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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1656Z Mar 17, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1256 PM EDT TUE MAR 17 2015

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

MODEL DISCUSSION (VALID FROM MARCH 17 AT 0000 UTC): GLOBAL MODELS
AND ENSEMBLE MEMBERS ARE IN MUCH BETTER AGREEMENT ON FORECAST
EVOLUTION. FORECAST CONFIDENCE IS HIGH THROUGH 96-108 HRS.

REMARKABLE SITUATION IS THE ACTIVATION OF CONVECTION IN COASTAL
AREAS OF ECUADOR AND NORTHERN PERU. THIS IS A RESPONSE TO THE
DEVELOPMENT OF MOIST ONSHORE FLOW AT LOW-LEVELS...FORMIDABLE
VENTILATION IN THE NORTHERN PERIPHERY OF THE SUBTROPICAL
RIDGE...AND THE ARRIVAL OF THE UPPER DIVERGENT PHASE OF THE
MADDEN-JULIAN OSCILLATION. THIS IS COINCIDING WITH A GRADUAL
DEVELOPMENT OF POSITIVE SEA SURFACE TEMPERATURE ANOMALIES DURING
THE PERIOD WITH CLIMATOLOGICALLY WARMEST SSTS. EXPECTING
LATE-AFTERNOON/NOCTURNAL CONVECTION TO PRODUCE ACCUMULATIONS OF
50-100MM/DAY THROUGH EARLY WEDNESDAY...40-80MM/DAY BY WEDNESDAY
INTO EARLY THURSDAY DECREASING SLIGHTLY BY THURSDAY INTO FRIDAY.
NOTE THAT ISOLATED HIGHER AMOUNTS ARE EXPECTED IN STRONGEST
THUNDERSTORMS AND IN INTERACTION WITH OROGRAPHY.

OTHER ACTIVE REGIONS IN TROPICAL SOUTH AMERICA INCLUDE THE CENTRAL
ANDES INTO THE ALTIPLANO. INITIALLY...STRONGEST DIURNAL
THUNDERSTORMS ARE TO PRODUCE MAXIMA OF 15-30MM/DAY EARLY IN THE
CYCLE. AS THE UPPER RIDGE RELOCATES...UPPER TROUGH TO THE EAST
WEAKENS...AND DIVERGENT PHASE OF THE MADDEN JULIAN OSCILLATION
STARTS TO RELOCATE TO THE EAST AS THE WEEKEND
APPROACHES...EXPECTING A GRADUAL DECREASE IN ACCUMULATIONS. ACROSS
THE AMAZON BASIN/EQUATORIAL SOUTH AMERICA...EXPECTING SCATTERED TO
WIDESPREAD DIURNAL CONVECTION WITH LOCALLY HEAVY AMOUNTS. AS FOCUS
FOR STRONGEST CONVECTION MOVES EAST THROUGH THE CYCLE...EXPECTING
STRONGEST THUNDERSTORMS TO ALSO RELOCATE FROM THE CENTRAL/WESTERN
EQUATORIAL AMAZON EARLY IN THE CYCLE TO
TOCANTINS/GOIAS/PARA/MARANHAO IN BRASIL BY LATE CYCLE.
INITIALLY...STRONGEST/MOST WIDESPREAD THUNDERSTORMS WILL LINGER
ACROSS EQUATORIAL CENTRAL/WESTERN SOUTH AMERICA. BY FRIDAY INTO
THE WEEKEND EXPECTING AN INCREASE IN MAXIMA FO 50-100MM/DAY WITH
FOCUS ACROSS NORTHEASTERN SOUTH AMERICA.

SEASONALLY ACTIVE DIURNAL CONVECTION IS ALSO EXPECTED IN PARTS OF
CENTRAL BRASIL IN ASSOCIATION WITH AN ILL-DEFINED SOUTH ATLANTIC
CONVERGENCE ZONE. UPPER TROUGH DIGGING IN THE SOUTHEASTERN
PERIPHERY OF SUBTROPICAL RIDGE WILL REACTIVATE THE SACZ ON
TUESDAY. THE CONVERGENCE ZONE WILL LINGER INTO THURSDAY WHEN IT
WILL START LOSING DEFINITION. THE SACZ WILL EXTEND FROM RIO DO
JANEIRO/SAO PAULO NORTHWESTWARD INTO MATO GROSSO-RONDONIA-NORTHERN
BOLIVIA. STRONGEST CONVECTION WILL EXTEND ALONG THE SACZ BUT
ESPECIALLY TO THE NORTHEAST...WHERE EXPECTING ACCUMULATIONS IN THE
ORDER OF 30-60MM/DAY THROUGH EARLY THURSDAY. CONVECTION WILL START
ACTIVATING ACROSS GOIAS/TOCANTINS/MARANHAO AFTERWARDS...WHILE
DECREASING TO THE WEST.

SOUTH ACROSS THE SOUTHERN CONE...LIMITED MOISTURE WILL RESULT IN
LIMITED RAINFALL AMOUNTS. EXPECTING PRECIPITATION TO CONFINE TO
TWO AREAS MAINLY. ONE WILL BE THE AYSEN AND MAGALLANES REGIONS IN
CHILE WHERE BRISK WEST-NORTHWESTERLY WINDS WILL LEAD TO NUMEROUS
COLD SHOWERS WITH MAXIMA OF 10-15MM/DAY. AMOUNTS ARE TO DECREASE
AFTER THURSDAY AS A STRONG UPPER RIDGE BUILDS OVER THE REGION. TO
THE EAST...EXPECTING ALSO SOME ACTIVITY ACROSS NORTHEASTERN
PATAGONIA IN ARGENTINA WHERE ISOLATED DIURNAL THUNDERSTORMS WILL
DEVELOP IN INTERACTION WITH SURFACE COLD FRONTS TO PRODUCE
ACCUMULATIONS OFTEN BELOW THE 10-15MM/DAY RANGE. ISOLATED MAXIMA
NEAR 20MM ARE POSSIBLE THROUGH EARLY WEDNESDAY.

ARREAGA.....INAMHI (ECUADOR)
SCHNEIDER...INMET (BRASIL)
GALVEZ......WPC (USA)