790
FXSA20 KWBC 081700
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
100 PM EDT FRI AUG 08 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 08). AFTER 60-72 HRS MODELS
EVOLVE INTO A HODGEPODGE OF SOLUTIONS...WITH MODELS CLEARLY
DIVERGING ON THE INTENSITY/AMPLITUDE AND SPEED OF PROPAGATION OF
SOUTHERN STREAM PERTURBATIONS ENTERING THE DOMAIN. CYCLE-TO-CYCLE
CONTINUITY IS NON-EXISTENT...SO SELECTING THE PROPER SOLUTION IS
NOT A TRIVIAL THING. THE MEAN OF THE GLOBAL ENSEMBLES...
CONSISTING OF THE GFS-CANADIAN-ECMWF MEMBERS...SIDE WITH THE GFS
SOLUTION. SO IN THE ABSENCE OF CLEAR GUIDANCE...WE ARE GOING WITH
THE STATISTICAL MEAN. CONFIDENCE IN THE FORECAST BEYOND 60-72 HRS
IS LOW.
AT LOW LEVELS...AN ELONGATED FRONT OVER THE SOUTH ATLANTIC MAKES
LANDFALL OVER SOUTHEAST BRASIL...THEN WEST-NORTHWEST ACROSS
MATO GROSSO DO SUL/PARAGUAY TO BOLIVIA. THE BOUNDARY WILL REMAIN
NEARLY STATIONARY THROUGH 36-48 HRS...THEN RETROGRESS TO SOUTHERN
BRASIL BY 72 HRS. AT 96 HRS A WAVE/LOW WILL FORM OVER SOUTHERN
BRASIL THAT IS FORECAST TO RAPIDLY DEEPEN WHILE EJECTING TO THE
SOUTHEAST...FORCING THE FRONT NORTH ACROSS PARAGUAY. A MOIST LOW
LEVEL NORTHERLY FLOW WILL HELP SUSTAIN BOUNDARY LAYER DEW POINT
TEMPERATURES OF 18-20C ALONG THE FRONT OVER THE CONTINENT...
WHICH WILL HELP FUEL ORGANIZED CONVECTION AS SHORT WAVE VORTICES
STREAM ACROSS MID SECTIONS OF SOUTH AMERICA. SCATTERED CONVECTION
OVER SOUTHEAST/SOUTHERN BRASIL AND CENTRAL/SOUTHEAST PARAGUAY
WILL AMOUNT TO 25-50MM/DAY THROUGH 36 HRS...AND 20-40MM/DAY AT
36-72 HRS. BY 96-120 HRS EXPECT A SURGE IN ACTIVITY ACROSS
URUGUAY-MESOPOTAMIA VALLEY/SOUTHEAST PARAGUAY-SOUTHERN BRASIL...
WITH MAXIMA OF 30-60MM.
A NORTHERN STREAM MID LEVEL TROUGH OVER THE EASTERN PACIFIC WILL
MOVE EAST TO EXTEND BETWEEN 80W-120W BY 24 HRS. THROUGH 48 HRS IT
WILL SHEAR A SHORT WAVE VORTEX EASTWARD THAT IS TO MOVE ACROSS
SOUTHERN/CENTRAL CHILE TO PATAGONIA-LA PAMPA IN ARGENTINA. AS IT
ENTERS CHILE/ARGENTINA...THE SHORT WAVE TROUGH WILL MERGE WITH AN
AMPLIFYING SOUTHERN STREAM TROUGH ENTERING THE SOUTHERN CONE. AT
72-96 HRS A BROAD TROUGH WILL THEN EXTEND OVER THE CONTINENT
BETWEEN 80W-40W AND SOUTH OF 20S. THIS IS TO SLOWLY MIGRATE TO
THE WESTERN ATLANTIC BY 120 HRS. AT LOW LEVELS...EXPECT AN
ELONGATED FRONT TO SLOWLY MOVE EAST...ENTERING CHILE BETWEEN
CONCEPCION-PUERTO MONTT...WHERE A SERIES OF FRONTAL WAVES ARE TO
MAKE LANDFALL OVER THE NEXT THREE DAYS. THIS WILL CONTRIBUTE TO
SCATTERED CONVECTION...WITH ACCUMULATION OF 05-10MM/DAY THROUGH
AT LEAST 36 HRS...AND 10-15MM/DAY AT 36-72 HRS. THROUGH 72-84 HRS
THE SURFACE FRONTS WILL PUSH EAST ACROSS LA PAMPA/BUENOS AIRES
PROVINCE...TO CONTINUE FARTHER NORTH ACROSS RIO DE LA PLATA-
CORDOBA BY 96 HRS. THIS IS TO THEN REINFORCE THE OLD SURFACE
FRONT OVER MID SECTIONS OF SOUTH AMERICA (SEE ABOVE)...TO SUSTAIN
ORGANIZED DEEP CONVECTION. FARTHER SOUTH...AS A BROAD TROUGH
SETTLES OVER THE DRAKE PASSAGE...OTHER SOUTHERN STREAM VORTICES
WILL ENTER TIERRA DEL FUEGO..WHERE THEY WILL SUSTAIN MODERATE
CONVECTION AND RISK OF SCATTERED SNOWFALL AMOUNTS.
AT 200 HPA...THE SUBTROPICAL RIDGE...WITH A CLOSED HIGH NEAR 10S
50W...EXTENDS OVER THE CONTINENT TO THE NORTH OF 20S. A SHORT
WAVE TROUGH OF THE COAST OF PERU WILL PUSH AGAINST THIS AXIS...
AND THROUGH 48 HRS IT IS TO GRADUALLY INDUCE THE COLLAPSE OF THE
RIDGE AS IT PUSHES FARTHER EAST/INLAND. THROUGH 72 HRS A NEW CELL
OF THE SUBTROPICAL RIDGE IS TO CLOSE OVER THE EASTERN PACIFIC...
WITH BROAD RIDGE PATTERN TO DOMINATE AREA NORTH OF 25S AND WEST
OF 70W. THE TROUGH...MEANWHILE...WILL MOVE ACROSS 65W BY 24
HRS...TO CONTINUE ACROSS 50W BY 72 HRS. AS THE TROUGH ENTERS
CHILE/BOLIVIA IT WILL SUPPORT A DIVERGENT PATTERN ALOFT...TO
ENHANCE CONVECTION ACROSS THE SOUTHERN JUNGLE/SIERRA OF PERU AND
THE ALTIPLANO OF BOLIVIA...WHERE IT WILL FAVOR MAXIMA OF 15-
30MM/DAY. LOCALLY HIGHER AMOUNTS ARE LIKELY DUE TO TOPOGRAPHICAL
FORCING. OVER SOUTHEAST BRASIL...IN INTERACTION WITH THE SURFACE
FRONT...IT WILL SUPPORT MAXIMA OF 30-60MM/DAY. OVER NORTHEAST
BRASIL...MODERATE TO HEAVY RAINS ARE EXPECTED ACROSS PERNAMBUCO-
RIO GRANDE DO NORTE THROUGH 48 HRS...AS A TROPICAL WAVE TO THE
NORTH ENHANCES SEA BREEZE CONVERGENCE TO SUSTAIN DAILY MAXIMA OF
20-35MM. NORTHERN PARA-AMAZONAS INTO SOUTHERN COLOMBIA WILL
CONTINUE EXPERIENCING SCATTERED CONVECTION IN CLUSTERS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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