SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1216 PM EDT MON SEP 27 2021

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

FORECAST DISCUSSION FROM SEP 27/16UTC: A BROAD MID/UPPER LEVEL
TROUGH IS INITIALIZED OVER THE SOUTHWEST ATLANTIC WITH AXIS
CENTERING ON A CLOSED LOW NEAR 50S 48W EARLY THIS MORNING. A
PROGRESSIVE TROUGH IN THE SOUTHERN STREAM CLOSELY FOLLOWS THIS
VORTEX...STREAMING ACROSS THE DRAKE PASSAGE/SOUTHERN CONE LATER
THIS EVENING. THROUGH TUESDAY AFTERNOON THIS WILL THEN INTERACT
WITH THE VORTEX TO THE EAST...FAVORING THE GENERATION OF A SHORT
WAVE TROUGH OFF THE COAST OF URUGUAY/SOUTHERN BRASIL. THE NORTHERN
STREAM PERTURBATION IS TO THEN MEANDER EAST ACROSS 40W WHILE
BOTTOMING OUT TO THE SOUTH OF 23S ON WEDNESDAY EVENING. THIS IS TO
THEN ORGANIZE INTO A CLOSED LOW NEAR 40S 30W LATER ON THURSDAY. 
AT LOW LEVELS...AN OCCLUDING LOW TO THE NORTH-NORTHWEST OF THE
SOUTH GEORGIA ISLANDS IS TO EXTEND A FRONT ACROSS THE WESTERN
ATLANTIC TO URUGUAY/CENTRAL ARGENTINA LATER THIS EVENING. ON
TUESDAY THE FRONT MEANDERS NORTH INTO SOUTHERN BRASIL WHILE
TRAILING ACROSS NORTHERN ARGENTINA. THE BOUNDARY IS TO BRIEFLY
STALL IN THIS AREA THROUGH WEDNESDAY EVENING. ACROSS CENTRAL
ARGENTINA THE FRONT IS TO INITIALLY FAVOR LIGHT CONVECTION...BUT
AS IT PUSHES NORTH INTO SOUTHERN BRASIL/NORTHERN ARGENTINA THE
BOUNDARY INTENSIFIES. ON THE SOUTHERN STATES OF BRASIL THIS WILL
SUSTAIN RAINFALL AMOUNTS OF 20-30MM ON TUESDAY...WHILE ON
WEDNESDAY IT WEAKENS TO 00-05MM/DAY. HIGHER AMOUNTS ARE EXPECTED
ACROSS CORRIENTES IN ARGENTINA...WHERE THIS WILL SUSTAIN RAINFALL
MAXIMA OF 20-45MM. ACTIVITY ALONG THIS BOUNDARY WEAKENS ON
WEDNESDAY...WHEN IT IS TO SUSTAIN LIGHT RAINFALL AMOUNTS OF
00-05MM/DAY.

SIMULTANEOUSLY...A PROGRESSIVE TROUGH ON THE SOUTHERN STREAM RACES
ACROSS THE SOUTHEAST PACIFIC TO SOUTHERN CHILE EARLY ON WEDNESDAY
MORNING. THIS IS TO THEN PULL AS IT ENTERS THE DRAKE
PASSAGE/PATAGONIA IN ARGENTINA AND CENTRAL REGIONS IN ARGENTINA ON
THURSDAY. AT LOW LEVELS THIS SUSTAINS A PROGRESSIVE FRONT ACROSS
SOUTHERN CHILE TO PATAGONIA IN ARGENTINA ON WEDNESDAY
MORNING...WITH BOUNDARY REACHING LA PAMPA/CENTRAL CHILE LATER IN
THE DAY AS A DEEPER PERTURBATION/STRONGER FRONT MAKES LANDFALL TO
THE SOUTH. THESE THEN COMBINE IN SUPPORT OF A PROGRESSIVE FRONT
THAT RACES ACROSS CENTRAL ARGENTINA/RIO DE LA PLATA BASIN LATER ON
THURSDAY...WITH BOUNDARY TO THEN INTERACT/REINFORCE THE
AFOREMENTIONED FRONT MEANDERING TO THE NORTH. OVER SOUTHERN CHILE
INITIALLY EXPECTING RAINFALL AMOUNTS OF 20-35MM...DECREASING TO
05-10MM ON TUESDAY. ON WEDNESDAY THIS SURGES AGAIN TO 20-35MM AS
THE STRONGER/DEEPER PERTURBATION MAKES LANDFALL OVER SOUTHERN
CHILE. OVER LA PAMPA/CENTRAL ARGENTINA THIS IS TO THEN SUSTAIN A
ROUND OF MODERATE/POTENTIALLY SEVERE CONVECTION ON WEDNESDAY
AFTERNOON...WITH MAXIMA OF 20-35MM. ON THURSDAY EXPECTING
ADDITIONAL RAINFALL AMOUNTS OF 15-20MM AS IT SURGES ACROSS THE
BUENOS AIRES PROVINCE TO CENTRAL ARGENTINA.

