Tropical Update: The Very Active Atlantic, Monitoring 5 Tropical Threats

Tropical Update Overview:
Hurricane Dorian: Dorian remains a category 5 hurricane and is nearly stationary across Grand Bahama, pummeling the island with winds of 270 KM/H, gusts to 320 KM/H and a storm surge of 18-23 feet. Widespread, catastrophic damage expected.
Invest 91L/Tropical Wave 43: This tropical wave, located south and west of the Cabo Verde Islands, are our next contender for a named tropical cyclone in the Atlantic Basin. Presently, this wave has high chances of tropical cyclone development over the next 48 hours and 5 days respectively. This system will be no threat to the Lesser Antilles as it moves into the Central Atlantic Ocean.
Invest 92L: This trough of low pressure continues to organize in the Central Atlantic, north of the Lesser Antilles. As of 8:00 AM, this area has a low chance of development over the next 48 hours and a medium chance of development over the next 5 days. Interests in Bermuda should monitor this system accordingly. This system will not pose a threat to the Lesser Antilles.
Invest 93L: A broad area of low pressure in the Gulf of Mexico continues to organize. Presently, this system has medium chances of formation over the next 48 hours and 5 days respectively. Interests in Western Mexico should monitor this system accordingly. No threat to the Lesser Antilles.
Tropical Wave 42: This tropical wave is located in the Central Atlantic, forecast to move across the Lesser Antilles on Tuesday. Note that this wave is not forecast to develop into a tropical cyclone at this time. However, it is forecast to drag the ITCZ across Trinidad and Tobago, bringing showers and thunderstorms to the region mainly on Tuesday. Street/flash flooding, gusty winds are possible, particularly across Northern and Eastern Trinidad, as well as Tobago. Afternoon thunderstorms are possible across Western Trinidad.
Tropical Wave 44: This tropical wave is still located on-land. However, models are in favor of developing this into a tropical cyclone shortly after it hits water in the Eastern Atlantic. It is too soon to tell where this storm may go as it moves across the Atlantic. We’ll be keeping a close eye on this.
— No direct tropical threats forecast for Trinidad and Tobago over the next five days.

Before we dive into the Tropical Update, a few notes:

  • Tropical waves are a normal part of the rainy season.
  • Not every tropical wave will form into a tropical cyclone.
  • Weaker tropical waves produce more rainfall across Eastern parts of the islands with mostly cloudy conditions and a few showers across western parts of the islands.
  • Rainfall will be more isolated and intermittent with weaker tropical waves that do not have ITCZ or upper-level support.
  • Saharan Dust may weaken tropical waves.

You can read more about the weather associated with Tropical Waves, as well as what to expect as these waves move through the region below.

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.

Hurricane Dorian

Hurricane Dorian churning across the NW Bahamas this morning. Credit: Tropical Tidbits

As of the 11:00 AM Advisory from the National Hurricane Center, Hurricane Dorian has weakened to a Category 4 Hurricane, though this means quite little for the Northwestern Bahamas as catastrophic impacts continue.

Dorian is moving west at 2 kilometers per hour, with sustained winds of 250 kilometers per hour and gusts to 305 kilometers per hour.

Storm surge of 18 to 23 feet is forecast to continue across the Grand Bahama Island, with large and destructive waves moving across the surge.

Up to 700 millimeters of rainfall is expected across the region, compounding widespread flooding across these low-lying islands.

Dorian is forecast to remain across the Grand Bahama Island over the next 24 hours, producing dangerous and life-threating hurricane conditions, taking a gradual northwestern and northern turn on Tuesday.

Though gradual weakening is expected, Dorian is forecast to remain a powerful hurricane over the next few days as it moves along the Southeastern United States, bringing life-threatening storm surge and swells to the region.  

For the latest, see the updated advisories from the National Hurricane Center.

Invest 91L/Tropical Wave 43

8:00 AM Tropical Update for Invest 91L
8:00 AM Tropical Update for Invest 91L

Shower and thunderstorm activity associated with a low-pressure system located about 300 miles west of the Cabo Verde Islands continues to show signs of organization. A tropical depression is expected to form during the next day or so while the system moves generally northwestward across the eastern tropical Atlantic Ocean. Locally heavy rainfall will be possible over the Cabo Verde Islands today.

Invest 91L/Tropical Wave 43 south and west of the Cabo Verde Islands, with increasing showers and thunderstorms. Credit: Weathernerds.org

A 1006 millibar low-pressure center is in the Atlantic Ocean near 15N27W. Scattered moderate to isolated strong showers, in clusters, are within 180 nm of the center in the N semicircle. Scattered moderate to strong showers are within 75 -240 nautical miles of the center in the SW quadrant. The precipitation pattern is beginning to show signs of organization.

