Increasing Showers, Thunderstorms Near Invest 93L, Low Chances of Tropical Cyclone Formation

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

As of the 8:00 PM Tropical Weather Outlook from the National Hurricane Center, a tropical wave we’ve been keeping an eye on has been unchanged, with low chances of tropical cyclone formation over the next 48 hours and 5 days, at 10% respectively. This wave is over 3000 kilometers east of Trinidad and Tobago, nearly 4-5 days away.

What We Know

Tropical Waves 19 and 21 clearly visible on the above satellite imagery, both with scattered showers and isolated thunderstorms occurring. Tropical Wave 19 is forecast to move across Trinidad and Tobago over the next 48 hours, while Tropical Wave 21/Invest 93L forecast to move across T&T by Monday into Tuesday of next week. Credit: Weathernerds.org

Tropical Wave 20 broke into two sections, with the northern part of the axis moving faster ahead of the southern axis, forming two tropical waves.

The northern axis was Tropical Wave 20, which has now dissipated, with the southern axis, embedded within the ITCZ is now Tropical Wave 21, or Invest 93L.

This tropical wave (20/21) moved off the African Coast on July 8th through the 9th and has been progressing westward between 15-25 knots. It has been located in an area of low wind shear, high total precipitable moisture and favorable conditions for showers and thunderstorms to flourish.

It also has well-stacked vorticity (spin at low and mid-levels) that are above one another, which is favorable in the development of tropical cyclones.

What We Expect

Track

The Takeaway: Forecast to move across the Windwards, including Trinidad and Tobago, Monday through Tuesday of the upcoming week.

Generally, this tropical wave (Invest 93L) is forecast to move west to west-northwestward over the next 5 days, moving across the Southern Windwards by Monday into Tuesday. Guidance from the 12Z and 18Z model runs and their ensembles are generally following the westward motion of this wave, forecasting it to move across the Windwards on Monday into Tuesday.

Presently, this wave is moving fairly rapidly westward at 24 KM/H, and is forecast to do so over the next 5 days due to strong easterly tradewinds.

The EMCWF and its ensembles keep this wave closer to Trinidad and Tobago, with the bulk of heavier showers and thunderstorms just passing north of Tobago. Both the UKMET (United Kingdom) and GFS (U.S.) models follow a similar track.

This wave is forecast to move across T&T Monday into Tuesday. Because of this amplified tropical wave, the ITCZ will be dragged northward both ahead and behind this wave, similarly to Tropical Wave 19, which is forecast to move across Trinidad and Tobago on Friday. The ITCZ will result in increased showers and thunderstorms ahead and behind of the passage of this wave, particularly late Sunday through Tuesday.

Intensity

Invest 93L churning in the Central/Eastern Atlantic over the course of Thursday 11th July 2019, as showers and thunderstorms develop near the low-pressure center of circulation, Credit: Tropical Tidbits.
Invest 93L churning in the Central/Eastern Atlantic over the course of Thursday 11th July 2019, as 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).

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 be 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 over the next five days.

The GFS and EMCWF ensembles have lessened in support for development, but most keep this system as a tropical wave as it moves westward, though there is a brief window over the next 24-48 hours where conditions remain fairly conducive for brief organization. The UKMET operational model keeps this system as a wave as it progresses westward.

Ensemble modeling means running two or more related but different models and then synthesizing the results into a single score or spread in order to improve the accuracy of predictive analytics and data mining applications.

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).

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.

Note: Street/Flash flooding, frequent lightning, gusty winds, landslides/mudslides are all possible beginning Sunday afternoon through Tuesday evening. In addition, locally heavy showers and thunderstorms may produce isolated areas, up to 150 millimeters of rainfall, across the Windward Islands, particularly north of Trinidad and Tobago.

Locally Heavy Rainfall & Flooding: Based on the latest model runs on Thursday, from Sunday (2:00 AM) through Wednesday (2:00 AM), generally, over the 72 hour period, models are in agreement of fewer than 20 millimeters across Western halves of Trinidad and Tobago and between 20-40 millimeters across the Eastern halves of both islands. Isolated rainfall totals up to 60 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 70 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.

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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