798
FXSA20 KWBC 191624
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1224 PM EDT FRI SEP 19 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 19). LOOKS LIKE THE GFS AND
ECMWF FINALLY REACHED AN AGREEMENT ON HOW THE MID LEVEL TROUGH
ENTERING CHILE/ARGENTINA IS GOING TO EVOLVE...WITH ONLY SUBTLE
DIFFERENCES LATER IN THE CYCLE. THE UKMET SUPPORTS THESE MODELS
ON THE NORTHERN STREAM SOLUTION...BUT ON THE SOUTHERN STREAM IT
DIVERGES AFTER 96/108 HRS...AS IT PROJECTS A FASTER/MORE
PROGRESSIVE TROUGH TO ENTER THE SOUTH ATLANTIC. DIVERGENCE AMONG
THE ENSEMBLE MEMBERS IS QUITE HIGH WITH RESPECT TO THIS
PERTURBATION...SO CONFIDENCE ON ITS FORECAST/EVOLUTION IS LOW.
AT MID LEVELS...MODELS CONTINUE TO FORECAST A 500 HPA TROUGH TO
MOVE ACROSS CHILE TO ARGENTINA DURING THE NEXT 12-18 HRS...WITH
AXIS TO EXTEND NORTH ALONG 60W/64W TO NORTHWEST PROVINCES OF
ARGENTINA/SOUTHERN BOLIVIA. AT 36-48 HRS A LOW WILL CLOSE JUST
EAST OF BUENOS AIRES PROVINCE...WHILE THE TROUGH EXTENDS ACROSS
URUGUAY/SOUTHERN BRASIL TO PARAGUAY. THE LOW THEN MOVES NORTH
ALONG THE COAST TO 32S 49W BY 72 HRS...CENTERING JUST EAST OF
URUGUAY/RIO GRANDE DO SUL BRASIL. THE MODELS GENERALLY AGREE ON
A SLOW EASTWARD PROGRESSION...WITH SYSTEM TO MOVE TO 30S 40W
THROUGH 120 HRS. NOTE THAT THIS IS CONSIDERABLY SLOWER THAN
PREVIOUSLY ANTICIPATED. AT LOW LEVELS...A FRONT EXTENDS ACROSS
THE BUENOS AIRES PROVINCE-A WAVE/LOW NEAR SIERRA DE CORDOBA...TO
NORTHERN MENDOZA. THE LOW/WAVE MOVES ACROSS BUENOS AIRES PROVINCE
BY 24 HRS...FORCING THE FRONT EAST AND NORTH ACROSS CORDOBA TO
NORTHWEST PROVINCES OF ARGENTINA. AT 36-48 HRS IT WILL THEN MERGE
WITH AN OLD FRONT TO THE NORTH...MEANWHILE SUSTAINING A SURFACE
OCCLUSION JUST EAST OF BUENOS AIRES PROVINCE/URUGUAY. AS THE
FRONT MOVES ACROSS CORDOBA-MENDOZA IN ARGENTINA IT WILL SUSTAIN
MODERATE TO HEAVY CONVECTION WITH MAXIMA OF 20-35MM. SIMILAR
AMOUNTS ARE EXPECTED OVER SOUTHERN BRASIL BY 36-48 HRS. AS THE
LOW OCCLUDES OFF THE COAST OF BUENOS AIRES PROVINCE/URUGUAY...IT
WILL SUSTAIN A MOIST EASTERLY FLOW THAT IS TO TRIGGER STRATIFORM
RAINFALL WITH MAXIMA OF 05-10MM.
A MEANDERING MID/UPPER LEVEL TROUGH IS INITIALIZED OVER THE
WESTERN ATLANTIC...WITH AXIS AT 500 HPA ENVELOPING AREA BETWEEN
50W-20W AND 40S-18S. EARLY THIS CYCLE THE TROUGH WILL DRIFT EAST
OF 25W/30W AS IT GRADUALLY MERGES WITH AN AMPLIFYING TROUGH ON
THE SOUTHERN STREAM FLOW. AT LOW LEVELS AN ELONGATED FRONT
EXTENDS ACROSS THE SOUTH ATLANTIC-ESPIRITO SANTO/SOUTHERN MINAS
GERAIS-MATO GROSSO DO SUL TO CHACO PARAGUAYO/EASTERN BOLIVIA. A
WAVE/LOW TO FORM ALONG THIS BOUNDARY OVER CENTRAL PARAGUAY EARLY
ON...AND THROUGH 24-36 HRS IT WILL MOVE TO SOUTHERN BRASIL WHERE
IT WILL BEGIN TO OCCLUDE. THROUGH 48 HRS IT WILL DEEPEN OFF THE
COAST OF BRASIL. AS IT INTENSIFIES/DEEPENS...IT WILL TIGHTEN THE
GRADIENT AND SUSTAIN A SOUTHEASTERLY LOW LEVEL JET ACROSS
NORTHERN ARGENTINA/PARAGUAY TO BOLIVIA. THIS WILL THEN FORCE THE
SURFACE FRONT NORTH INTO SOUTHERN BRASIL-MATO GROSSO DO SUL TO
NORTHERN BOLIVIA BY 30-36 HRS. AT 48-72 HRS THE FRONT MEANDERS
NORTH ACROSS SAO PAULO-MATO GROSSO DO SUL TO NORTHERN BOLIVIA.
