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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1625Z Mar 25, 2020)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1225 PM EDT WED MAR 25 2020

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

FORECAST DISCUSSION FROM MARCH 25/17UTC: THE GFS AND EUROPEAN
MODELS CONTINUE TO DISAGREE ON HOW PATTERN EVOLVES DURING THE
MEDIUM RANGE PERIOD...WITH THE GFS FAVORING A MORE AGRESSIVE
SOLUTION OVER THE SOUTHWEST ATLANTIC THAN WHAT THE EUROPEAN MODELS
SUGGEST. IN THIS AREA THE GFS IS FORECASTING A DEEPER TROUGH TO
STREAM ACROSS THE MALVINAS/FALKLAND ISLANDS TO THE SOUTH GEORGIA
ISLANDS DURING THE LATTER PART OF THE FORECAST. ALTHOUGH THE MEAN
OF THE NCEP ENSEMBLES GIVE SOME SUPPORT TO THE GFS...THE
VARIABILITY IS TOO HIGH TO ESTABLISH CONFIDENCE IN THE FORECAST.
THUS...ONCE AGAIN...OUR CONFIDENCE IN THE FORECAST BEYOND 84 HRS
REMAINS LOW.

AN UPPER LEVEL CELL OF THE SUBTROPICAL RIDGE OVER PERU/BOLIVIA IS
STEERING A SHORT WAVE TROUGH ON THE NORTHERN STREAM EAST ACROSS
THE NORTHERN ANDES OF CHILE TO NORTHERN ARGENTINA. THE MID LEVEL
TROUGH IS TO MEANDER OVER NORTHERN ARGENTINA DURING THE NEXT THREE
DAYS...THEN NEARLY FILL AS IT PULLS ACROSS SRN BRASIL/URUGUAY
LATER ON SATURDAY. AT LOW LEVELS...THIS INTERACTED WITH A SURFACE
FRONT OVER CENTRAL ARGENTINA EARLY THIS MORNING... TRIGGERING A
HEAVY RAINFALL PRODUCING MESO SCALE CONVECTIVE SYSTEM. DURING THE
DAY TODAY THROUGH EARLY MORNING ON THURSDAY THIS IS TO TRIGGER A
SECOND ROUND OF CONVECTION...WITH FOCUS OF THE ACTIVITY SHIFTING
TO MENDOZA-CORDOBA IN ARGENTINA. IN THIS AREA EXPECTING MAXIMA OF
30-60MM EARLY IN THE CYCLE...WHILE ON WEDNESDAY-THURSDAY EXPECT
ADDITIONAL RAINFALL AMOUNTS OF 10-15MM. OTHER CONVECTION IS
EXPECTED TO THE NORTH OVER JUJUY/SALTA IN NORTHWEST
ARGENTINA...WITH MAXIMA IN THIS AREA TO PEAK AT 30-60MM EARLY IN
THE CYCLE.

ON THE SOUTHERN STREAM...A MID LEVEL TROUGH LIFTS ACROSS THE
EASTERN PACIFIC EARLY IN THE CYCLE...WITH AXIS TO THEN SLOW DOWN
ON THURSDAY AS IT CLASHES WITH A SKEWED RIDGE OVER
ARGENTINA/WESTERN ATLANTIC. AS THE TROUGH AND RIDGE COLLIDE...THE
TROUGH WILL START TO SHEAR UNDER THIS AXIS. ON FRIDAY THIS THEN
EVOLVES INTO A MID LEVEL SHORT WAVE TROUGH OVER SOUTHERN CHILE. ON
SATURDAY THIS WILL PULL ACROSS SOUTHEN CHILE TO THE WESTERN
ATLANTIC...WHERE THE EUROPEAN MODELS FORECAST THIS SYSTEM TO
QUICKLY WEAKEN. THE GFS...IN-CONTRAST...CONTINUES TO FORECAST A
DEEPER TROUGH TO STREAM ACROSS THE MALVINAS TO THE FALKLAND
ISLANDS. IT...HOWEVER...CORRECTED IN FAVOR OF A SLIGHTLY SLOWER
EVOLUTION. AT LOW LEVELS...AN ELONGATED FRONT IS TO EXTEND ACROSS
TIERRA DEL FUEGO TO SOUTHERN CHILE LATER TODAY...WITH AN OCCLUDED
LOW MOVING ACROSS SOUTHERN CHILE LATER ON FRIDAY/EARLY ON SATURDAY
MORNING. THIS LOW WILL PULL ACROSS PATAGONIA IN ARGENTINA TO THE
WRN ATLANTIC ON SATURDAY...DRIVING THE FRONT NORTH ACROSS
PATAGONIA TO LA PAMPA. AS IT ENTERS SOUTHERN CHILE THIS WILL FAVOR
STRONG BOUNDARY LAYER WINDS OF 30-40KT...WHILE ALSO TRIGGERING
MODERATE TO HEAVY RAINFALL AMOUNTS. OVER SOUTHERN CHILE TO SRN
PATAGONIA/TIERRA DEL FUEGO...INITIALLY EXPECTING MAXIMA OF
05-10MM. ON THURSDAY THIS INCREASES TO 15-20MM IN DEEPER
CONVECTION... WHILE ON FRIDAY EXPECTING ADDITIONAL RAINFALL
AMOUNTS OF 10-15MM. OVER LA PAMPA/BUENOS AIRES PROVINCE MOST
ACTIVE CONVECTION WITH THIS FEATURE IS EXPECTED ON SUNDAY...WHEN
MAXIMA INCREASES TO 25-50MM...WITH LOCALLY HIGHER AMOUNTS PROBABLE
DUE TO STRONG TOPOGRAPHICAL FORCING.

MEANDERING UPPER LEVEL CELL OF THE SUBTROPICAL RIDGE OVER
PERU/BOLIVIA...IS INDUCING THE NORTHWARD AMPLIFICATION OF A SHORT
WAVE TROUGH ACROSS NORTHEAST BRASIL. THIS REACHES MAXIMUM
AMPLITUDE EARLY IN THE CYCLE..THEN RAPIDLY WEAKENS. EARLY IN THE
CYCLE THIS IS TO SUSTAIN HEAVY CONVECTION ACROSS THE NORTHEAST
STATES OF BRASIL...WITH MAXIMA OF 35-70MM. THROUGH SATURDAY THIS
DECREASES TO 20-40MM WHILE BUILDING WESTWARD ACROSS PARA IN
BRASIL. OVER MATO GROSSO-RONDONIA TO THE WEST...ACTIVITY IS TO
CLUSTER ALONG A LOW LEVEL TROUGH OVER BRASIL...WITH MAXIMA OF
20-35MM/DAY. OVER CENTRAL BOLIVIA-SOUTHERN PERU...MEANWHILE...AN
MCS IS LIKELY FORM ON THURSDAY-FRIDAY...WITH CONVECTION CLUSTERING
OVER CENTRAL BOLIVIA. IN THIS AREA EXPECTING MAXIMA OF 40-80MM. ON
THURSDAY TO FRIDAY THIS DECREASES TO 25-50MM WHILE BUILDING NORTH
ACROSS CENTRAL PERU/ACRE IN WESTERN BRASI.

DAVISON...WPC (USA)
GALVEZ...WPC (USA)