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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1535Z Apr 14, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1135 AM EDT TUE APR 14 2015

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

MODEL DISCUSSION (VALID FROM APRIL 14 AT 0000 UTC): GLOBAL MODELS
FAILED TO REACH A CONSENSUS ON MEDIUM RANGE PATTERN
EVOLUTION...WITH THE GLOBAL ENSEMBLE MEMBERS ALSO DIVERGING ON HOW
PATTERN ACROSS THE SOUTHERN CONE IS GOING TO EVOLVE. AS A
RESULT...OUR CONFIDENCE IN THE FORECAST BEYOND 72 HRS IS LOW.

DURING THE SHORT RANGE PERIOD...SYSTEM OF CONCERN IS A MID
LATITUDE TROUGH THAT IS TO MEANDER OFF THE SOUTHERN-CENTRAL COAST
OF CHILE THROUGH 48-54 HRS. BY 72-84 HRS IT SPILLS ACROSS THE
ANDES INTO THE CENTRAL PROVINCES IN ARGENTINA...WHERE IT IS TO
SUSTAIN HEIGHT FALLS OF 75-150GPM. AS IT SPILLS ACROSS THE
ANDES...A SUBTROPICAL JET MAXIMA IS TO ACCOMPANY THIS TROUGH. THE
JET ALOFT IS TO FAVOR A DIVERGENT PATTERN ACROSS RIO DE LA PLATA
BASIN/BUENOS AIRES PROVINCE TO ENTRE RIOS IN ARGENTINA/URUGUAY ON
DAY 03...AND ACROSS PARAGUAY-CHACO ARGENTINO/CORRIENTES ON DAY 04.
AT LOW LEVELS THE TROUGH ALOFT IS TO INTERACT WITH A MEANDERING
SURFACE FRONT OVER CENTRAL ARGENTINA. THIS IS TO TRIGGER
CYCLOGENESIS ALONG THIS BOUNDARY BY 72-84 HRS. THROUGH 96-108 HRS
THE LOW IS TO GRADUALLY OCCLUDE AS IT DEEPENS OVER THE WESTERN
SOUTH ATLANTIC. AS THE LOW DEEPENS...THE SURFACE FRONT IS TO MOVE
NORTH INTO SOUTHERN BRASIL-PARAGUAY/ SOUTHERN BOLIVIA. THIS IS
FARTHER SOUTH AND NOT AS FAST AS PREVIOUSLY FORECASTED.
FURTHERMORE...A LOW LEVEL NORTHERLY JET ACROSS PARAGUAY TO
NORTHERN ARGENTINA IS TO SUSTAIN A WARM/MOIST ADVECTIVE
PATTERN...WITH MOISTURE TO CONVERGE ALONG THE SURFACE FRONT TO THE
SOUTH. AS MOISTURE ADVECTS SOUTH AND SHORT WAVE ENERGY SHEARS
ACROSS THE ANDES...THIS IS TO SUSTAIN SCATTERED DEEP CONVECTION ON
THE CENTRAL PROVINCES IN ARGENTINA...WITH MAXIMA TO PEAK AT
20-30MM/DAY. BY 72-84 HRS THIS IS TO INCREASE TO 50-100MM...WITH
MODELS SHOWING POTENTIAL FOR SEVERE CONVECTION. THROUGH 96-108 HRS
THE FOCUS OF THE HEAVY CONVECTION SHIFTS TO NORTHERN PROVINCES IN
ARGENTINA-PARAGUAY AND THE SOUTHERN STATES OF BRASIL.

AS PREVIOUSLY INDICATED...THE GLOBAL MODELS FAILED TO REACH A
CONSENSUS ON HOW THE PATTERN IS TO EVOLVE DURING THE MEDIUM RANGE
PERIOD. HOWEVER...ALTHOUGH THEY DIVERGE ON THE TIMING/AMPLITUDE OF
THE SHORT WAVE TROUGHS ENTERING THE CONTINENT...THEY GENERALLY
AGREE ON A PERTURBED PATTERN TO PREVAIL. THUS EXPECTING POTENTIAL
FOR SCATTERED DEEP CONVECTION ACROSS SOUTHERN TO CENTRAL PORTIONS
OF CHILE DURING THE SECOND HALF OF THE FORECAST...WITH RAINFALL
MAXIMA TO PEAK AT 20-30MM AS DEEP MID LEVEL PERTURBATIONS
ESTABLISH A SUBTROPICAL CONNECTION.

NORTH OVER THE CONTINENT...AT 200 HPA...THE UPPER FLOW CONTINUES
TO DIVERGE BETWEEN THE SUBTROPICAL AND THE SUBEQUATORIAL RIDGE.
THE SUBTROPICAL RIDGE IS TO DOMINATE MOST OF BRASIL-PERU AND
BOLIVIA WHILE ANCHORING ON A CLOSED HIGH OVER MATO GROSSO IN
BRASIL. THE SUBEQUATORIAL RIDGE TO THE NORTH IS TO CENTER OVER THE
CENTRAL TROPICAL ATLANTIC AND IT IS TO EXTEND WEST ALONG 05N TO
NORTHERN SOUTH AMERICA. BEST DIFFLUENCE ALOFT IS EXPECTED BETWEEN
PARA AND RIO GRANDE DO NORTE IN BRASIL. THIS...IN-TURN...IS TO
ENHANCE CONVECTIVE DEVELOPMENT ACROSS EQUATORIAL PORTIONS OF SOUTH
AMERICA. OVER NORTH COAST OF BRASIL AND PARA...UNDER FAVORABLE MJO
CONDITIONS...EXPECTING RAINFALL MAXIMA OF 30-60MM THROUGH 36 HRS.
THROUGH 84 HRS IT IS TO DECREASE TO 20-40MM/DAY...AND LATER IN THE
CYCLE IT PEAKS AT 20-30MM. OVER NORTHERN AMAZONAS-SOUTHERN
COLOMBIA AND NORTHERN PERU INITIALLY EXPECTING RAINFALL MAXIMA OF
20-45MM. THROUGH 72-84 HRS THIS INCREASES TO 30-60MM. OTHER
CONVECTION IS TO CLUSTER ACROSS WESTERN ECUADOR...WHERE UNDER
FAVORABLE MJO CONDITIONS AND A BRISK WESTERLY FLOW THE DAILY
MAXIMA IS TO PEAK AT 30-60MM OVER THE NEXT TWO DAYS. OTHER TRADE
WIND CONVERGENCE IS EXPECTED OVER EASTERN BAHIA IN BRASIL...WHERE
DAILY MAXIMA IS TO PEAK AT 15-25MM/DAY EARLY IN THE CYCLE. THROUGH
96 HRS THIS DECREASES TO 05-10MM/DAY.

SCHNEIDER...INMET (BRASIL)
CALLE...UNALM (PERU)
CHARPENTIER...DMC (CHILE)
DAVISON/GALVEZ...WPC (USA)