Invest 93L – Heavy Rainfall Threat to T&T, Southern Windwards. Low Chance of Tropical Cyclone Development.

Trinidad, Tobago, and the Eastern Caribbean are NOT under any tropical storm or hurricane watch or warning at this time.

As of the 2:00 PM Tropical Weather Outlook from the National Hurricane Center, a tropical wave we’ve been keeping an eye on still has low chances of tropical cyclone formation over the next 48 hours and 5 days, at 10% respectively. This wave is over 1800 kilometers east of Trinidad and Tobago, nearly 2-3 days away.

What We Know

Tropical Wave 21 (Invest 93L) clearly visible on the above satellite imagery with a concentrated area, just west and north of the low-pressure center, of showers and thunderstorms. Tropical Wave 21/Invest 93L forecast to move across T&T by Monday into Tuesday of next week. Credit: Weathernerds.org
Tropical Wave 21 (Invest 93L) clearly visible on the above satellite imagery with a concentrated area, just west and north of the low-pressure center, of showers and thunderstorms. Tropical Wave 21/Invest 93L forecast to move across T&T by Monday into Tuesday of next week. Credit: Weathernerds.org

Tropical Wave 21 (Invest 93L) is located at 10N, 44W. Over the last 18-24 hours, Invest 93L has suffered some dry air inhibiting persistent convection near the low-pressure center.

However, a few showers and thunderstorms have developed yet again west of the low-pressure center. It is located in an area of marginally favorable low-level convergence and upper-level divergence as well as low wind shear.

An ASCAT-C pass at 12:30 Z on Saturday showed winds up to 46 KM/H, mainly north of the low-pressure center.

Invest 93L and the tropical wave associated with this low-pressure is moving west-northwestward at 28 KM/H into an area of warmer sea surface temperatures of 28 – 29°C. However, significant Saharan Dust remains north and west of this wave, which likely will be one of the two major detrimental factors in allowing this wave to organize and intensify.

Invest 93L located in an area of concentrated Saharan Dust surrounding the wave and low-pressure center, inhibiting shower, and thunderstorm formation. Credit: UW-CIMSS
Invest 93L located in an area of concentrated Saharan Dust surrounding the wave and low-pressure center, inhibiting shower, and thunderstorm formation. Credit: UW-CIMSS

Because it is presently located in an area of low wind shear and high total precipitable moisture, marginally favorable conditions are in place for showers and thunderstorms to develop as mid-level moisture remains limited.

Vorticity associated with this system, though still present, has reduced. Generally, Invest 93L is located in a generally unfavorable environment for the development of a tropical depression. Any development of this wave will be slow to occur.

What We Expect

Track

Track model consensus for Invest 93L (it's remnants) have the minimum low pressure now moving across Trinidad and Tobago as of 12Z Saturday, July 13, 2019. Note that this system is forecast to move across T&T and the Southern Windwards as a tropical wave, not a tropical depression or tropical storm due to unfavorable conditions for tropical cyclone formation. Regardless, we expect to see heavy rainfall across T&T resulting from this system's passage. Image: Tropical Tidbits
Track model consensus for Invest 93L (it’s remnants) have the minimum low pressure now moving across Trinidad and Tobago as of 12Z Saturday, July 13, 2019. Note that this system is forecast to move across T&T and the Southern Windwards as a tropical wave, not a tropical depression or tropical storm due to unfavorable conditions for tropical cyclone formation. Regardless, we expect to see heavy rainfall across T&T resulting from this system’s passage. Image: Tropical Tidbits

The Takeaway: Forecast to move across the Windwards, including Trinidad and Tobago, Monday through Tuesday of the upcoming week. 06Z & 12Z global models (not pictured above) are keeping the remnants of this low-pressure system moving across, or just north of Tobago.

Generally, this tropical wave (Invest 93L) is forecast to move west (varying between west-northwestward to west-southwestward at times) over the next 2-3 days, moving across the Southern Windwards by Monday into Tuesday. Presently, this wave is moving fairly rapidly westward at 28 KM/H and is forecast to do so over the next 2-3 days due to strong easterly tradewinds.

