947
FXSA20 KWBC 161615
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1214 PM EDT TUE SEP 16 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 16). THE GLOBAL MODELS
REMAIN IN GOOD AGREEMENT ON THEIR FORECAST OF A MID/UPPER LEVEL
TROUGH TO SLOWLY MOVE ACROSS CENTRAL CHILE/ARGENTINA STARTING ON
DAY 02 AND PERSISTING ONTO DAY 05. MINOR CORRECTIONS TO THIS
FEATURE ARE EVIDENT...WITH MODELS RESOLVING A SLIGHTLY WEAKER
PERTURBATION THAN ON THE PREVIOUS RUN. OVERALL...THE CONFIDENCE
IN THE FORECAST REMAINS HIGH.
AT MID LEVELS...A TROUGH IS FORECAST TO MOVE ALONG 80W/85W
AND SOUTH OF 30S BY 24 HRS...WITH ITS SHORT WAVE ENERGY FOCUSING
ACROSS CHILE TO THE SOUTH OF CONCEPCION. THROUGH 48-60 HRS THE
TROUGH WILL START TO MOVE ACROSS 70W...AND IT IS TO START PUSHING
AGAINST A BLOCKING RIDGE OVER THE WESTERN ATLANTIC/ARGENTINA.
UNDER INFLUENCE OF THE BLOCKING RIDGE...SOME OF THE SHORT WAVE
ENERGY WILL SHEAR SOUTHEASTWARD ACROSS THE DRAKE PASSAGE/
PATAGONIA TO THE SOUTH ATLANTIC. MEANWHILE...ON THE NORTHERN
STREAM...A SHORT WAVE TROUGH WILL SEGREGATE OFF THE COAST OF
CHILE ALONG 76W/78W. BETWEEN 60-72 HRS THE NORTHERN STREAM TROUGH
WILL BEGIN TO MOVE ACROSS 70W...FOCUSING ITS SHORT WAVE ENERGY ON
CONTINENTAL AREA BETWEEN 25S-35S...WHERE IT WILL SUSTAIN HEIGHT
FALLS OF 40-80GPM. BY 96 HRS A SHORT WAVE TROUGH WILL ENVELOP
AREA BETWEEN 72W-54W AND SOUTH OF 20S...WHILE CENTERING ON A
CLOSED LOW NEAR 32S 62W. THROUGH 120 HRS THE MEAN AXIS MOVES
ALONG 58W/60W...WHILE THE MID LEVEL LOW RELOCATES OVER
URUGUAY/SOUTHERN BRASIL...WHERE IT WILL FAVOR HEIGHT FALLS OF 40-
80 GPM. AS THE MID/UPPER TROUGH MOVES ACROSS CHILE IT WILL FAVOR
A DIVERGENT PATTERN ALOFT...MOST INTENSE BETWEEN 36-72 HRS. THIS
WILL FAVOR DEEP CONVECTION ON THE ANDES AND ACROSS THE COASTAL
PLAINS...FAVORING RAINFALL AMOUNTS OF 10-15MM. SNOWFALL WILL
CONCENTRATE ON ELEVATIONS ABOVE 4.5-5.0 KM. AT LOW LEVELS...A
FRONT EXTENDS ACROSS CENTRAL PATAGONIA...AND THROUGH 48 HRS IT
ADVANCES ACROSS NORTHERN PATAGONIA-LA PAMPA TO SOUTHERN CUYO. BY
72 HRS A LOW/WAVE WILL FORM ALONG THE FRONT OVER NORTHERN
PATAGONIA...WHICH OCCLUDES INLAND BY 96 HRS. THE OCCLUDING LOW
WILL SUSTAIN A PAMPERO JET ACROSS CENTRAL ARGENTINA. THIS WILL
DRIVE THE ASSOCIATED FRONT NORTH ACROSS CENTRAL ARGENTINA/
MENDOZA BY 72 HRS...AND THROUGH 96 HRS IT PUSHES ACROSS THE
BUENOS AIRES PROVINCE/CORDOBA. IT THEN MERGES WITH AN OLD FRONT
TO THE NORTH. AS THE FRONT ENTERS SOUTHERN CHILE IT WILL SUSTAIN
MAXIMA OF 10-15MM/DAY BY 24 HRS...WITH MOST INTENSE OVER CENTRAL
CHILE BY 36-72 HRS. AS THE LOW OCCLUDES OVER CENTRAL ARGENTINA IT
WILL SUSTAIN MAXIMA OF 15-25MM...WITH STRATIFORM PRECIPITATION
OVER NORTHERN PATAGONIA/LA PAMPA AND CONVECTIVE PRECIPITATION
ACROSS CORDOBA-BUENOS AIRES PROVINCE. ALTHOUGH STRONG DYNAMICS
ARE TO ACCOMPANY THIS SYSTEM AS IT ENTERS ARGENTINA...AVAILABLE
MOISTURE IS LACKING. HIGHER MOISTURE...HOWEVER...MIGHT BE PRESENT
BETWEEN CORDOBA-CORRIENTES/CHACO ARGENTINO...AND THIS MIGHT BE
THE BEST AREA FOR ISOLATED SEVERE CONVECTIVE CELLS TO DEVELOP.
