246  
FXSA20 KWBC 141558  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST THU DEC 14 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 14 AT 0000 UTC): IN THIS CYCLE  
THE GLOBAL MODELS MADE MINOR CHANGES TO PREVIOUS FORECAST...WITH  
THE GFS AND ECMWF GLOBAL MODELS FOLLOWING A SIMILAR EVOLUTION  
THROUGH DAY 06. THE UKMET AGREES WITH THESE MODELS THROUGH 108-120  
HRS...THEN DIVERGES ON EVOLUTION OF UPSTREAM SYSTEMS ENTERING THE  
EASTERN PACIFIC LATER IN THE CYCLE. THE GLOBAL ENSEMBLE MEMBERS  
FOLLOW A SIMILAR EVOLUTION TO WHAT THE DETERMINISTIC MODELS  
SUGGEST...LEADING TO A HIGHER CONFIDENCE IN THE FORECAST.  
 
SHORT WAVE TROUGH ON THE NORTHERN STREAM FLOW MOVES ACROSS  
NORTHERN ARGENTINA-SOUTHERN BRASIL EARLY IN THE CYCLE...PULLING  
ACROSS 50W LATER THIS EVENING. AS IT STREAMS ACROSS MID SECTIONS  
OF SOUTH AMERICA THIS IS TO INTERACT WITH MEANDERING FRONT OVER  
SOUTHERN BRASIL-NORTHERN ARGENTINA...TRIGGERING SCATTERED DEEP  
CONVECTION WITH ACCUMULATION OF 10-15MM. ON FRIDAY...AS THE SHORT  
WAVE TROUGH PULLS AWAY...THE FRONT IS FORECAST TO FRONTOLIZE.  
 
ON THE SOUTHERN STREAM FLOW...PROGRESSIVE MID LEVEL TROUGH MOVES  
ACROSS THE SOUTH PACIFIC TO SOUTHERN REGIONS OF CHILE LATER ON  
FRIDAY. AS IT PULLS ACROSS THE SOUTHERN CONE TO THE WESTERN  
ATLANTIC THE TROUGH IS TO AMPLIFY ACROSS LA PAMPA TO CENTRAL  
PROVINCES IN ARGENTINA LATER ON SATURDAY. LATER IN THE CYCLE THE  
TROUGH IS FORECAST TO DAMPEN AS IT PRESSES AGAINST A RIDGE TO THE  
EAST. AT LOW LEVELS THIS WILL SUSTAIN A DEEPENING TROUGH ACROSS  
PATAGONIA IN ARGENTINA/SOUTHERN CHILE LATER ON FRIDAY...MEANWHILE  
SUSTAINING A SURFACE FRONT ACROSS PATAGONIA IN ARGENTINA/SOUTHERN  
REGIONS OF CHILE. ON SATURDAY...BUILDING POLAR RIDGE OVER SOUTHERN  
ARGENTINA IS TO SUSTAIN A PAMPERO LOW LEVEL JET OF 25-35KT.  
THIS...IN-TURN DISPLACES THE FRONT NORTH ACROSS LA PAMPA/BUENOS  
AIRES PROVINCE TO RIO DE LA PLATA BASIN-CORDOVA. ON SUNDAY IT  
MIGRATES FARTHER NORTH TO THE SOUTHERN STATES OF BRASIL-PARAGUAY.  
AS IT MAKES LANDFALL OVER SOUTHERN CHILE ON FRIDAY THIS WILL  
SUSTAIN MODERATE CONVECTION WITH ACCUMULATION OF 15-20MM. HIGHER  
AMOUNTS ARE EXPECTED ON SATURDAY...WITH SURGING FRONTAL BOUNDARY  
TRIGGERING MODERATE TO HEAVY CONVECTION ACROSS URUGUAY-ENTRE RIOS  
IN ARGENTINA...WITH MAXIMA OF 30-60MM IN POTENTIALLY SEVERE  
CONVECTION. NOTE THAT LOCALLY HIGHER AMOUNTS ARE HIGHLY PROBABLE.  
ON SUNDAY...AS THE FRONT MEANDERS OVER SOUTHERN  
BRASIL-PARAGUAY...MOST ACTIVE CONVECTION IS EXPECTED ACROSS  
SOUTHERN BOLIVIA-NORTHWEST ARGENTINA AND THE CHACO  
PARAGUAYO...WITH MAXIMA OF 30-60MM.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A CELL OF THE SUBTROPICAL  
RIDGE EXTENDS OVER WESTERN BRASIL-PERU-BOLIVIA...WHILE CENTERING  
ON A CLOSED HIGH OVER NORTHERN BOLIVIA. CONSISTENT WITH PREVIOUS  
FORECASTS THIS RIDGE IS TO GRADUALLY BUILD THROUGH SUNDAY...WHEN  
IT IS TO ENVELOP CONTINENTAL AREA TO THE NORTH OF 35S AND WEST OF  
50W. AS THE RIDGE BUILDS...A SHORT WAVE TROUGH TO THE EAST IS TO  
GRADUALLY AMPLIFY. EARLY IN THE CYCLE THE TROUGH WILL EXTEND TO  
THE SOUTHERN/SOUTHEAST STATES OF BRASIL. ON SATURDAY...IN RESPONSE  
TO THE BUILDING RIDGE TO THE WEST...THE TROUGH IS TO AMPLIFY  
ACROSS SAO PAULO TO TOCANTINS/GOIAS IN NORTHEAST BRASIL. THIS IS  
TO THEN SETTLE OVER THE NORTHEAST STATES OF BRASIL LATER IN THE  
CYCLE. THE MID LEVEL TROUGH IS TO VENT CONVECTION ACROSS MINAS  
GERAIS-RIO DE JANEIRO-ESPIRITO SANTO IN BRASIL...WITH MAXIMA OF  
20-35MM/DAY EXPECTED OVER THE NEXT COUPLE OF DAYS...DECREASING TO  
15-20MM ON SATURDAY-SUNDAY. HIGHER AMOUNTS ARE EXPECTED TO THE  
NORTH ACROSS TOCANTINS-GOIAS-PARA TO MATO GROSSO...WITH MAXIMA OF  
40-80MM IN SCATTERED DEEP CONVECTION. AS THE UPPER LEVEL RIDGE  
STRENGTHENS...FOCUS OF THE ORGANIZED CONVECTION SHIFTS WEST TO  
AMAZONAS-PARA-RONDONIA IN BRASIL...AND THROUGH MONDAY IT IS TO  
THEN CONVERGE OVER SOUTHERN COLOMBIA-NORTHERN JUNGLE OF PERU AND  
AMAZONAS IN BRASIL WITH MAXIMA OF 35-70MM.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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