635  
FXSA20 KWBC 131756  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1255 PM EST MON NOV 13 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM NOV 13 AT 0000 UTC): CONFIDENCE IN  
THE FORECAST IS HIGH THROUGH 96-108 HRS...WITH GLOBAL MODELS  
FOLLOWING SIMILAR EVOLUTION DURING THE SHORT RANGE PERIOD. LATER  
IN THE CYCLE THEY DISAGREE ON THE SPEED/AMPLITUDE OF SHORT WAVE  
PERTURBATIONS STREAMING ACROSS THE SOUTHEAST PACIFIC TO THE DRAKE  
PASSAGE...WITH DIVERGENCE AMONG THE ENSEMBLE MEMBERS LEADING TO A  
LOWER CONFIDENCE IN THE FORECAST BY 120 HRS AND ONWARD.  
 
EARLY IN THE CYCLE...AT MID LEVELS...A SHORT WAVE RIDGE EXTENDS  
ACROSS ARGENTINA. THIS...IN-TURN...IS TO STEER A SHORT WAVE TROUGH  
ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA LATER  
TODAY...CROSSING LAS MALVINAS/FALKLAND ISLANDS LATER ON TUESDAY. A  
SECONDARY MID LEVEL TROUGH FOLLOWS...CROSSING CENTRAL CHILE ON  
TUESDAY EVENING AND THEN ACROSS CENTRAL ARGENTINA ON WEDNESDAY.  
THESE ARE TO THEN MERGE OVER THE WESTERN ATLANTIC LATER ON  
WEDNESDAY. AT LOW LEVELS THE MID LEVEL PERTURBATIONS SUSTAIN AN  
ELONGATED FRONT THAT EXTENDS FROM AN OCCLUDED LOW OVER PATAGONIA  
IN ARGENTINA...ACROSS LA PAMPA-CENTRAL CHILE INTO THE EASTERN  
PACIFIC. THE FRONT MOVES NORTH ACROSS CENTRAL ARGENTINA ON  
TUESDAY....REACHING URUGUAY/SOUTHERN BRASIL-NORTHERN ARGENTINA ON  
WEDNESDAY. OVER SOUTHERN CHILE THIS IS TO TRIGGER LIGHT TO  
MODERATE CONVECTION WITH ACCUMULATION OF 10-15MM...WHILE OVER THE  
CENTRAL ANDES OF CHILE THE MAXIMA ON TUESDAY PEAKS AT 05-10MM. AS  
THE FRONT MEANDERS NORTH ACROSS LA PAMPA-BUENOS AIRES PROVINCE ON  
TUESDAY-WEDNESDAY THIS IS TO THEN TRIGGER MODERATE CONVECTION WITH  
ACCUMULATION OF 15-30MM IN SCATTERED CONVECTION. SIMILAR AMOUNTS  
ARE EXPECTED OVER URUGUAY-CORRIENTES IN ARGENTINA ON  
WEDNESDAY-THURSDAY.  
 
PROGRESSIVE SHORT WAVE TROUGH ON THE SOUTHERN STREAM IS TO  
FOLLOW...WITH MID LEVEL VORTEX REACHING SOUTHERN CHILE EARLY ON  
THURSDAY MORNING. BUILDING RIDGE TO THE WEST IS TO THEN INDUCE THE  
NORTHWARD AMPLIFICATION OF THIS TROUGH ACROSS PATAGONIA TO CENTRAL  
ARGENTINA ON THURSDAY. ACROSS CENTRAL ARGENTINA THIS WILL SUSTAIN  
HEIGHT FALLS OF 150-250GPM...WHILE OVER NORTHERN ARGENTINA  
EXPECTING HEIGHT FALLS OF 50-100GPM. ON THURSDAY-FRIDAY THE TROUGH  
BECOMES THE DOMINANT FEATURE ACROSS MID SECTIONS OF SOUTH AMERICA.  
AT LOW LEVELS THE ASSOCIATED FRONT SURGES ACROSS SOUTHERN CHILE TO  
PATAGONIA IN ARGENTINA ON THURSDAY MORNING...AND THROUGH FRIDAY  
EVENING IT RACES ACROSS URUGUAY TO NORTHERN ARGENTINA. THIS IS TO  
THEN MERGE WITH/REINFORCE THE OLD BOUNDARY TO THE NORTH. AS IT  
ENTERS SOUTHERN CHILE THIS WILL TRIGGER SCATTERED CONVECTION TO  
THE SOUTH OF ISLA DE CHILOE...WITH MAXIMA OF 10-15MM. ON THURSDAY  
THIS BUILDS NORTH ACROSS TEMUCO TO CONCEPCION WHERE IT IS TO FAVOR  
SIMILAR RAINFALL AMOUNTS. AS THE FRONTS COMBINE TO THE NORTH THESE  
ARE TO TRIGGER HEAVY CONVECTION OVER SOUTHERN BRASIL-MISIONES IN  
ARGENTINA TO SOUTHEAST PARAGUAY...TO TRIGGER MAXIMA OF 35-70MM ON  
THURSDAY TO FRIDAY. ON FRIDAY TO SATURDAY THE ENVIRONMENTAL  
CONDITIONS BECOME FAVORABLE FOR AN MCS TO DEVELOP...WITH MAXIMA  
INCREASING TO 50-100MM IN POTENTIALLY SEVERE CONVECTION.  
 