THROUGH 96 HRS THE FRONT MOVES NORTH INTO RIO DE JANEIRO/ESPIRITO
SANTO-MINAS GERAIS THEN WEST/SOUTHWEST TO CENTRAL BOLIVIA. AS
THE WAVE/LOW DEEPENS OVER SOUTHERN BRASIL...AND UNDER FAVORABLE
JET DYNAMICS...IT IS TO TRIGGER SCATTERED CONVECTION WITH
MAXIMA OF 25-50MM BY 36 HRS. THROUGH 48 HRS WE NOW EXPECT MAXIMA
OF 20-35MM/DAY AS THE FRONT MOVES INTO SOUTHEAST BRASIL-MATO
GROSSO DO SUL TO EASTERN BOLIVIA. AFTER 60 HRS MAXIMA WILL
DECREASE TO 10-20MM/DAY AS IT BUILDS ACROSS SAO PAULO...WITH
LESSER AMOUNTS OVER RIO DE JANEIRO/ESPIRITO SANTO BY 96-108
HRS...WITH MAXIMA OF 05-10MM. FURTHERMORE...UNDER INFLUENCE OF A
WIND SURGE ACROSS BOLIVIA/TO PERU...SCATTERED CONVECTION WILL
BUILD AHEAD OF THE FRONT OVER NORTHERN BOLIVIA TO SOUTHERN/
CENTRAL PERU...WITH MAXIMA OF 20-35MM BY 48-60 HRS. BY 72-84 HRS
MOST INTENSE CONVECTION WILL BUILD FARTHER NORTH INTO NORTHERN
PERU/ECUADOR WITH MAXIMA OF 25-50MM...TO DECREASE TO 20-40MM/DAY
BY 96-120 HRS.
THE MODELS THEN FORECAST ANOTHER TROUGH TO MOVE ACROSS THE
EASTERN PACIFIC ALONG 100W/110W TO 20S BY 24 HRS. THROUGH 72 HRS
IT WILL MOVE ACROSS 85W. IT WILL THEN SLOW DOWN AS IT PUSHES
AGAINST A BLOCKING RIDGE OVER THE SOUTHERN CONE/WESTERN ATLANTIC.
THE TROUGH IS NOT TO ENTER THE CONTINENT UNTIL 96-108 HRS.
AFTERWARDS...MOST OF THE ENERGY WILL SHEAR SOUTH-SOUTHEAST ACROSS
PATAGONIA TO THE SOUTH ATLANTIC. THE ASSOCIATED FRONT WILL ENTER
SOUTHERN CHILE BY 72 HRS...INTO PATAGONIA-CENTRAL CHILE THROUGH
96-108 HRS. AS IT ENTERS SOUTHERN CHILE/TIERRA DEL FUEGO IT WILL
FAVOR SCATTERED CONVECTION WITH ACCUMULATION OF 05-10MM.
AT 200 HPA...THE UPPER FLOW PATTERN OVER NORTHERN SOUTH AMERICA
CONTINUES TO EVOLVE AS A RIDGE BUILDS OVER THE EASTERN TROPICAL
PACIFIC. THIS WILL FAVOR/SUSTAIN THE NORTHWARD AMPLIFICATION OF A
TROUGH ACROSS ARGENTINA-BOLIVIA TO SOUTHERN PERU BY 48 HRS...WITH
TROUGH TO SLOWLY MOVE EAST ALONG 50W BY 96 HRS. THIS WILL ENHANCE
CONVECTION OVER THE BOLIVIAN JUNGLE/SOUTHERN PERU BY 24-60
HRS...WITH MAXIMA OF 20-350MM/DAY. MOST INTENSE...HOWEVER...IS
EXPECTED OVER PARA-AMAZONAS TO NORTHERN PERU/SOUTHERN
COLOMBIA...WHERE DAILY MAXIMA WILL PEAK AT 25-50MM/DAY.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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