797  
FXSA20 KWBC 211717  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1216 PM EST THU DEC 21 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 21 AT 0000 UTC): GLOBAL MODELS  
AGREE ON PATTERN EVOLUTION OVER THE CONTINENT AND WESTERN SOUTH  
ATLANTIC THROUGH 132 HRS. UPSTREAM...OVER THE EASTERN  
PACIFIC...THEY FOLLOW SIMILAR EVOLUTION THROUGH 84-96 HRS...THEN  
DISAGREE ON AMPLITUDE/SPEED OF SHORT WAVE PERTURBATIONS ENTERING  
THE DOMAIN. THE GFS IS THE ONE CLOSER TO THE MEAN OF THE GLOBAL  
ENSEMBLE MEMBERS...AND AS SUCH IS FAVORED OVER THE EUROPEAN MODELS.  
 
DEEPENING MID LEVEL TROUGH ENTERS THE DRAKE PASSAGE/SOUTHERN CONE  
OF SOUTH AMERICA LATER TODAY...TO QUICKLY PULL ACROSS PATAGONIA TO  
THE WESTERN ATLANTIC ON FRIDAY AFTERNOON. THIS IS TO THEN  
ACCELERATE ACROSS THE SOUTH ATLANTIC...CROSSING 20W LATER ON  
SUNDAY. AT LOW LEVELS THIS SUSTAINS A DEEP TROUGH OVER THE  
BELLINGSHAUSEN SEA/SOUTHERN CHILE-PATAGONIA IN ARGENTINA LATER  
THIS AFTERNOON THAT MOVES TO THE WEDDELL SEA/SOUTHWEST ATLANTIC  
LATER ON FRIDAY/EARLY SATURDAY MORNING. THE ASSOCIATED FRONT MOVES  
ACROSS PATAGONIA IN ARGENTINA THIS AFTERNOON. ON FRIDAY...A  
BUILDING POLAR RIDGE TO THE SOUTH WILL SUSTAIN A PAMPERO LOW LEVEL  
JET THAT DISPLACES THE FRONT NORTH ACROSS LA PAMPA TO THE BUENOS  
AIRES PROVINCE-MENDOZA IN ARGENTINA LATER ON FRIDAY. AS IT RACES  
ACROSS CENTRAL ARGENTINA THIS IS TO TRIGGER A PREFRONTAL SQUALL  
LINE TO THE NORTH. THIS WILL BE ACCOMPANIED BY HEAVY/POTENTIALLY  
SEVERE CONVECTION. ON SATURDAY THE FRONT MOVES TO SOUTHERN  
BRASIL-SOUTHEAST PARAGUAY-NORTHERN PROVINCES IN ARGENTINA...WITH  
RIDGE TO THE SOUTH FAVORING STRONG LOW LEVEL WINDS ACROSS NORTHERN  
ARGENTINA TO SOUTHERN BOLIVIA. AS IT ENTERS SOUTHERN CHILE EARLY  
IN THE CYCLE THIS WILL SUSTAIN SCATTERED CONVECTION WITH MAXIMA OF  
15-30MM THROUGH FRIDAY MORNING. OVER LA PAMPA/BUENOS AIRES THIS IS  
TO TRIGGER RAINFALL MAXIMA OF 25-50MM. ON FRIDAY-SATURDAY THE  
SQUALL LINE WILL TRIGGER SEVERE CONVECTION ACROSS SOUTHERN  
BRASIL-PARAGUAY/NORTHERN ARGENTINA-SOUTHERN BOLIVIA...WITH  
RAINFALL MAXIMA OF 40-80MM ON FRIDAY AND 50-100MM ON SATURDAY. 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 TO THE SOUTHERN JUNGLE OF PERU. OVER PARANA-SANTA  
CATARINA-MISIONES IN ARGENTINA THE MAXIMA WILL THEN PEAK AT  
30-60MM.  
 
A LOWER AMPLITUDE MID LEVEL TROUGH ON THE SOUTHERN STREAM WILL  
THEN FOLLOW...REACHING THE DRAKE PASSAGE-SOUTHERN CHILE SATURDAY.  
AT LOW LEVELS THIS WILL SUSTAIN A DEEP OCCLUDED LOW OVER THE  
SOUTHEAST PACIFIC ON SATURDAY-SUNDAY...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.  
 
A RIDGE AND A TROUGH ARE TO DOMINATE THE UPPER LEVEL FLOW OVER  
NORTHERN SOUTH AMERICA...WITH RIDGE AT 200 HPA TO ENVELOP  
CONTINENTAL AREA TO THE NORTH OF 30S AND WEST OF 50W WHILE  
CENTERING ON A CLOSED HIGH OVER THE ALTIPLANO OF BOLIVIA. THE  
TROUGH LIES TO THE EAST OF THIS RIDGE...CENTERING ON A CLOSED LOW  
OVER EASTERN BAHIA. UNDER THE INFLUENCE OF THE RIDGE TO THE WEST  
THE LOW/TROUGH WILL RETROGRESS TO NORTHEAST BRASIL LATER ON  
SATURDAY/EARLY ON SUNDAY MORNING. MEANWHILE...THE UPPER LEVEL  
TROUGH WILL ENHANCE SEA BREEZE RELATED CONVERGENCE ACROSS NORTHERN  
BRASIL TO FAVOR MAXIMA OF 20-45MM/DAY. 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 SOUTHEAST BRASIL...WITH MAXIMA OF 20-40MM  
EXPECTED OVER THE NEXT COUPLE OF DAYS...DECREASING TO 15-20MM ON  
SATURDAY-SUNDAY. OVER AMAZONAS-ACRE-RONDONIA TO THE WEST INITIALLY  
EXPECTING MAXIMA OF 25-50MM. 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 TO AFFECT THE  
CENTRAL/SOUTHERN JUNGLE DURING THE NEXT TWO TO THREE DAYS. ON  
SUNDAY IT SHIFTS TO THE CENTRAL/NORTHERN JUNGLE.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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