619  
FXSA20 KWBC 221544  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1043 AM EST FRI DEC 22 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 22 AT 0000 UTC): THE GFS AND  
EUROPEAN MODELS REMAIN IN GOOD AGREEMENT OVER THE CONTINENT AND  
SOUTH ATLANTIC. THEY...HOWEVER...CONTINUE TO STRUGGLE WITH THE  
UPSTREAM PATTERN...MAKING CORRECTIONS TO THE SPEED/AMPLITUDE OF  
MID LEVEL PERTURBATIONS ENTERING THE SOUTH AMERICAN DOMAIN. AS A  
RESULT...FORECAST CONFIDENCE IN THIS AREA IS LOW BEYOND 96 HRS.  
 
DEEP MID LEVEL TROUGH DOMINATES THE FLOW ACROSS THE DRAKE PASSAGE  
TO THE SOUTHERN CONE OF SOUTH AMERICA. DURING THE DAY TODAY THE  
TROUGH PULLS ACROSS PATAGONIA IN ARGENTINA TO THE WESTERN  
ATLANTIC. THIS IS TO THEN ACCELERATE ACROSS THE SOUTH  
ATLANTIC...CROSSING 20W LATER ON SUNDAY. AT LOW LEVELS THIS  
SUSTAINS A DEEP TROUGH THAT MOVES ACROSS THE DRAKE  
PASSAGE/ANTARCTIC PENINSULA TO THE WEDDELL SEA/SOUTHWEST ATLANTIC  
LATER TODAY/EARLY SATURDAY MORNING. THE ASSOCIATED FRONT...UNDER  
INFLUENCE OF A BUILDING RIDGE AND ENSUING PAMPERO LOW LEVEL JET  
RACES ACROSS LA PAMPA TO THE BUENOS AIRES PROVINCE-MENDOZA IN  
ARGENTINA LATER TODAY. AS IT ACCELERATES ACROSS CENTRAL ARGENTINA  
THIS IS TO TRIGGER A PREFRONTAL SQUALL LINE. THIS WILL BE  
ACCOMPANIED BY HEAVY/POTENTIALLY SEVERE CONVECTION AND STRONG  
GUSTY WINDS. ON SATURDAY THE FRONT MOVES TO SOUTHERN  
BRASIL-SOUTHEAST PARAGUAY/PROVINCES IN ARGENTINA...WHILE THE RIDGE  
TO THE SOUTH FAVORS STRONG LOW LEVEL WINDS ACROSS NORTHERN  
ARGENTINA TO SOUTHERN BOLIVIA. OVER URUGUAY-NORTHERN ARGENTINA THE  
SQUALL LINE WILL SUPPORT HEAVY RAINFALL AMOUNTS WITH MAXIMA OF  
40-80MM THIS AFTERNOON THROUGH SATURDAY MORNING. ON SATURDAY TO  
SUNDAY FOCUS OF THE CONVECTION SHIFTS TO  
PARAGUAY-MISIONES/CORRIENTES-CHACO IN ARGENTINA-SOUTHERN STATES OF  
BRASIL...WITH MAXIMA OF 50-100MM. OVER SOUTHERN-CENTRAL BOLIVIA  
THIS IS TO THEN SUSTAIN MAXIMA OF 30-60MM. ON SUNDAY THE FOCUS OF  
THE HEAVY CONVECTION SHIFTS TO NORTHERN BOLIVIA-RONDONIA-MATO  
GROSSO IN BRASIL WITH MAXIMA OF 35-70MM...TO SLOWLY BUILD NORTH TO  
THE SOUTHERN JUNGLE OF PERU. OVER PARANA-SANTA CATARINA-MISIONES  
IN ARGENTINA THE MAXIMA WILL THEN PEAK AT 30-60MM...DECREASING TO  
25-50MM ON MONDAY.  
 
