935
FXSA20 KWBC 141639
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1239 PM EDT THU AUG 14 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
COR
MODEL COMPARISON (VALID FROM 00Z AUG 14). GFS AND UKMET GLOBAL
MODELS CONTINUE TO MAKE CORRECTIONS TO NORTHERN STREAM TROUGH
SYSTEM ENTERING THE SOUTHERN CONE THROUGH 30-36 HRS. THEY NOW
FOLLOW A SIMILAR EVOLUTION AS THE ECMWF WITH MOST OF THE ENERGY
FOCUSING SOUTH OF TEMUCO CHILE. BUT ENOUGH ENERGY/MOISTURE WILL
REACH CENTRAL CHILE DURING THE NEXT 48-60 HRS TO SUSTAIN EXTENDED
PERIODS OF HEAVY RAINFALL AMOUNTS. CONFIDENCE IN THE FORECAST IS
HIGH.
MODELS NOW AGREE QUITE WELL ON THE EVOLUTION OF A NORTHERN STREAM
TROUGH OVER THE EASTERN PACIFIC...WITH THE 500 HPA CIRCULATION TO
MOVE ALONG 42S 85W BY 24 HRS. AT 30-36 HRS IT IS TO NEAR THE
COAST OF CHILE...WHERE IT WILL PHASE WITH AN AMPLIFYING SOUTHERN
STREAM TROUGH OVER THE SOUTHERN CONE. AS THEY COMBINE...THEY WILL
INCITE HEIGHT FALLS IN EXCESS OF 200 GPM...FIRST OVER SOUTHERN
CHILE/PATAGONIA AND THEN INTO LA PAMPA/BUENOS AIRES PROVINCE BY
48-60 HRS. AT 250 HPA A SUBTROPICAL-NORTHERN POLAR JET PAIR WILL
ACCOMPANY THESE PERTURBATIONS...WHICH WILL TEND TO MAXIMIZE
DIVERGENCE ALOFT AS THEY DIVERGE ACROSS CENTRAL CHILE-ARGENTINA.
THE SLOW TO EVOLVE TROUGH PATTERN WILL TAKE ADVANTAGE OF ABOVE
NORMAL SEA SURFACE TEMPERATURES OVER THE PACIFIC TO SUSTAIN A
MOIST FLUX...WHICH WILL CONTRIBUTE TO BOUNDARY LAYER MIXING RATIO
VALUES OF 08-10G/KG ACROSS CHILE SOUTH OF LA SERENA. POOLS OF
HIGHER MOISTURE CONTENT WILL PEAK AT 11-12G/KG...WHICH ARE
EXTREMELY HIGH FOR THIS PORTION OF THE CONTINENT. THE HEIGHT OF
THE 0C DEG ISOTHERM STARTS AT 3.0-3.5KM...BUT THROUGH 48 HRS IT
WILL DROP TO 2.5KM...PRESENTING A HIGH RISK OF ORGANIZED SNOWFALL
AMOUNTS. AT LOW LEVELS MODELS PROJECT A SIMILAR EVOLUTION...AS A
BROAD TROUGH PATTERN ENTERS CHILE BETWEEN CONCEPCION-PUERTO MONTT
EARLY THIS PERIOD. THROUGH 24 HRS THE TROUGH WILL SPILL ACROSS
THE SOUTHERN ANDES TO PATAGONIA...DRAWING A FRONT NORTH ACROSS
NORTHERN PATAGONIA/LA PAMPA-CONCEPCION CHILE TO THE EASTERN
PACIFIC. THROUGH 48 HRS THE FRONT WILL MOVE ALONG THE COAST TO
NORTHERN CHILE...AND BY 96 HRS IT WILL MOVE ACROSS URUGUAY TO RIO
GRANDE DO SUL BRASIL-NORTHERN PROVINCES OF ARGENTINA/PARAGUAY.
