Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
154 PM EDT Tue Apr 28 2026

Forecast Bulletin 28 April 2026 at 1800 UTC:

The majority of the forecast domain is experiencing the effects of
a convergent phase of the Madden-Julian Oscillation, favoring
subsident conditions aloft. Forecast guidance is suggesting this
pattern could continue for the rest of the work week, while a
Kelvin Wave is moving over the Pacific Ocean on Tuesday and
potentially affecting Mexico and northern Central America by the
end of the work week.

Northern South America and South Central America...
The region of tropical South America is seeing the presence of an
upper level high pressure system that is centered over the Central
Atlantic and extends over northern Venezuela, with a ridge axis
entering into southern Colombia. In the lower levels, the a
propagating trough is moving over the Andean region of Colombia,
associated with high levels of precipitable water. This moist flow
will see orographic lift and enhanced moisture convergence,
favoring deep convection throughout the day on Tuesday. This
release of latent heat into the atmosphere will favor an
enhancement of the upper level ridge, and the trough will build
its own outflow in the upper levels. This system will then enhance
the ridge over Colombia and Venezuela, restructuring the upper
level pattern into Wednesday. The low level trough will interact
with the semi-stationary cyclonic rotation over the Gulf of
Panama, favoring moist onshore flow into into western Colombia and
northwest Ecuador on Tuesday. Moderate to locally heavy
precipitation is expected in along the Pacific coastal regions. On
Wednesday, the low level trough is moving slowly over the Andean
region of Colombia, continuing to enhance the cyclonic rotation
over the region, while providing ample moisture convergence in the
lower levels as this flow interacts with the valleys and orography
of the region. Expect the mid to upper level ridging to enhance
and reorganize over the Caribbean and into the northern South
America. The heaviest precipitation is expected along the western
Colombia and Ecuador coasts. Some of the cyclonic circulation
associated with the wider troughing regime in the region will
bring moist onshore into northern Panama and east Costa Rica by
Wednesday evening. This region can expect moderate to locally
heavy precipitation. By Thursday, the propagating trough is
expected to enter the Gulf of Panama and be introduced to warm sea
surface temperatures, favoring enhanced convection over Costa
Rica, Panama, Colombia, and portions of Ecuador. The region could
continue to see moderate to heavy precipitation.

Caribbean and the Bahamas...
In the upper levels, an upper trough is present over the west
Caribbean and entering into Central America, weakening throughout
the day on Tuesday as the northern portion of the trough
propagates east and the southern portion over Central America is
weakened due to the upper level ridging developing over
northwestern South America. By Tuesday evening, a frontal boundary
over the Central Atlantic Ocean extends just north of Puerto Rico
and weakening over Hispaniola and portions of Cuba. A weak
prefrontal trough develops over Puerto Rico/VI, enhancing unstable
conditions throughout the day and favoring light to moderate
precipitation on Tuesday. Instability is also expected over
Hispaniola with the eastern portion of the weak trough over the
region. Expect light to moderate precipitation as moisture levels
are expected to remain moderate. On Wednesday, the remnants of the
frontal boundary enter Hispaniola throughout the day, enhanced by
orographic lift, favoring moderate precipitation totals throughout
the Dominican Republic and Haiti. By Thursday, a restructuring of
the mid to upper levels of the atmosphere enhances the presence of
dry air and trade wind inversion caps over much of the Caribbean.
This will favor drier conditions throughout the region, with light
precipitation expected over the Greater Antilles.

Mexico and Northern Central America...
The region is expected to be dominated by a potent mid to upper
level ridge fueling subsidence and trade wind inversions
throughout for the next three days. In addition, the lack of
moisture in the region will favor localized diurnal showers, with
totals remaining as a trace to light totals. The potent ridge
meanders to the east continuing to favor similar conditions over
much of the region on Wednesday. By Thursday, a mid to upper level
shortwave trough over the southwestern United States extends into
northwest Mexico, disrupting the mid to upper level ridge over
Mexico. This will begin weakening the mid to upper ridge, favoring
light to moderate precipitation over the northern Mexico region,
enhanced by the divergence of the upper trough propagating during
the day.

Castellanos...(WPC)