729
FXUS20 KWBC 291735
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
134 PM EDT Tue Sep 29 2026

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

Forecast Bulletin 29 September 2026 at 1800 UTC:

Persistent heavy precipitation is expected throughout the forecast
period across Ecuador and Colombia, driven by a combination of
atmospheric factors. These include diurnal convection,
precipitable water (PWAT) values exceeding 50 mm, and persistent
low-level cyclonic flow advecting deep moisture from the Pacific
onshore. Elevated sea surface temperatures (SSTs) will also
continue supplying moisture to the region. Today through Thursday,
low-level moisture convergence will serve as the primary driver of
precipitation. Accumulations over southwestern Colombia are
anticipated to be around 40-80mm, and around 30-60mm over western
Ecuador. From Thursday through Friday, thunderstorm activity will
increase as upper-level divergence strengthens and enhances
instability. Accumulations will range from 34-70mm. Current model
guidance suggests that rainfall will persist beyond the end of the
forecast period.

Relatively low precipitation totals are expected across the Amazon
Basin due to limited precipitable water (PWAT) in the region.
However, daily rainfall and thunderstorms remain likely due to
diurnal heating, orographic enhancement, low-level troughs, and
local effects. An increase in rainfall is expected from Thursday
through Friday as PWAT increases across the tropics. Totals are
forecasted to reach 20-45mm, over the Northwest Amazon and Western
Amazon basin

A cold front currently extending from Bolivia into southern Brazil
will enhance low-level convergence across the region. Low-level
flow will also favor tropical moisture advection, further
supporting rainfall. At upper levels, a trough reaching Peru and
Chile will enhance divergence throughout the forecast period. The
heaviest precipitation today is expected over Bolivia, Paraguay,
portions of northern Argentina where totals will reach 50-100mm
and 25-50mm respectively. Additional rainfall is anticipated along
the frontal boundary in Brazil. A risk of severe weather exists
for Bolivia, Paraguay, and Brazil today. As the frontal boundary
propagates from Wednesday through Thursday, precipitation will
continue, along with a slight risk for severe weather over Brazil.
Accumulation totals are forecasted to reach 30-60mm. Additional
precipitation will affect Brazil from Thursday through Friday,
though totals will be lower as precipitable water decreases. The
highest precipitation is anticipated to be over the State of Goias
and the Central-West region of Brazil, with totals ranging from
20-45mm.

Light precipitation is expected over Uruguay today through
Wednesday morning due to the frontal boundary, followed by drier
conditions. Similarly, light rainfall is expected over Argentina
from Wednesday through Thursday, particularly in the Crdoba and
La Pampa regions. From Thursday through Friday, a mid- to
upper-level trough will increase instability, bringing a risk for
severe weather on Friday.

Over Chile, light rainfall and mountain snow are expected
throughout the forecast period as frontal boundaries cross the
region alongside upper-level troughs and jets. However,
accumulations are likely to remain low due to limited precipitable
water availability. From Thursday through Friday, an increase in
PWAT with a front will increase precipitation. Total accumulations
are expected to range from 15-25mm over Central Chile, and around
20-35mm over Southern Chile.


Rivera-Torres...(WPC)





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