630
FXUS20 KWBC 271833
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
233 PM EDT Wed May 27 2026

GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/

Forecast Bulletin 27 May 2026 at 1830 UTC:

Mid-Latitude South America...
The southern portion of the continent is seeing a ridging pattern
throughout the atmospheric column in the beginning of the day on
Wednesday, while an upper trough is making its way into the
continent throughout the day and into the evening hours of
Wednesday. In the mid levels, the trough is vertically stacked,
favoring a weaker system entering the southern regions of Chile
from Wednesday into Thursday. This in turn will weaken the extra
tropical cyclone associated with the upper trough, leading to
light to locally moderate precipitation totals in the Sur region
of Chile on Wednesday. As the trough continues to enter the
continent, the lower atmosphere is expected to react by lowering
pressures in the Patagonia region, activating weak moisture
transport over Bolivia into Paraguay and Argentina from the north.
A series of shortwave troughs entering northern Argentina into
Paraguay throughout the evening on Thursday into the early morning
hours of Friday will interact with the enhanced moisture coming
from the northern portions of the continent, enhancing the risk of
severe weather from the eastern Chaco region into southeast
Paraguay on Thursday into the early hours of Friday. This region
can also expect moderate to locally heavy precipitation on
Thursday. On Friday, the upper level trough enters into Paraguay
and Uruguay, favoring lower pressures into the lower levels, where
enhanced moisture content is expected with the presence of the
tropical air mass in the region. However, rainfall for Friday
could extend from Paraguay through southern Brasil, with moderate
totals expected for the region.

Tropical South America...
Various tropical waves will be moving into the tropical region of
South America, bringing much of the rainy conditions to the
northern Amazon Basin and into Colombia, Venezuela, and the
Guianas for the next three days. With the enhanced moisture
content associated with the waves, expect localized heavy
precipitation and generalized moderate precipitation over much of
the northern region over the forecast period. On Wednesday, two
tropical waves enter the Guianas and favor locally heavy
precipitation throughout the day. To the west, the enhanced
moisture and interaction with the mountain range in Colombia will
favor heavy precipitation. On Thursday, the tropical waves
propagate into eastern Venezuela and Suriname, favoring heavy
convection and precipitation totals. The rotation in the Gulf of
Panama will favor moist onshore flow, favoring locally heavy
precipitation in western Colombia. On Friday, the entry of a new
tropical wave into the Amazon Delta and French Guiana will favor
heavy precipitation, while the advance of the previous tropical
waves continue to affect Venezuela throughout the day on Friday.

Castellanos...(WPC)






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