058  
FXSA20 KWBC 011738  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1238 PM EST THU FEB 01 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM FEB 01 AT 0000 UTC): GLOBAL MODELS  
ARE IN GENERAL AGREEMENT THROUGH 72-96 HRS...DEVELOPING SHORT WAVE  
PATTERN DIFFERENCES LATER IN THE CYCLE. OVERALL CONFIDENCE IN THE  
FORECAST IS HIGH THROUGH THE MEDIUM RANGE PERIOD.  
 
BROAD MID/UPPER LEVEL TROUGH DOMINATES THE FLOW OVER THE SOUTHWEST  
ATLANTIC...WITH AXIS ANCHORING ON A CLOSED LOW TO THE  
SOUTH-SOUTHWEST OF THE SOUTH GEORGIA ISLANDS. THE BROAD MID/UPPER  
LEVEL TROUGH IS TO REMAIN NEARLY STATIONARY THROUGH THIS EVENING.  
STARTING ON FRIDAY...THE TROUGH IS TO SLOWLY MIGRATE EAST...MOVING  
ACROSS 10W LATER ON SUNDAY. AT LOW LEVELS THIS SUSTAINS A BROAD  
TROUGH THAT CENTERS ON A DEEP 962 HPA OCCLUDED LOW THAT MEANDERS  
BETWEEN THE SOUTH ORKNEY AND THE SOUTH GEORGIA ISLANDS. THIS IS TO  
ALSO PULL TO THE EAST-SOUTHEAST AS THE CYCLE PROGRESSES. THE  
ASSOCIATED FRONT TRAILS ACROSS THE BUENOS AIRES PROVINCE TO  
CENTRAL ARGENTINA....TO MEANDER NORTH ACROSS URUGUAY TO SOUTHERN  
BRASIL LATER ON FRIDAY. BOUNDARY IS TO THEN FRONTOLIZE WHILE  
MEANDERING OFF THE SOUTH COAST OF BRASIL ON SATURDAY.  
 
THIS IS FOLLOWED BY A BROAD MID/UPPER LEVEL TROUGH...THAT MOVES TO  
THE SOUTHEAST PACIFIC LATER TODAY. UNDER INFLUENCE OF THIS FEATURE  
A RIDGE BUILDS SOUTH ACROSS CHILE/ARGENTINA TO THE DRAKE  
PASSAGE-ANTARCTIC PENINSULA. THE TROUGH...AS IT PRESSES AGAINST  
THIS RIDGE...IS TO THEN SHEAR SHORT WAVE PERTURBATIONS THAT ARE TO  
THEN PROPAGATE ACROSS EXTREME SOUTHERN CHILE-TIERRA DEL FUEGO  
DURING THE NEXT FEW DAYS. THE STRONG RIDGE PATTERN IS TO PERSIST  
THROUGH SATURDAY EVENING. BUT LATER ON SUNDAY...UNDER PRESSURE  
FROM THE TROUGH...IT IS FORECAST TO WEAKEN/NEARLY COLLAPSE TO THE  
SOUTH OF 40S. AS IT WEAKENS...SHORT WAVE PERTURBATIONS ARE TO THEN  
STREAM ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA. AT LOW  
LEVELS THE TROUGH ALOFT WILL SUSTAIN A BROAD TROUGH OVER THE  
SOUTHEAST PACIFIC/BELLINGSHAUSEN SEA...WITH POLAR FRONTS REVOLVING  
AROUND THIS AXIS TO STREAM ACROSS SOUTHERN CHILE/PATAGONIA IN  
ARGENTINA. DURING THE NEXT THREE DAYS THIS IS TO FAVOR LIGHT  
CONVECTION OVER CHILE TO THE SOUTH OF ISLA DE CHILOE...WITH DAILY  
MAXIMA OF 05-10MM. ON SUNDAY...AS THE MID LEVEL RIDGE WEAKENS AND  
THE TROUGH ESTABLISHES A SUBTROPICAL CONNECTION...CONVECTION OVER  
SOUTHERN CHILE IS TO INTENSIFY. DURING THIS PERIOD THE MAXIMA  
INCREASES TO 15-20MM.  
 
