South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
325 PM EDT Wed Sep 30 2026

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

Forecast Bulletin 30 September 2026 at 1800 UTC:

Heavy rainfall is expected to continue over the next three days
along the Pacific coast of Colombia and Ecuador, as low-level flow
continues to advect deep moisture onshore from the Pacific.
Additional support from diurnal convection is anticipated,
resulting in higher totals. From today through Thursday, expect
accumulations around 30-60 mm, with isolated regions over northern
Ecuador reaching 50-100 mm. From Thursday through Friday,
thunderstorm activity will increase as upper-level divergence
strengthens, enhancing convective activity. Accumulations will
range from 35-70 mm, particularly over Colombia. Then from Friday
through Saturday, a tropical wave is forecasted to approach the
region, increasing moisture and cyclonic flow and supporting
additional precipitation. An additional 35-70 mm of rainfall is
expected.

Across the tropical region of the continent, light to moderate
rainfall is anticipated today across the Amazon Basin, where
low-level winds are expected to converge and advect moisture.
Today, some rainfall is also expected over the Roraima region due
to moisture transport and advection by low-level winds from the
Orinoquia and Canaima regions. An increase in precipitable water
(PWAT) is forecasted from Thursday through Friday as a tropical
wave propagates. The cyclonic flow associated with the tropical
wave will also support low-level convergence, enhancing rainfall
over the Amazon. Rainfall accumulations will range from 20-45 mm.
Then from Friday through Saturday, a decrease in PWAT and the
presence of the nocturnal jet will result in lower precipitation
totals.

A cold front continues to propagate over Brazil, which, combined
with the presence of an upper-level trough, will bring heavy
rainfall and thunderstorms along the frontal boundary,
particularly from Acre into Parana. Additionally, instability
associated with the system is expected over Serra do Mar, between
Rio de Janeiro and Sao Paulo. The heaviest precipitation is
anticipated over Santa Catarina and Parana, with totals reaching
70 mm, supported by another stationary frontal boundary extending
over Santa Catarina into Mesopotamian Argentina and the Chaco
region. From western Parana to Mato Grosso, precipitation will
reach 40 mm, with a risk for severe weather in the region. The
front is anticipated to enhance low-level convergence over the
southwestern Amazon. Rainfall accumulations of 25 mm are
anticipated for Peru, while upper-level divergence could bring
totals up to 50 mm over northern Bolivia and 30 mm over Yungas and
Chiquitania. As the front propagates north, rainfall is
anticipated to continue along the boundary in Brazil, with the
highest accumulations expected over Serra do Mar. From Friday
through Saturday, remnants of the front will continue to cause
rainfall over Brazil.

In Peru, rainfall is expected today through Thursday along the
Selva Alta region and the Andes due to northerly flow convergence.
Rainfall is forecasted to reach 30 mm.

A frontal boundary will bring light rainfall over Cordoba in
Argentina, with accumulations around 25 mm today through Thursday.

Over Chile, the remnants of a front will promote rainfall and
mountain snow that will remain below 15mm today through Thursday.
An increase in precipitation is expected over southern Chile from
Thursday through Friday as a front reaches the area and PWAT
increases. Accumulation will range between 15-30mm.


Silva..(INMET)
Rivera-Torres...(WPC)