355
FXSA20 KWBC 211637
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1237 PM EDT THU AUG 21 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 21). THE GLOBAL MODELS
REMAIN IN GOOD AGREEMENT...ONLY MAKING MINOR CORRECTIONS TO
PREVIOUS PATTERN. THIS IS REFLECTED ON THE GLOBAL ENSEMBLES...AS
THE MEMBERS CLUSTER QUITE WELL ACROSS THE EASTERN PACIFIC AND
SOUTHERN HALF OF SOUTH AMERICA. ENSEMBLE DIFFERENCES LIMIT TO THE
CENTRAL/WESTERN ATLANTIC...AS THE MEMBERS DIVERGE ON SPEED OF
PROPAGATION OF AN AMPLIFYING TROUGH. ON THE MEDIUM-LONG RANGE
TIME FRAME...THE MODELS CONTINUE TO FORECAST A HIGH AMPLITUDE/
LONG WAVE TROUGH TO ESTABLISH OFF THE COAST OF CHILE. THIS SYSTEM
IS TO ESTABLISH A SUBTROPICAL CONNECTION...AND SUSTAIN ADVECTION
OF DEEP/LAYERED MOISTURE...FIRST INTO SOUTHERN CHILE AND THEN
CENTRAL CHILE. THIS IS TO ONCE AGAIN PRESENT A RISK OF HEAVY
RAINFALL AMOUNTS OVER THIS AREA.
AT 500 HPA...A NORTHERN STREAM TROUGH EXTENDS ACROSS CHILE
AND ARGENTINA. THIS TROUGH MOVES EAST TO ALIGN ALONG 50W AND
SOUTH OF 25S BY 24 HRS...AND THROUGH 48 HRS IT WILL ADVANCE
ACROSS 40W...WHERE IT WILL MERGE WITH AN AMPLIFYING SOUTHERN
STREAM TROUGH. THESE THEN EVOLVE INTO A LONG WAVE TROUGH BETWEEN
45W-15W AND SOUTH OF 20S. LATER IN THE CYCLE A LOW WILL CLOSE
ALONG THIS AXIS NEAR 42S 19W. AT LOW LEVELS...PROGRESSIVE FRONTS
WILL STREAM ACROSS SOUTHERN CHILE AND PATAGONIA DURING THE NEXT
24-36 HRS...TO SUSTAIN SCATTERED CONVECTION WITH ACCUMULATION OF
05-10MM/DAY. ACROSS PATAGONIA THEY WILL FAVOR MOSTLY LIGHT
COASTAL CONVECTION...WITH BEST FOR PRECIPITATION OVER LA PAMPA-
SOUTHERN BUENOS AIRES PROVINCE. THERE IS ANOTHER FRONT FARTHER TO
THE NORTH...WITH AXIS ALONG AN OCCLUDING LOW OVER RIO GRANDE DO
SUL BRASIL/URUGUAY-TO PARAGUAY/SOUTHERN BOLIVIA. THE OCCLUDING
LOW WILL RAPIDLY DEEPEN THROUGH 36-48 HRS...IN THIS PROCESS
DRAWING THE FRONT NORTH ACROSS SANTA CATARINA IN SOUTHERN BRASIL
TO SAO PAULO...THEN WEST OVER MATO GROSSO DO SUL/PARAGUAY...WHERE
IT WILL REMAIN THROUGH 72-84 HRS. AT 250 HPA...THE SUBTROPICAL
JET TRAVERSES THE CONTINENT ALONG 24S/26S...AND IT IS TO
GRADUALLY MIGRATE OFF THE COAST OF BRASIL BY 72-84 HRS. JET
MAXIMA WILL PROPAGATE ALONG THIS AXIS...MEANWHILE SUSTAINING
AREAS OF DIVERGENCE ALOFT THAT ARE TO ENHANCE CONVECTION ALONG
THE SURFACE FRONT. SCATTERED RAINS WILL CONCENTRATE OVER SOUTHERN
BRASIL-UPPER MESOPOTAMIA...WITH MAXIMA OF 10-15MM/DAY.
ANOTHER NORTHERN STREAM MID LEVEL SHORT WAVE TROUGH MOVES INTO
CENTRAL CHILE BY 36-48 HRS. THROUGH 60-72 HRS THE PERTURBATION
WILL MOVE ACROSS THE ANDES OF CHILE INTO ARGENTINA...TO ALIGN
ALONG 63W. BY 96 HRS IT WILL MOVE OFF THE COAST OF URUGUAY/
SOUTHERN BRASIL. AS IT ENTERS CENTRAL CHILE...IT WILL FAVOR
WIDELY ISOLATED SNOW SHOWERS OVER THE ANDES...MAINLY SOUTH OF
30S. BUT AS IT ENTERS ARGENTINA...IT WILL INTERACT WITH THE
AFOREMENTIONED FRONT OVER BRASIL/PARAGUAY...FORCING THE FRONT TO
RETROGRESS WHILE INDUCING A NEW WAVE/LOW OVER URUGUAY/MESOPOTAMIA
VALLEY. THROUGH 72-96 HRS THE WAVE/LOW MOVES OFF THE COAST OF
URUGUAY...BUT IN THIS CYCLE THE MODELS FAVOR A WEAKER LOW THAN ON
THE PREVIOUS RUNS. THIS WILL ALSO TRIGGER SCATTERED RAINFALL
ACROSS URUGUAY-RIO GRANDE DO SUL BRASIL AND PORTIONS OF
MESOPOTAMIA...WITH ACCUMULATION OF 10-15MM/DAY.
