Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
322 PM EDT Wed May 20 2026

Forecast Bulletin 20 May 2026 at 1922 UTC:

Mexico and Northern Central America...
On Wednesday, a deepening upper level trough extends into
northwest Mexico throughout the day, reaching the Mexican Plateau
by the evening. Embedded in this trough is mid to upper level
short wave trough propagating over Coahuila and Nuevo Leon in the
evening into the early morning hours of Thursday, fueling deep
convection along the border between Coahuila/Nuevo Leon and
Texas-United States. With the moist onshore flow in the low levels
from the south due to a strengthening front over the central
United States, the North American Low Level Jet (LLJ) will favor
moderate to heavy precipitation in the aforementioned regions of
Mexico on Wednesday. In northern Central America, the ridging in
the upper levels begins to weaken, while a ridge remains in the
mid levels, limiting convection in the region and favoring the
presence of moderate trade wind inversion caps. Expect locally
light precipitation totals on Wednesday. On Thursday, the mid to
upper level trough over the northern Mexico/southern United States
region propagates into the central region of the United States,
giving way to zonal flow aloft, with a short wave trough embedded
in the mid levels. This short wave trough will favor severe
weather for northern Coahuila, Nuevo Leon, and Tamaulipas. In
terms of precipitation, moisture is advected into the region and
will favor moderate totals for Coahuila and the northern Sierra
Madre Oriental. To the south, the mid level ridge that was located
over south Mexico and northern South America weakens and allows
for weakened trade wind caps throughout the region, primarily in
southern Mexico, into the Pacific coasts of Guatemala and El
Salvador. With ridging still present in the lower levels, favoring
winds from the west over the East Pacific, expect moist onshore
flow to enter along the Pacific coasts from Oaxaca through El
Salvador. This will support moisture convergence along the higher
elevation and maxima could reach moderate totals on Thursday into
early Friday. On Friday, a new upper level trough axis enters
northwest Mexico, favoring divergence aloft from Coahuila through
Tamaulipas. Conditions are expected to continue favoring severe
weather in the northern region of Mexico as well as moderate to
locally heavy precipitation. To the south, the moist onshore is
expected to continue into south Mexico through northern Central
America, favored by the weakened ridge over the region.

Caribbean and the Bahamas...
On Wednesday afternoon, a retrograding Tropical Tropospheric Upper
Trough (TUTT) is center just of the Bahamas, with its base over
Hispaniola but it is expected to stall over the region by the
evening hours. As this TUTT meanders over night, expect the region
to see weakening upper level divergence. Through the day, expect
precipitable water values of less than 40mm in the northern
Bahamas while values of 45 to 50mm in the southern Bahamas and
portions of east Cuba and northern Hispaniola. The areas with
higher available moisture could expect moderate maxima, while the
regions with less available moisture expect regions to see trace
to light totals on Wednesday. The east Caribbean can expect trace
amount of precipitation as moisture decreases to further east you
go in the Caribbean, favored by the presence of a potent ridge in
the Central Atlantic. On Thursday, the TUTT meanders to the east
as it weakens over the region. From the east, the mid to upper
level extends from the Central Atlantic further west into the
western Caribbean, bringing with it dry conditions and a potential
for enhanced Saharan Air Layer. These conditions will limit
precipitation and favor trace to light precipitations over the
eastern Caribbean and into Hispaniola. From Cuba through the
Bahamas, expect the residual moisture to favor light to locally
moderate precipitation totals on Thursday. By Friday, the TUTT
over the Bahamas has weakened and gives way to generally zonal
flow in the upper levels, but the remnants of the TUTT remains in
the mid levels. The drier conditions from the east will continue
to enter into the Caribbean and limit the development of deep
convection. Over the Caribbean, expect trace to locally light
precipitation totals on Friday.

Southern Central America and Northern South America...
A series of troughs in the lower levels are propagating over the
northern South America region and over Panama and Costa Rica.
These troughs are favoring deep convection and contain large
amounts of available moisture, which will in turn favor heavy
precipitation over the region. From the Central Atlantic, dry air
is anticipated to enter from east into the Guianas later in the
work week, decreasing the precipitation totals by Friday. The
heaviest rainfall totals are expected in the northwestern sector
of South America as moist air interacts with the topography of the
region, enhancing low level convergence, while supported by upper
level diffluence from interacting upper level ridges over northern
South America. Enhanced moisture over Central America with a
passing of a tropical wave on Wednesday will favor moderate
precipitation. This moisture will remain over Thursday and Friday,
favoring moderate precipitation on both days.

TYP SOF INIT 21/00 21/12 22/00 22/12 23/00 23/12 24/00 24/12
TW  08N  41W  43W  45W   49W   51W   54W   57W   59W   63W
TW  12N  63W  67W  68W   70W   73W   75W   78W   81W   82W
TW  12N  82W  84W  87W   89W   91W   93W   95W   97W  100W
TW  13N 109W 111W 114W  116W   --   --  --  --

Castellanos..(WPC)