The environmental conditions appear to be conducive for a tropical depression to form during the next day or so, while the system moves generally northwestward across the eastern tropical Atlantic Ocean. Locally heavy rainfall will be possible in the Cabo Verde Islands through Tuesday. The chance of a tropical cyclone to develop during the next 48 hours is high at 80 %, and 90% over the next 5 days.

Based on present modeling, this system is forecast to move into the Central Atlantic, remaining far away from the Lesser Antilles and any landmasses.

Invest 92L

8:00 AM Tropical Update for Invest 92L

A trough of low pressure is located several hundred miles south-southeast of Bermuda. Although the associated showers and thunderstorms continue to show signs of organization, the system does not have a well-defined surface center. The gradual development of this system is possible during the next few days while the disturbance moves slowly northward or north-northwestward. Interests in Bermuda should monitor the progress of this disturbance.

Invest 92L, north of the Lesser Antilles, with increasing showers and thunderstorms. Credit: Weathernerds.org

A broad middle-level cyclonic wind flow, related to an inverted trough, covers the area that is from 20N northward between 50W and 60W. A surface trough is from 28N59W to 23N62W. Scattered moderate convection is from 24N-28N between 58W-64W.

Now the technical stuff aside, models are not fairly enthusiastic with development of Invest 92L, with very few of the ensemble members of the EMCWF and GFS showing development.

Regardless of development, showers and thunderstorms are forecast to track near Bermuda later this week as this system progresses north to north-northwestward.

Invest 93L

A broad area of low pressure located over the south-central Gulf of Mexico is producing a wide area of showers and thunderstorms. This system has become better organized since yesterday, and a tropical depression could form during the next few days while the low moves slowly westward across the south-central and southwestern Gulf of Mexico toward the coast of Mexico.

Invest 93L, in the Central Gulf of Mexico, with increasing showers and thunderstorms. Credit: Weathernerds.org

This system has become better organized since yesterday, and a tropical depression could form during the next few days while the low moves slowly westward across the south-central and southwestern Gulf of Mexico toward the coast of Mexico. The chance of a tropical cyclone to develop during the next 48 hours is medium.

Based on the latest tropical outlook from the National Hurricane Center, this tropical wave has a medium, 40%, chance of tropical cyclone formation over the next 48 hours and a medium, 50% chance of tropical cyclone formation over the next 5 days.

Tropical Wave 42

Tropical Wave 43 approaching the Lesser Antilles, with the ITCZ being dragged northward. Credit: Weathernerds.org

An active tropical wave is located along 50 W, dragging the ITCZ northward. Today across Trinidad and Tobago, conditions are forecast to remain mostly sunny with isolated to scattered showers due to daytime heating and sea breeze convergence.

A few thunderstorms are possible, which may bring street/flash flooding and gusty winds to parts of Trinidad on Monday.

However, overnight through Tuesday, the ITCZ, combined with the Tropical Wave, is forecast to bring showers and thunderstorms to Trinidad and Tobago.

Based on the present model guidance, showers and thunderstorms are forecast after midnight, favoring Tobago and the Northern and Eastern halves of Trinidad.

Isolated to scattered showers are forecast to interrupt partly cloudy skies. Isolated thunderstorms are forecast to occur through the late afternoon.

Based on the latest model guidance, isolated showers and thunderstorms are forecast to move across both islands, with heavier activity favoring much of Eastern and Southern Trinidad through the weekend.

There are no alerts, watches or warnings in effect for Trinidad and Tobago.

Locally Heavy Rainfall & Flooding: Based on the latest model runs on Monday, from Monday (8:00 PM) through Wednesday morning (2:00 AM), generally, over the 36 hour period, models are in agreement of up to 20 millimeters of rainfall is forecast across the northern and eastern halves of Trinidad, as well as Tobago. Less than 15 millimeters of accumulation is forecast across parts of Southern Trinidad. Isolated totals up to 50 millimeters possible in thunderstorm activity.

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.

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

Gusty Winds: Gusty winds, generally between 40 and 55 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 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.

High Wind Event Precautions
High Wind Event Precautions

Landslides: Heavy showers across the Northern Range may trigger landslides in landslide-prone areas. Particularly in areas that receive heavy rainfall, landslides and/or mudflows may occur across both islands. These landslides, in addition to gusty winds, may down trees, utility poles and impede traffic on roadways.

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 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.

Tropical Wave 44

A tropical wave is forecast to emerge over the far eastern tropical Atlantic between Africa and the Cabo Verde Islands in a few days. Some gradual development of this disturbance will be possible late this week or over the weekend while it moves westward to west-northwestward.

There is some discrepancy in where this wave may go, and could potentially track near the Leewards. However, this system is beyond 7 days away, and there is little value in models beyond this time frame. Still, it’s worth keeping an eye on.

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