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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1759Z Sep 05, 2018)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EDT WED SEP 05 2018

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

MODEL DISCUSSION (VALID FROM SEP 04 AT 0000 UTC): THE GLOBAL
MODELS ARE IN EXCELLENT AGREEMENT THROUGH 132-144 HRS. CONFIDENCE
IN THE FORECAST IS HIGH.

A SLOW TO EVOLVE SHORT WAVE TROUGH ON THE NORTHERN STREAM FLOW
MEANDERS OFF THE SOUTHEAST COAST OF BRASIL...WITH AXIS AT 500 HPA
EXTENDING TO THE NORTH-NORTHWEST FROM 30S 40W TO SOUTHERN MINAS
GERAIS IN BRASIL. ON WEDNESDAY THIS IS TO MEANDER EAST ACROSS
35W...NEARING 30W LATER ON THURSDAY. AT LOW LEVELS THIS SUSTAINS A
SURFACE FRONT WITH AXIS NORTHWEST ACROSS RIO DE JANEIRO-SOUTHERN
MINAS GERAIS TO MATO GROSSO/RONDONIA IN WESTERN BRASIL. ON
WEDNESDAY THE FRONT MEANDERS NORTH ACROSS RIO DE JANEIRO TO
ESPIRITO SANTO...WHILE TRAILING PORTION OVER THE CONTINENT
FRONTOLIZES. AS IT MEANDERS NORTH...IT IS TO SUSTAIN A PREFRONTAL
SHEAR LINE ACROSS EASTERN BAHIA. THIS SHEAR LINE WILL ENHANCE
COASTAL CONVERGENCE STARTING LATER ON WEDNESDAY THROUGH FRIDAY. ON
WEDNESDAY THIS WILL FAVOR RAINFALL AMOUNTS OF 15-20MM...INCREASING
TO 15-30MM ON THURSDAY...WHILE ON FRIDAY THE MAXIMA IN THIS AREA
WILL PEAK AROUND 20-40MM.

ANOTHER SHORT WAVE TROUGH ON THE NORTHERN STREAM FLOW IS TO MOVE
ACROSS THE EASTERN PACIFIC TO SOUTHERN REGIONS OF CHILE EARLY ON
WEDNESDAY. THIS IS TO THEN PUSH EAST ACROSS THE SOUTHERN ANDES TO
PATAGONIA IN ARGENTINA...WITH DEEP LOW CLEARING THE COAST EARLY ON
THURSDAY. ON FRIDAY TO SATURDAY THE LOW/TROUGH IS TO MEANDER
ACROSS 40W. AT LOW LEVELS THIS SUSTAINS A DEEP OCCLUDED LOW OFF
THE SOUTH COAST OF CHILE LATER THIS EVENING...TO MOVE EAST ACROSS
PATAGONIA TO THE WESTERN ATLANTIC ON WEDNESDAY. THIS IS THEN
FORECAST TO OCCLUDE WHILE DEEPENING OFF THE COAST OF ARGENTINA.
THE DEEPENING LOW WILL SUSTAIN THE ADVECTION OF COLD MOIST AIR
ACROSS SOUTHERN PATAGONIA. SURFACE FRONT WILL THEN SURGE ACROSS LA
PAMPA TO CENTRAL ARGENTINA LATER ON THURSDAY...TO STALL OVER THE
RIO DE LA PLATA BASIN ON FRIDAY. AS IT ENTERS SOUTHERN CHILE ON
WEDNESDAY THIS IS TO FAVOR MODERATE CONVECTION BETWEEN CONCEPCION
AND ISLA DE CHILOE...WITH MAXIMA OF 20-35MM. ACROSS SOUTHERN
PATAGONIA THIS WILL FAVOR RAINFALL AMOUNTS OF 20-30MM...WITH
POTENTIAL FOR HEAVY SNOWFALL AMOUNTS ACROSS THE SOUTHERN
ANDES/WESTERN PORTIONS OF SOUTHERN PATAGONIA.

FARTHER UPSTREAM...A HIGH AMPLITUDE TROUGH IS TO DEEPEN BETWEEN
130W-85W TO 20S LATER ON SATURDAY. THIS...IN-TURN...WILL INDUCE
THE SOUTHWARD AMPLIFICATION OF A RIDGE ACROSS CHILE/ARGENTINA. THE
HIGH AMPLITUDE/LONG WAVE RIDGE IS TO THEN HOLD THROUGHOUT THE
WEEKEND. THIS IS A HIGH CONFIDENCE FORECAST THAT HAS THE SUPPORT
OF THE GLOBAL ENSEMBLE MEMBERS.

NORTH OVER THE CONTINENT...AT 200 HPA...LOW AMPLITUDE RIDGE
CONFINES TO NORTHERN PARA-AMAZONAS IN BRASIL-PARAGUAY. SOUTH OF
THIS RIDGE...A LONG WAVE TROUGH IS STREAMING ACROSS THE SOUTHEAST
STATES OF BRASIL. AS THE TROUGH PULLS AWAY....THE RIDGE IS TO
BUILD SOUTH ACROSS PERU TO BOLIVIA...WITH AXIS TO GRADUALLY BECOME
WELL ENTRENCHED ACROSS NORTHERN SOUTH AMERICA DURING THE WEEKEND.
EARLY IN THE CYCLE LIGHT DIURNAL CONVECTION WILL CONFINE TO
EQUATORIAL SOUTH AMERICA. LATER ON THURSDAY...AS DEEPER MOISTURE
POOLS ACROSS NORTHERN BRASIL...EXPECTING SCATTERED CONVECTION TO
BUILD ACROSS NORTHERN PARA-AMAZONAS/RORAIMA IN BRASIL...WITH DAILY
MAXIMA OF 15-20MM. THIS DECREASES TO 10-15MM LATER ON
FRIDAY-SATURDAY. FURTHERMORE...AS THE RIDGE BUILDS...IT WILL THEN
STEER A SHORT WAVE TROUGH ACROSS SOUTHERN PERU/NORTHERN CHILE TO
BOLIVIA DURING THE WEEKEND. THIS WILL LEAD TO A GRADUAL INCREASE
IN DIURNAL CONVECTION ON THE SOUTHERN SIERRA OF PERU AND THE
ALTIPLANO OF BOLIVIA.

SANCHEZ...SMN (ARGENTINA)
SALAZAR...SENAMHI (BOLIVIA)
DAVISON...WPC (USA)