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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1617Z Oct 16, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1217 PM EDT THU OCT 16 2014

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

MODEL DISCUSSION (VALID FROM OCTOBER 16 AT 00UTC): GLOBAL MODELS
AND ENSEMBLE MEMBERS SHOW GOOD AGREEMENT ON FORECAST EVOLUTION AS
THE PATTERN TRANSITIONS FROM A SHORT WAVE ONE INTO A LONGER WAVE
PATTERN. FORECAST CONFIDENCE IS HIGH. MODELS CONTINUE SUGGESTING
ACTIVE PATTERN OF CONVECTION FROM NORTHEASTERN ARGENTINA INTO
NORTHERN URUGUAY/SOUTHERN BRASIL THROUGH THE NEXT 3 DAYS TO
PRODUCE SEVERAL ROUNDS OF LOCALLY HEAVY RAINFALL.

SHORT WAVE TROUGH EXITING INTO THE ATLANTIC ALONG 50W IS
SUSTAINING A PROGRESSIVE SURFACE FRONT. THE FRONT IS RAPIDLY
MOVING ACROSS CENTRAL URUGUAY/ENTRE RIOS IN ARGENTINA...BUT IS
FORECAST TO STALL IN THE REGION LATER TODAY AND REMAIN AS AN
ILL-DEFINED BOUNDARY THROUGH EARLY SUNDAY. DURING THIS
PERIOD...THE ILL-DEFINED BOUNDARY WILL BE FOCUS OF MOISTURE
CONVERGENCE AIDED BY THE NORTHERLY LOW-LEVEL JET. MODELS ALSO
CONTINUE SUGGESTING INSTABILITY TO REMAIN HIGH...WITH SEVERAL
PERIODS WITH LIFTED INDEX VALUES UNDER -9 AND GALVEZ-DAVISON INDEX
VALUES APPROACHING +50. THIS...AIDED BY MODERATE VENTILATION IN
THE UPPER TROPOSPHERE...WILL SUSTAIN SEVERAL ROUNDS WITH STRONG
THUNDERSTORMS AND LOCALLY HEAVY RAINS. THROUGH EARLY SATURDAY
LARGEST AMOUNTS ARE EXPECTED BETWEEN CORRIENTES/NORTHERN ENTRE
RIOS INTO THE NORTHEASTERN HALF OF URUGUAY AND RIO GRANDE DO SUL.
EXPECTING ACCUMULATIONS OF 30-60MM/DAY WITH THE RISK OF MCS
FORMATION AND SEVERE WEATHER DURING BOTH DAYS. BY SATURDAY INTO
EARLY SUNDAY...EXPECTING MAXIMUM ACCUMULATIONS OF 25-50MM/DAY WITH
THE RISK OF SEVERE WEATHER TO CONTINUE. ISOLATED HIGHER AMOUNTS
ARE ALWAYS EXPECTED IN STRONGEST STORMS.

NEXT SHORT WAVE TO APPROACH THE CONTINENT IS FORECAST TO ENTER
CENTRAL CHILE DURING THE DAY ON FRIDAY...TO TRANSVERSE CENTRAL
ARGENTINA DURING SATURDAY AND EXTEND INTO URUGUAY/RIO GRANDE DO
SUL/MISIONES DURING SUNDAY. THIS WILL PRODUCE CYCLOGENESIS AND
UNDULATE/RESTRUCTURE EXISTING SURFACE BOUNDARIES ACROSS THE
REGION. A NEW ORGANIZING COLD FRONT WILL START MOVING NORTHWARD
LATE ON SATURDAY TO EXTEND ACROSS PARANA IN BRASIL-EASTERN
PARAGUAY BY SUNDAY EVENING WHERE IT IS TO STALL. AS FRONT MOVES
BY...IT WILL LEAD TO A DECREASE IN ACTIVITY IN URUGUAY/PARTS OF
NORTHERN ARGENTINA TO MOVE THE FOCUS FOR CONVECTION TO BRASIL. BY
SUNDAY INTO EARLY MONDAY EXPECTING MAXIMA OF 20-30MM/DAY EXTENDING
FROM CORRIENTES/MISIONES INTO SANTA CATARINA/NORTHERN RIO GRANDE
DO SUL AND SOUTHERN PARANA IN BRASIL. BY MONDAY INTO EARLY TUESDAY
EXPECTING MAXIMA OF 15-20MM/DAY ACROSS SAO PAULO/NORTHERN PARANA
INTO PARTS OF 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 AMAZON BASIN. THE TROUGH IS FORECAST TO RETROGRESS AS
THE SUBTROPICAL HIGH MOVES CONTINUES MIGRATING INTO THE PACIFIC
EARLY IN THE CYCLE. THIS WILL PLACE THE AREA OF BEST VENTILATION
FOR DIURNAL CONVECTION ACROSS NORTHWESTERN AMAZONIA. EXPECTING
MAXIMUM DIURNAL AMOUNTS IN THE ORDER OF 20-30MM/DAY EXTENDING FROM
WESTERN AMAZONAS IN BRASIL INTO CENTRAL/NORTHERN PERU...ECUADOR
AND SOUTHERN COLOMBIA. MODELS ARE THEN UNDULATING A NORTHERN
STREAM TROUGH SOUTH OF THE ALTIPLANO. THE TROUGH WILL AMPLIFY BY
SUNDAY AFTERNOON TO EXTEND FROM NORTHWESTERN ARGENTINA INTO 20S
80W. THIS WILL VENTILATE CONVECTION ACROSS THE EASTERN
PERUVIAN/BOLIVIAN ANDES AND ACROSS SOUTHERN BOLIVIA. IN THE ANDEAN
REGIONS EXPECTING DIURNAL MAXIMA IN THE ORDER OF 10-15MM/DAY BY
SUNDAY AFTERNOON-EARLY MONDAY TO CONTINUE INTO EARLY TUESDAY.
ACROSS SOUTHERN BOLIVIA...EXPECTING MAXIMA OF 20-40MM/DAY BY
SUNDAY INTO EARLY MONDAY.

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