Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
151 PM EST Wed Dec 17 2025

Forecast Bulletin 17 December 2025 at 1850 UTC:

North and east of The Bahamas and the Caribbean region, a frontal
boundary extends from the Central Atlantic and is weakening just
north of Cuba, where it will remain weak into Wednesday evening.
The frontal boundary is accompanied by a weak shear line that is
located just north of Hispaniola and will reach the north Haiti by
Wednesday night. This feature is being assisted by a mid to upper
level trough that is weakening over the Central Atlantic Ocean,
where cyclogenesis is expected over the ocean from Wednesday to
Thursday, with no significant impact to the forecast region. The
amount of available moisture in the region will favor light
precipitation throughout, ranging from maxima of 10-15mm. On
Thursday, light precipitation continues to be the pattern as the
mid to upper level trough has moved away from the region, and a
weak ridging pattern develops in the upper levels, but a ridge in
the mid level is centered just north of Hispaniola and expands
over The Bahamas, and reaches into the ABC islands. It also
reaches Central America and into the Lesser Antilles by Thursday
evening. The presence of a trade wind inversion cap at around from
the 900-850 hPa level is possible over the region, meaning that
the development of deep convection could be limited on Thursday.
Relative humidity is expected to remain below 40% over most of the
region, continuing to contribute to the dry conditions. In the
lower levels, the aforementioned frontal boundary is no longer
expected in the region, but a weak shear line is expected over
Puerto Rico/VI which may favor some precipitation, although light
due to the limited amount of available moisture in the region. A
moisture plume embedded in the easterly trade winds is entering
the Lesser Antilles, where expect maxima of 15-25mm. Elsewhere in
the Caribbean and the Bahamas, expect maxima to range from
10-15mm. By Friday, a potent mid to upper level trough is located
along the eastern seaboard of the United States and extends just
north of the Bahamas by Friday night. This trough will disturb the
mid-upper level ridge over the region, but will maintain similar
conditions from Thursday. Trace to light amounts of precipitation
are expected over the region, ranging from 10-15m in the Bahamas,
due to a prefrontal trough associated with an approaching frontal
boundary. Similar amounts are expected in the Republican Republic
and the Lesser Antilles.

In Mexico and Central America, a general dry pattern has developed
over the region. The lack of moisture in Mexico will decrease the
potential for heavy precipitation in the region, and in Central
America, there exit region of a branch of the Subequatorial Jet in
the upper levels is favoring subsidence over the region, and
localized light precipitation is expected over the region. On
Wednesday, moisture plumes in northeast Mexico and north Central
America will favor precipitation with maxima of 15-20mm. The
monsoon trough over Costa Rica and Panama will favor precipitation
totals of 15-25mm. On Thursday, moist plumes in the trade winds
entering the Yucatan Peninsula and troughing extending into
Guatemala will favor generalized maxima of 15-25mm. In Costa Rica
and Panama, the resurgence of the Caribbean LLJ (CLLJ) will
transport moist plumes in the trades into the region, interacting
with the terrain upon their arrival. As the monsoon trough
continues to provide circulation over the region, the region can
expect maxima of 15mm on Thursday. On Friday, the CLLJ will
continue to interact with the Caribbean basins of Costa Rica and
Panama, pooling moisture in the northern region of Panama, where
expect maxima of 20-35mm, while less moisture is expected from
Nicaragua to Costa Rica, and thus maxima of 15-20mm are expected
on Friday. To the north, a weaker jet stream is located over
central and southern Mexico, reflecting in the mid levels as a
trough is developing on its divergent side, over the Yucatan
Peninsula and north Central America. This instability will enhance
the convection developing from orographic lift in Guatemala,
Belize, and portions of the south Yucatan, where expect maxima of
20-35mm. Elsewhere from the south of Veracruz, into Chiapas, south
Guatemala and west Honduras, as well as in the north Yucatan,
expect generalized maxima of 15-25mm.

In north south America, the advance of a frontal boundary into the
western Amazon Basin will favor interaction between the easterly
trade winds and the southerly flow associated with the frontal
boundary. As these features interact over the moist environment of
the Amazon Basin on Wednesday, prefrontal features are developing
along the Amazon Basin and into Central Brasil. Expect regions
from east Ecuador to west Amazonas-Brasil to see maxima of
30-60mm, with a potential for locally higher totals of
precipitation. Para and east Amazonas-Brasil can expect maxima of
20-45mm, while from south Venezuela/Colombia into north
Amazonas-Brasil, expect maxima of 20-35mm. In the Guianas,
moisture over the region will favor maxima of 20-35mm on
Wednesday. On Thursday, an increase in moisture and in wind speeds
in the trade winds will enhance the potential for heavier
precipitation in the Guianas, with a potential of maxima of
30-60mm, enhanced by the base of a trough axis of the Northeast
Brasilian Upper Trough extending into the region on Thursday. A
decrease of moisture being advected over the Amazon Basin will
favor lower amounts of precipitation. In the northern portion of
the Amazon Basin, expect maxima of 15-25mm, while the southern
portion of the region can expect generalized maxima of 20-35mm. By
Friday, dry air continues to be advected in the trade winds, now
that the aforementioned frontal boundary is no longer present, the
region will see the incursion of this dry air impact the total
precipitation. The central Amazon Basin can expect maxima of
15-25mm, while east Ecuador into central Peru can expect maxima of
20-35mm with the moisture converging along the Andes Mountains, as
the nocturnal LLJ continue to advect air southward. Over the
eastern Amazon Basin into central Brasil, troughing in the lower
levels will interact with the periphery of the Bolivian High and
the Northeast Brasilian Upper Trough, where enhanced divergence is
expected. While moisture convergence is increased, deep convection
will favor maxima of 30-60mm on Friday. In the Guianas, moisture
continues to enter the region, interacting with the terrain, and
favoring maxima of 20-45mm.

Positions of Tropical/Easterly Waves Initialized at 12 UTC
TYP SOF  INIT  18/00  18/12  19/00  19/12  20/00  20/12  21/00
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For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/crb_day1-3.shtml

Castellanos...(WPC)