Prognostic Meteorological Discussion
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1019 AM EDT FRI MAY 26 2017

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

LAST NO AMENDS...NEXT BULLETIN ON MAY 30.

MODEL DISCUSSION (VALID FROM MAY 26 AT 0000 UTC): THE GLOBAL
MODELS REMAIN IN GOOD AGREEMENT DURING THE FORECAST
PERIOD...FORESEEING A SIMILAR EVOLUTION ACROSS THE SOUTH AMERICAN
DOMAIN. THIS IS WELL SUPPORTED BY THE MEAN OF THE GLOBAL
ENSEMBLES...LEADING TO A HIGH CONFIDENCE FORECAST THROUGH DAY 06.

A DEEP/HIGH AMPLITUDE TROUGH IS TO AMPLIFY ACROSS THE WESTERN
ATLANTIC EARLY IN THE CYCLE. THIS IS TO THEN PULL ACROSS 20W/30W
LATER ON SATURDAY. A MEANDERING SHORT WAVE TROUGH OFF THE CENTRAL
COAST OF CHILE TRAILS THIS SYSTEM...WHILE ANCHORING ON A CLOSED
LOW NEAR THE ARCHIPELAGO DE JUAN FERNANDEZ. ON SATURDAY IT IS TO
SPILL ACROSS THE ANDES INTO THE CENTRAL/NORTHERN PROVINCES IN
ARGENTINA...AND EARLY ON SUNDAY IT IS TO STREAM ACROSS THE RIO DE
LA PLATA BASIN TO THE WESTERN ATLANTIC. MEANWHILE...AS IT MEANDERS
OFF THE COAST OF CHILE...THE MID LEVEL VORTEX IS TO SHEAR SHORT
WAVE PERTURBATIONS ACROSS THE CENTRAL/NORTHERN ANDES INTO NORTHERN
ARGENTINA/PARAGUAY-SOUTHERN STATES OF BRASIL/URUGUAY. THESE ARE TO
PROVIDE DYNAMICAL SUPPORT TO ORGANIZED DEEP CONVECTION OVER THE
NEXT TWO DAYS. AT 250 HPA...THE TROUGH ALOFT IS TO FOCUS A
SUBTROPICAL JET ACROSS THE NORTHERN ANDES OF CHILE/ARGENTINA INTO
MID SECTIONS OF SOUTH AMERICA. THE JET ALOFT WILL FAVOR GENERATION
OF LEESIDE PERTURBATIONS ACROSS THE NORTHERN PROVINCES IN
ARGENTINA WHILE ALSO FAVORING AN UPPER DIVERGENT PATTERN.

AT LOW LEVELS...DEEPENING POLAR TROUGH OVER THE WESTERN ATLANTIC
WILL SUSTAIN A POLAR FRONT ACROSS CENTRAL ARGENTINA EARLY IN THE
CYCLE...THAT MIGRATES NORTH ACROSS RIO DE LA PLATA BASIN/NORTHERN
ARGENTINA LATER TODAY. AS IT MEANDERS NORTH THIS WILL THEN MERGE
WITH REMNANTS OF AN OLD FRONT OVER MIDSECTIONS OF SOUTH AMERICA.
THE MEANDERING FRONT WILL REMAIN FOCUS OF LOW LEVEL MOISTURE
CONVERGENCE ACROSS SOUTHERN BRASIL-PARAGUAY/MISIONES IN ARGENTINA
AND SOUTHEAST PARAGUAY. IN THIS AREA EXPECTING ORGANIZED DEEP
CONVECTION OVER THE NEXT TWO DAYS TO TRIGGER RAINFALL AMOUNTS OF
35-70MM THROUGH SATURDAY MORNING...AND 30-60MM ON SATURDAY TO
SUNDAY...WITH LESSER AMOUNTS EXPECTED ON SUNDAY AS MID LEVEL
VORTEX SHEARS AWAY.

CONSISTENT WITH PREVIOUS FORECASTS...AN AMPLIFYING MID LEVEL
TROUGH WILL PULL ACROSS 95W LATER TODAY...NEARING THE COAST OF
CHILE ON SATURDAY EVENING. ON SUNDAY...UNDER FRICTIONAL EFFECT OF
THE ANDES...THE TROUGH IS TO THEN SPLIT IN TWO. ITS SOUTHERN HALF
WILL THEN PULL ACROSS PATAGONIA IN ARGENTINA TO THE WESTERN
ATLANTIC...WHILE THE NORTHERN HALF STALLS OFF THE CENTRAL COAST OF
CHILE. ON MONDAY...THE REMNANTS WILL THEN SPILL ACROSS THE CENTRAL
ANDES INTO CUYO IN ARGENTINA...AND LATER ON TUESDAY IT WILL MOVE
ACROSS ENTRE RIOS IN ARGENTINA TO URUGUAY/SOUTHERN BRASIL. THE
DEEP MID LEVEL VORTEX IS TO THEN FAVOR HEIGHT FALL OF 75-125GPM AS
IT MOVES ACROSS PARAGUAY/SOUTHERN BRASIL ON TUESDAY TO WEDNESDAY.
AT LOW LEVELS THIS WILL THEN INTERACT WITH MEANDERING FRONT OVER
NORTHERN ARGENTINA...TO TRIGGER A FRONTAL WAVE/LOW OVER
CORRIENTES/ENTRE RIOS. THIS WILL THEN DRIVE THE FRONT NORTH ACROSS
PARAGUAY TO MATO GROSSO DO SUL IN BRASIL/SANTA CRUZ IN EASTERN
BOLIVIA EARLY NEXT WEEK...WHILE ALSO TRIGGERING ORGANIZED DEEP
CONVECTION. NOTE THAT THE EXPECTED MESO-SYNOPTIC FORCING IS LIKELY
TO TRIGGER SEVERE CONVECTION IN THIS REGION.

POLAR TROUGH STREAMING ACROSS NORTHERN ARGENTINA EARLY IN THE
CYCLE WILL THEN PRESS AGAINST THE SUBTROPICAL RIDGE TO THE EAST
AND NORTH. AT 200 HPA THE RIDGE IS EXPECTED TO GRADUALLY FLATTEN
EARLY ON SATURDAY MORNING...WHEN IT IS TO THEN CONFINE TO
CONTINENTAL AREA TO THE NORTH OF 20S. AS IT MEANDERS OVER THE
CONTINENT THIS WILL VENT DEEP CONVECTION ON THE PERUVIAN
JUNGLE-WESTERN AMAZONAS/ACRE IN BRASIL AND EASTERN PORTIONS OF
ECUADOR. IN THIS AREA EXPECTING RAINFALL AMOUNTS OF 05-10MM/DAY
AND MAXIMA OF 15-20MM IN ISOLATED TO SCATTERED CONVECTION. OTHER
CONVECTION IS TO CLUSTER OVER NORTHEAST BRASIL...TO SUSTAIN
RAINFALL AMOUNTS OF 20-30MM/DAY AND MAXIMA OF 20-45MM. ACTIVITY IS
TO GRADUALLY SPREAD WEST ALONG THE NORTH COAST THROUGH SUNDAY
EVENING...TO SUSTAIN RAINFALL AMOUNTS OF 10-15MM IN ISOLATED TO
SCATTERED CONVECTION.

ABDEMUR...FAA (ARGENTINA)
ESPINOLA...DNM (PARAGUAY)
DAVISON...WPC (USA)
$$





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