396
FXSA20 KWBC 051629
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1228 PM EDT TUE AUG 05 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 05). QUITE SURPRISINGLY...
THE GFS MAINTAINS CONTINUITY ON THE SOUTHERN STREAM FLOW ON ITS
FORECAST OF A STRONG POLAR SURGE ACROSS THE DRAKE PASSAGE TO
TIERRA DEL FUEGO BY 48-72 HRS. THE ECMWF-UKMET ARE NOW STARTING
TO RESOLVE THIS FEATURE...ALTHOUGH NOT AS STRONG AS THE GFS
SUGGESTS. BUT UNCERTAINTY REMAINS HIGH AS DEPICTED BY THE GLOBAL
ENSEMBLES...WITH HIGH VARIABILITY AMONG THE MEMBERS ON THE
EVOLUTION OF THIS PERTURBATION. SO UNCERTAINTY ON THE SOUTHERN
STREAM FLOW REMAINS HIGH. MEDIUM-LONG RANGE FORECAST SUGGESTS
POSSIBILITY OF A HIGH AMPLITUDE TROUGH PATTERN TO DOMINATE THE
SOUTHERN CONE BEYOND 144 HRS...SO WINTER IS NOT OVER.
ON THE NORTHERN STREAM...A PROGRESSIVE 500 HPA SHORT WAVE TROUGH
MOVES ACROSS ARGENTINA ALONG 60W EARLY THIS PERIOD...AND THROUGH
24 HRS IT IS TO MOVE ACROSS 40W. AT LOW LEVELS...A
QUASISTATIONARY/ELONGATED FRONT EXTENDS ACROSS THE SOUTH ATLANTIC
TO SAO PAULO-MATO GROSSO DO SUL IN BRASIL-CHACO PARAGUAYO-
BOLIVIA...WHERE IT MEANDERS DURING THE NEXT FEW DAYS. FARTHER
SOUTH...A POLAR FRONT OVER URUGUAY-CORDOBA ARGENTINA IS TO PUSH
NORTH INTO SOUTHERN BRASIL-SOUTHEAST PARAGUAY BY 24 HRS. THIS
FRONT IS TO CONTINUE FARTHER NORTH AND THROUGH 36-48 HRS IT IS TO
MERGE WITH THE OLD FRONT OVER SOUTHEAST BRASIL-PARAGUAY. AS THEY
COMBINE...THE ENSUING BOUNDARY WILL THEN PERSIST THROUGH 96-120
HRS...WHILE UNDULATING ACROSS SOUTHEAST BRASIL-PARAGUAY-BOLIVIA.
AT 250 HPA...AN ANTICYCLONICALLY CURVED SUBTROPICAL JET IS TO
ENTER SOUTHERN BOLIVIA-PARAGUAY EARLY THIS PERIOD...WITH ITS
RIGHT (DIVERGENT) EXIT REGION TO POSITION OVER EL PANTANAL/
SOUTHERN BRASIL-PARAGUAY BY 18-24 HRS. THIS JET IS TO SLOWLY MOVE
EAST ACROSS MID SECTIONS OF SOUTH AMERICA...MEANWHILE SUSTAINING
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. ACROSS PARAGUAY-SAO PAULO/SANTA
CATARINA-MATO GROSSO DO SUL WE EXPECT RAINFALL MAXIMA OF 30-60MM
THROUGH 36 HRS...AND 25-50MM/DAY AT 36-60 HRS. CONVECTION WILL
SURGE ONCE AGAIN BETWEEN 84-108 HRS WITH MAXIMA OF 30-60MM...WITH
MODERATE TO HEAVY RAINFALL TO PERSIST WELL INTO 120-132 HRS. ON
THE TAIL END OF THE FRONT...OVER NORTHERN BOLIVIA/SOUTHERN
PERU...EXPECT MAXIMA OF 15-35MM/DAY THROUGH 36 HRS...TO GRADUALLY
WEAKEN THROUGH 48-60 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 IS INITIALIZED OVER THE
FAR EASTERN PACIFIC...WHILE A RIDGE LIES OVER THE SOUTHERN
ATLANTIC/NORTH OF THE POLAR ICE CAP. THROUGH 24-48 HRS THE TROUGH
WILL MOVE EAST TO EXTEND BETWEEN 90W-40W...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 120 HRS. DURING THE NEXT 48 HRS THIS WILL
ALLOW FOR SHORT WAVE PERTURBATIONS TO FOCUS ACROSS CHILE/
ARGENTINA BETWEEN 35S-45S. 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 OVER THE SOUTHERN CONE THROUGH 60-72 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 CONTINUE FAVORING ORGANIZED SNOWFALL
ACROSS SOUTHERN CHILE AND THE SOUTHERN ANDES OF ARGENTINA. DURING
THE NEXT 48-60 HRS 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 48-60 HRS. DURING THIS PERIOD...A MAXIMA OF 20-
35MM/DAY IS TO AFFECT CHILE BETWEEN TEMUCO-PUERTO MONTT...TO
GRADUALLY DECREASE TO 10-20MM/DAY LATE ON DAY 02.
AT 200 HPA...THE SUBTROPICAL RIDGE REMAINS OVER THE CONTINENT TO
NORTH OF 10S. THIS RIDGE WILL REMAIN NARROW AND ELONGATED THROUGH
24 HRS. AFTERWARD IT IS FORECAST TO GRADUALLY BUILD/INTENSIFY...
WITH AXIS TO EXPAND SOUTH TO ENVELOP CONTINENTAL AREA NORTH OF
20S/25S. AS IT BUILDS...A HIGH WILL CLOSE OVER BRASIL NEAR 10S
60W BY 48 HRS...FORECAST TO MOVE EAST TO 10S 40W BY 96 HRS. 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.
ALSO NOTE THAT AN UPPER LEVEL SHORT WAVE TROUGH IS TO ENTER
NORTHERN CHILE/SOUTHERN PERU BY 72-96 HRS. THIS WILL PUSH EAST
ACROSS 60W BY 120 HRS. THERE IS A RISK OF SCATTERED SNOWFALL ON
THE SOUTHERN SIERRA OF PERU INTO THE ALTIPLANO OF BOLIVIA IN
ASSOCIATION WITH THIS FEATURE. THE MODELS HAVE BEEN FAIRLY
CONSISTENT ON THIS FORECAST...AND CONFIDENCE IS HIGH.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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