Prognostic Meteorological Discussion
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FXSA20 KWBC 271625
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1224 PM EDT TUE SEP 27 2016

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 27 AT 0000 UTC): MINOR
CORRECTIONS TO PREVIOUS FORECAST PACKAGE...WITH GLOBAL MODELS
GENERALLY FOLLOWING A SIMILAR EVOLUTION ACROSS THE SOUTH AMERICAN
DOMAIN THROUGH DAY 05. THIS FORECAST IS WELL SUPPORTED BY THE MEAN
OF THE GLOBAL ENSEMBLES.

A DEEPENING MID LEVEL SHORT WAVE TROUGH CROSSED THE DRAKE PASSAGE/
SOUTHERN CONE...WITH AXIS TO EXTEND FROM THE WEDDELL SEA TO
EASTERN PATAGONIA LATER TODAY. A BUILDING RIDGE TO THE WEST IS TO
THEN SUSTAIN RAPID AMPLIFICATION OF THIS FEATURE THROUGH THURSDAY
MORNING...WITH AXIS TO BOUND BETWEEN 50W-20W WHILE REMAINING TO
THE SOUTH OF 20S. IT IS TO THEN ANCHOR ON A CLOSED LOW NEAR 45S
30W LATER IN THE DAY. AS THE TROUGH DEEPENS...AND RIDGE BUILDS TO
THE WEST...THE ASSOCIATED FRONT WILL SURGE NORTH ACROSS ARGENTINA.
THROUGH THIS EVENING IT WILL MOVE TO THE BUENOS AIRES
PROVINCE-SOUTHERN CUYO...AND ON WEDNESDAY AFTERNOON IT REACHES
SOUTHERN BRASIL-URUGUAY-CENTRAL PROVINCES IN ARGENTINA. BROAD
SURFACE RIDGE TO THE SOUTH IS TO THEN ANCHOR ON AN UNUSUALLY
STRONG 1031-1033 HPA HIGH OFF THE COAST OF PATAGONIA. THIS WILL
THEN DRIVE THE SURFACE FRONT FARTHER NORTH ACROSS SAO PAULO/RIO DE
JANEIRO-PARAGUAY LATER ON FRIDAY. AS THE FRONT SURGES ACROSS
SOUTHERN TO SOUTHEAST BRASIL...AND UNDER INFLUENCE OF A SOUTHERLY
JET OVER THE WESTERN ATLANTIC...EXPECTING SCATTERED COASTAL
CONVECTION BETWEEN NORTHERN SAO PAULO AND RIO DE JANEIRO TO RESULT
IN RAINFALL AMOUNTS OF 05-10MM. HIGHER AMOUNTS ARE EXPECTED ON
THURSDAY TO FRIDAY AS COUPLED JETS ALOFT VENT ORGANIZED CONVECTION
ALONG THE TRAILING END OF THE FRONT. ACTIVITY WILL THEN CLUSTER
BETWEEN NORTHERN ARGENTINA AND PARAGUAY WITH MAXIMA OF 15-20MM.
THIS IS TO INTENSIFY THROUGH SATURDAY WHEN MODELS SUGGEST
FAVORABLE ENVIRONMENTAL CONDITIONS FOR THE GENERATION MESO SCALE
CONVECTIVE SYSTEM.

AS THE AFOREMENTIONED MID LEVEL TROUGH DEEPENS OVER THE WESTERN
ATLANTIC IT IS TO INTERACT WITH A BROAD TROUGH TO THE NORTH. THE
TROUGH IS TO EXTEND BETWEEN 50W-10W AND TO THE SOUTH OF 20S
THROUGH THIS EVENING. IT IS TO THEN RAPIDLY WEAKEN WHILE SHEARING
EAST ACROSS 15W ON WEDNESDAY. AT LOW LEVELS THIS SUSTAINS A BROAD
TROUGH OVER THE WESTERN SOUTH ATLANTIC...AND A MEANDERING FRONT
ACROSS ESPIRITO SANTO/SOUTHERN MINAS GERAIS. THROUGH WEDNESDAY
EVENING/THURSDAY MORNING THE FRONT IS THEN FORECAST TO FRONTOLIZE.
MEANWHILE...THIS WILL ENHANCE MOISTURE CONVERGENCE BETWEEN
SOUTHERN PARA-BAHIA IN BRASIL...TO FAVOR MODERATE CONVECTION WITH
ACCUMULATION OF 15-30MM ON TUESDAY TO WEDNESDAY...DECREASING TO
05-10MM LATER ON WEDNESDAY.

