791  
FXSA20 KWBC 191730  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1229 PM EST FRI JAN 19 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM JAN 19 AT 0000 UTC): THE  
ENVIRONMENTAL CONDITIONS OVER MID SECTIONS OF SOUTH AMERICA REMAIN  
FAVORABLE FOR THE FORMATION OF HEAVY RAINFALL PRODUCING MESO SCALE  
CONVECTIVE SYSTEMS DURING THE NEXT FOUR DAYS. THIS FORECAST IS  
WELL SUPPORTED BY THE DETERMINISTIC MODELS AND THE GLOBAL ENSEMBLE  
MEMBERS. CONFIDENCE IN THE FORECAST IS HIGH.  
 
DEEP MID LEVEL TROUGH DOMINATES THE FLOW ACROSS THE FAR WESTERN  
ATLANTIC. THIS TROUGH IS TO MOVE EAST ACROSS THE CENTRAL SOUTH  
ATLANTIC LATER ON SATURDAY. A SHORT WAVE TROUGH ON THE NORTHERN  
STREAM TRAILS THIS PERTURBATION...MEANDERING OFF THE CENTRAL COAST  
OF CHILE. AS IT MEANDERS OVER THE EASTERN PACIFIC THIS IS TO SHEAR  
SHORT WAVE ENERGY ACROSS THE CENTRAL ANDES INTO CENTRAL  
ARGENTINA-RIO DE LA PLATA BASIN/SOUTHERN STATES OF BRASIL. ON  
SATURDAY-SUNDAY THE SHORT WAVE TROUGH IS TO THEN MEANDER OVER  
NORTHERN ARGENTINA-URUGUAY...AND LATER ON MONDAY IT WILL START TO  
MOVE FARTHER TO THE EAST. AS IT MEANDERS OVER THE CONTINENT...AT  
250 HPA...A SUBTROPICAL JET MAXIMA IS TO ACCOMPANY THE SHORT WAVE  
TROUGH. 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.  
THIS WILL PROVIDE THE VENTILATION ALOFT TO ORGANIZED DEEP  
CONVECTION DURING THE NEXT FEW DAYS.  
 
AT LOW LEVELS...A MEANDERING FRONT EXTENDS ACROSS SOUTHERN BRASIL  
TO NORTHERN ARGENTINA...WHILE A SECONDARY BOUNDARY LIES TO THE  
SOUTH. THE LATTER IS TO GRADUALLY MERGE WITH THE FORMER ON  
SATURDAY...TO HELP SUSTAIN THE MEANDERING FRONT OVER THE SOUTH  
ATLANTIC-CENTRAL SOUTH AMERICA DURING THE NEXT FEW DAYS. AS THE  
MID LEVEL TROUGH SPILLS ACROSS THE ANDES INTO NORTHERN ARGENTINA  
IT IS TO INDUCE CYCLOGENESIS ALONG THE SURFACE FRONT OVER ENTRE  
RIOS IN ARGENTINA/URUGUAY ON SUNDAY-MONDAY. NOTE THAT BOTH  
EUROPEAN MODELS CONTINUE TO FORESEE A DEEPER PERTURBATION THAN  
WHAT THE GFS FORECASTS. THE LOW IS TO THEN TRACK TO THE  
SOUTH-SOUTHEAST ACROSS URUGUAY TO THE WESTERN SOUTH ATLANTIC LATER  
ON MONDAY/EARLY TUESDAY MORNING. AS IT MEANDERS OVER THE  
CONTINENT...MOISTURE IS TO CONVERGE ALONG THE SURFACE FRONT...WITH  
THE 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. ACROSS  
PARAGUAY-CORRIENTES-MISIONES IN ARGENTINA TO SOUTHERN BRASIL  
INITIALLY EXPECTING RAINFALL MAXIMA OF 35-70MM. ON  
SATURDAY-SUNDAY...AS THE LOW LEVEL CYCLONE FORMS ALONG THE SURFACE  
FRONT...THIS INCREASES TO 50-100MM. DURING THAT PERIOD SOME OF THE  
MODELS SHOW LOCALIZED MAXIMA OF 150-200MM. ON SUNDAY-MONDAY THE  
MAXIMA IS TO PEAK AT 40-80MM...WHILE ON MONDAY TO TUESDAY  
EXPECTING MAXIMA OF 25-50MM. ALSO... STRONG MESO-SYNOPTIC FORCING  
IS LIKELY TO RESULT IN SEVERE CONVECTION DURING THE NEXT FEW DAYS.  
 
FARTHER WEST...A BROAD MID LEVEL TROUGH IS TO BECOME THE DOMINANT  
FEATURE OVER THE SOUTHEAST PACIFIC LATER TODAY...WITH BROAD TROUGH  
TO SLOWLY MOVE ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA TO THE  
WESTERN ATLANTIC ON SUNDAY-MONDAY. AS IT NEARS THE SOUTHERN  
CONE...THE DEEP TROUGH WILL SUSTAIN A TIGHT GRADIENT AND STRONG  
LOW/MID LEVEL WINDS ACROSS SOUTHERN CHILE-PATAGONIA IN ARGENTINA.  
AT LOW LEVELS THIS SUSTAINS A BROAD TROUGH OVER THE SOUTHEAST  
PACIFIC...FORECAST TO SLOWLY MOVE ACROSS THE ANTARCTIC  
PENINSULA-DRAKE PASSAGE EARLY IN THE CYCLE. POLAR FRONTS ARE TO  
REVOLVE AROUND THIS AXIS...WITH ONE TO MEANDER EAST ACROSS  
SOUTHERN CHILE-PATAGONIA IN ARGENTINA EARLY IN THE CYCLE...WHILE A  
SECONDARY BOUNDARY IS TO ENTER TIERRA DEL FUEGO LATER ON SATURDAY.  
OVER SOUTHERN CHILE THESE ARE TO GENERALLY FAVOR LIGHT TO MODERATE  
CONVECTION WITH ACCUMULATION OF 05-10MM/DAY AND ISOLATED MAXIMA OF  
15MM. LESSER AMOUNTS ARE EXPECTED OVER TIERRA DEL FUEGO...WITH  
RAINFALL ACCUMULATION OF 00-05MM/DAY OVER THE NEXT FEW DAYS.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...AN EAST-TO-WEST RIDGE OVER  
MID SECTIONS OF SOUTH AMERICA IS TO CENTER ON A CLOSED HIGH OVER  
NORTHERN CHILE-WESTERN BOLIVIA DURING THE NEXT FEW DAYS. 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 TODAY...MOVING TO AMAZONAS IN NORTHWEST BRASIL LATER ON  
SATURDAY/SUNDAY MORNING. AS IT MEANDERS WEST...THE UPPER LEVEL  
TROUGH 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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