418  
FXSA20 KWBC 011723  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1222 PM EST FRI DEC 01 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 01 AT 0000 UTC): AT MID/UPPER  
LEVELS...SHORT WAVE PERTURBATIONS CONTINUE TO POPULATE THE SOUTH  
AMERICAN DOMAIN. THIS IS RESULTING IN CYCLE-TO-CYCLE  
INCONSISTENCIES AMONG THE GLOBAL MODEL GUIDANCE AS THEY MAKE  
TIMING/AMPLITUDE CORRECTIONS. OVERALL CONFIDENCE IN THE FORECAST  
IS HIGH THROUGH MONDAY MORNING.  
 
IN A SPLIT FLOW PATTERN...THE FLOW OVER THE WESTERN ATLANTIC  
SPLITS BETWEEN A RIDGE ON THE SOUTHERN STREAM AND A SHORT WAVE  
TROUGH TO THE NORTH. UNDER PRESSURE FROM SUCCESSIVE SHORT WAVE  
PERTURBATIONS THE RIDGE IS TO FLATTEN/NEARLY COLLAPSE LATER  
TODAY...WITH A HIGHER AMPLITUDE TROUGH TO TAKE ITS PLACE EARLY ON  
SUNDAY MORNING. THE SHORT WAVE TROUGH TO THE  
NORTH...MEANWHILE...IS TO MEANDER OFF THE SOUTHEAST COAST OF  
BRASIL WHILE CENTERING ON A CLOSED LOW NEAR 35S 39W. AS THE RIDGE  
TO THE SOUTH WEAKENS...AND THE HIGHER AMPLITUDE TROUGH ESTABLISHES  
OVER THE SOUTHWEST ATLANTIC...THE NORTHERN STREAM SHORT WAVE  
TROUGH IS TO GRADUALLY ACCELERATE TO THE EAST-SOUTHEAST WHILE  
SLOWLY MERGING INTO THE LONG WAVE PATTERN. AT LOW LEVELS THIS  
SUSTAINS AN ELONGATED FRONT OVER THE SOUTH ATLANTIC THAT EXTENDS  
TO THE WEST-NORTHWEST TO RIO DE JANEIRO/SOUTHERN MINAS GERAIS IN  
BRASIL...AND ACROSS MATO GROSSO DO SUL TO PARAGUAY. THE BOUNDARY  
IS TO REMAIN NEARLY STATIONARY THROUGH SUNDAY EVENING...THEN  
DISSIPATE ON MONDAY. MOST ACTIVE CONVECTION IS EXPECTED ALONG AND  
NORTH OF THIS BOUNDARY. OVER MATO GROSSO DO SUL-MINAS GERAIS/RIO  
DE JANEIRO-ESPIRITO SANTO INITIALLY EXPECTING RAINFALL MAXIMA OF  
40-80MM. ON SATURDAY-SUNDAY THIS DECREASES TO 30-60MM/DAY. ON  
MONDAY...AS THE FRONT WEAKENS...THE MAXIMA IS TO PEAK AT 20-40MM.  
 
FARTHER WEST...AT 500 HPA...A MEANDERING SHORT WAVE TROUGH ON THE  
NORTHERN STREAM FLOW LIES OFF THE CENTRAL/NORTHERN COAST OF  
CHILE...WITH MID LEVEL AXIS TO EXTEND NORTH ALONG 75W/80W. THIS IS  
TO SHEAR SHORT WAVE ENERGY EASTWARD ACROSS THE CENTRAL ANDES INTO  
NORTHERN ARGENTINA. INFLOW OF SHORT WAVE ENERGY WILL ENHANCE  
DIURNAL CONVECTION ACROSS THE NORTHWEST PROVINCES IN ARGENTINA AND  
ALONG THE LEE OF THE ANDES. IN THIS AREA THE DAILY MAXIMA WILL  
GRADUALLY INCREASE TO 20-30MM/DAY ON SATURDAY.  
 
