587
FXUS20 KWBC 081806
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
205 PM EDT Wed Oct 08 2025

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

South American Forecast Bulletin from 08 October 2025 at 1805 UTC

On Wednesday, the southern cone of the continent, an atmospheric
river and a frontal boundary is impacting the southern region of
Chile and into Argentina, where precipitation totals are expected
to be high. This system is accompanied by available moisture
ranging from 25-30mm in precipitable water, as it enters from the
west, favoring orographic lift as it interacts with the terrain on
Wednesday into Thursday. In the upper levels, this will have the
enhancement of the divergent jet stream exit region and the base
of a shortwave trough passing over Austral and Sur regions of
Chile by the evening on Wednesday and into Thursday. On Wednesday,
expect maxima of 30-60mm in Los Lagos and north Aysen with a risk
of cumulonimbus formation. Expect totals of 15-30mm in south Aysen
and Magallanes, where south Magallanes could see snow totals
ranging from 5-10cm. On Thursday, the amount of available moisture
decreases and reaches the central portions of Chile, while
remaining moisture in the Austral regions enters the region with
increased wind speeds from the west. This will favor precipitation
values of 20-35mm in Magallanes, and 15-20mm in Aysen, with the
chance of snow in higher elevations. The initial frontal boundary
is expected to favor maxima of 15-20mm in north Los Lagos. On
Friday, similar conditions persist with an entrance of a new warm
frontal boundary favoring maxima of 20-35mm in Magallanes with a
chance of rain changing to snow in the higher elevations. The
initial cold front has continued to progress east and northward
into the Cuyo and La Pampa regions of Argentina. An increase in
moisture from the north, and the passing of shortwave troughs over
the region in the upper levels will favor moderate severe weather
from the Cordoba region to the north Patagonia region of
Argentina, extending into the Buenos Aires province by Friday
evening into early Saturday. Buenos Aires can expect maxima of
20-35mm, while the central Argentina region can expect maxima of
15mm.

In south Brasil, a persistent stationary frontal boundary is
expected to enhance moist onshore flow into the Serra do Mar
region, while the weaker extent of the frontal boundary reaches
the Parana Basin and Pantanal regions on Wednesday. The region is
seeing the presence of a trough axis in the upper levels,
sustaining much of the baroclinicity that will favor the continued
presence of a weak trough or front during the forecast period.
While this front meanders from Espirito Santo on Thursday, it will
favor moist onshore flow on Wednesday and Thursday along the Serra
do Mar region. On Wednesday expect maxima of 20-45mm with risk of
thunderstorms from Rio de Janeiro, along Sao Paulo and into
portions of central Parana. On Thursday, expect maxima of 25-50mm
in south Sao Paulo, while central and north Sao Paulo/Parana can
expect maxima of 20-35mm. Generalized maxima ranging from 15-25mm
could be expected in the south and eastern regions of Brasil. On
Friday, a reorganization of the frontal boundary is due to a
weakening of the upper trough, while a decrease in available
moisture is expected over the Parana Basin. The onshore flow is
expected to contribute to the maxima of 20-35mm in south Brasil.
Although a weak upper trough is expected in the region,
baroclinicity remains where a slight risk of severe weather is
expected.

In the northern portion of the continent, increased level of
moisture and diurnal processes will assist with heavy
precipitation in the region, particularly in the western portions
of the Amazon Basin. In addition the presence of low level troughs
in the trade winds will favor areas of moisture convergence, where
moderate precipitation will be expected over the forecast period.
On Wednesday expect maxima of 20-45mm in the western Amazon Basin
and east Venezuela into the Guianas. Elsewhere in the Amazon
Basin, expect maxima of 15-25mm. On Thursday, expect maxima of
25-50mm in south Colombia/Venezuela into Amazonas-Brasil. And
maxima of 20-40mm are expected in the western Amazon Basin. The
Central Amazon basin can expect maxima of 20-35mm. On Friday, a
slight decrease in moisture will favor maxima of 20-35mm from
Amazonas-Brasil into south Colombia and Venezuela, reaching
portions of north Peru and east Ecuador.

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Castellanos...(WPC)



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