ON THE NORTHERN STREAM...AT MID LEVELS...A SHORT WAVE TROUGH IS TO
THEN STREAM ACROSS CENTRAL CHILE TO ARGENTINA EARLY ON THURSDAY
MORNING. AT 250 HPA A SUBTROPICAL JET MAXIMA IS TO ACCOMPANY THIS
PERTURBATION AS IT MAKES LANDFALL OVER CHILE...WITH UPPER LEVEL
JET MAXIMA TO THEN SUSTAIN A DIVERGENT PATTERN ALOFT AS IT STREAM
ACROSS CENTRAL ARGENTINA ON THURSDAY. THE MID LEVEL ENERGY AND
FAVORABLE UPPER LEVEL JET DYNAMICS WILL INCITE ORGANIZED DEEP
CONVECTION ALONG THE MEANDERING FRONT OVER SOUTHERN
BRASIL/NORTHERN ARGENTINA...WITH MAXIMA OF 35-70MM IN POTENTIALLY
SEVERE CONVECTION. ON FRIDAY-SATURDAY EXPECTING ADDITIONAL
RAINFALL AMOUNTS OF 25-50MM ALONG THIS BOUNDARY.

NORTH OVER THE DOMAIN...AN UPPER LEVEL HIGH TO THE SOUTH OF THE
GALAPAGOS ISLANDS EXTENDS A RIDGE TO THE SOUTHEAST ACROSS NORTHERN
CHILE TO BOLIVIA/NORTHERN ARGENTINA/PARAGUAY. UNDER THE INFLUENCE
OF THIS RIDGE A TROUGH IS TO THEN AMPLIFY ACROSS SOUTHEAST TO
CENTRAL BRASIL EARLY IN THE CYCLE. THROUGH THURSDAY...AS THE
AFOREMENTIONED PERTURBATIONS STREAM TO THE SOUTH...THE RIDGE IS
EXPECTED TO FLATTEN. IN RESPONSE TO THIS CHANGE THE TROUGH IS TO
THEN PULL WHILE MEANDERING EAST ACROSS SOUTHEAST BRASIL TO THE
WESTERN ATLANTIC. MEANWHILE...UNDER THE INFLUENCE OF THIS
TROUGH...SCATTERED DEEP CONVECTION WILL BUILD ACROSS CENTRAL TO
NORTHWEST BRASIL...WITH CONVECTION ACROSS TOCANTINS/GOIAS TO PARA
TO SUSTAIN RAINFALL AMOUNTS OF 15-20MM/DAY. OVER AMAZONAS TO THE
NORTHWEST INTO NORTHERN JUNGLE OF PERU/EASTERN ECUADOR THIS WILL
FAVOR RAINFALL MAXIMA OF 20-35MM. BUT AS THE TROUGH PULLS...THIS
WILL GRADUALLY WEAKEN TO 15-20MM/DAY LATER IN THE WEEK.

DAVISON...WPC (USA)