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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1455Z Sep 15, 2020)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1055 AM EDT TUE SEP 15 2020

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

FORECAST DISCUSSION FROM SEP 15/15UTC: CONSISTENT WITH THE
PREVIOUS FORECAST...A MID/UPPER LEVEL RIDGE ON THE NORTHERN STREAM
WILL DOMINATE THE FLOW OVER THE EASTERN PACIFIC THROUGH MOST OF
THE CYCLE...WITH AXIS TO INITIALLY EXTEND BETWEEN 115W-80W AND TO
THE NORTH OF 50S. THROUGH THE END OF THE WORKWEEK THE RIDGE IS
EXPECTED TO GRADUALLY BUILD SOUTH ALONG 90W TO 70S...WHERE IT IS
TO THEN HOLD ITS GROUND THROUGH THE WEEKEND. MEANWHILE...OVER THE
NEXT COUPLE OF DAYS...THE RIDGE ALOFT IS TO STEER PROGRESSIVE
SHORT WAVE PERTURBATIONS EASTWARD ACROSS THE SOUTHEAST PACIFIC TO
THE DRAKE PASSAGE/SOUTHERN CONE OF SOUTH AMERICA. BUT AS THE RIDGE
BUILDS TO THE WEST ON THURSDAY-FRIDAY...IT IS TO THEN INDUCE THE
NORTHWARD AMPLIFICATION OF A LONG WAVE TROUGH OVER THE WESTERN
ATLANTIC TO MID SECTIONS OF SOUTH AMERICA ON SATURDAY...WITH AXIS
BOTTOMING OUT TO THE SOUTH OF 30S.

AT LOW LEVELS...A DEEPENING LOW STREAMS ACROSS THE SOUTHEAST
PACIFIC EARLY IN THE CYCLE...WITH WITH CLOSED LOW RAPIDLY
DEEPENING TO A 958 HPA LOW AS IT NEARS TIERRA DEL FUEGO/DRAKE
PASSAGE LATER ON WEDNESDAY. THE TIGHTENING GRADIENT WILL SUSTAIN
STRONG BOUNDARY LAYER WINDS ACROSS CHILE AND ARGENTINA TO THE
SOUTH OF 40S...WITH 850 HPA WINDS FORECAST TO PEAK AT 35-50KT. ON
THURSDAY TO FRIDAY THIS EVOLVES INTO A BROAD TROUGH OVER THE
SOUTHWEST PACIFIC...WITH ASSOCIATED FRONT SURGING ACROSS SOUTHERN
CHILE/PATAGONIA TO CENTRAL PROVINCES IN ARGENTINA/RIO DE LA PLATA
BASIN LATER ON FRIDAY. DURING THE WEEKEND...A BUILDING POLAR RIDGE
OVER PATAGONIA/CENTRAL ARGENTINA WILL SUSTAIN A SOUTHERLY PAMPERO
LOW LEVEL JET. THIS WILL DRIVE THE FRONT FARTHER NORTH ACROSS
SOUTHERN BRASIL-PARAGUAY ON SATURDAY. ACROSS SOUTHERN CHILE THIS
IS TO SUSTAIN MODERATE TO HEAVY CONVECTION...WITH MAXIMA PEAKING
AT 20-30MM ON WEDNESDAY AND THURSDAY. ACROSS TIERRA DEL FUEGO AND
THE SOUTHERN ANDES THIS IS TO TRIGGER HEAVY SNOWFALL
AMOUNTS...WITH ACCUMULATION OF 10-15CM ON THURSDAY...WHILE ON
FRIDAY EXPECTING ADDITIONAL SNOWFALL AMOUNTS OF 05-10CM. AS THE
BOUNDARY SURGES ACROSS CENTRAL SOUTH AMERICA ON SATURDAY...IT IS
TO TRIGGER MODERATE CONVECTION ACROSS CHACO PARAGUAYO-SOUTHERN
BOLIVIA...WITH ACCUMULATION OF 20-30MM...WHILE OVER
CENTRAL/NORTHER BOLIVIA THE MAXIMA PEAKS AT 25-50MM AS ENHANCED BY
THE LOW LEVEL JET. OTHER CONVECTION IS TO CLUSTER ALONG THE COAST
OF SAO PAULO-RIO DE JANEIRO...WITH ACCUMULATION OF 20-30MM.
LOCALLY HIGHER AMOUNTS ARE LIKELY DUE TO A STRONG ONSHORE FLOW
SETTING LATER IN THE CYCLE.

