132
FXUS20 KWBC 111838
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
238 PM EDT Mon May 11 2026

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

Forecast Bulletin 11 May 2026 at 1840 UTC:

On Monday, a potent cold air mass that propagated into tropical
South America over the weekend weakens by the early morning hours
over the western Amazon Basin. By the evening hours, the frontal
boundary is expected over northern Peru, extending into the
southern Amazon Basin, south Mato Grosso, and exiting into the
south Atlantic Ocean through Espirito Santo. This cold air mass
will favor unstable conditions throughout much of the tropical
South America region, especially with the passing of moist
propagating troughs in the lower levels. One of these waves is
expected to enter eastern Venezuela and extend into Roraima and
northern Amazonas-Brasil, and as these low level features
aggregate, deep convection and maxima of 40-80mm will be favored
on Monday. To the west, along the Andes Mountains in Colombia and
Ecuador, the easterly trade winds are advecting moisture along the
foothills and enhanced orographic lift will favor heavy
precipitation from Monday into early Tuesday morning. Similarly,
along the Amazon Delta and into French Guiana, moisture entering
the coastal region will persist throughout the day, favor moderate
to heavy precipitation. Similar conditions will continue on
Tuesday over much of the tropical region of South America. Remnant
troughs from the frontal boundary are expected along the central
portions of the continent, while the rest of the cold front
extends from the Espirito Santo/south Bahia region and into the
south Atlantic. A tropical wave is expected to enter the northern
coast of Brasil and favor light to moderate rainfall totals over
the region, with locally higher amounts possible. As moisture
continues entering the tropical region from the Atlantic, expect
moisture convergence over Venezuela and central Amazonas from a
tropical wave that is moving slowly and favors maxima of 40-80mm.
Over the Amazon Delta the moisture from the Atlantic will interact
with the terrain and its persistence will favor maxima of 30-60mm,
with some localized higher accumulations possible. On Wednesday, a
decrease in available moisutre in the mid levels over the
northwest Amazon Basin will interact with the propagating wave
entering east Colombia, favoring a decrease in total precipitation
accumulations. Moisture over south Venezuela, through Roraima and
south Guyana will favor heavy precipitation, while a tropical wave
entering the Amazon Delta region will favor coastal heavy
precipitation as available moisture is expected to exceed 65mm in
a large portion of the region.

In the central and southern portions of the continent, the passing
of the potent cold airmass over the weekend favored a migratory
high pressure system to settle in the lower levels, favoring dry
conditions over much of the region. The regions of interest that
will see precipitation over the next three days are expected in
Austral Chile and the Patagonia region due to a frontal bounday
accompanied by an atmospheric river. However, much of the
strongest synoptic conditions are expected to remain away from the
region as well as the amount of available moisture is not expected
to be high. Austral Chile can expect maxima below 15mm daily with
a chance of mountain snow. The Patagonia region could expect trace
to light amounts of precipitation after the front enters the
region since it will weaken and decrease in available moisture.

Castellanos...(WPC)




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