FXUS20 KWBC 271800

PMDSA 



South America Forecast Discussion - International Desks

NWS Weather Prediction Center College Park MD

159 PM EDT Mon Oct 27 2025



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



South American Forecast Bulletin for 27 October 2025 at 18000 UTC



A broad upper trough crossing the southern cone will drive an

active weather pattern across northern Argentina, Paraguay,

Uruguay, and southern Brazil during the next three days.

Upper-level divergence and deep moisture ahead of a frontal

boundary will sustain heavy rainfall with maxima between 40 and 80

mm/day, along with a risk of severe convection including strong

winds and hail. Across the southern Andes, successive troughs will

cause some snow showers with accumulations up to 10â€20 cm

through Wednesday. Farther north, the Amazon Basin will observe

widespread convection, with rainfall maxima near 25â€50 mm/day is

possible, while northeast Brazil stays generally dry under

subsident flow.



Over the next three days, a frontal boundary will move north from

northern Argentina and Uruguay today, and by Thursday, the frontal

boundary will progress into southern Brazil and north of Paraguay,

focusing on heavy convection over the area. At upper levels,

divergence and convective development is forecast across much of

tropical South America, especially over the Amazon Basin, northern

Bolivia and eastern Peru. To the south, a robust trough over the

southeast Pacific moves eastward across central Chile and

Argentina on Tuesday and Wednesday. The interaction of this trough

with moist low-level flow from northern Argentina and Paraguay

enhances the chances of widespread convective development. By

Thursday, the trough exits into the South Atlantic, while another

short-wave disturbance approaches from the Pacific, maintaining

unsettled conditions along southern Chile and the Patagonian

Andes.



Ahead of the front across central South America, relatively deep

moisture will be moving south ahead of the front, with

precipitable water values around 45â€60 mm. This moisture aligns

with strong upper divergence and low-level convergence, favoring

mesoscale convective development. Behind the front, a strong

anticyclone builds across southern Chile and Patagonia,

reinforcing subsidence and stable post-frontal conditions.



Southern Andes and Southern Chile:

As the upper trough approaches, scattered precipitation develops

along southern Chile and the Andean region between 38S and 45S.

Through Tuesday and Wednesday, snow accumulations of 10â€20 cm

are expected in higher terrain. Light post-frontal precipitation

persists into Thursday, generally below 15 mm/day.



Northern Patagonia and Central Argentina:

During Tuesday and early Wednesday, convection increases as the

upper trough enhances large-scale ascent. Expect moderate rainfall

with maxima between 15 and 25 mm, mainly over La Pampa and

northern Patagonia. By late Wednesday, convection strengthens

across central and northern Argentina. Organized thunderstorms

will produce rainfall maxima between 35 and 70 mm/day, with

locally heavier amounts. Severe weather is possible, including

damaging winds and hail due to strong instability and favorable

jet coupling.



Paraguay, Uruguay, and Southern Brazil:

By late Tuesday into Wednesday, the frontal system shifts

northeastward, interacting with abundant low-level moisture.

Scattered to widespread convection develops from Paraguay into

southern Brazil and Uruguay, with maxima of 35â€70 mm/day and

embedded thunderstorms capable of producing locally heavy rainfall

and hail.



Brazil and the Western and Central Amazon Basin:

The ridge aloft and deep moisture convergence supports convection

across Colombia, northern Peru, western Brazil, and Bolivia

throughout the period. Daily rainfall maxima ranges from 25 to 50

mm, locally higher near the Andean foothills and zones of strong

diurnal heating. Subsident flow will maintain a dry pattern across

most of eastern Brazil. Only isolated light showers are expected

along easternmost coastal areas, with accumulations generally

below 15 mm/day.



For the latest available charts please visit:

https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml



Alamo...(WPC)







$$