Invest 93L has no model support from the top three global models from tropical development, and minimal support from its ensemble runs. Hence, a strong tropical wave is forecast to move across the Southern Windwards in 48-72 hours.

This wave is forecast to exit the region on Tuesday. However, Invest 93L is forecast to drag the ITCZ across Trinidad and Tobago beginning Sunday night through Tuesday, bringing a heavy rainfall threat to the islands, particularly T&T.

Intensity

Invest 93L churning in the Central Atlantic over the course of Saturday 13th July 2019, as minimal showers and thunderstorms develop near the low-pressure center of circulation, Credit: Tropical Tidbits.
Invest 93L churning in the Central Atlantic over the course of Saturday 13th July 2019, as minimal showers and thunderstorms develop near the low-pressure center of circulation, Credit: Tropical Tidbits.

The Takeaway: A strong tropical wave is forecast to move across the Southern Windwards, generally with scattered heavy showers and thunderstorms but no tropical cyclone is forecast (at this time). This wave will cause the ITCZ to move across T&T, producing showers and thunderstorms mainly on Monday into Tuesday.

All operational runs of the top global models (EMCWF, GFS, UKMET, ICON, etc.) keep this system as a tropical wave as it moves across the region on Monday into Tuesday of the upcoming week.

This lack of development is likely due to the detriment of its fast forward motion as mentioned above. Fast moving systems and tropical waves have more difficulty aligning themselves vertically and getting organized.

Furthermore, dense Saharan Dust north and forecast to remain ahead of this wave as it progresses westward will keep mid levels of the atmosphere fairly dry, inhibiting convection required for strengthening and organization.

Though wind shear is predicted to be low to moderate and SSTs will be adequate for development, near 26.5 – 27°C (80- 81°F), this dry air will create hostile enough conditions that development into a tropical depression is unlikely, particularly beyond Sunday.

As mentioned above, there is no model support for tropical development from the top three global models. In fact, conditions have become largely unfavorable for the development of a tropical depression according to the National Hurricane Center. Any development of this system will be slow to occur.

Because of the large moisture field associated with this tropical wave, showers and isolated thunderstorms are forecast to begin on Sunday afternoon, with peak activity forecast Monday evening into Tuesday (July 15th into July 16th).

Although the low associated with this wave eventually dissipate as this wave progresses westward, instability will be maintained from the interaction with a low-level easterly Jet. Cloudy skies, scattered showers, possible thunderstorms, windy conditions, and occasional gusts are expected across mainly Tobago, Northern and Eastern Trinidad.

Impacts

The Takeaway: Increased cloudiness by late Sunday, showers to begin overnight Sunday into Monday, with thunderstorms possible Monday into Tuesday, generally interrupting overcast to mostly cloudy skies. Street/Flash flooding, gusty winds up to 60 KM/H, landslides and downed trees/utility poles possible.

Generally, increasing cloudiness is forecast to begin late Sunday, with isolated showers occurring through Sunday night into Monday morning.

On Monday, during the late morning through the afternoon, scattered showers and isolated thunderstorms are possible across parts of Trinidad and northeastern Venezuela. The latter activity will likely cause mostly cloudy skies across the southern half of Trinidad. This activity will be directly associated with Invest 93L and is forecast to subside by the late afternoon into the evening.

However, as Invest 93L progresses westward, the ITCZ is forecast to remain across the Northern half of Trinidad and Tobago through Tuesday. This will facilitate scattered to possibly widespread showers across the aforementioned areas. The heaviest rainfall is likely across Eastern parts of Trinidad and Tobago.

Based on the latest model guidance, this activity is likely to linger into Wednesday with intermittent showers across both islands.

Note: Street/Flash flooding, frequent lightning, gusty winds, landslides/mudslides are all possible beginning Sunday evening through Tuesday evening. In addition, locally heavy showers and thunderstorms may produce isolated areas, in excess of 100 millimeters of rainfall, across the Windward Islands, particularly Eastern areas of Trinidad and Tobago and north of T&T.

