Tropical Wave 01 – Where Did The Rainfall Go?

Tropical weather is a fickle thing and Tropical Wave 01 is just the latest example.

The atmosphere is a fluid environment as we try our best to capture the ever-changing situation with in situ observations and remote sensing.

On the ground, automated weather stations collect winds, temperature, humidity, and rainfall, usually at airports, and atmospheric soundings give us properties of a slice of the atmosphere at one time. High above Earth, satellites give us an abundance of data on the dynamic atmosphere. All of this data is then fed into supercomputers to produce numerical weather prediction models.

Even with all of that, the forecast can be busted. That’s what happened today. The forecast was wrong. We’re not happy about it either because we lose credibility, but it happens when you live in a tropical area. Why? The atmosphere was just too stable (for now).

We’ve been monitoring this tropical wave for a few days and all indications (even until Tuesday night) showed periods of showers and isolated thunderstorms affecting T&T on Wednesday. Yet, we awoke to mostly sunny skies.

The Tropical Wave

A tropical wave has an axis, but it’s really a larger atmospheric feature that brings a myriad of weather – sunshine, showers, thunderstorms, and Saharan Dust may follow.

According to the National Hurricane Center, the axis of this wave moved across T&T overnight. However, based on the characteristic wind shifts (east, then east-northeast, then briefly east, then east-southeast to the southeast, returning to east) and satellite imagery, the axis of this wave finally moved across the Lesser Antilles this morning.

The typical model of a tropical wave, with generally stable conditions ahead of the wave axis and active weather following – similar to Tropical Wave 01
The typical model of a tropical wave, with generally stable conditions ahead of the wave axis and active weather following – similar to Tropical Wave 01.

Ahead of a tropical wave, atmospheric moisture surges but the overall atmosphere is not optimal for shower or thunderstorm development. Following the wave axis, the active weather follows.

Fighting Wind Shear

Over the last 24 hours, wind shear across the Eastern Caribbean has been strong, above 60 knots, the highest in the Caribbean Region. This westerly wind shear kept showers east of the Lesser Antilles even as the wave axis moves westward.

Strong wind shear across the Lesser Antilles, up to 60 knots. This was one of the many detriments to our lack of showers and thunderstorms today associated with Tropical Wave 01 Image: CIMSS
Strong wind shear across the Lesser Antilles, up to 60 knots. This was one of the many detriments to our lack of showers and thunderstorms today associated with Tropical Wave 01 Image: CIMSS

Wind shear also limited the development of the showers or thunderstorms, and any that developed, it did not persist.

A Stable Atmosphere

This morning’s atmospheric sounding from Piarco by the Trinidad and Tobago Meteorological Service gave the best picture of what our atmosphere looks like today.

The Atmospheric Sounding, a snapshot of the atmosphere at one moment in time, taken at 8:00 AM Wednesday 20th May 2020, as Tropical Wave 01 moves west of T&T. Image: COD Meteorology/Trinidad and Tobago Meteorological Service
The Atmospheric Sounding, a snapshot of the atmosphere at one moment in time, shown on a SKEW-T Chart, taken at 8:00 AM Wednesday 20th May 2020, as Tropical Wave 01 moves west of T&T. Image: COD Meteorology/Trinidad and Tobago Meteorological Service

The atmosphere is stable. To measure instability, a number of indices are used, but utilizing a calculation called the lapse rate (the rate at which temperature falls with altitude) can tell us how unstable the atmosphere is at a point in time. Presently, the lapse rate indicates the atmosphere is stable. However, with our sunny skies, this stability could destabilize, which will allow for showers to develop.

The atmosphere is slightly too warm and dry. While the energy for convection (showers or thunderstorms) is there, the temperature of the atmosphere does not cool as it rises. In fact, there is a fairly large area of warm air aloft (between 1,045 meters and 2,607 meters), which limits air from rising. This means that convection will struggle to develop.

It needs to get hot for showers to develop. Because of all the inhibiting factors, temperatures near the ground need to get hot before convection can begin. How hot? 36.06° to be exact. This means that it is possible air temperatures could reach this number in cities, but nowhere near the widespread heat needed to initiate convection.

What’s Next?

Tropical Wave 01 continues to move westward, with its large plume of moisture continuing to traverse the region. Showers and thunderstorms will continue to favor areas well north of Trinidad and Tobago due to favorable upper level conditions (even with the pervasive wind shear).

Southeasterly wind flow across Trinidad and Tobago, with cloudiness favoring Western and Northern areas due to sea breeze convergence and orographic lift. Image: Weathernerds
Southeasterly wind flow across Trinidad and Tobago, with cloudiness favoring Western and Northern areas due to sea breeze convergence and orographic lift. Image: Weathernerds

Across Trinidad and Tobago, due to the tropical wave’s influence, winds are now from the southeast. This means moist air will move across warm land, and then forced upwards by hilly areas. This is called orographic uplift. This may be the saving grace for the few showers T&T experiences, favoring Northern Trinidad and Tobago.

Orographic precipitation occurs when warm, moist air is forced upward by hilly or mountainous topography. This can trigger shower development, or intensify shower and/or thunderstorms. Diagram: Trinidad and Tobago Weather Center

Another effect – sea breeze convergence. As winds from the Gulf of Paria collide with the prevailing southeasterly winds, localized convergence becomes possible across Western Trinidad, triggering showers.

Sea breeze convergence mechanisms for the Gulf of Paria and the Atlantic Ocean across Trinidad which can produce thunderstorm activity.. Note that sea breeze convergence is possible across all coasts of both islands. Diagram: Trinidad and Tobago Weather Center

All of the above mentions what could develop. What’s already existing – isolated to scattered showers just east of Tobago.

This activity is forecast to begin affecting Tobago by 3:00 PM and Trinidad by 5:00 PM and continue through the night with isolated to scattered showers and a low chance of a thunderstorm, favoring coastal areas.

These showers will interrupt variably cloudy skies later this afternoon, tonight and through tomorrow.

Gusty winds up to 55 KM/H are possible, with street flooding remaining the main hazard following locally heavy rainfall.

Approaching showers and isolated thunderstorms associated with the favorable area of Tropical Wave 01. Image: Weathernerds
Approaching showers and isolated thunderstorms associated with the favorable area of Tropical Wave 01. Image: Weathernerds

An important detail to note – during the evening through the early night (6:00 PM through 8:00 PM), convective activity tends to be at its minimum, meaning the existing activity can dissipate, and the prevailing current atmosphere is not in its favor. The chance for showers and thunderstorms still remain this evening through tomorrow morning.

Winds: Sustained surface winds between 25 KM/H and 45 KM/H with gusts up to 55 KM/H are possible in heavy showers or thunderstorms across Trinidad and Tobago.

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, bringing down utility poles and lines.

High Wind Event Precautions
High Wind Event Precautions

Rainfall: Accumulated totals are forecast to be between 10-15 millimeters, with 20-40 millimeters overall in areas that experience isolated thunderstorms or heavy downpours.

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 forecast thunderstorms, frequent lightning is likely. Lightning can cause power outages, voltage dips, damage to life and property, particularly during cloud-to-ground lightning strikes.

Why I May Not/Will Not See Constant Rainfall?

A frequent complaint is the forecast is wrong because 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.

This week, isolated to scattered activity is forecast.

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