A LOWER AMPLITUDE MID LEVEL TROUGH ON THE SOUTHERN STREAM WILL  
THEN FOLLOW...REACHING THE DRAKE PASSAGE-SOUTHERN CHILE ON  
SATURDAY EVENING. THIS IS ALSO FORECAST TO SLOWLY MOVE ACROSS THE  
DRAKE PASSAGE TO THE SOUTHWEST ATLANTIC ON SUNDAY-MONDAY.  
SECONDARY PERTURBATIONS WILL FOLLOW...WITH ONE ENTERING SOUTHERN  
CHILE ON SUNDAY AND CROSSING PATAGONIA EARLY N MONDAY. AT LOW  
LEVELS THIS WILL SUSTAIN A DEEP OCCLUDED LOW OVER THE SOUTHEAST  
PACIFIC-ANTARCTIC PENINSULA ON SATURDAY-MONDAY...WHILE THE  
ASSOCIATED FRONT STREAMS ACROSS SOUTHERN CHILE TO PATAGONIA IN  
ARGENTINA. AS IT ENTERS THE SOUTHERN CONE THE TROUGH WILL SUSTAIN  
A SUBTROPICAL CONNECTION...TO FAVOR MODERATE RAINFALL AMOUNTS WITH  
MAXIMA OF 20-30MM ON SATURDAY TO SUNDAY.  
 
MODELS THEN DIVERGE ON THE NEXT PERTURBATION...DISAGREEING ON THE  
AMPLITUDE/SPEED OF MID LEVEL TROUGH STREAMING ACROSS THE SOUTHEAST  
PACIFIC ON MONDAY-TUESDAY. BOTH EUROPEAN MODELS FAVOR A FASTER  
SOLUTION THAN WHAT THE GFS SUGGESTS...FORECASTING THE MID LEVEL  
AXIS TO REACH SOUTHERN CHILE ON TUESDAY. ASSOCIATED SURFACE FRONT  
IS TO THEN EXTEND ACROSS EXTREME SOUTHERN PORTIONS OF  
CHILE-PATAGONIA IN ARGENTINA...WHERE WE EXPECT MODERATE RAINFALL  
AMOUNTS.  
 
AN UPPER LEVEL RIDGE AND A TROUGH DOMINATE THE FLOW OVER NORTHERN  
SOUTH AMERICA...WITH THE RIDGE ENVELOPING CONTINENTAL AREA TO THE  
NORTH OF 30S AND WEST OF 50W WHILE CENTERING ON A CLOSED HIGH THAT  
IS TO MEANDER OVER THE ALTIPLANO OF BOLIVIA. THE TROUGH LIES TO  
THE EAST OF THIS RIDGE... CENTERING ON A CLOSED LOW NEAR 10S 38W.  
OVER THE NEXT TWO DAYS THE LOW/TROUGH IS TO REMAIN NEARLY  
STATIONARY. ON SUNDAY-MONDAY...UNDER THE INFLUENCE OF THE RIDGE TO  
THE WEST...THE LOW/TROUGH WILL THEN RETROGRESS ACROSS THE  
NORTHEAST STATES OF BRASIL...NEARING EASTERN PARA LATER ON  
TUESDAY. AS IT MEANDERS OVER NORTHEAST BRASIL...THE UPPER LEVEL  
TROUGH WILL ENHANCE SEA BREEZE RELATED CONVERGENCE TO FAVOR  
RAINFALL MAXIMA OF 20-45MM/DAY IN SCATTERED DEEP CONVECTION. ON  
SUNDAY THIS DECREASES TO 20-30MM WHILE FOCUS OF THE HEAVY  
CONVECTION SHIFTS TO AMAPA-NORTHERN PARA TO THE WEST. THE RIDGE  
ALOFT IS TO VENT DEEP CONVECTION ACROSS WESTERN BRASIL-PERU AND  
BOLIVIA. OVER AMAZONAS-ACRE-RONDONIA TO THE WEST INITIALLY  
EXPECTING MAXIMA OF 20-45MM. THROUGH SATURDAY THIS DECREASES TO  
20-35MM. ON THE JUNGLE OF PERU...MEANWHILE... DIURNAL CONVECTION  
IS TO FAVOR MAXIMA OF 15-20MM/DAY. MOST ACTIVE IS EXPECTED ON  
SUNDAY-MONDAY UNDER INFLUENCE OF THE AFOREMENTIONED FRONT OVER  
NORTHERN ARGENTINA/SOUTHERN BRASIL.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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