Tropical waves are beginning to roll off the African Coast, and they are expected in May. With these early-season tropical waves, showers and thunderstorms tend to be quite spare, and the wind shifts associated with them are quite weak. While the first tropical wave analyzed by the National Hurricane Center for 2024 may not be the strongest, it certainly is well-defined and is forecast to move across T&T this weekend, bringing scattered showers, thunderstorms, and gusty winds, particularly on Sunday.
Meanwhile, the NHC is monitoring an area of low pressure in the North Atlantic Ocean with low chances of tropical cyclone formation. This area has produced significant rainfall across the Greater Antilles over the last few days and is responsible for a damaging tornado in Haiti.
What you need to know
— Tropical Wave 01: The axis of this tropical wave is well-defined, located along 56W on Friday night, moving west at 10-15 knots (18-27 KM/H). It is forecast to move across Trinidad and Tobago later on Saturday, May 25th, 2024. Rainfall is forecast for T&T, which will mark the start of the 2024 Wet Season.
— AOI in the Southwestern Atlantic: An area of low-pressure being monitored by the NHC is producing disorganized showers and thunderstorms. The area is located in an area with unfavorable environmental conditions and has low chances of development.
— Hazards: While no widespread severe weather is forecast across Trinidad and Tobago, mainly following the passage of Tropical Wave 01, locally heavy rainfall and gusty winds are possible, favoring Trinidad.
Numbering Tropical Waves
Before delving into the tropical update for this year’s “first” tropical wave, let’s clarify why it’s the “first.”
Different meteorological entities also produce maps of weather features based on wind, pressure and temperature called surface analyses. The Barbados Meteorological Service makes its public and has adopted a numbering convention for tropical waves, while the Trinidad and Tobago Meteorological Service (TTMS), on the other hand, keeps its surface analyses private.
As a result, there may be differences in the number of waves analyzed. As of 00Z (8 PM local time) May 25th, 2024, the Barbados Meteorological Service has the first tropical wave analyzed by the National Hurricane Center as its third for 2024, while the TTMS has this wave as its second for the year. The first tropical wave, according to the TTMS, actually moved across T&T on May 22nd, and based on data at the nation’s main airports at Crown Point and Piarco, did produce measurable rainfall, but the 2024 Wet Season was not declared then.
As a reminder, the Wet Season is declared when measurable rainfall is recorded at T&T’s climate reference sites (Piarco International Airport at Piarco and A.N.R. Robinson International Airport at Crown Point) due to the passage of a Tropical Wave or the Intertropical Convergence Zone.
Generally, we look towards the NHC’s Tropical Analysis and Forecast Branch’s Surface Pressure Analysis of the Atlantic for tropical waves. In fact, in our annual tropical wave numbering, we use the NHC’s analysis as our baseline for counting, as it is the most visible product and accessible to all – the public and meteorologists alike. Tropical waves are sometimes dropped and added to their analysis, which is done every six hours. At times, features that were analyzed as troughs initially can be reanalyzed into a tropical wave by the NHC as well. We take these into account and generally number waves accordingly.
Tropical Wave 01
The first tropical wave analyzed by the National Hurricane Center moved off the African Coast on May 19th, 2024. While convection pulsed with this wave as it moved across the Atlantic, it generally remained well-defined at the atmospheric levels where we look for a tropical wave signature.
Tropical waves’ signature is best seen at the 700-millibar level, approximately three kilometers above ground. Outside of this 700-millibar signature of a wind shift, a vorticity maximum, and an increase in atmospheric moisture. All of these features were tracked across the Atlantic. This tropical wave has been moving westward at approximately 10 to 15 knots (18-27 KM/H). Based on its current trajectory, it is forecast to move across T&T on Saturday (May 25th) night.
Across the wave axis, total precipitable water (TPW) values, which is a measure of atmospheric moisture, are high, between 2.5 and 3 inches. Wind shear is moderate to strong across the Lesser Antilles, with speeds up to 30 knots.
Following the passage of this wave on Saturday evening, cloudy periods with shower/thunderstorm activity are forecast. Winds will be veering through the atmosphere, meaning winds will be turning clockwise as they increase in height through the atmosphere. This veering wind and an increase in moisture are forecast to support thunderstorms and isolated to scattered heavy showers Saturday night through Sunday, favoring Trinidad.
Not Every Tropical Wave Brings Severe Weather
Tropical waves vary in intensity as they travel westward across the Atlantic. This variability can be on the order of days to as short term as hours. Sometimes, you may see satellite imagery that shows heavy thunderstorms and deep convective activity east of Trinidad and Tobago during the late night and early morning, but by daybreak – nothing.
This is due to diurnal (daily) variations in convective showers whereby convection peaks overnight and wanes during the day for already occurring convective systems (i.e., not triggered by daytime heating).
Activity can be enhanced by its proximity to upper-level troughs and the ITCZ but can also be degraded by hostile upper-level conditions and Saharan Dust.