ON THE SOUTHERN STREAM...A BROAD MID LEVEL TROUGH EXTENDS ACROSS
THE DRAKE PASSAGE/ANTARCTIC PENINSULA...WITH THE FRONT TO REMAIN
BETWEEN 100W-40W AND SOUTH OF 50S THROUGH AT LEAST 36 HRS. BY
48-60 HRS THE TROUGH WILL MOVE TO EXTEND BETWEEN 40W/60W...AND BY
72-96 HRS IT WILL MOVE ACROSS 25W/30W WHILE AMPLIFYING NORTH TO
30S. IN A TIGHT PRESSURE GRADIENT...THIS WILL SUSTAIN STRONG
BOUNDARY LAYER WINDS OF 35-55KT ACROSS THE SOUTH PACIFIC TO THE
DRAKE PASSAGE AND TIERRA DEL FUEGO DURING THE NEXT 36-48 HRS. AT
LOW LEVELS...THE ASSOCIATED FRONT EXTENDS ACROSS TIERRA DEL
FUEGO/SOUTHERN PATAGONIA. BUT IT THEN MIGRATES FARTHER SOUTH BY
48-72 HRS. THIS WILL SUSTAIN A WET PATTERN OVER CHILE SOUTH OF
47S...TO ALSO INCLUDE MOST OF TIERRA DEL FUEGO...WITH DAILY
RAINFALL AMOUNTS OF 05-10MM. SNOWFALL IS EXPECTED ON THE SOUTHERN
ANDES.
FARTHER UPSTREAM...AT 500 HPA...A HIGH AMPLITUDE TROUGH WILL MOVE
TO EXTEND BETWEEN 120W-80W TO 20S. A LOW IS TO GRADUALLY CLOSE
ALONG THIS AXIS NEAR 40S 100W BY 96-108 HRS. THE LOW WILL MOVE TO
44S 90W BY 120 HRS. BY THE END OF THE PERIOD SHORT WAVE ENERGY
WILL START TO SHEAR SOUTHEASTWARD ACROSS CENTRAL AND SOUTHERN
CHILE...WITH SYSTEM TO INDUCE HEIGHT FALLS OF 75-150 GPM. THE
HIGH AMPLITUDE TROUGH IS TO ESTABLISH A SUBTROPICAL CONNECTION...
AND IT WILL ADVECT WARM/MOIST AIR FIRST INTO SOUTHERN CHILE...AND
THEN CENTRAL CHILE BY 108-120 HRS. UNDER INFLUENCE OF THIS
SYSTEM...THE BOUNDARY LAYER MIXING RATIO VALUES WILL INCREASE TO
07-08G/KG BETWEEN CONCEPCION AND PUERTO MONTT CHILE. THE MEDIUM/
LONG RANGE PROGS THEN SHOW A HIGH RISK OF ORGANIZED HEAVY
RAINFALL AMOUNTS TO DEVELOP IN ASSOCIATION WITH THIS FEATURE...IN
A REPEAT OF SIMILAR SYSTEM THAT WAS OBSERVED NEARLY A WEEK AGO.
RAINFALL MAXIMA OF 25-50MM WILL CONCENTRATE BETWEEN PUERTO MONTT
AND CONCEPCION CHILE...WHILE BETWEEN PUERTO MONTT AND ISLA DE
CHILOE MAXIMA WILL PEAK AT 20-35MM.
THE SUBTROPICAL RIDGE...AT 200 HPA...WILL DOMINATE CONTINENTAL
AREA NORTH OF 20S. MIGRATORY HIGHS/ANTICYCLONES WILL ANCHOR THIS
RIDGE OVER THE CONTINENT. CONVECTION OVER NORTHERN AMAZONAS INTO
COLOMBIA-ECUADOR WILL REMAIN ACTIVE...WITH CELLS SPREADING INTO
THE NORTHERN JUNGLE OF PERU AND ECUADOR. DAILY MAXIMA WILL PEAK
AT 25-35MM. SEA BREEZE CONVERGENCE ALONG THE NORTHERN COAST...TO
AFFECT WEST OF FORTALEZA WILL SUSTAIN MAXIMA OF 15-30MM. OVER
BAHIA-RIO GRANDE DO NORTE...TRADE WIND CONVERGENCE WILL FAVOR
MAXIMA OF 10-15MM/DAY IN SHALLOW CONVECTION.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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