703
FXSA20 KWBC 041632
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1232 PM EDT MON AUG 04 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 04). GLOBAL MODELS AGREE
QUITE WELL ON THE NORTHERN STREAM PATTERN THROUGH 120-132 HRS.
DIFFERENCES ON THE SOUTHERN STREAM...HOWEVER...BECOME EVIDENT AS
EARLY AS 72 HRS...WITH THE GFS PROJECTING A STRONGER MID LEVEL
TROUGH TO ENTER THE DRAKE PASSAGE/ANTARCTIC PENINSULA. THIS IS A
CHANGE FROM THE PREVIOUS FORECAST WHEN THE MODEL WAS FAVORING A
WEAKER PERTURBATION. FIRST IMPRESSION IS THAT THE MODEL
OVERCORRECTED...SO WE EXPECT FUTURE RUNS TO CHANGE IN SUPPORT OF
A LOWER AMPLITUDE TROUGH.
ON THE NORTHERN STREAM...A SHORT WAVE TROUGH ENTERS CENTRAL CHILE
EARLY THIS PERIOD...AND THROUGH 24 HRS IT WILL MOVE ACROSS 60W.
THE PROGRESSIVE SHORT WAVE TROUGH WILL QUICKLY MOVE ACROSS
38W/40W BY 48 HRS. AT LOW LEVELS...AN ELONGATED FRONT EXTENDS
ACROSS THE SOUTH ATLANTIC TO RIO DE JANEIRO-MATO GROSSO BRASIL-
NORTHERN BOLIVIA. THROUGH 24-36 HRS THE BOUNDARY WILL RETROGRESS
TO MATO GROSSO DO SUL/PARAGUAY...WHILE SIMULTANEOUSLY A POLAR
FRONT PUSHES NORTH ACROSS URUGUAY/RIO DE LA PLATA-LA SERENA
CHILE. THE LATTER IS TO CONTINUE FARTHER NORTH INTO SOUTHERN
BRASIL-PARAGUAY BY 42-48 HRS...AND THROUGH 60-72 HRS IT IS TO
MERGE WITH THE OLD FRONT OVER SOUTHEAST BRASIL-PARAGUAY. THIS
BOUNDARY WILL THEN PERSIST THROUGH 96-120 HRS...MEANWHILE
UNDULATING ACROSS SAO PAULO-MATO GROSSO DO SUL-PARAGUAY-BOLIVIA.
AT 250 HPA...AN ANTICYCLONICALLY CURVED SUBTROPICAL JET IS TO
ENTER SOUTHERN BOLIVIA-PARAGUAY BY 36 HRS...WITH ITS RIGHT
(DIVERGENT) EXIT REGION OVER EL PANTANAL/SOUTHERN BRASIL-PARAGUAY
BY 36-48 HRS. THIS JET IS TO SLOWLY MOVE EAST ACROSS MID SECTIONS
OF SOUTH AMERICA...AND IT IS TO MEANWHILE HELP SUSTAIN THE
BAROCLINICITY OF THE QUASISTATIONARY FRONTAL BOUNDARY. THE
FAVORABLE JET DYNAMICS...PERSISTENT SURFACE BOUNDARIES...MOIST
NORTHERLY FLOW (OVERRUNNING SITUATION) WILL FAVOR PERIODS
OF DENSE/DEEP CLOUD COVER WITH FORMATION OF NIMBOSTRATUS AND
EMBEDDED CUMULUS NIMBUS. OBJECTIVE MODEL ANALYSIS SHOWS BEST
DYNAMICS FOR THIS AT 33-60 HRS AND AT 84-120 HRS. ACROSS
PARAGUAY-SAO PAULO/SANTA CATARINA-MATO GROSSO DO SUL WE EXPECT
RAINFALL MAXIMA OF 15-35MM THROUGH 36 HRS...AND 30-60MM/DAY AT
36-60 HRS. AT 60-108 HRS DAILY MAXIMA WILL PEAK AT 20-40MM. LATER
IN THE CYCLE...ON DAY 05...WE ONCE AGAIN EXPECT MAXIMA OF 30-
60MM. ON THE TAIL END OF THE FRONT...OVER NORTHERN
BOLIVIA/SOUTHERN PERU...EXPECT MAXIMA OF 15-35MM/DAY THROUGH 36-
48 HRS...TO GRADUALLY WEAKEN THROUGH 60-72 HRS. LATER IN THE
PERIOD A SLIGHT INCREASE IN CONVECTION IS EXPECTED...WITH MAXIMA
TO PEAK AT 10-15MM.