OVER CENTRAL CHILE IT IS TO INITIALLY SUSTAIN MAXIMA OF 20-35MM
/DAY...FORECAST TO INCREASE TO 30-60MM/DAY WHILE EXPANDING NORTH
ACROSS CHILE THROUGH 60 HRS. NOTE THAT DURING THIS PERIOD THE
ENSEMBLE MEMBERS CONSISTENTLY FORECAST MAXIMA IN EXCESS OF
75MM...SO LOCALIZED HEAVY RAINFALL AMOUNTS ARE QUITE POSSIBLE.
BETWEEN SANTIAGO AND LA SERENA MAXIMA WILL THEN PEAK AT 10-15
MM/DAY BY 72 HRS AS A SECONDARY UPPER LEVEL VORTEX INTERACTS WITH
THE SURFACE FRONT. OVER PATAGONIA-LA PAMPA EXPECT MOSTLY LIGHT
CONVECTION WITH THE ENTRANCE OF THE MID LEVEL PERTURBATION...WITH
SCATTERED SNOWFALL TO AFFECT TIERRA DEL FUEGO AND THE SOUTHERN
ANDES. ACROSS MESOPOTAMIA VALLEY-URUGUAY INTO SOUTHERN BRASIL THE
APPROACHING FRONT WILL ALSO FAVOR SCATTERED CONVECTION BY MID
CYCLE WITH MAXIMA OF 15-30MM.
ON THE SOUTHERN STREAM...A PROGRESSIVE TROUGH WILL MOVE ALONG
100W AND SOUTH OF 45S BY 48 HRS...AND THROUGH 72-84 HRS IT WILL
ENTER THE DRAKE PASSAGE/SOUTHERN CONE. THE ASSOCIATED FRONT
ENTERS SOUTHERN CHILE/PATAGONIA BY 72-84 HRS...AND MOVES NORTH
INTO LA PAMPA BY 96 HRS. THIS IS TO ALSO FAVOR SCATTERED
PRECIPITATION OVER SOUTHERN CHILE WITH ACCUMULATION OF 05-
10MM/DAY.
AT 200 HPA...A NARROW SUBTROPICAL RIDGE CONFINES TO AREA NORTH OF
20S. THE RIDGE REMAINS DIVIDED IN TWO CELLS...WITH ONE OVER THE
EASTERN PACIFIC NEAR 14S 90W AND THE OTHER OVER NORTHEAST BRASIL.
ALTHOUGH THE PACIFIC CELL IS TO GENERALLY PERSIST THROUGH 96-120
HRS...THE CONTINENTAL CELL OF THE SUBTROPICAL RIDGE NEARLY
COLLAPSES BY 72-84 HRS AS A SHORT WAVE TROUGH PUSHES EAST ACROSS
PARAGUAY TO SOUTHERN/SOUTHEAST BRASIL. BUT AS THE TROUGH MOVES
OFF THE COAST LATER IN THE PERIOD...THE RIDGE IS TO ONCE AGAIN
BUILD OVER THE CONTINENT. TROPICAL CONVECTION OVER NORTHERN
BRASIL WILL REMAIN SPARSE...WITH MOST ACTIVE EXPECTED ALONG AND
NORTH OF THE EQUATOR...WITH DAILY MAXIMA TO PEAK AT 15-30MM IN
CLUSTERS. ACROSS ECUADOR...WESTERN PORTIONS...DAILY MAXIMA WILL
PEAK AT 05-10MM/DAY IN SEA BREEZE CONVERGENCE. AT LOW LEVELS A
FRONT ENTERS SOUTHERN BRASIL-PARAGUAY...WHERE IT WILL MEANDER
THROUGH 48-60 HRS. THIS BOUNDARY WILL REMAIN ACTIVE FROM SOUTHERN
PARAGUAY-UPPER MESOPOTAMIA TO SOUTHERN BRASIL...WHERE IT WILL
SUPPORT MAXIMA OF 20-35MM THROUGH 36 HRS...AND 05-10MM/DAY AT 36-
60 HRS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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