ON THE NORTHERN STREAM...AT 500 HPA...A SHORT WAVE TROUGH IS
INITIALIZED OFF THE NORTHERN COAST OF CHILE. AS THE AFOREMENTIONED
RIDGE STRENGTHENS ITS FOOTHOLD OVER THE EASTERN PACIFIC THIS IS TO
DISPLACE THE SHORT WAVE TROUGH EAST ACROSS THE ANDES TO NORTHERN
ARGENTINA LATER TODAY...WITH AXIS FOCUSING ITS SHORT WAVE ENERGY
ACROSS PARAGUAY-MESOPOTAMIA VALLEY IN ARGENTINA-RIO GRANDE DO SUL.
THROUGH THURSDAY MORNING THIS IS TO SLOWLY PULL ACROSS SOUTHERN
BRASIL TO THE WESTERN ATLANTIC. FURTHERMORE...A 250 HPA
SUBTROPICAL JET MAXIMA IS TO ACCOMPANY THIS TROUGH AS IT STREAMS
ACROSS CENTRAL PORTIONS OF THE CONTINENT...WITH THE JET ALOFT
SUSTAINING AN UPPER DIVERGENT PATTERN AS IT STREAMS ACROSS
PARAGUAY-SOUTHERN BRASIL. AT LOW LEVELS THIS IS TO INTERACT WITH A
MEANDERING FRONT OVER MID SECTIONS OF SOUTH AMERICA...TRIGGERING
CYCLOGENESIS OVER MISIONES/CORRIENTES IN ARGENTINA. THE LOW IS TO
MEANDER EAST INTO THE WESTERN SOUTH ATLANTIC ON WEDNESDAY...TO
CONTINUE EAST AS THE WEEK WEARS ON. FRONTAL BOUNDARY LOSES ITS
UPPER LEVEL SUPPORT ON THURSDAY...TO DISSIPATE/FRONTOLIZE ON
FRIDAY-SATURDAY. MEANWHILE...OVER CORRIENTES IN
ARGENTINA/SOUTHEAST PARAGUAY-SOUTHERN BRASIL THIS IS TO SUSTAIN
MODERATE TO HEAVY RAINFALL AMOUNTS WITH MAXIMA OF 20-40MM IN
POTENTIALLY SEVERE CONVECTION. ON WEDNESDAY EXPECTING ADDITIONAL
RAINFALL AMOUNTS OF 20-30MM WITH MOST ACTIVE AFFECTING RIO GRANDE
DO SUL IN BRASIL-MISIONES IN ARGENTINA.

NORTH OVER THE CONTINENT...AT 250 HPA...AN ILL ORGANIZED CELL OF
THE SUBTROPICAL RIDGE DOMINATES THE FLOW TO THE NORTH OF 20S.
ALONG THIS AXIS...A CLOSED HIGH IS TO ROLL FROM THE
EAST...ENTERING CEARA/PARAIBA IN NORTHEAST BRASIL LATER TODAY.
THROUGH THURSDAY THIS CONTINUES WEST TO CENTRAL BRASIL...THEN
SLOWLY RELOCATES TO AMAZONAS TO THE WEST LATER IN THE CYCLE. EARLY
IN THE CYCLE THE EVOLVING RIDGE PATTERN ALOFT IS TO VENT DEE
CONVECTION OVER EQUATORIAL SOUTH AMERICA-AMAZONAS IN
BRASIL/PERUVIAN JUNGLE...WITH MAXIMA OF 15-25MM. BUT AS THE RIDGE
PATTERN ALOFT EVOLVES...THIS DECREASES TO 10-15MM/DAY...WITH MOST
ACTIVE CLUSTERING OVER NORTHERN PARA/AMAZONAS-RORAIMA. ALTHOUGH
ACTIVITY OVER BRASIL WANES...CONVECTION ALONG THE LEE OF THE ANDES
BETWEEN PERU AND ECUADOR IS TO GRADUALLY INTENSIFY ON
THURSDAY-FRIDAY...WITH MAXIMA IN THIS AREA PEAKING AT 15-20MM.

DAVISON/GALVEZ...WPC (USA)