ON THE SOUTHERN STREAM...A BROAD TROUGH OVER THE SOUTHEAST
PACIFIC IS TO SLOWLY MOVE BETWEEN 120W-60W AND SOUTH OF 40S BY 24
HRS. THROUGH 72 HRS THIS TROUGH WILL MIGRATE BETWEEN 85W-
35W...WHILE THE AXIS AMPLIFIES NORTH INTO THE SOUTHERN CONE TO
25S/30S. A BROAD TROUGH PATTERN IS TO THEN PERSIST OVER THE
WESTERN ATLANTIC/SOUTHERN CONE THROUGH AT LEAST 108-120
HRS...WITH SHORT WAVE PERTURBATIONS QUICKLY REVOLVING/ROTATING
AROUND THIS AXIS. AT LOW LEVELS IT WILL SUSTAIN A BROAD TROUGH
AND A TIGHT PRESSURE GRADIENT...WHICH IN-TURN WILL FAVOR STRONG
LOW LEVEL WINDS OF 30-45KT ACROSS CHILE/PATAGONIA SOUTH OF 40S.
THIS WILL PERSIST THROUGH 84-96 HRS WHEN THE SURFACE TROUGH MOVES
FARTHER EAST INTO THE SOUTH ATLANTIC. UNDER THE INFLUENCE OF THIS
TROUGH...THE -30C ISOTHERM IS TO REMAIN AROUND 530 HPA DURING
MOST OF THE CYCLE. THE ENSUING DEEP/COLD AIR MASS WILL FAVOR
ORGANIZED SNOWFALL ACROSS SOUTHERN CHILE AND THE SOUTHERN ANDES
OF ARGENTINA. DURING THE NEXT THREE DAYS EXPECT PROGRESSIVE POLAR
FRONTS TO LIMIT TO CONTINENTAL AREA SOUTH OF 40S...WITH OCCLUDED
LOWS ENTERING SOUTH OF 45S/50S. IN THIS PATTERN MIXING RATIO
VALUES OVER SOUTHERN CHILE WILL REMAIN AROUND 04-06 G/KG. MOST
ACTIVE CONVECTION IS EXPECTED THROUGH 72-84 HRS...WITH DAILY
MAXIMA OF 10-20MM. MOST INTENSE...HOWEVER...IS EXPECTED AT 24-48
HRS...WHEN THE DAILY MAXIMA WILL PEAK AT 20-40MM.
AT 200 HPA...THE SUBTROPICAL RIDGE IS INITIALIZED OVER
CONTINENTAL AREA NORTH OF 10S. THE RIDGE WILL REMAIN NARROW AND
ELONGATED THROUGH 36-48 HRS. AFTERWARD IT IS FORECAST TO
GRADUALLY BUILD/INTENSIFY...WITH AXIS TO EXPAND SOUTH TO ENVELOP
AREA NORTH OF 20S/25S. AS IT BUILDS...A HIGH WILL CLOSE OVER
BRASIL NEAR 10S 60W BY MID CYCLE. THE INTENSIFICATION OF THE
RIDGE WILL FOLLOW A GRADUAL INCREASE IN CONVECTION ACROSS PARA-
AMAZONAS IN WESTERN BRASIL...WHERE DAILY MAXIMA WILL PEAK AT 15-
30MM. ACROSS NORTHEAST-NORTHERN BRASIL...MEANWHILE...EXPECT
WIDELY ISOLATED CONVECTION...WITH PERIODS OF MODERATE TO HEAVY
RAINFALL TO ASSOCIATED WITH TROPICAL WAVES. ONE IN
PARTICULAR...IS EXPECTED TO AFFECT NORTHEAST BRASIL BY 72-96
HRS...WITH MAXIMA OF 10-20MM. THIS IS TO GRADUALLY SPREAD ALONG
THE NORTHERN COAST TO THE AMAZON RIVER DELTA REGION/AMAPA LATER
IN THE PERIOD.
GUZMAN...SENAMHI (BOLIVIA)
DAVISON...HPC (USA)
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