387  
FXSA20 KWBC 271653  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1252 PM EDT FRI OCT 27 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM OCT 27 AT 0000 UTC): THE GFS AND THE  
ECMWF FOLLOW A SIMILAR EVOLUTION THROUGH 132 HRS...A SOLUTION WELL  
SUPPORTED BY THE CONSENSUS OF THE GLOBAL ENSEMBLE MEMBERS. THE  
UKMET INITIALLY FOLLOWS A SIMILAR EVOLUTION...THEN DISAGREES ON  
HOW A TROUGH LIFTING ACROSS THE EASTERN PACIFIC EVOLVES. THIS IS  
ADDING SOME UNCERTAINTY TO THE LONG RANGE FORECAST...BUT FOR THE  
MOST PART ONLY EXPECTING MINOR CORRECTIONS.  
 
A MID LEVEL SHORT WAVE TROUGH ON THE NORTHERN STREAM MOVES OFF THE  
SOUTH COAST OF BRASIL-URUGUAY EARLY IN THE CYCLE. THROUGH SATURDAY  
EVENING IT IS TO PULL ACROSS 40W WHILE MERGING WITH AN AMPLIFYING  
TROUGH TO THE SOUTH. THROUGH SUNDAY THIS EVOLVES INTO A BROAD  
TROUGH OVER THE CENTRAL SOUTH ATLANTIC. THE TROUGH ALOFT SUSTAINS  
A POLAR FRONT THAT MOVES NORTH ACROSS SOUTHERN BRASIL-PARAGUAY  
LATER TODAY. THE BOUNDARY WILL THEN MERGE WITH/REPLACE AN OLD  
FRONT TO THE NORTH OVER SOUTHEAST BRASIL. AT 250 HPA...A  
PERSISTENT JET MAXIMA IS TO TRAVERSE THE CONTINENT FROM WEST TO  
EAST ALONG 22S/24S...TO FAVOR AN UPPER DIVERGENT PATTERN THAT IS  
TO VENT DEEP CONVECTION ACROSS MID SECTIONS OF SOUTH AMERICA.  
INITIALLY EXPECTING MODERATE CONVECTION ACROSS SOUTHEAST  
BRASIL-MATO GROSSO DO SUL TO RESULT IN MAXIMA OF 20-40MM.  
 
ANOTHER MID LEVEL PERTURBATION IS TO THEN STREAM ACROSS NORTHERN  
CHILE EARLY ON SATURDAY...CROSSING PARAGUAY TO SOUTHERN BRASIL ON  
SUNDAY. THIS IS TO ALSO INTERACT WITH MEANDERING SURFACE FRONT TO  
FAVOR A SURGE IN CONVECTION. ON SUNDAY MAXIMA ACROSS PARAGUAY-MATO  
GROSSO DO SUL AND PORTIONS OF SAO PAULO IS TO PEAK AT 75-125MM IN  
POTENTIALLY SEVERE CONVECTION. ON MONDAY EXPECTING ADDITIONAL  
RAINFALL AMOUNTS OF 40-80MM AS RISK OF AN MCS PERSISTS OVER MID  
SECTIONS OF SOUTH AMERICA.  
 
