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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1626Z Sep 07, 2018)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1226 PM EDT FRI SEP 07 2018

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

MODEL DISCUSSION (VALID FROM SEP 07 AT 0000 UTC): MODELS CONTINUE
IN GOOD AGREEMENT ON THE EVOLUTION OF THE FORECAST. THE CONFIDENCE
HAS INCREASED PARTICULARLY OVER THE SOUTHEASTERN PACIFIC BY 96 HRS
AND ON. SOME DISCREPANCIES APPEAR AFTER 120 HRS IN THE ATLANTIC
BUT TO THE EAST OF 40W...WITH LITTLE IMPACT IN CONTINENTAL AREAS.
FORECAST CONFIDENCE IS HIGH.

BROAD MID-UPPER RIDGE PATTERN IS FORECAST TO PERSIST ACROSS
MID-SECTIONS OF SOUTH AMERICA INTO CENTRAL ARGENTINA INTO EARLY
MID-WEEK...WHILE WEAK TROUGHS IN THE WESTERLIES PROPAGATE TO THE
SOUTH OF 40S. IN THE MEAN TIME...A LONG WAVE TROUGH IN THE
SOUTHERN STREAM IS SLOWLY MEANDERING EASTWARD ACROSS THE SOUTH
PACIFIC TO APPROACH THE CONTINENT ON MONDAY. EMBEDDED IN THIS
TROUGH...A SHORT WAVE AXIS IS FORECAST TO CROSS THE ANDES OF CHILE
AND ARGENTINA LATE ON MONDAY...WHILE A SECOND TROUGH AXIS CROSSES
THE ANDES ON TUESDAY-EARLY WEDNESDAY. THIS WILL SUSTAIN MODERATE
PRECIPITATION IN CENTRAL/NORTHEASTERN ARGENTINA AND
URUGUAY/SOUTHERN BRASIL INTO MID-NEXT WEEK.

IN CHILE...LIMITED PRECIPITABLE WATER THROUGH THE CYCLE WILL
RESULT IN LIGHT RAINFALL ACCUMULATIONS. A WEAK SUBTROPICAL
MOISTURE CONNECTION IS FORECAST TO ARRIVE LATE ON SUNDAY...BUT IT
WILL RAPIDLY MEANDER NORTHWARD FROM MAGALLANES ON SUNDAY TO CHILOE
BY MONDAY EVENING. THIS WILL SUSTAIN A SHORT PERIOD FOR
ACCUMULATING PRECIPITATION THUS EXPECTING MAXIMA OF 05-10MM/DAY ON
SUNDAY AND MONDAY. BY MONDAY-TUESDAY...ALTHOUGH AVAILABLE MOISTURE
WILL BE LIMITED...STRONG WINDS EXCEEDING 40KT IN EXTREME SOUTHERN
CHILE WILL ENHANCE MOISTURE CONVERGENCE TO SUSTAIN MAXIMA OF
05-10MM/DAY IN WESTERN PORTIONS OF THE REGION.

TO THE EAST OF THE ANDES...IN THE MEAN TIME...A SURFACE FRONT
EXTENDS ACROSS SOUTHERN URUGUAY INTO CENTRAL ARGENTINA WHERE IT
WILL MEANDER AT LEAST THROUGH SUNDAY. AS SHORT WAVE PERTURBATIONS
CROSS THE ANDES...THEY WILL INTERACT WITH THE SURFACE FRONT TO
TRIGGER CONVECTION. ACTIVITY IS FORECAST TO DEVELOP LATE ON
FRIDAY/EARLY ON SATURDAY TO SUSTAIN MAXIMA OF 10-15MM/DAY BY
SATURDAY-EARLY SUNDAY FROM SAN LUIS-LA PAMPA INTO THE CENTRAL
BUENOS AIRES PROVINCE. ACTIVITY IS FORECAST TO PEAK DURING THE DAY
ON SUNDAY TO SUSTAIN LOCALIZED MAXIMA OF 20-35MM...DECREASING
AFTER. THE NEW ROUND OF ACTIVITY WILL START ORGANIZING ON TUESDAY.
GIVEN THE PERSISTENCE OF A LOW-LEVEL JET FROM THE NORTH THROUGH
THE CYCLE...A MOISTURE POOL OF PRECIPITABLE WATER NEAR OR SLIGHTLY
ABOVE 30MM. AS A SURFACE FRONT STARTS PROPAGATING
NORTHWARD...EXPECTING MAXIMA OF 15-20MM/DAY DEVELOPING ON TUESDAY
BETWEEN LA PAMPA/NORTHERN PATAGONIA.

NORTH ACROSS THE CONTINENT...AT UPPER LEVELS...AN ELONGATED RIDGE
IS FORECAST TO MEANDER FROM NORTHEASTERN BRASIL TO NORTHERN PERU
THROUGH THE CYCLE...TO PROVIDE SUFFICIENT VENTILATION TO SUSTAIN
CONVECTION IN AREAS TO THE NORTH OF 05S...WHERE INSTABILITY AND
MOISTURE REMAIN UNSEASONABLY HIGH. A WEAK PERTURBATION IN THE
TRADES IS PROPAGATING ACROSS THE AMAZON DELTA REGION ON
FRIDAY...NORTHEASTERN PARA/AMAPA ON SATURDAY...TO RORAIMA/EASTERN
MAAZONAS ON SUNDAY AS IT WEAKENS. THIS WILL SUSTAIN LOCAL
ENHANCEMENT OF DIURNAL CONVECTION TO SUSTAIN MAXIMA OF 20-40MM
ACROSS THE AMAZON DELTA ON FRIDAY-EARLY SATURDAY...AND
ACCUMULATIONS OF 20-35MM/DAY AS IT MEANDERS WESTWARD. ELSEWHERE IN
THE AMAZON BASIN...EXPECTING ACCUMULATIONS IN THE ORDER OF
15-25MM/DAY ON A DAILY BASIS NORTHEAST OF A LINE FROM SOUTHEAST
AMAZONAS INTO EXTREME NORTHEASTERN PERU THROUGH THE CYCLE.

ALSO IN TROPICAL SOUTH AMERICA...A FRONT AND A SHEAR LINE WILL BE
MEANDERING ALONG THE COAST OF BRASIL TO SUSTAIN TO ENHANCE TRADE
WIND CONVECTION THROUGH MONDAY. THIS WILL SUSTAIN AMOUNTS OF
15-20MM/DAY IN BAHIA/ALAGOAS/SERGIPE ON FRIDAY...DECREASING TO
10-15MM/DAY FROM SATURDAY-MONDAY...FURTHER DECREASING AFTER.
ALSO...ISOLATED DIURNAL THUNDERSTORMS ARE EXPECTED IN THE ANDES OF
SOUTHERN PERU TO SUSTAIN MAXIMA OF 05-10MM/DAY. ISOLATED HIGHER
AMOUNTS ARE EXPECTED ON MONDAY-TUESDAY ALONG THE SELVA ALTA OF
SOUTHERN PERU.

SANCHEZ...SMN (ARGENTINA)
SALAZAR...SENAMHI (BOLIVIA)
GALVEZ...WPC (USA)