Locally Heavy Rainfall & Flooding: Based on the latest model runs on Saturday, from Sunday (2:00 AM) through Wednesday (2:00 AM), generally, over the 72 hour period, models are in agreement of fewer than 30 millimeters across Western halves of Trinidad and Tobago and between 30-50 millimeters across the Eastern halves of both islands. Isolated rainfall totals up to 75 millimeters are possible across Eastern coastal Trinidad and Tobago. As mentioned above, much of the heaviest showers and thunderstorms are forecast to move north of Trinidad and Tobago, particularly between Tobago and Barbados.

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, based on the above-mentioned forecast rainfall totals.

Frequent Lightning: In addition, with thunderstorms, frequent lightning is likely.

Gusty Winds: Gusty winds, generally up to 60 KM/H are possible Gusty winds are most likely prior to, in the vicinity of, or occur during heavy showers or thunderstorms. With wind gusts up to 60 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.

Across parts of Grenada, St. Vincent and the Grenadines, winds may gust up to 65 KM/H at times, particularly in the vicinity of heavy showers and thunderstorms.

High Wind Event Precautions
High Wind Event Precautions

Landslides: In landslide-prone areas, particularly in areas that receive heavy rainfall, landslides and/or mudflows may occur. These landslides, in addition to gusty winds, may down trees, utility poles and impede traffic on roadways.

Other Concerns on Invest 93L
(A Tropical Wave)

Why I May Not/Will Not See Constant Rainfall?

Tropical waves typically have fair weather ahead of the wave axis, though other features in the area such as surface troughs, increased atmospheric moisture, and local climatic effects, to name a few, may trigger showers and the odd thunderstorm. Following the passage of the wave, much of the active weather typically associated with a tropical wave occurs.
Tropical waves typically have fair weather ahead of the wave axis, though other features in the area such as surface troughs, increased atmospheric moisture, and local climatic effects, to name a few, may trigger showers and the odd thunderstorm. Following the passage of the wave, much of the active weather typically associated with a tropical wave occurs.

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. Generally, because of winds associated with weak tropical waves, showers and thunderstorms tend to follow a west-northwest track, generally missing areas across Southwestern Trinidad and even Northwestern Trinidad – though activity due to local climatic effects such as daytime heating may trigger shower and thunderstorm development across these areas. See the below graphic for a simple explanation.

With this tropical wave, mostly scattered showers are forecast. Hence, we are likely to see intermittent showers and thunderstorms, interrupting mostly cloudy to overcast skies between late Sunday and Tuesday. In addition, most of the heavier showers, thunderstorms, much like the tropical wave moving across the region on Friday through Saturday, will occur between Tobago and Barbados.

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, especially since this tropical wave is more than 5 days away from moving across the Windwards.

Always check the National Hurricane Center for the latest information for tropical cyclones and your local meteorological offices for country-specific advisories concerning Invest 93L.

What is an Invest?

It sounds ominous, but from the outset, it really isn’t. Invest is short for investigation, followed by the numbers 90 through 99 and either the letter “L” for the Atlantic basin systems or “E” for the Eastern Pacific Systems.

This naming convention is used by the National Hurricane Center to identify features they are monitoring for potential future development into a tropical depression or a tropical storm.

According to the National Hurricane Center, by designating a tropical weather system as an “Invest”, the collection of specialized data sets and computer model guidance on the area of interest can begin. This collection and processing of data are shown on a number of government and academic websites for analyzing.

That said, the “Invest” assignment does not correspond to how likely a system may develop into a tropical depression or storm.

What should I do?

Firstly, don’t panic. There is a high likelihood of Invest 93L remaining a tropical wave moving across Trinidad, Tobago and the Southern Windwards by the beginning of next week.

Secondly, if you are a risk-averse person, now is a good time to check your inclement weather or hurricane season plan, ensuring your preparedness supplies are not expired, stocked and in a safe location.

The Office of Disaster Preparedness and Management has put together a comprehensive guide for preparing for the 2019 Wet and Hurricane Season.

How often can I expect updates?

The National Hurricane Center issues Tropical Weather Outlooks every 6 hours, at 2 AM, 8 AM, 2 PM and 8 PM daily.

We generally issue a tropical update once daily, but with a system that may threaten our region, two updates will be posted, following the major model updates (after 10:00 AM and after 4:00 PM daily).

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