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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1500Z Sep 09, 2020)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1100 AM EDT WED SEP 09 2020

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

FORECAST DISCUSSION FROM SEP 09/15UTC: THE GLOBAL MODELS FOLLOW A
SIMILAR EVOLUTION THROUGHOUT THE SHORT RANGE PERIOD...WITH
SUCCESSIVE MID LEVEL TROUGHS EXPECTED TO STREAM ACROSS THE
SOUTHERN CONE DURING THE NEXT TWO DAYS...WITH ONE MOVING ACROSS
PATAGONIA/CENTRAL ARGENTINA EARLY IN THE CYCLE AS ANOTHER MAKES
LANDFALL OVER SOUTHERN CHILE ON THURSDAY MORNING. AT LOW LEVELS
THESE WILL COMBINE IN SUPPORT OF PROGRESSIVE FRONTS THAT LIFT
ACROSS CHILE/ARGENTINA TO MID SECTIONS OF SOUTH AMERICA...WHILE
ALSO SUSTAINING A COLD ADVECTIVE PATTERN. OVER SOUTHERN CHILE THE
DEEP FRONTAL PERTURBATIONS WILL BE ACCOMPANIED BY STRONG BOUNDARY
LAYER WINDS...ENHANCING TOPOGRAPHICALLY INDUCED CONVECTION TO
RESULT IN ACCUMULATION OF 15-20MM DURING THE DAY TODAY AND 10-15MM
ON THURSDAY. OVER THE SOUTHERN ANDES THESE WILL FAVOR HEAVY
SNOWFALL AMOUNTS...WITH STORM TOTAL OF 15-20CM EXPECTED OVER THE
NEXT TWO DAYS. AS THEY LIFT ACROSS ARGENTINA...THESE WILL
INITIALLY BE DEVOID OF SIGNIFICANT PRECIPITATION. BUT AS THEY
PRESS FARTHER NORTH DEEPER MOISTURE WILL MEET THESE
BOUNDARIES...ALLOWING FOR CONVECTION TO BECOME MORE ACTIVE AND
ORGANIZED. OVER THE BUENOS AIRES PROVINCE THIS WILL SUSTAIN A
ROUND OF POTENTIALLY SEVERE CONVECTION ON THURSDAY-FRIDAY WITH
MAXIMA OF 10-15MM. HIGHER AMOUNTS ARE EXPECTED ACROSS RIO GRANDE
DO SUL IN BRASIL-NORTHERN URUGUAY AND CORRIENTES IN
ARGENTINA...WITH MAXIMA OF 35-70MM ON FRIDAY AND 20-40MM/DAY ON
SATURDAY AND SUNDAY. FAVORABLE UPPER LEVEL JET DYNAMICS WOULD
LIKELY RESULT IN SEVERE CONVECTION...WITH CONDITIONS APPEARING
HIGHLY FAVORABLE FOR GENERATION OF LARGE HAIL PRODUCING
THUNDERSTORMS DURING THE WEEKEND.

DURING THE MEDIUM RANGE PERIOD THE UPSTREAM PATTERN OVER THE
EASTERN PACIFIC IS FORECAST TO CHANGE...WITH A LONG WAVE RIDGE TO
BUILD SOUTH BETWEEN 110-80W TO 50S-60S. CONSISTENT WITH PREVIOUS
FORECASTS...PROGRESSIVE PERTURBATIONS ARE TO STREAM TO THE SOUTH
OF THIS RIDGE...WITH ONE BOTTOMING OUT TO THE SOUTH OF 40S AS IT
STREAMS ACROSS SOUTHERN CHILE/ARGENTINA ON FRIDAY EVENING. LACKING
STRONG MID LEVEL SUPPORT...THE ASSOCIATED FRONT WILL CONFINE TO
CONTINENTAL AREA TO THE SOUTH OF 40S AS IT MOVES ACROSS THE
SOUTHERN CONE ON SATURDAY. DEEP PLUME OF MOISTURE IS TO ACCOMPANY
THIS FEATURE AS IT ENTERS SOUTHERN CHILE...FAVORING MODERATE
CONVECTION WITH MAXIMA OF 15-20MM.

VARIABILITY AMONG THE GLOBAL MODELS AND GLOBAL ENSEMBLE MEMBERS
INCREASES DURING THE LONG RANGE PERIOD...WITH THE GFS AND EUROPEAN
MODELS FAILING TO REACH A CONSENSUS ON HOW THE PATTERN IS GOING TO
EVOLVE. DURING THE LATER PERIODS A STRONG MID LEVEL PERTURBATION
IS EXPECTED TO LIFT ACROSS THE SOUTHEAST PACIFIC TO THE SOUTHERN
CONE...FAVORING A DEEP SURFACE TROUGH OVER SOUTHERN CHILE DURING
THE WEEKEND. THIS IS TO ALSO TRIGGER MODERATE TO HEAVY CONVECTION
ALONG THE SOUTH COAST OF CHILE...WITH ACCUMULATION OF 15-20MM.
LIKEWISE...HEAVY SNOWFALL AMOUNTS ARE TO AFFECT THE SOUTHERN
ANDES...WITH ACCUMULATION OF 15-20CM.

NORTH OVER THE CONTINENT...AT 250 HPA...A SHORT WAVE RIDGE OVER
SOUTHEAST BRASIL WILL DRIVE A WANING INVERTED TROUGH WEST ACROSS
AMAZONAS IN BRASIL TO CENTRAL/NORTHERN PERU-EASTERN ECUADOR EARLY
IN THE CYCLE. THIS WILL THEN YIELD TO A LIGHT AND MEANDERING UPPER
FLOW PATTERN DURING THE WEEKEND...WITH AN ILL ORGANIZED UPPER
LEVEL RIDGE TO CONTROL THE FLOW. LACKING STRONG UPPER LEVEL
SUPPORT...DIURNAL HEATING DRIVEN CONVECTION ARE TO PREVAIL TO THE
NORTH OF 10S. OVER NORTHWEST SOUTH AMERICA INITIALLY EXPECTING
ACCUMULATION OF 15-20MM. THROUGH THE END OF THE WEEK THIS IS TO
BECOME LESS INTENSE...WITH MAXIMA OF 05-10MM.

DAVISON/GALVEZ...WPC (USA)