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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1621Z Apr 15, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1221 PM EDT TUE APR 15 2014

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

MODEL DISCUSSION (VALID FROM 15 APRIL 00UTC): MINOR CORRECTIONS TO
PREVIOUS PACKAGE AS MODELS CONTINUE TO AGREE ON SHORT RANGE
PATTERN EVOLUTION ACROSS THE SOUTH AMERICAN DOMAIN.
THEY...HOWEVER...START TO DIVERGE DURING THE MEDIUM RANGE
PERIOD...WITH DIFFERENCES GROWING AT A NEARLY EXPONENTIAL RATE
THROUGH DAY 07. AS A RESULT...OUR CONFIDENCE IN THE FORECAST
BEYOND 84/96 HRS IS LOW.

MID/UPPER LEVEL TROUGH OVER THE SOUTH ATLANTIC CONTINUES TO WEAKEN
WHILE MIGRATING TO THE EAST-SOUTHEAST. ASSOCIATED SURFACE FRONT
EXTENDS TO THE WEST-NORTHWEST ACROSS THE SOUTH ATLANTIC-RIO DE
JANEIRO/NORTHERN SAO PAULO TO NORTHERN BOLIVIA. THIS BOUNDARY IS
LOSING UPPER LEVEL SUPPORT...AND AS A RESULT IT IS TO GRADUALLY
WEAKEN/FRONTOLIZE OVER THE CONTINENT THROUGH 30-36 HRS. IT...
HOWEVER...IS TO PERSIST OVER THE SOUTH ATLANTIC...WITH TRAILING
END MEANDERING ALONG THE SOUTHEAST COAST OF BRASIL. THIS IS TO
ENHANCE MOISTURE CONVERGENCE ACROSS SAO PAULO-RIO DE JANEIRO AND
MATO GROSSO IN BRASIL...WITH RAINFALL AMOUNTS OF 15-25MM THROUGH
36 HRS...AND 10-15MM AT 36-60 HRS.

A BROAD TROUGH OVER THE EASTERN PACIFIC FOLLOWS. THIS IS TO ENTER
SOUTHERN CHILE EARLY IN THE CYCLE...FOCUSING ITS SHORT WAVE ENERGY
ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA...WHERE IT WILL
SUSTAIN HEIGHT FALLS OF 150-175GPM. THIS IS TO SLOWLY PULL ACROSS
PATAGONIA TO THE WESTERN ATLANTIC THROUGH 54-60 HRS AS ANOTHER
PERTURBATION ENTERS SOUTHERN CHILE. THE LATTER IS TO CLEAR THE
COAST BY 84-96 HRS...DRAWING THE TROUGH EASTWARD INTO THE WESTERN
ATLANTIC. NOTE THAT THIS IS SLOWER AND CONSIDERABLY DEEPER THAN
WHAT THE MODELS PREVIOUSLY SUGGESTED. AT LOW LEVELS THIS IS TO
SUSTAIN A BROAD TROUGH ACROSS THE BELLINGSHAUSEN SEA TO THE
SOUTHERN CONE. POLAR FRONTS ARE TO REVOLVE AROUND THIS AXIS...WITH
ONE THAT PULLS ACROSS SOUTHERN CHILE TO PATAGONIA EARLY IN THE
CYCLE. THE FRONT WILL THEN MOVE NORTH ACROSS LA PAMPA TO THE
BUENOS AIRES PROVINCE/CORDOBA IN ARGENTINA THROUGH 48 HRS...INTO
RIO DE LA PLATA BASIN/URUGUAY BY 72 HRS. BOUNDARY IS TO THEN MOVE
NORTH INTO SOUTHERN BRASIL-PARAGUAY BY 96 HRS...THEN FARTHER NORTH
ACROSS SAO PAULO BY 120 HRS. ACROSS SOUTHERN CHILE THIS IS TO
FAVOR SCATTERED CONVECTION WITH ACCUMULATION OF 10-15MM. AS IT
PRESSES NORTH ACROSS ARGENTINA BY 48 HRS...AND AS A MOIST
NORTHERLY JET ESTABLISHES ACROSS BOLIVIA/PARAGUAY THIS IS LIKELY
TO TRIGGER ORGANIZED DEEP CONVECTION ACROSS THE NORTHERN PROVINCES
IN ARGENTINA/PARAGUAY-MESOPOTAMIA VALLEY AND OVER URUGUAY/RIO
GRANDE DO SUL IN BRASIL. IN THIS AREA THE RAINFALL MAXIMA WILL
PEAK AT 25-50MM BY 72-84 HRS...AND 20-40MM AT 84-108 HRS. BUILDING
POLAR RIDGE IS TO THEN SUSTAIN A SOUTHERLY WIND SURGE ACROSS
PARAGUAY-BOLIVIA...WHICH IN TURN TRIGGERS CONVECTION ALONG THE
EASTERN AND NORTHERN SLOPES OF THE BOLIVIAN ALTIPLANO. IN THIS
AREA EXPECTING RAINFALL AMOUNTS OF 20-40MM

NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBTROPICAL RIDGE
CONTINUES TO WEAKEN AS COOLER/DRYER AIR ADVECTS ACROSS
PARAGUAY-BOLIVIA TO NORTHERN PORTIONS OF SOUTH AMERICA. THROUGH 48
HRS THE ILL ORGANIZED ANTICYCLONE IS EXPECTED TO WEAKEN INTO AN
OPEN RIDGE OVER NORTHERN BRASIL. DIURNAL CONVECTION ACROSS BRASIL
IS TO ALSO WEAKEN...WITH ACTIVITY BETWEEN THE EQUATOR AND 10S TO
PEAK AT 15-20MM. HIGHER AMOUNTS ARE EXPECTED TO THE NORTH...ACROSS
VENEZUELA-COLOMBIA TO NORTHERN PORTIONS OF AMAZONAS IN BRASIL WITH
ACCUMULATION OF 20-35MM/DAY ALONG AND NORTH OF THE NEAR EQUATORIAL
TROUGH.

ITA...SENAMHI (PERU)
FERNANDES...INMET (BRASIL)
DAVISON...WPC (USA)