474
FXSA20 KWBC 171718
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
118 PM EDT WED SEP 17 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 17). MODELS DEVIATE FROM
PREVIOUS FORECAST...AS THEY MAKE CORRECTIONS TO A NORTHERN STREAM
PERTURBATION ENTERING CENTRAL CHILE/ARGENTINA. THE GFS AND UKMET
IN PARTICULAR NOW FORESEE A WEAKER PERTURBATION TO CROSS THE
CONTINENT. THE ECMWF...IN CONTRAST...MAINTAINS BETTER CONTINUITY
AS IT CONTINUES TO FORECAST A STRONGER/DEEPER MID LEVEL TROUGH.
THE ENSEMBLE MODELS...ALTHOUGH SHOWING SOME VARIABILITY...SIDE
WITH THE ECMWF SOLUTION. SO WE ARE CORRECTING THE GFS FORECAST
CHARTS TO SHOW DEEPER PERTURBATIONS ACROSS PARAGUAY/SOUTHERN
BRASIL ON DAYS 03-05.
OVER THE PACIFIC...MODELS INITIALIZE A LONG WAVE TROUGH ALONG THE
COAST OF CHILE. THE MID LEVEL TROUGH IS TO PUSH AGAINST A
BLOCKING RIDGE OVER ARGENTINA/WESTERN ATLANTIC. UNDER INFLUENCE
OF THIS 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 36-48 HRS THE NORTHERN STREAM TROUGH WILL BEGIN TO MOVE
ACROSS 70W...FOCUSING ITS SHORT WAVE ENERGY ON CONTINENTAL AREA
BETWEEN 20S-35S...WHERE IT WILL SUSTAIN/INDUCE HEIGHT FALLS OF
40-80GPM. BY 72 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 96 HRS THE MEAN AXIS MOVES ALONG 58W/60W...AND
BY 120 HRS IT WILL CONTINUES INTO SOUTHERN BRASIL/WESTERN
ATLANTIC. AS THE MID/UPPER TROUGH MOVES ACROSS CENTRAL CHILE TO
CUYO IN ARGENTINA IT WILL TRIGGER ISOLATED TO SCATTERED
CONVECTION...WITH ACCUMULATION OF 05-10MM...WITH MOST INTENSE
OVER THE ANDES. AT LOW LEVELS...A FRONT EXTENDS ACROSS NORTHERN
PATAGONIA-TEMUCO CHILE TO THE EASTERN PACIFIC. THIS FRONT MOVES
NORTH INTO LA PAMPA/CENTRAL CUYO ARGENTINA-CENTRAL CHILE BY 24
HRS. A WAVE/LOW FORMS ALONG THIS FRONT BY 48 HRS...FORCING ITS
NORTHWARD PROGRESSION ACROSS BUENOS AIRES-CORDOBA-MENDOZA IN
ARGENTINA BY 72 HRS. IT WILL THEN MERGE WITH AN OLD FRONT TO THE
NORTH. AS THE LOW/WAVE FORMS ALONG THE FRONT OVER ARGENTINA...IT
WILL SUSTAIN TWO AREAS OF RAINFALL...WITH STRATIFORM
PRECIPITATION OVER NORTHERN PATAGONIA/LA PAMPA-CUYO AND
CONVECTIVE PRECIPITATION OVER CORDOBA/CORRIENTES-LOWER
MESOPOTAMIA-BUENOS AIRES PROVINCE...WITH RAINFALL MAXIMA OF 15-
25MM. CONSIDERING THE CHANGES/CORRECTIONS MADE BY THE MODELS...
IT IS HARD TO GAUGE POTENTIAL FOR SEVERE CONVECTION. BUT
NEVERTHELESS...A SLIGHT RISK REMAINS BETWEEN CORDOBA-CORRIENTES-
SOUTHERN MESOPOTAMIA.
THE MODELS ARE MORE CONSISTENT ON THEIR EVOLUTION/FORECAST OF A
MID LEVEL TROUGH OVER THE WESTERN ATLANTIC. THE 500 HPA TROUGH
WILL EXTEND BETWEEN 55W-20W AND SOUTH OF 20S THROUGH 24-36 HRS.
BY 48-72 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 24-36 HRS...THEN DRIFTS SOUTH INTO
PARAGUAY BY 36-48 HRS...WITH A WAVE/LOW TO FORM ALONG THIS
BOUNDARY OVER CENTRAL PARAGUAY BY 48-60 HRS AS THE AFOREMENTIONED
MID LEVEL TROUGH OVER ARGENTINA INTERACTS WITH THIS BOUNDARY. THE
SURFACE LOW DEEPENS AS IT MOVES TO SOUTHEAST PARAGUAY THROUGH 72
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 84-96 HRS. BY 108-120 HRS THE LOW WILL OCCLUDE OFF THE
COAST OF URUGUAY/RIO GRANDE DO SUL BRASIL...WHILE THE FRONT
MEANDERS ACROSS SAO PAULO-MATO GROSSO DO SUL TO NORTHERN BOLIVIA.
CONVECTION IS EXPECTED OVER NORTHERN SAO PAULO-RIO DE JANEIRO-
ESPIRITO SANTO...WITH DAILY MAXIMA OF 10-20MM/DAY THROUGH 72-84
HRS. AS THE WAVE/LOW FORMS ALONG THE SURFACE FRONT BY 48-60
HRS...EXPECT A SURGE IN ACTIVITY ACROSS CENTRAL-SOUTHEAST
PARAGUAY-CHACO ARGENTINO/CORRIENTES-MESOPOTAMIA VALLEY...WHERE WE
EXPECT MAXIMA OF 25-50MM/DAY BY 60-84 HRS...TO AFFECT SOUTHERN
BRASIL-MATO GROSSO DO SUL AND EASTERN/NORTHERN BOLIVIA BY 84-108
HRS.
AT 200 HPA...AN ILL ORGANIZED SUBTROPICAL FLOW PATTERN IS
INITIALIZED ACROSS NORTHERN SOUTH AMERICA. THIS IS TO GRADUALLY
CHANGE THROUGH 72-84 HRS...WHEN A HIGH IS TO CLOSE OVER THE
EASTERN PACIFIC NEAR 13S 90W. THIS HIGH WILL BUILD A RIDGE SOUTH
INTO THE EASTERN PACIFIC...MEANWHILE INDUCING/FAVORING THE
NORTHWARD AMPLIFICATION OF TROUGH INTO SOUTHERN PERU/BOLIVIA.
THIS WILL ENHANCE THE AFOREMENTIONED CONVECTION OVER THE BOLIVIAN
JUNGLE/SOUTHERN PERU BY MID CYCLE...WITH MAXIMA OF 10-20MM/DAY.
MOST INTENSE...HOWEVER...IS EXPECTED OVER PARA-AMAZONAS TO
NORTHERN PERU/SOUTHERN COLOMBIA...WHERE DAILY MAXIMA WILL PEAK AT
20-40MM THROUGH AT LEAST 60 HRS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)Q'
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