038  
FXSA20 KWBC 091659  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1158 AM EST TUE JAN 09 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM JAN 09 AT 0000 UTC): GLOBAL MODEL  
GUIDANCE FOLLOW SIMILAR EVOLUTION OVER THE CONTINENT/SOUTH  
ATLANTIC THROUGH 132-144 HRS. OVER THE SOUTH PACIFIC...THE  
CONTINUITY WITH THE PREVIOUS RUN IS GOOD THROUGH 84-96 HRS. THE  
MODELS THEN START TO DIVERGE ON HOW THE PATTERN IS GOING TO  
EVOLVE. IN THIS CYCLE THE GFS AND THE UKMET NO LONGER FORESEE A  
PERSISTENT RIDGE TO PREVAIL ON THE SOUTHERN STREAM FLOW.  
INSTEAD...IN A TIGHT BELT OF MID LATITUDE WESTERLIES...THESE  
MODELS SHOW THE RIDGE RAPIDLY FLATTENING AS PROGRESSIVE MID LEVEL  
PERTURBATIONS STREAM ACROSS THE SOUTHEAST PACIFIC. AMPLITUDE AND  
SPEED DIFFERENCES AMONG THE MODELS GRADUALLY BECOME MORE  
PRONOUNCED BY 144-168 HRS...LEADING TO A LOWER CONFIDENCE IN THE  
FORECAST LATER IN THE CYCLE.  
 
OVER THE SOUTHEAST PACIFIC...THE GLOBAL MODELS INITIALIZE A BROAD  
MID/UPPER LEVEL TROUGH THAT DOMINATES AREA BETWEEN 130W-60W AND TO  
THE SOUTH OF 30S. THE TROUGH PRESSES AGAINST A RIDGE THAT LIES TO  
THE EAST...WITH AXIS SOUTH ALONG 40W/50W TO THE WEDDELL SEA. THE  
RIDGE IS TO INITIALLY HOLD ITS GROUND...BUT EARLY ON THURSDAY IT  
WILL WEAKEN ENOUGH TO ALLOW THE TROUGH TO PUSH EAST ACROSS THE  
SOUTHERN CONE OF SOUTH AMERICA. THROUGH FRIDAY EVENING THE TROUGH  
IS TO THEN PULL ACROSS 60W/65W TO CENTRAL PROVINCES IN  
ARGENTINA...TO CONTINUE ACROSS 40W/50W LATER ON SATURDAY. AT LOW  
LEVELS THE TROUGH ALOFT SUSTAINS A BROAD OCCLUDED TROUGH OVER THE  
SOUTHEAST PACIFIC... WITH AXIS TO MEANDER EAST ACROSS THE  
ANTARCTIC PENINSULA TO THE WEDDELL SEA ON WEDNESDAY-THURSDAY.  
POLAR FRONTS REVOLVE AROUND THIS AXIS...WITH ONE TO MOVE ACROSS  
NORTHERN PATAGONIA-LA PAMPA IN ARGENTINA LATER TODAY. THIS IS TO  
FRONTOLIZE LATER ON WEDNESDAY. MEANWHILE...IT WILL SUSTAIN LIGHT  
CONVECTION WITH ACCUMULATION OF 05-10MM. ANOTHER FRONT TO THE  
SOUTH WILL EXTEND ACROSS THE ANTARCTIC PENINSULA-TIERRA DEL FUEGO  
INTO THE SOUTHEAST PACIFIC LATER THIS EVENING...WHERE IT IS TO  
REMAIN THROUGH WEDNESDAY EVENING. AS IT MEANDERS OVER THE EXTREME  
SOUTHERN CHILE/TIERRA DEL FUEGO THIS BOUNDARY WILL ESTABLISH A  
SUBTROPICAL CONNECTION...TO TRIGGER MODERATE CONVECTION ON  
WEDNESDAY-THURSDAY. ON WEDNESDAY THE MAXIMA WILL PEAK AT  
15-20MM... WHILE ON THURSDAY IT INCREASES TO 20-30MM.  
 
AS THE MID LEVEL TROUGH STREAMS ACROSS THE SOUTHERN CONE THIS WILL  
THEN DRIVE THE SURFACE FRONT NORTH ACROSS PATAGONIA TO LA PAMPA IN  
ARGENTINA LATER ON FRIDAY. ON SATURDAY...AS A RIDGE BUILDS TO THE  
SOUTH...THE FRONT IS TO MOVE FARTHER NORTH ACROSS THE BUENOS AIRES  
PROVINCE TO RIO DE LA PLATA-ENTRE RIOS IN ARGENTINA TO MENDOZA.  
OVER LA PAMPA THIS IS TO INITIALLY FAVOR RAINFALL AMOUNTS OF  
05-10MM EARLY ON FRIDAY. LATER IN THE DAY IN INCREASES TO 10-15MM  
WHILE BUILDING NORTH ACROSS THE BUENOS AIRES PROVINCE. OVER  
MENDOZA IN ARGENTINA THE MAXIMA WILL THEN PEAK AROUND 20-30MM IN  
SCATTERED DEEP CONVECTION. ON SATURDAY ENVIRONMENTAL CONDITIONS  
APPEAR CONDUCIVE FOR AN MCS TO DEVELOP ACROSS CENTRAL  
ARGENTINA...WITH MAXIMA OF 40-80MM IN POTENTIALLY SEVERE  
CONVECTION.  
 
