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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1509Z Oct 17, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1109 AM EDT FRI OCT 17 2014

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

MODEL DISCUSSION (VALID FROM OCTOBER 17 AT 00UTC): GLOBAL MODELS
AND ENSEMBLE MEMBERS CONTINUE SHOWING A TRANSITION INTO A LONGER
WAVE PATTERN. YET...DETAILS ON THE EVOLUTION ARE BECOMING LESS
CLEAR AS EARLY AS 96-108 HOURS ESPECIALLY FOR THE EASTERN PACIFIC.
FORECAST CONFIDENCE IS HIGH THROUGH 84-96 HRS...AND STARTS
DECREASING GRADUALLY AFTERWARDS.

CHILE IS EXPECTING A TRANSIENT INCREASE IN PRECIPITATION AS A
SUBTROPICAL MOISTURE CONNECTION ARRIVES LATE ON SATURDAY INTO
SUNDAY. IN THE MEAN TIME...ACTIVITY WILL CONCENTRATE BETWEEN
CONCEPCION AND THE AYSEN REGION...AS A WEAKENING OCCLUDED LOW
ACCOMPANIED BY MODERATE NORTHWESTERLY WINDS ENTERS IN THE REGION.
THIS WILL LEAD TO DENSE CLOUD COVER AND ISOLATED MAXIMA OF
10-15MM/DAY THROUGH SATURDAY MORNING. TRAIN OF FRONTS AND
SUBTROPICAL MOISTURE CONNECTION WILL REACH THE AYSEN/PUERTO MONTT
REGIONS VERY EARLY SUNDAY...WITH A SECONDARY INCREASE IN MOISTURE
AND WIND SPEEDS LATE SUNDAY INTO EARLY MONDAY NIGHT. THIS WILL
LEAD TO NUMEROUS SHOWERS WITH LOCALLY MODERATE RAINFALL TO PRODUCE
SCATTERED MAXIMA OF 15-20MM/DAY. ISOLATED HIGHER AMOUNTS ARE
POSSIBLE IN INTERACTION WITH OROGRAPHY. AMOUNTS WILL RAPIDLY
DECREASE DURING MONDAY.

ILL DEFINED SURFACE FRONT CONTINUES LINGERING ACROSS RIO GRANDE DO
SUL-NORTHERN URUGUAY AND INTO NORTHEASTERN ARGENTINA. THE FRONT
CONTINUES TO BE A GOOD FOCUS FOR MOISTURE CONVERGENCE AIDED BY AN
ACTIVE LOW-LEVEL JET THAT ORIGINATES IN BOLIVIA. THIS TOGETHER
WITH SEASONAL DIURNAL HEATING...IS ALLOWING FOR DEEP-LAYER
INSTABILITY TO CONTINUE LINGERING IN THE AREA. AT UPPER
LEVELS...WEAK SHORT WAVE PERTURBATIONS ARE STREAMING ACROSS THE
NORTHERN ARGENTINEAN ANDES INTO MID-SECTIONS OF THE CONTINENT.
ALTHOUGH THE PERTURBATIONS ARE WEAK...THE INSTABILITY IS
SUFFICIENTLY LARGE TO ALLOW THE DEVELOPMENT OF STRONG
THUNDERSTORMS. THROUGH EARLY SATURDAY...LARGEST RAINFALL AMOUNTS
ARE EXPECTED ONCE AGAIN FROM CORRIENTES INTO NORTHERN URUGUAY/RIO
GRANDE DO SUL WITH MAXIMA OF 30-60MM/DAY AND RISK OF MCS FORMATION
AND SEVERE CONVECTION. PATTERN IS TO CONTINUE DURING
SATURDAY...BUT THIS TIME ADDITIONAL AID WILL BE PRESENT AS A
BETTER DEFINED NORTHERN STREAM TROUGH CROSSES THE ANDES. THE
TROUGH WILL ENHANCE CYCLOGENESIS AND SYNOPTIC-SCALE ASCENT...WHICH
WILL LEAD TO LARGER RAINFALL AMOUNTS AND A STRENGHTENING/NORTHWARD
ADVANCE OF THE SURFACE FRONT. SATURDAY EVENING...SURFACE FRONT
WILL RESTRUCTURE ACROSS NORTHERN URUGUAY INTO CHACO/SANTIAGO DEL
ESTERO IN ARGENTINA. EXPECTING SCATTERED THUNDERSTORMS TO DEVELOP
FROM THE ANDEAN FOOTHILLS INTO THE PARAGUAY RIVER WITH MAXIMA OF
20-40MM/DAY. STRONGER THUNDERSTORMS ARE EXPECTED TO THE EAST
EXTENDING FROM CORRIENTES INTO NORTHERN URUGUAY/RIO GRANDE DO SUL
WITH MAXIMUM ACCUMULATIONS OF 35-70MM/DAY AND THE RISK FOR SEVERE
CONVECTION. FRONT WILL RAPIDLY ADVANCE NORTHEASTWARD THROUGH
SUNDAY AFTERNOON TO EXTEND ACROSS NORTHERN PARANA IN BRASIL...WITH
TAIL STALLING ACROSS CENTRAL PARAGUAY/EXTREME SOUTHERN BOLIVIA. AS
UPPER TROUGH EXITS INTO THE ATLANTIC...EXPECTING A DECREASE IN
ACCUMULATIONS IN SOUTHERN BRASIL...WITH SCATTERED THUNDERSTORMS
PRODUCING ISOLATED MAXIMA OF 20-30MM/DAY. FURTHER WEST ACROSS
SOUTHERN BOLIVIA...VENTILATION WILL HOWEVER CONTINUE FAVORABLE FOR
A FEW STRONG THUNDERSTORMS...AS A NEW UPPER TROUGH STARTS
ORGANIZING TO THE WEST. THIS WILL LEAD TO ISOLATED TO SCATTERED
MAXIMA OF 20-40MM/DAY. BY MONDAY AFTERNOON THE SURFACE FRONT WILL
EXTEND ACROSS RIO DO JANEIRO AND SOUTHERN MINAS GERAIS. EXPECTING
SCATTERED THUNDERSTORMS TO PRODUCE MAXIMA OF 15-20MM/DAY. SIMILAR
AMOUNTS ARE EXPECTED ACROSS THE ANDEAN SLOPES OF BOLIVIA...WITH
ISOLATED HIGHER AMOUNTS IN INTERACTION WITH OROGRAPHY. BY TUESDAY
AFTERNOON INTO EARLY WEDNESDAY...EXPECTING SCATTERED MAXIMA OF
15-30MM/DAY FROM ESPIRITO SANTO INTO PARTS OF SOUTHEASTERN MINAS
GERAIS/EASTERN RIO DO JANEIRO.

