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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1608Z Sep 16, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1208 PM EDT TUE SEP 16 2014

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 16 AT 00UTC): GOOD
CONTINUITY WITH PREVIOUS FORECAST PACKAGE THROUGH DAY 04...THEN
MODELS START TO DIVERGE ON HOW UPSTREAM PATTERN OVER THE EASTERN
PACIFIC IS GOING TO EVOLVE. BOTH THE GFS AND UKMET TREND TO A
DEEPER TROUGH TO REACH THE DOMAIN LATER IN THE CYCLE...WHILE THE
EUROPEAN MODEL IS FORECASTING A WEAKER/MORE PROGRESSIVE PATTERN.
ALTHOUGH THERE IS A LOT OF VARIABILITY...THE MEAN OF THE GLOBAL
ENSEMBLES SUPPORT THE DEEPER GFS/UKMET SOLUTION. DUE TO THE HIGH
VARIABILITY...CONFIDENCE IN THE FORECAST BEYOND 108/120 HRS IS LOW.

OVER THE SOUTHEAST PACIFIC...A DEEP LOW/TROUGH IS TO CENTER OVER
THE BELLINGSHAUSEN SEA. THIS...IN-TURN...IS TO SUSTAIN A TIGHT
BELT OF MID LATITUDE WESTERLIES. EMBEDDED IN THIS
FLOW...PROGRESSIVE SHORT WAVE TROUGHS ON THE SOUTHERN STREAM ARE
TO RACE ACROSS THE SOUTHERN CONE OF SOUTH AMERICA/DRAKE PASSAGE.
ONE IS TO PULL ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA TO THE
WESTERN ATLANTIC EARLY IN THE CYCLE. ANOTHER IS TO CLOSELY
FOLLOW...ENTERING SOUTHERN CHILE/TIERRA DEL FUEGO BY 36-42
HRS...AND ACROSS PATAGONIA TO THE WESTERN ATLANTIC BY 48-54 HRS.
AT LOW LEVELS...THIS SUSTAINS A BROAD OCCLUDED TROUGH ACROSS THE
BELLINGSHAUSEN SEA/DRAKE PASSAGE TO THE SOUTHERN CONE OF SOUTH
AMERICA. THE ENSUING PRESSURE GRADIENT WILL RESULT IN BOUNDARY
LAYER WINDS OF 25-40KT. THIS WILL COMBINE WITH PROGRESSIVE FRONTS
TO SUSTAIN SCATTERED RAINFALL ACROSS CHILE SOUTH OF ISLA DE
CHILOE. ON DAY 01 THE MAXIMA IS TO PEAK AT 05-10MM...WHILE ON DAY
02 THE MAXIMA PEAKS AT 15-30MM AS SURFACE FRONT IS TO ESTABLISH A
SUBTROPICAL CONNECTION.

A SHORT WAVE TROUGH ON THE NORTHERN NEARS CENTRAL/NORTHERN CHILE
EARLY IN THE CYCLE...WITH MID LEVEL VORTEX TO MEANDER JUST WEST OF
LA SERENA CHILE THROUGH 48 HRS. AS IT SPILLS ACROSS THE ANDES INTO
ARGENTINA...THIS IS TO TRIGGER HEIGHT FALLS OF 50-75GPM...WITH
FOCUS OF THE SHORT WAVE ENERGY ACROSS URUGUAY-MESOPOTAMIA
VALLEY/SOUTHERN BRASIL BY 72-84 HRS. AS IT SPILLS ACROSS THE
ANDES...THE TROUGH ALOFT IS TO INTERACT WITH A SURFACE FRONT OVER
SOUTHERN BRASIL/ PARAGUAY. BY 48-72 HRS THIS IS TO TRIGGER
CYCLOGENESIS OVER CORRIENTES IN ARGENTINA/RIO GRANDE DO SUL.
THROUGH 96 HRS THIS LOW IS FORECAST TO OCCLUDE AS IT MOVES OFF THE
COAST OF BRASIL INTO THE SOUTH ATLANTIC. THE DEEPENING LOW WILL
REINFORCE THE BAROCLINIC BOUNDARY AS IT MEANDERS NORTH INTO SAO
PAULO-SANTA CRUZ IN EASTERN BOLIVIA...WITH FRONT TO MOVE NORTH
ACROSS RIO DE JANEIRO/MATO GROSSO IN BRASIL TO NORTHERN BOLIVIA BY
96-120 HRS. UNDER FAVORABLE JET DYNAMICS THIS IS TO SUSTAIN
ORGANIZED HEAVY CONVECTION....WITH MODELS SHOWING RISK OF AN MCS
FORMING BETWEEN SOUTHERN BRASIL AND SOUTHEAST PARAGUAY BY 66-102
HRS...WITH MAXIMA NOW FORECAST TO PEAK AT 40-80MM. OVER
CENTRAL/NORTHWEST PARAGUAY TO SOUTHERN BOLIVIA EXPECTING
ACCUMULATION OF 10-15MM. AS THE FRONT MEANDERS NORTH ACROSS
SOUTHEAST BRASIL IT IS TO SUSTAIN SCATTERED CONVECTION WITH
ACCUMULATION OF 20-30MM...WHILE OVER BOLIVIA-SOUTHERN/CENTRAL
JUNGLE OF PERU AND ACRE IN BRASIL EXPECTING RAINFALL AMOUNTS OF
15-30MM.

DEEP MID LEVEL TROUGH IS TO PULL ACROSS THE SOUTH PACIFIC...WITH
MID LEVEL TROUGH MOVING ACROSS 110W/115W BY 72 HRS...AND ACROSS
80W BY 96 HRS. THE HIGH AMPLITUDE TROUGH IS TO ESTABLISH A
SUBTROPICAL CONNECTION AS IT PULLS ACROSS THE EASTERN PACIFIC...TO
SUSTAIN A POLAR FRONT ACROSS SOUTHERN CHILE TO PATAGONIA IN
ARGENTINA BY 108-120 HRS. STRONG BOUNDARY LAYER WINDS ARE TO
ACCOMPANY THIS BOUNDARY AS IT MAKES LANDFALL OVER THE SOUTHERN
CONE...AND IT IS TO TRIGGER SCATTERED CONVECTION WITH ACCUMULATION
OF 15-25MM.

NORTH OVER THE CONTINENT...AT 200 HPA...GLOBAL MODELS CONTINUE TO
FORECAST A CELL OF THE SUBTROPICAL RIDGE TO CONFINE TO NORTHWEST
SOUTH AMERICA/EASTERN TROPICAL PACIFIC. THE SUBEQUATORIAL
RIDGE...MEANWHILE...WILL LIMIT TO THE EQUATORIAL ATLANTIC/NORTH
COAST OF BRASIL. AS THE UPPER LEVEL RIDGE HOLDS TO THE WEST...THIS
WILL SUSTAIN AND INDUCED TROUGH ACROSS THE NORTHEAST STATES OF
BRASIL. THE RIDGE ALOFT IS TO VENT DEEP CONVECTION ALONG THE
SIERRA/JUNGLE OF PERU TO ECUADOR AND NORTHERN BOLIVIA. MOST ACTIVE
IS EXPECTED OVER THE NEXT COUPLE OF DAYS...WHEN MAXIMA IN THIS
AREA IS TO PEAK AT 20-35MM. THROUGH 72 HRS THIS DECREASES TO
15-25MM.

JALFIN...SMN (ARGENTINA)
CAROCA...DMC (CHILE)
DAVISON...WPC (USA)