768  
FXSA20 KWBC 181810  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
110 PM EST THU JAN 18 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM JAN 18 AT 0000 UTC): NORTHERN  
ARGENTINA-SOUTHERN BRASIL IS THE AREA OF CONCERN...WHERE  
ENVIRONMENTAL CONDITIONS ARE LIKELY TO BECOME FAVORABLE FOR THE  
GENERATION OF MESO SCALE CONVECTIVE SYSTEMS DURING THE NEXT  
SEVERAL DAYS. THE GFS IS SLOWLY TRENDING TOWARDS A MORE  
ACTIVE/BETTER ORGANIZED SYSTEM...FOLLOWING THE TREND OF THE  
EUROPEAN AND CANADIAN MODELS.  
 
AN AMPLIFYING MID LEVEL TROUGH IS TO DOMINATE THE FLOW OVER THE  
WESTERN ATLANTIC DURING THE NEXT COUPLE OF DAYS...WITH MID LEVEL  
TROUGH TO SLOWLY PULL ACROSS THE CENTRAL SOUTH ATLANTIC LATER ON  
SATURDAY. A SHORT WAVE TROUGH ON THE NORTHERN STREAM CLOSELY  
FOLLOWS...NEARING CENTRAL CHILE LATER TODAY. THIS IS TO MEANDER  
OFF THE COAST OF CHILE THROUGH FRIDAY EVENING...MEANWHILE SHEARING  
SHORT WAVE ENERGY ACROSS THE CENTRAL ANDES INTO CENTRAL  
ARGENTINA-RIO DE LA PLATA BASIN. ON SATURDAY THE SHORT WAVE TROUGH  
SPILLS ACROSS THE ANDES AND MOVES TO CENTRAL/NORTHERN  
ARGENTINA-RIO DE LA PLATA BASIN/SOUTHERN STATES OF BRASIL...WHERE  
IT IS TO REMAIN THROUGH SUNDAY EVENING. AS IT SPILLS ACROSS THE  
ANDES...AT 250 HPA A SUBTROPICAL JET MAXIMA IS TO ACCOMPANY THIS  
FEATURE. THE JET ALOFT IS TO FAVOR AN UPPER DIVERGENT PATTERN  
ACROSS NORTHERN ARGENTINA STARTING DURING THE EARLY MORNING HOURS  
ON SATURDAY...TO PERSIST THROUGH SUNDAY EVENING/MONDAY MORNING. AT  
LOW LEVELS...A POLAR FRONT SURGES ACROSS THE BUENOS AIRES PROVINCE  
TO RIO DE LA PLATA/MENDOZA IN ARGENTINA LATER TODAY. ON FRIDAY IT  
PUSHES FARTHER NORTH...MERGING WITH AN OLD BOUNDARY TO THE NORTH.  
THE LATTER EXTENDS ACROSS THE SOUTH ATLANTIC TO SOUTHERN  
BRASIL-NORTHERN ARGENTINA/PARAGUAY. THE MID LEVEL TROUGH IS TO  
INDUCE CYCLOGENESIS ALONG THIS BOUNDARY...WITH THE  
CANADIAN/EUROPEAN MODELS FAVORING A DEEPER/STRONGER CYCLONE THAN  
WHAT THE GFS. THIS IS TO TRACK TO THE SOUTH-SOUTHEAST ACROSS  
URUGUAY LATER ON SUNDAY...DEEPENING AS IT MOVES OFF THE COAST  
EARLY ON MONDAY. AS IT MEANDERS OVER THE CONTINENT...THIS BOUNDARY  
IS THE FOCUS OF LOW LEVEL MOISTURE CONVERGENCE ACROSS MID SECTIONS  
OF SOUTH AMERICA...WITH PRECIPITABLE WATER CONTENT EXPECTED TO  
PEAK AROUND 50-60MM DURING THE NEXT SEVERAL DAYS. HIGH MOISTURE  
CONTENT WILL COMBINE WITH FAVORABLE MID/UPPER DYNAMICS TO LIKELY  
FAVOR GENERATION OF HEAVY RAINFALL PRODUCING MESO SCALE CONVECTIVE  
SYSTEMS. STARTING TODAY EXPECTING MAXIMA OF 25-50MM OVER CHACO  
ARGENTINO-CORRIENTES-PARAGUAY. ON FRIDAY TO SATURDAY THIS  
INCREASES TO 35-70MM WHILE BUILDING EAST ACROSS MISIONES IN  
ARGENTINA TO SOUTHERN BRASIL. PEAK IN CONVECTION IS EXPECTED ON  
SATURDAY TO SUNDAY...AS THE LOW LEVEL CYCLONE FORMS ALONG THE  
MEANDERING FRONT...WITH MAXIMA OF 50-100MM. DURING THIS PERIOD  
LOCALLY HIGHER AMOUNTS HIGHLY PROBABLE...WITH SOME OF THE MODELS  
SHOWING MAXIMA OF 150-200MM. OVER THE NEXT FEW DAYS STRONG  
MESO-SYNOPTIC FORCING IS ALSO LIKELY TO RESULT IN SEVERE  
CONVECTION.  
 