FARTHER EAST...OVER THE SOUTH ATLANTIC...A MID LEVEL TROUGH  
EXTENDS BETWEEN 50W-10W AND TO THE SOUTH OF 30S. AS THE RIDGE TO  
THE WEST FLATTENS...THE TROUGH IS FORECAST TO GRADUALLY PULL TO  
THE EAST-SOUTHEAST THROUGH THURSDAY. AT LOW LEVELS...THIS SUPPORTS  
AN ELONGATED FRONT ACROSS THE SOUTH ATLANTIC TO SOUTHERN  
BRASIL-ENTRE RIOS IN ARGENTINA/NORTHERN URUGUAY. THE BOUNDARY IS  
TO REMAIN NEARLY STATIONARY THROUGH WEDNESDAY AFTERNOON...AND ON  
WEDNESDAY-THURSDAY...AS THE UPPER TROUGH PULLS AWAY...THE SURFACE  
FRONT IS FORECAST TO FRONTOLIZE. ALSO AT LOW LEVELS...A TROUGH  
EXTENDS FROM NORTHERN BOLIVIA...ACROSS MATO GROSSO DO SUL TO  
SOUTHERN STATES OF BRASIL. THIS TROUGH IS THE FOCUS OF LOW LEVEL  
CONVERGENCE...FAVORING MODERATE TO HEAVY CONVECTION WITH MAXIMA OF  
30-60MM. THROUGH THURSDAY EVENING ACTIVITY OVER MATO GROSSO DO  
SUL-SOUTHERN BRASIL WILL DECREASE TO 20-35MM/DAY...WHILE OVER  
NORTHERN BOLIVIA/ACRE-RONDONIA IN BRASIL IT WILL CONTINUE PEAKING  
AROUND 30-60MM. LESSER AMOUNTS ARE THEN EXPECTED LATER IN THE  
WEEK.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBTROPICAL RIDGE OVER  
NORTHWEST SOUTH AMERICA IS TO ANCHOR ON A CLOSED HIGH THAT IS TO  
MEANDER BETWEEN NORTHERN CHILE-SOUTHERN PERU AND LAKE TITICACA.  
EAST OF THIS RIDGE...A SHORT WAVE TROUGH EXTENDS ACROSS THE  
NORTHEAST STATES OF BRASIL WHILE CENTERING ON A CLOSED LOW NEAR  
08S 40W. UNDER INFLUENCE OF THE RIDGE TO THE WEST THE TROUGH/LOW  
IS TO MEANDER FARTHER EAST ACROSS NORTHEAST BRASIL...WITH CLOSED  
LOW NEARING NORTHERN PARA/ILHA DE MARAJO ON WEDNESDAY-THURSDAY. AS  
THE LOW/TROUGH MEANDERS WEST IT IS TO ENHANCE CONVECTION ON THE  
NORTHEAST STATES OF BRASIL. EARLY IN THE CYCLE THIS IS TO FAVOR  
MAXIMA OF 10-15MM. THROUGH WEDNESDAY/THURSDAY THE MAXIMA IN THIS  
AREA INCREASES TO 15-25MM IN SCATTERED DEEP CONVECTION. THROUGH  
WEDNESDAY THIS INCREASES TO 20-35MM/DAY. OTHER CONVECTION IS  
EXPECTED ALONG THE NEAR EQUATORIAL TROUGH OVER PARA-AMAZONAS IN  
BRASIL...WITH MAXIMA OF 20-45MM/DAY. AS THE LOW RELOCATES ACROSS  
NORTHEAST BRASIL...FOCUS OF THE CONVECTION SHIFTS ACROSS PARA TO  
AMAZONAS IN BRASIL LATER IN THE WEEK...WITH MAXIMA OF 20-45MM/DAY.  
ACROSS NORTHERN PERU-ECUADOR THIS IS TO INITIALLY FAVOR AN  
INCREASE IN CONVECTION...WITH MAXIMA OF 20-40MM ON TUESDAY. ON  
THURSDAY TO FRIDAY THE MAXIMA IN THIS AREA WILL PEAK AT 15-25MM.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
DAVISON...WPC (USA)  

 
 
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