657
FXUS20 KWBC 271759
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
158 PM EDT Tue Oct 27 2026

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

Forecast Bulletin 27 October 2026 at 1800 UTC:

A weakening Bolivian High remains present in northwest South
America, as a potent and deep upper level trough extends from the
south into Peru and south Ecuador during the day on Tuesday. With
this trough digging into the continent, an upper level ridge is
expected to develop in east Brasil on Tuesday. Further northeast a
High Level Cyclonic Vortex (VCAN, for its abbreviation in
Portuguese) strengthens in the region, favoring dry air advection
in northeast Brasil. The mid to upper level set up is expected to
produce ventilation over the central portions of the continent,
and favoring the moisture convergence in the low levels from the
southern Amazon Basin through the southern Parana Basin. The
troughing in the upper levels will favor the lowering of
pressures, where much of the convection will initiate daily on
Tuesday and Wednesday. By Thursday, the upper trough becomes zonal
over the central portions of the continent, while the Bolivian
High reactivates from the southern Amazon Basin into central
Bolivia and the Parana Basin.

In western Ecuador and northwest Peru, moist onshore flow entering
from the Pacific Ocean and interacting with the mountainous
terrain. This will persist throughout the forecast period, and
change in intensity. On Tuesday, winds from the north will
propagate into the higher terrain and fuel the convection. Expect
moderate to heavy precipitation from the enhanced upper divergence
from the interacting mid to upper level systems. On Wednesday, a
trough develop along the west coast of Ecuador and north Peru,
favoring moist onshore floe from the northwest, continuing the
pattern of moisture convergence along the Pacific coasts of the
region. Expect as slight decrease in total precipitation. On
Thursday, the heavy precipitation will pick back up as the trade
winds from the north converge with the trade winds of the south,
meeting just off-shore south Colombia and Ecuador. Enhanced upper
divergence is expected again as the Bolivian High returns and its
periphery interacts with an upper trough that has entered north
Colombia.

In the southern cone of the continent, expect precipitation to
remain light to moderate, but progressive as a variety of upper
level systems propagate into Chile and Argentina over the next
three days. A broad trough with shortwave troughs embedded will
propagate into the region on Tuesday and early Wednesday providing
some ventilation for convection from Cordoba through the north
Patagonia. Expect a slight risk of severe weather and light
precipitation totals. On Wednesday, a ridge axis enters Chile,
while the divergence propagates to the north and east Argentina.
To the south, a frontal boundary with enhanced moisture brushes
the coast of Austral Chile, while much of the heavier
precipitation remains offshore on Wednesday. On Thursday, the cold
front crosses the Andes in Austral Chile, favoring moisture
convergence. Expect maxima to reach moderate totals with a chance
of mountain snow.

Castellanos...(WPC)



$$