CONSISTENT WITH PREVIOUS FORECASTS...OTHER SHORT WAVE  
PERTURBATIONS ARE TO STREAM ACROSS SOUTHERN CHILE TO PATAGONIA/LA  
PAMPA IN ARGENTINA. HOWEVER...IN THIS CYCLE THE MODELS ARE  
TRENDING IN SUPPORT OF WEAKER PERTURBATIONS THAN WHAT WAS  
PREVIOUSLY SUGGESTED. THE INFLOW OF SHORT WAVE ENERGY WILL SUSTAIN  
CYCLOGENESIS OVER LA PAMPA/NORTHERN PATAGONIA LATER DURING THE  
WEEKEND. ON FRIDAY-SUNDAY THIS IS TO ALSO TRIGGER SCATTERED  
CONVECTION ACROSS LA PAMPA-NORTHERN PATAGONIA...WITH ACCUMULATION  
OF 05-10MM/DAY.  
 
FARTHER NORTH...A MEANDERING SHORT WAVE TROUGH LIES OFF THE  
SOUTH/SOUTHEAST COAST OF BRASIL. A SHORT WAVE TROUGH LIFTING ALONG  
THE COAST OF ARGENTINA IS TO INTERACT WITH THE VORTEX TO THE  
NORTH. THE INFLOW OF SHORT WAVE ENERGY WILL THEN SUSTAIN A MID  
LEVEL LOW OFF THE COAST OF BRASIL...TO MEANDER NEAR 32S 37W LATER  
ON SATURDAY. NOTE THAT BOTH EUROPEAN MODELS CONTINUE TO FAVOR A  
SLIGHTLY DEEPER/LONGER LASTING CIRCULATION THAN WHAT THE GFS  
SUGGESTS. AT LOW LEVELS...THE MID LEVEL TROUGH CONTINUES TO  
INTERACTING WITH A MEANDERING FRONT OVER THE SOUTH  
ATLANTIC-SOUTHEAST STATES OF BRASIL. THIS IS DRAWING THE FRONT  
ACROSS SAO PAULO-SANTA CATARINA TO MISIONES IN ARGENTINA/SOUTHEAST  
PARAGUAY...WHERE IT IS TO REMAIN WHILE GRADUALLY WEAKENING ON  
SUNDAY-MONDAY. THE SOUTH AMERICA CONVERGENCE ZONE (SACZ) LIES TO  
THE NORTH...EXTENDING OVER RIO DE JANEIRO-SOUTHERN MINAS GERAIS TO  
MATO GROSSO DO SUL IN BRASIL/SANTA CRUZ IN EASTERN BOLIVIA. OVER  
THE NEXT COUPLE OF DAYS IT MEANDERS NORTH ACROSS ESPIRITO  
SANTO-MINAS GERAIS WHILE TRAILING TO MATO GROSSO IN BRASIL/  
NORTHERN BOLIVIA. FURTHERMORE...THE CLIMATE MODELS SHOW DIVERGENCE  
PHASE OF THE MADDEN-JULIAN OSCILLATION (MJO) ENVELOPING NORTHERN  
SOUTH AMERICA THROUGH THE MIDDLE OF NEXT WEEK. THIS IS TO ENHANCE  
CONVECTIVE DEVELOPMENT ALONG THE SACZ OVER BRASIL. EARLY IN THE  
CYCLE MOST ACTIVE CONVECTION IS EXPECTED ACROSS CHACO  
PARAGUAYO-SOUTHEAST-NORTHERN BOLIVIA-MATO GROSSO DO SUL/MATO  
GROSSO-RONDONIA TO WESTERN MINAS GERAIS...WITH MAXIMA OF 40-80MM.  
ON FRIDAY TO SATURDAY THE MAXIMA IN THIS AREA INCREASES TO  
40-80MM/DAY...WHILE BUILDING NORTH ACROSS PARA. HIGHER AMOUNTS ARE  
EXPECTED OVER BAHIA-MINAS GERAIS-ESPIRITO SANTO ON FRIDAY...WITH  
MAXIMA OF 75-150MM. THIS BUILDS WEST AND NORTH ACROSS  
TOCANTINS-GOIAS...WITH MAXIMA OF 75-150MM/DAY TO PERSIST THROUGH  
THE WEEKEND/EARLY NEXT WEEK. NOTE THAT SOME OF THE MODELS CONTINUE  
TO SHOW LOCALIZED MAXIMA OF 200-250MM/DAY IN THIS AREA. THIS IS  
HIGHLY PROBABLE DUE TO STRONG TOPOGRAPHICAL FORCING. CONVECTION IS  
TO SIMULTANEOUSLY BUILD WEST AND NORTH ACROSS ACRE-SOUTHERN  
AMAZONAS IN BRASIL. INITIALLY EXPECTING MAXIMA OF  
20-30MM...INCREASING TO 20-45MM ON SUNDAY-MONDAY.  
 