A SHORT WAVE TROUGH ON THE NORTHERN STREAM FOLLOWS...WITH THE MID
LEVEL PERTURBATION EXPECTED TO MOVE ACROSS 90W LATER ON TUESDAY.
AS IT NEARS CENTRAL CHILE THIS WILL SUSTAIN AN INDUCED RIDGE TO
THE EAST...THAT IS TO EXTEND OVER ARGENTINA TO THE DRAKE PASSAGE.
AS IT PRESSES AGAINST THE RIDGE TO THE EAST THE TROUGH IS FORECAST
TO WEAKEN AS SHORT WAVE ENERGY SHEARS SOUTH. THE RIDGE IS TO THEN
STEER THE REMNANTS OF THE WANING SHORT WAVE TROUGH ACROSS
SOUTHERN/CENTRAL CHILE TO PATAGONIA IN ARGENTINA ON THURSDAY TO
FRIDAY. AT LOW LEVELS THIS WILL SUSTAIN A BROAD TROUGH OFF THE
CENTRAL COAST OF CHILE LATER ON WEDNESDAY...WITH AXIS TO MOVE
INLAND EARLY ON THURSDAY. ACROSS CENTRAL CHILE THIS WILL FAVOR
LIGHT RAINFALL AMOUNTS WITH ACCUMULATION OF 05-10MM...WHILE OVER
SOUTHERN CHILE EXPECTING LIGHT TO MODERATE AMOUNTS WITH
ACCUMULATION OF 10-15MM. ACROSS CENTRAL ARGENTINA MAXIMA WILL
INCREASE TO 15-20MM AS ENHANCED BY A POLAR JET MAXIMA STREAMING
ACROSS CENTRAL CHILE. MODERATE CONVECTION WILL THEN PERSIST
THROUGH SATURDAY...FOCUSING ACROSS PATAGONIA WHERE THE DAILY
MAXIMA IS TO PEAK AT 10-15MM.

NORTH OVER THE CONTINENT...AT 200 HPA...AN ILL ORGANIZED CELL OF
THE SUBTROPICAL RIDGE WILL DOMINATE AREA TO THE NORTH OF 10S
DURING THE NEXT FIVE DAYS. A TROUGH TO THE SOUTH...MEANWHILE...IS
TO INITIALLY EXTEND ACROSS SOUTHEAST BRASIL TO MATO GROSSO.
CONSISTENT WITH PREVIOUS FORECAST...THIS IS EXPECTED TO MEANDER
EAST ACROSS MINAS GERAIS/RIO DE JANEIRO THROUGH THURSDAY
AFTERNOON. AS IT MEANDERS OVER THE CONTINENT...THE RIDGE ALOFT
WILL VENT CONVECTION ACROSS NORTHWEST SOUTH AMERICA...WITH MAXIMA
OF 15-20MM EXPECTED ACROSS RONDONIA-ACRE-AMAZONAS IN BRASIL.
LOCALLY HIGHER AMOUNTS ARE TO INITIALLY CLUSTER ON THE CENTRAL
JUNGLE OF PERU...WITH MAXIMA IN THIS AREA TO PEAK AT 20-35MM. BUT
THROUGH WEDNESDAY IT IS TO ALSO DECREASE TO 15-20MM. OVER ECUADOR
MOST ACTIVE IS EXPECTED ON THE JUNGLE...WITH DAILY RAINFALL
AMOUNTS OF 10-15MM. MODERATE RAINFALL AMOUNTS ARE EXPECTED BETWEEN
PARA-MINAS GERAIS IN BRASIL...WITH MAXIMA IN THIS AREA TO PEAK AT
15-30MM AS ENHANCED BY UPPER TROUGH PATTERN. BUT AS THE TROUGH
WEAKENS THIS WILL DECREASE TO 10-15MM LATER IN THE CYCLE.

OSORIO...DMC (CHILE)
ALVA...SENAMHI (PERU)
DAVISON...WPC (USA)
$$





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