cloudiness associated with showers and thunderstorms that develop across western Trinidad or Tobago moving eastward. This wind shear will also keep heavier rainfall south and east of the country.

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