NORTH OVER THE CONTINENT...THE UPPER FEATURE TO DOMINATE IS A
SLOWLY RETROGRESSING TROUGH/CAVADO DO NORDESTE THAT WILL DOMINATE
MOST OF THE WESTERN AMAZON BASIN THROUGH SUNDAY EVENING. PATTERN
IS FORECAST TO EVOLVE AFTERWARDS WITH UPPER TROUGH REFORMING
ACROSS BAHIA IN BRASIL...AND RIDGE GRADUALLY BUILDING ACROSS THE
SOUTHWESTERN AMAZON BASIN INTO MID-SECTIONS OF THE CONTINENT.
EXPECTING LARGEST ACCUMULATIONS ACROSS THE NORTHWESTERN AMAZON
THROUGH EARLY SATURDAY...WITH ISOLATED MAXIMA OF 20-30MM/DAY.
AMOUNTS AND COVERAGE ARE TO DECREASE AFTERWARDS...WITH SHORT-LIVED
DIURNAL THUNDERSTORMS DOMINATING AREAS NORTH OF 10S. LARGEST
AMOUNTS ARE EXPECTED WEST OF 60W...WITH ISOLATED MAXIMA
CONSTRAINING TO THE 15-30MM/DAY RANGE. ACROSS THE CENTRAL
ANDES...EXPECTING A GRADUAL INCREASING TREND AS UPPER TROUGH EXITS
INTO THE PACIFIC AND STIMULATES THE ADVECTION OF AMAZONIAN
MOISTURE. THIS WILL LEAD TO SCATTERED MAXIMA OF 10-15MM/DAY ACROSS
THE PERUVIAN ANDES STARTING ON SUNDAY AFTERNOON-EARLY MONDAY INTO
EARLY NEXT WEEK. EXPECTING ALSO AN ACTIVATION OF THE BOLIVIAN
ANDES DURING MONDAY INTO EARLY TUESDAY WITH AFTERNOON
THUNDERSTORMS PRODUCING SCATTERED MAXIMA OF 05-10MM/DAY.

CAROCA...DMC (CHILE)
DURAN...SENAHMI (PERU)
GALVEZ...WPC (USA)