FARTHER EAST...OVER THE WESTERN SOUTH ATLANTIC...A BROAD MID/UPPER  
LEVEL TROUGH EXTENDS OFF THE COAST OF ARGENTINA-BRASIL...WITH AXIS  
NORTH ALONG 40W TO 20S. THE TROUGH IS TO SLOWLY MOVE EAST ACROSS  
25W/30W LATER ON TUESDAY...AND ON WEDNESDAY IT IS TO CROSS 15W. AT  
LOW LEVELS THIS SUSTAINS AN ELONGATED FRONT THAT EXTENDS TO THE  
NORTHWEST ACROSS THE SOUTH ATLANTIC TO SOUTHEAST BRASIL...WITH  
BOUNDARY THEN MEANDERING TO THE SOUTHWEST TO MID SECTIONS OF SOUTH  
AMERICA. THROUGH WEDNESDAY EVENING BOUNDARY OVER THE CONTINENT IS  
FORECAST TO DISSIPATE...WHILE REMNANTS OVER THE ATLANTIC REMAIN.  
MOST ACTIVE CONVECTION WITH THIS FEATURE IS EXPECTED TO THE NORTH  
ACROSS BAHIA-TOCANTINS/GOIAS IN BRASIL...WITH MAXIMA OF 50-100MM  
THROUGH TUESDAY MORNING AND 40-80MM ON TUESDAY-WEDNESDAY. THIS  
THEN DECREASES TO 15-20MM LATER IN THE WEEK.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...ILL ORGANIZED CELL OF THE  
SUBTROPICAL RIDGE DOMINATES FLOW TO THE NORTH OF 20S. LATER ON  
TUESDAY THIS BECOMES SOMEWHAT BETTER ORGANIZED WHILE CENTERING ON  
A CLOSED HIGH OVER NORTHERN BOLIVIA/ACRE IN BRASIL...WHERE IT IS  
TO THEN HOLD THROUGH THE END OF THE WEEK. AS THE HIGH/RIDGE  
BECOMES BETTER ORGANIZED...A SHORT WAVE TROUGH IS TO AMPLIFY  
ACROSS THE NORTHEAST STATES OF BRASIL THROUGH WEDNESDAY MORNING.  
THE RIDGE ALOFT IS VENTING CONVECTION ACROSS NORTHWEST SOUTH  
AMERICA...WHERE THE MAXIMA IS TO INITIALLY PEAK AT 25-50MM. ON  
TUESDAY-WEDNESDAY THIS DECREASE TO 20-35MM/DAY. ON THE PERUVIAN  
JUNGLE-EASTERN ECUADOR EXPECTING ACCUMULATION OF 15-20MM/DAY WHILE  
ON THE SIERRA TO THE SOUTH THE MAXIMA PEAKS AT 10-15MM. ON THE  
ALTIPLANO OF BOLIVIA INITIALLY EXPECTING RAINFALL AMOUNTS OF  
05-10MM. PEAK IN ACTIVITY IS EXPECTED ON WEDNESDAY...WHEN GDI  
SHOWS POTENTIAL FOR MODERATE RAINFALL AMOUNTS. DURING THAT PERIOD  
THE MAXIMA WILL PEAK AT 15-30MM. THROUGH FRIDAY IT DECREASES TO  
15-20MM.  
 
OVIEDO...SHN (ARGENTINA)  
VANNUCCI...SMN (ARGENTINA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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