UPSTREAM OVER THE EASTERN SOUTH PACIFIC...A MID LEVEL TROUGH IS TO  
EXTEND NORTH BETWEEN 120W-100W TO 34S LATER TODAY. THE TROUGH IS  
TO PRESS AGAINST A RIDGE TO THE EAST...WITH THE LATTER TO EXTEND  
SOUTH ALONG 80W TO 60S. ON FRIDAY THE RIDGE RELOCATES TO THE  
WESTERN ATLANTIC AS THE TROUGH MOVES TO THE SOUTHEAST  
PACIFIC/SOUTHERN CONE OF SOUTH AMERICA. ON SATURDAY TO SUNDAY THE  
TROUGH IS TO RAPIDLY WEAKEN WHILE ITS SHORT WAVE ENERGY SHEARS  
UNDER THE RIDGE TO THE EAST. AT LOW LEVELS THIS SUSTAINS A BROAD  
TROUGH OVER THE SOUTHEAST PACIFIC...FORECAST TO SLOWLY MOVE ACROSS  
THE ANTARCTIC PENINSULA-DRAKE PASSAGE LATER TODAY/EARLY FRIDAY  
MORNING. POLAR FRONTS ARE TO REVOLVE AROUND THIS AXIS...WITH ONE  
TO MEANDER EAST ACROSS SOUTHERN CHILE-PATAGONIA IN ARGENTINA  
THROUGH SUNDAY. OVER SOUTHERN CHILE THIS IS TO GENERALLY FAVOR  
LIGHT TO MODERATE CONVECTION WITH ACCUMULATION OF 05-10MM/DAY AND  
MAXIMA OF 15MM. OVER TIERRA DEL FUEGO EXPECTING LESSER  
AMOUNTS...WITH ACCUMULATION OF 00-05MM/DAY OVER THE NEXT FEW DAYS.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...AN EAST-TO-WEST RIDGE  
EXTENDS ACROSS SOUTHERN BRASIL-PARAGUAY-BOLIVIA TO THE EASTERN  
PACIFIC....WITH AXIS ANCHORING ON CLOSED HIGHS OVER BOLIVIA AND  
PARAGUAY/MATO GROSSO DO SUL IN BRASIL. LATER ON FRIDAY THESE ARE  
TO COMBINE INTO A SINGLE HIGH THAT IS TO MEANDER BETWEEN THE  
ALTIPLANO OF BOLIVIA AND NORTHERN CHILE/SOUTHERN SIERRA OF PERU.  
AS THE RIDGE MEANDERS OVER MID SECTIONS OF SOUTH AMERICA IT IS TO  
STEER AN UPPER LEVEL LOW/SHORT WAVE TROUGH ACROSS NORTHEAST BRASIL  
TO PARA LATER ON FRIDAY. AS IT MEANDERS WEST...THIS WILL INTERACT  
WITH THE NEAR EQUATORIAL TROUGH OVER NORTHERN BRASIL TO FAVOR  
ORGANIZED DEEP CONVECTION. ALONG THE CONVERGENCE ZONE OVER  
NORTHERN AMAZONAS-PARA/RORAIMA EXPECTING RAINFALL AMOUNTS OF  
25-50MM/DAY...WITH LOCALIZED MAXIMA OF 80MM. THROUGH SUNDAY THIS  
DECREASES TO 10-15MM/DAY AND MAXIMA OF 20-35MM...WHILE OVER  
NORTHEAST BRASIL THE MAXIMA IS TO PEAK AT 25-50MM. MODERATE TO  
HEAVY RAINS ARE ALSO EXPECTED ACROSS ECUADOR ON  
FRIDAY-SATURDAY...WITH MAXIMA OF 20-35MM/DAY. OVER THE  
ALTIPLANO-SOUTHERN SIERRA OF PERU THE DAILY MAXIMA WILL RANGE  
BETWEEN 10-15MM/DAY THROUGH SATURDAY...INCREASING TO 15-20MM ON  
SUNDAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
DAVISON...WPC (USA)  

 
 
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