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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1617Z Dec 03, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1117 AM EST WED DEC 03 2014

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

MODEL DISCUSSION (VALID FROM DECEMBER 03 AT 00UTC): GLOBAL MODELS
GENERALLY AGREE ON PATTERN EVOLUTION OVER THE NEXT FEW DAYS. ONLY
AREA OF DISCORD IS OVER THE SOUTHERN CONE ON DAYS 3-5...WITH THE
MODELS DISAGREEING IN HOW FAST A SHORT WAVE TROUGH IS TO SPILL
ACROSS THE ANDES. THE GFS IS THE SLOWEST AND THE ECMWF IS THE
FASTEST. THE UKMET LIES IN BETWEEN THESE MODELS...OFFERING THE
COMPROMISE SOLUTION. ALTHOUGH THE MEAN OF THE GLOBAL ENSEMBLES
FAVORS THE SLOWER GFS SOLUTION...THE VARIABILITY AMONG THE MEMBERS
IS TOO HIGH TO ESTABLISH CONFIDENCE IN THE FORECAST. SO EXPECT
FURTHER CORRECTIONS IN FUTURE RUNS.

AT 500 HPA...A SHORT WAVE TROUGH ON THE NORTHERN STREAM MOVES EAST
ACROSS RIO DE LA PLATA BASIN/URUGUAY EARLY IN THE CYCLE. AS IT
ENTERS THE WESTERN ATLANTIC...IT IS TO THEN MERGE WITH AN
AMPLIFYING TROUGH TO THE SOUTH. AT LOW LEVELS...THIS SUSTAINS AN
ELONGATED FRONT ACROSS THE SOUTH ATLANTIC TO THE SOUTHERN STATES
OF BRASIL. BOUNDARY IS TO REMAIN NEARLY STATIONARY THROUGH 36-42
HRS...THEN GRADUALLY WEAKEN TO AN OPEN TROUGH OVER CENTRAL BRASIL.
EARLY IN THE CYCLE...UNDER FAVORABLE JET DYNAMICS...THIS IS TO
TRIGGER ORGANIZED CONVECTION ACROSS PARANA-SANTA CATARINA-SOUTHERN
SAO PAULO...WITH MAXIMA TO PEAK AT 20-40MM. AT 36-84 HRS...AS AN
OPEN TROUGH REMAINS OVER BRASIL...THIS IS TO THEN RESULT IN
SCATTERED CONVECTION WITH ACCUMULATION OF 20-35MM.

A HIGH AMPLITUDE LONG WAVE TROUGH IS TO FOLLOW...WITH MID LEVEL
PERTURBATION TO REACH THE SOUTHERN/CENTRAL COAST OF CHILE BY 42-48
HRS. UNDER INFLUENCE OF A RIDGE TO THE EAST...THE TROUGH IS TO
START SHEARING SHORT WAVE ENERGY SOUTH AND EAST ACROSS SOUTHERN
CHILE/PATAGONIA-THE DRAKE PASSAGE. IN THIS PROCESS THE TROUGH IS
TO SPLIT IN TWO...WITH THE SOUTHERN HALF OF THE TROUGH TO QUICKLY
MOVE ACROSS THE DRAKE PASSAGE/ANTARCTIC PENINSULA TO THE SOUTH
ATLANTIC BY 72 HRS. THE NORTHERN HALF...MEANWHILE...EVOLVES INTO A
CLOSED LOW OFF THE CENTRAL COAST OF CHILE NEAR THE CITY OF
CONCEPCION. THE MODELS CONTINUE TO DISAGREE ON WHEN THIS IS TO
SPILL ACROSS THE ANDES...WITH THE ECMWF FORECASTING A FASTER
PROGRESSION THAN THE GFS/UKMET. AS SUGGESTED BY THE MEAN OF THE
GLOBAL ENSEMBLES...WE ARE LEANING TOWARDS THE SLOWER GFS/UKMET
MODELS. AT 96-108 HRS THE TROUGH IS TO THEN SPILL ACROSS THE ANDES
INTO CUYO IN ARGENTINA...TO PULL ACROSS THE BUENOS AIRES PROVINCE
LATER IN THE CYCLE. A SUBTROPICAL JET IS TO ACCOMPANY THIS TROUGH
AS IT MOVES ACROSS CHILE TO ARGENTINA...TO SUSTAIN AN UPPER
DIVERGENT PATTERN AS IT PULLS ACROSS THE CONTINENT. AT LOW LEVELS
THE LONG WAVE TROUGH IS TO SUSTAIN AN ELONGATED FRONT ACROSS
SOUTHERN CHILE/PATAGONIA IN ARGENTINA EARLY IN THE CYCLE. THROUGH
72 HRS THIS IS TO UNDULATE NORTH ACROSS LA PAMPA-SOUTHERN
CUYO...AND BY 96 HRS A FRONTAL LOW IS TO DEVELOP OVER BUENOS AIRES
PROVINCE/RIO DE LA PLATA. THIS DEEPENS OVER THE WESTERN
ATLANTIC...DRAWING THE FRONT FARTHER NORTH ACROSS URUGUAY/RIO
GRANDE DO SUL TO THE NORTHERN PROVINCES IN ARGENTINA LATER IN THE
CYCLE. MOST ACTIVE CONVECTION OVER CHILE IS EXPECTED TO THE SOUTH
OF ISLA DE CHILOE...WHERE FRONT IS TO ENHANCE MOISTURE
CONVERGENCE. THIS IS TO PERSIST OVER THE NEXT THREE DAYS...WITH
RAINFALL AMOUNTS TO PEAK AT 10-15MM/DAY. OVER ARGENTINA MOST
ACTIVE IS EXPECTED ACROSS BUENOS AIRES PROVINCE-CORDOBA LATER ON
DAY 04...WITH MODERATE CONVECTION EXPECTED TO SUSTAIN RAINFALL
AMOUNTS OF 20-30MM IN POTENTIALLY SEVERE CONVECTION.

AT 200 HPA...THE SUBTROPICAL HIGH IS TO MEANDER WEST ACROSS
BOLIVIA TO SOUTHERN PERU/NORTHERN CHILE THROUGH 60-72 HRS...TO
THEN RELOCATE TO THE EASTERN PACIFIC OFF THE SOUTHERN COAST OF
PERU BY 96-108 HRS. AS THE HIGH RELOCATES...ASSOCIATED RIDGE OVER
THE CONTINENT IS TO WEAKEN. EARLY IN THE CYCLE THIS IS TO SUSTAIN
AN EASTERLY FLOW ACROSS AMAZONAS IN BRASIL TO CENTRAL/NORTHERN
PERU AND ECUADOR. THIS IS TO ENHANCE MOISTURE
TRANSPORT/CONVERGENCE ALONG THE LEE OF THE ANDES. EARLY IN THE
CYCLE THIS IS TO RESULT IN MODERATE TO HEAVY RAINFALL AMOUNTS WITH
ACCUMULATION OF 25-50MM. THROUGH 72-84 HRS THIS DECREASES TO
20-30MM. OTHER ACTIVITY IS EXPECTED ACROSS THE SOUTHERN SIERRA OF
PERU...WITH ACCUMULATION OF 05-10MM/DAY. THROUGH 96-120 HRS THIS
INCREASES TO 20-40MM.

ARREAGA...INAMHI (ECUADOR)
DURAN...SENAHMI (PERU)
DAVISON...WPC (USA)
GALVEZ...WPC (USA)