AT 200 HPA...BROAD CELL OF THE SUBTROPICAL RIDGE DOMINATES  
CONTINENTAL AREA TO THE NORTH OF 40S...WHILE ANCHORING ON A CLOSED  
HIGH OVER MATO GROSSO DO SUL IN BRASIL-CHACO PARAGUAYO. THROUGH  
FRIDAY AFTERNOON THE HIGH WILL DRIFT WEST ACROSS SOUTHERN  
BOLIVIA/NORTHWEST ARGENTINA...TO SETTLE OVER NORTHERN CHILE. AS  
THE RIDGE PULLS AWAY THE AFOREMENTIONED TROUGH OVER SOUTHEAST  
BRASIL IS TO DEEPEN/AMPLIFY ACROSS NORTHEAST SOUTH  
AMERICA...INTERACTING WITH THE SACZ IN SUPPORT OF ORGANIZED DEEP  
CONVECTION. THE RIDGE ALOFT...MEANWHILE...IS TO VENT ORGANIZED  
DEEP CONVECTION ACROSS SOUTHEASTERN BOLIVIA-CHACO PARAGUAYO AND  
NORTHWEST ARGENTINA...TO SUSTAIN MAXIMA OF 40-80MM/DAY DURING THE  
NEXT 24-36 HRS. BUT AS THE UPPER RIDGE PATTERN EVOLVES...ACTIVITY  
IS TO WEAKEN. OTHER CONVECTION IS TO BUILD ACROSS THE ALTIPLANO OF  
BOLIVIA TO SOUTHERN PERU...WITH MAXIMA TO PEAK AT 20-30MM OVER THE  
NEXT 48 HRS. ON SATURDAY TO SUNDAY THE MAXIMA IS TO THEN PEAK AT  
15-25MM/DAY. ON THE SOUTHERN/CENTRAL SIERRA OF PERU EXPECTING  
ACCUMULATION OF 05-10MM/DAY. OVER ECUADOR...UNDER THE INFLUENCE OF  
THE PANAMANIAN TROUGH...A MOIST NORTHWESTERLY FLOW IS TO PERSIST  
DURING THE NEXT FIVE DAYS TO GENERALLY FAVOR RAINFALL AMOUNTS OF  
05-10MM/DAY. LOCALLY HIGHER AMOUNTS ARE HIGHLY PROBABLE...WITH  
MAXIMA OF 15-20MM EXPECTED ON FRIDAY TO SATURDAY.  
 
BALBINO...INMET (BRASIL)  
POMIER...SENAMHI (BOLIVIA)  
DAVISON...WPC (USA)  

 
 
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