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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1636Z Mar 16, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1236 PM EDT MON MAR 16 2015

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

MODEL DISCUSSION (VALID FROM MARCH 16 AT 0000 UTC): GLOBAL MODELS
AND ENSEMBLE MEMBERS ARE IN GOOD AGREEMENT ON FORECAST EVOLUTION
THROUGH 96 HRS. AFTER THIS PERIODS...DISCREPANCIES IN THE SPEED
AND ESPECIALLY THE AMPLITUDE OF TROUGHS CROSSING INTO THE SOUTH
ATLANTIC TO THE SOUTH OF 25-30S ARE UNCLEAR. CONFIDENCE IS HIGH
THROUGH 84-96 HRS.

VERY ACTIVE CONVECTION IS ORGANIZING ACROSS SOUTH AMERICA AIDED BY
THE ARRIVAL OF THE DIVERGENT PHASE OF THE MADDEN-JULIAN
OSCILLATION. AT UPPER LEVELS IN TROPICAL SOUTH AMERICA...THE
SUBTROPICAL RIDGE CENTERS OVER SOUTHWESTERN BOLIVIA AND IS
FORECAST TO MEANDER WESTWARD WHILE LOSING ORGANIZATION AND
SPLITTING INTO TWO CELLS. THE RIDGE IS LEADING TO FORMIDABLE
VENTILATION TO THE NORTH AND NORTHEAST OF THE CENTER...WHICH IS
FORECAST TO CONTINUE THROUGH THE CYCLE. THIS WILL BE VENTILATING
STRONG CONVECTION IN PARTS OF EQUATORIAL SOUTH AMERICA AND
ESPECIALLY THE WESTERN TIER OVER ECUADOR/SOUTHERN
COLOMBIA/NORTHERN PERU AND AMAZONAS IN BRASIL. AREA OF CONCERN ARE
THE COASTS OF ECUADOR AND NORTHERN PERU...WHERE LOW-LEVEL
WESTERLIES IN A DEEP MOISTURE POOL WILL CONTINUE ENHANCING EVENING
MOISTURE CONVERGENCE AND TRIGGER SCATTERED SHOWERS AND
THUNDERSTORMS WITH LARGE ACCUMULATIONS. THROUGH EARLY TUESDAY
EXPECTING SCATTERED MAXIMA OF 30-60MM/DAY. AMOUNTS ARE TO INCREASE
TO 50-100MM/DAY BY TUESDAY INTO EARLY WEDNESDAY...TO DECREASE TO
MAXIMA OF 35-70MM/DAY BY WEDNESDAY INTO EARLY THURSDAY. AMOUNTS
ARE TO DECREASE AFTERWARDS. NOTE THAT ISOLATED LARGER VALUES ARE
EXPECTED IN INTERACTION WITH OROGRAPHY. TO THE EAST ACROSS THE
AMAZON BASIN EXPECTING VERY ACTIVE CONVECTION TO PRODUCE SCATTERED
MAXIMA OF 40-80MM/DAY THROUGH THE CYCLE...WITH REGION OF MOST
WIDESPREAD CONVECTION CENTERING NORTH OF 10S AND TO THE WEST OF
55W. INITIALLY...HOWEVER...ENHANCED VENTILATION WILL LEAD TO HEAVY
THUNDERSTORMS ALONG THE AMAZON DELTA WHERE ALSO EXPECTING MAXIMA
OF 40-80MM/DAY.

LARGE TROUGH MEANDERING TO THE EAST OF THE SUBTROPICAL RIDGE WILL
ALSO PROVIDE VENTILATION FOR DEEP CONVECTION ALONG ITS
NORTHEASTERN PERIPHERY. THIS WILL LEAD TO THE DEVELOPMENT OF THE
SOUTH ATLANTIC CONVERGENCE ZONE...WHICH WILL ESTABLISH DURING
TUESDAY AND LAST INTO EARLY THURSDAY...GRADUALLY LOSING DEFINITION
AFTERWARDS AS TROUGH MEANDERS WESTWARD. THIS WILL BE ALSO THE
FOCUS FOR SOME LOCALLY HEAVY AMOUNTS. EXPECTING MAXIMA OF
30-60MM/DAY DEVELOPING ON TUESDAY INTO EARLY THURSDAY ACROSS
GOIAS/MATO GROSSO. TO THE SOUTHEAST ACROSS SAO PAULO/RIO DO
JANEIRO/MINAS GERAIS EXPECTING ISOLATED MAXIMA OF 20-40MM/DAY IN
ASSOCIATION WITH THE SACZ. ISOLATED HIGHER AMOUNTS ARE EXPECTED
ALONG THE SERRA DO MAR AS UPPER TROUGH ORGANIZES.

IN EXTRATROPICAL PARTS OF SOUTH AMERICA...HEAVY RAINFALL EVENT IN
SOUTHERN CHILE IS WANING AS UPPER TROUGH APPROACHES THE ANDES AND
SUBTROPICAL MOISTURE CONNECTION RAPIDLY WANES. THIS WILL STILL
PRODUCE SCATTERED MAXIMA OF 15-20MM/DAY IN PARTS OF SOUTHERN CHILE
THROUGH EARLY TUESDAY. AMOUNTS ARE TO DECREASE AFTER BUT AS POTENT
TROUGH MOVES INTO THE CONTINENT...EXPECTING A TIGHT BELT OF
WEST-NORTHWESTERLIES TO PRODUCE WINDY AND SHOWERY CONDITIONS
THROUGH EARLY THURSDAY WITH 850 WINDS IN THE ORDER OF 35-50KT AND
ACCUMULATIONS IN THE ORDER OF 10-15MM/DAY DUE TO ENHANCED
OROGRAPHIC UPLIFT.

ACROSS ARGENTINA MOISTURE WILL BE SOMEWHAT LIMITED WHICH WILL
LIMIT RAINFALL AMOUNTS WITH POTENT TROUGHS ARRIVING IN THE
WESTERLIES. ACCUMULATIONS WILL CONSTRAIN TO NORTHEASTERN PATAGONIA
THROUGH THURSDAY WITH DIURNAL THUNDERSTORMS PRODUCING ISOLATED
MAXIMA OF 10-15MM/DAY. EXPECTING LARGEST AMOUNTS DURING THURSDAY
AS SHORT WAVE TROUGH AXIS ORGANIZES AND MOVES INTO THE RIO DE LA
PLATA BASIN. YET...THE EXACT EVOLUTION OF THIS AXIS IS UNCLEAR AND
MODELS DIVERGE IN RAINFALL AMOUNTS AND LOCATIONS. EXPECTING AN
INCREASE TO MAXIMA OF 15-20 DURING THURSDAY INTO EARLY FRIDAY WITH
AMOUNTS LIKELY TO DECREASE AFTERWARDS.

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