OVER THE ATLANTIC...MEANWHILE...A 500 HPA TROUGH IS TO EXTEND
BETWEEN 55W-20W AND SOUTH OF 20S THROUGH 48-60 HRS. BY 72-96 HRS
THE TROUGH WILL DRIFT EAST OF 45W/50W AS IT GRADUALLY MERGES WITH
A BROAD TROUGH ON THE SOUTHERN STREAM FLOW. AT LOW LEVELS AN
ELONGATED FRONT EXTENDS ACROSS THE SOUTH ATLANTIC-RIO DE
JANEIRO/SOUTHERN MINAS GERAIS-MATO GROSSO DO SUL TO NORTHERN
BOLIVIA/SOUTHERN JUNGLE OF PERU. THE FRONT REMAINS NEARLY
STATIONARY THROUGH 48-60 HRS...THEN DRIFTS SOUTH INTO PARAGUAY BY
60-72 HRS...WITH A WAVE/LOW TO FORM ALONG THIS BOUNDARY OVER
CENTRAL PARAGUAY BY 72-84 HRS AS THE AFOREMENTIONED MID LEVEL
TROUGH OVER ARGENTINA INTERACTS WITH THIS BOUNDARY. THE SURFACE
LOW DEEPENS AS IT MOVES TO SOUTHEAST PARAGUAY THROUGH 96 HRS.
THIS...IN-TURN...WILL FORCE THE FRONT TO ONCE AGAIN MIGRATE NORTH
INTO SOUTHERN BRASIL-MATO GROSSO DO SUL-NORTHERN JUNGLE OF
BOLIVIA BY THE END OF THE CYCLE. INITIALLY CONVECTION WILL FORM
ALONG THE TAIL END OF THE FRONT OVER SOUTHERN JUNGLE OF
PERU...WHERE WE EXPECT DAILY MAXIMA OF 10-20MM IN ISOLATED TO
SCATTERED CONVECTION. OTHER CONVECTION IS EXPECTED OVER NORTHERN
SAO PAULO-RIO DE JANEIRO...TO CONCENTRATE ALONG THE COAST...WITH
DAILY MAXIMA OF 15-30MM/DAY THROUGH 24 HRS...AND 10-20MM/DAY
THEREAFTER. AS THE WAVE/LOW FORMS ALONG THE SURFACE FRONT BY 72-
84 HRS...EXPECT A SURGE IN ACTIVITY ACROSS CENTRAL-SOUTHEAST
PARAGUAY-CHACO ARGENTINO/CORRIENTES-MESOPOTAMIA VALLEY...WHERE WE
EXPECT MAXIMA OF 25-50MM/DAY BY 84-108 HRS...TO AFFECT SOUTHERN
BRASIL-MATO GROSSO DO SUL AND EASTERN/NORTHERN BOLIVIA BY 108-132
HRS.
AT 200 HPA THE SUBTROPICAL PATTERN OVER NORTHERN SOUTH AMERICA
WILL REMAIN ILL DEFINED DURING THE NEXT 03-04 DAYS...WITH SHORT
WAVE PERTURBATIONS TO DOMINATE FLOW BETWEEN 05S-20S. BY THE END
OF THE CYCLE THE SUBTROPICAL RIDGE TRIES TO ESTABLISH/BUILD OVER
THE EASTERN PACIFIC. MEANWHILE...THIS IS TO CONTINUE FAVORING
DEEP CONVECTION ACROSS AMAZONAS-PARA IN BRASIL...WITH MAXIMA OF
20-40MM/DAY THROUGH 24 HRS...AND 10-20MM/DAY THEREAFTER.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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