A DEEP TROUGH ON THE SOUTHERN STREAM MOVES TO SOUTHERN  
CHILE/PATAGONIA IN ARGENTINA LATER THIS EVENING...MEANWHILE  
CENTERING ON A CLOSED LOW JUST WEST OF ISLA DE CHILOE IN SOUTHERN  
CHILE. THE TROUGH IS TO FOCUS ITS SHORT WAVE ENERGY OVER THE  
CONTINENT TO THE SOUTH OF 35S...SUSTAINING HEIGHT FALLS OF  
150-200GPM AS IT STREAMS ACROSS SOUTHERN CHILE TO PATAGONIA IN  
ARGENTINA. LATER ON SUNDAY/EARLY MONDAY MORNING THE TROUGH WILL  
MEANDER EAST TO THE WESTERN ATLANTIC...WITH AXIS TO SLOWLY  
RELOCATE ACROSS 45W TO 20S LATER ON TUESDAY. AT LOW LEVELS THIS  
SUSTAINS A DEEP OCCLUDED LOW OFF THE SOUTH COAST OF CHILE LATER  
TODAY...WITH THE ASSOCIATED FRONT MOVING EAST ACROSS PATAGONIA IN  
ARGENTINA. THROUGH SUNDAY THE LOW MOVES JUST EAST OF CHUBUT/SANTA  
CRUZ. THIS WILL DRIVE THE FRONT NORTH ACROSS BUENOS AIRES  
PROVINCE-MENDOZA IN ARGENTINA. BUILDING POLAR RIDGE WILL THEN  
DISPLACE THE FRONT FARTHER NORTH ACROSS URUGUAY TO SOUTHERN  
BRASIL-NORTHERN ARGENTINA LATER ON MONDAY. OVER SOUTHERN CHILE THE  
DEEP LOW IS TO SUSTAIN MODERATE CONVECTION WITH MAXIMA OF 20-30MM  
ON FRIDAY...AND AN ADDITIONAL 15-20MM ON SATURDAY. ACROSS  
PATAGONIA IN ARGENTINA THE DEEP OCCLUDED LOW WILL SUSTAIN A MOIST  
NORTHEASTERLY FLOW THAT IS TO CONVERGE/LIFT ON THE ARGENTINEAN  
SIDE OF THE ANDES....TO TRIGGER RAINFALL MAXIMA OF 15-20MM TODAY  
THROUGH SATURDAY MORNING...AND 20-35MM ON SATURDAY TO SUNDAY. AS  
THE FRONT SURGES ACROSS LA PAMPA-CENTRAL ARGENTINA ON  
SATURDAY...UNDER FAVORABLE JET DYNAMICS...IT WILL SUPPORT MODERATE  
TO HEAVY CONVECTION WITH ACCUMULATION OF 30-60MM IN POTENTIALLY  
SEVERE CONVECTION. ON SUNDAY IT DECREASES TO 15-20MM WHILE  
FOCUSING ACROSS THE BUENOS AIRES PROVINCE/CENTRAL ARGENTINA AND  
URUGUAY.  
 
MODELS THEN AGREE ON A SECONDARY PERTURBATION TO LIFT ACROSS THE  
EASTERN PACIFIC...WITH MID LEVEL VORTEX CENTERING ON A CLOSED LOW  
NEAR 45S 95W LATER ON SUNDAY. UNDER INFLUENCE OF A RIDGE TO THE  
WEST THIS IS TO THEN CUTOFF FROM THE FLOW...WITH THE CLOSED LOW  
MEANDERING EAST TO 40S 80W LATER ON TUESDAY. AT LOW LEVELS THIS  
SUSTAINS A DEEPENING LOW OVER THE EASTERN PACIFIC...FORECAST TO  
MOVE JUST WEST OF TEMUCO-CONCEPCION CHILE LATER ON MONDAY. AS IT  
ENTERS THE SOUTHERN CONE THE LOW/TROUGH WILL SUSTAIN A SUBTROPICAL  
CONNECTION...WITH DEEP MOISTURE CLUSTERING OVER CHILE TO THE SOUTH  
OF CHILOE. THIS IS TO TRIGGER MODERATE RAINFALL AMOUNTS ON MONDAY  
WITH ACCUMULATION OF 15-20MM... WHILE ON TUESDAY EXPECT RAINFALL  
AMOUNTS OF 20-30MM IN DEEP CONVECTION.  
 
AT 200 HPA...OVER NORTHERN SOUTH AMERICA...BROAD CELL OF THE  
SUBTROPICAL RIDGE DOMINATES THE FLOW TO THE NORTH OF 20S AND WEST  
OF 50W WHILE CENTERING ON A CLOSED HIGH OVER AMAZONAS IN BRASIL. A  
TROUGH TO THE EAST...MEANWHILE...IS TO EXTEND OVER THE NORTHEAST  
STATES OF BRASIL. THE BEST INSTABILITY ALOFT IS EXPECTED BETWEEN  
THE RIDGE TO THE WEST AND THE TROUGH TO THE EAST. OVER NORTHWEST  
SOUTH AMERICA EXPECTING SCATTERED CONVECTION TO FAVOR RAINFALL  
AMOUNTS OF 20-35MM/DAY. THROUGH SATURDAY THIS INCREASES TO  
20-45MM/DAY...WITH MODERATE TO HEAVY CONVECTION TO PERSIST  
THROUGHOUT THE WEEKEND. EARLY NEXT WEEK THE MAXIMA DROPS TO  
20-30MM. ON THE SOUTHERN/CENTRAL JUNGLE OF PERU THE DAILY MAXIMA  
IS TO PEAK AT 15-20MM/DAY...WITH SIMILAR AMOUNTS ACROSS THE  
SOUTHERN SIERRA. ON THE CENTRAL/NORTHERN SIERRA THE MAXIMA WILL  
PEAK AT 10-15MM.  
 
OVIEDO...SHIN (ARGENTINA)  
VANNUCCI...SMN (ARGENTINA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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