AS PREVIOUSLY INDICATED...A MID/UPPER LEVEL TROUGH ON THE SOUTHERN  
STREAM FLOW IS TO PULL ACROSS THE DRAKE PASSAGE TO THE SOUTH  
ATLANTIC LATER THIS AFTERNOON. BUILDING RIDGE TO THE WEST IS TO  
THEN INDUCE THE NORTHWARD AMPLIFICATION OF THIS TROUGH BETWEEN  
55W-25W TO 30S LATER ON SUNDAY. AT LOW LEVELS A SURFACE FRONT  
MEANDERS ACROSS LA PAMPA TO CENTRAL CUYO IN ARGENTINA...WHILE  
ANOTHER IS TO EXTENT ACROSS TIERRA DEL FUEGO TO SOUTHERN PATAGONIA  
LATER TODAY. THROUGH SUNDAY AFTERNOON THESE ARE TO COMBINE AS A  
BROAD RIDGE BUILDS OVER THE SOUTHERN CONE OF SOUTH AMERICA. AS  
THEY MERGE...AND THE RIDGE BUILDS TO THE WEST...THE SURFACE FRONT  
WILL THEN MOVE NORTH ACROSS RIO DE LA PLATA BASIN-CORDOBA IN  
ARGENTINA LATER ON SATURDAY/EARLY SUNDAY MORNING. ON MONDAY IT  
WILL MOVE FARTHER NORTH ACROSS THE SOUTHERN STATES OF  
BRASIL-PARAGUAY. EARLY IN THE CYCLE THIS IS TO FAVOR SCATTERED  
CONVECTION ACROSS CENTRAL ARGENTINA...WITH MAXIMA OF 20-45MM/DAY  
EXPECTED TODAY AND SATURDAY. ON SUNDAY...FOCUS OF THE CONVECTION  
SHIFTS TO ENTRE RIOS-CORRIENTES IN ARGENTINA-URUGUAY...WHERE THE  
MAXIMA IS TO PEAK AT 15-20MM. AS THE FRONT SURGES ACROSS SOUTHERN  
BRASIL ON SUNDAY MORNING IT WILL FAVOR SCATTERED CONVECTION WITH  
MAXIMA OF 15-20MM. OVER PARAGUAY-MATO GROSSO DO SUL IN BRASIL THE  
MAXIMA IS TO PEAK AT 25-50MM. ACROSS BOLIVIA THIS IS TO FAVOR AN  
MCS ON MONDAY-TUESDAY...WITH MAXIMA OF 35-70MM.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...BROAD CELL OF THE  
SUBTROPICAL RIDGE REMAINS THE DOMINANT FEATURE TO THE NORTH OF  
30S. THE RIDGE IS TO CENTER ON A CLOSED HIGH THAT MEANDERS BETWEEN  
RONDONIA IN BRASIL AND NORTHERN BOLIVIA. THIS IS TO PERSIST DURING  
THE NEXT FEW DAYS. AS THE RIDGE HOLDS...IT IS TO VENT DIURNAL  
CONVECTION ACROSS BRASIL-PERU-ECUADOR/SOUTHERN COLOMBIA. THE MOST  
ACTIVE IS TO CLUSTER ALONG AND NORTH OF THE AFOREMENTIONED SURFACE  
FRONT OVER BRASIL-PARAGUAY/BOLIVIA. OTHER CONVECTION IS EXPECTED  
ACROSS PARA IN BRASIL TO FAVOR MAXIMA OF 20-30MM/DAY IN SCATTERED  
DEEP CONVECTION. OVER ACRE-AMAZONAS IN WESTERN BRASIL TO THE  
NORTHERN JUNGLE OF PERU/EASTERN ECUADOR EXPECTING MAXIMA OF  
10-15MM/DAY. THROUGH SATURDAY THIS IS TO INTENSIFY...WITH MAXIMA  
OVER AMAZONAS-SOUTHERN COLOMBIA-NORTHERN JUNGLE OF PERU TO PEAK AT  
20-40MM. ON THE HIGH JUNGLE OF PERU...MEANWHILE...THE DAILY MAXIMA  
IS TO PEAK AT 20-30MM ON SATURDAY.  
 
OVIEDO...SHN (ARGENTINA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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