994
FXSA20 KWBC 121612
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1211 PM EDT FRI SEP 12 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 12). GENERAL AGREEMENT AMONG
THE DETERMINISTIC SOLUTION OF THE GLOBAL MODELS THROUGH 96-108
HRS...WITH DIFFERENCES LIMITING TO LATE IN THE PERIOD ON HOW THEY
RESOLVE/DEVELOP A SHORT WAVE PERTURBATION ENTERING THE SOUTHERN
CONE/DRAKE PASSAGE. IN THIS REGION BOTH EUROPEAN MODELS FAVOR A
SIGNIFICANTLY FASTER AND FARTHER SOUTH SOLUTION THAN THE GFS.
BUT...THE GFS SOLUTION HAS THE SUPPORT OF THE GLOBAL ENSEMBLES
(NCEP-EUROPEAN-CANADIAN). SO WE ARE GOING TO FOLLOW THE GFS
GUIDANCE.
ON THE NORTHERN STREAM...AT 500 HPA...A SHORT WAVE TROUGH
DOMINATES THE SOUTHERN CONE/MID SECTIONS OF SOUTH AMERICA. THIS
PERTURBATION WILL DRIFT EAST-NORTHEAST OVER THE NEXT FIVE DAYS.
AT 24 HRS IT WILL CENTER ON A DEEPENING LOW NEAR 40S 57W...TO
MOVE TO 39S 41W THROUGH 72 HRS. AS IT DRIFTS EAST...THE
ASSOCIATED SHORT WAVE TROUGH WILL GRADUALLY SWEEP FROM NORTHERN
ARGENTINA/PARAGUAY/SOUTHERN BOLIVIA EARLY THIS CYCLE TO SOUTHERN
BRASIL BY 60-72 HRS. BETWEEN 96-108 HRS THE LOW WILL FILL...BUT
THE TROUGH WILL PERSIST OVER SOUTHEAST BRASIL THROUGH AT LEAST
120 HRS. AT 250 HPA A SUBTROPICAL JET IS TO CROSS NORTHERN CHILE-
PARAGUAY INTO SOUTHERN BRASIL DURING THE NEXT 84-96 HRS. BY THE
END OF THE CYCLE THE JET STREAK IS TO WEAKEN. AT LOW LEVELS AN
OCCLUDING LOW MOVED OFF THE COAST OF ARGENTINA...WHICH IS TO
QUICKLY MERGE WITH A BROAD TROUGH PATTERN OFF THE SOUTHERN COAST
OF BRASIL. BETWEEN 24-48 HRS THIS SYSTEM DEEPENS/INTENSIFIES OVER
THE WESTERN ATLANTIC. THE ASSOCIATED SURFACE FRONT MOVES OVER SAO
PAULO/SANTA CATARINA IN BRASIL-MATO GROSSO DO SUL TO CENTRAL/
NORTHERN BOLIVIA...WHERE IT WILL REMAIN THROUGH AT LEAST 108 HRS.
MOST INTENSE CONVECTION OVER SOUTHERN/SOUTHEAST BRASIL IS
EXPECTED THROUGH 36 HRS...WHEN IT WILL SUSTAIN MAXIMA OF 15-30MM.
THIS IS TO GRADUALLY DECREASE TO 05-10MM/DAY THROUGH 72-84 HRS
WHILE CONFINING TO NORTHERN SAO PAULO/RIO DE JANEIRO. BUT BY 96
HRS ACTIVITY ONCE AGAIN SURGES AS AN UPPER LEVEL TROUGH INTERACTS
WITH THE FRONT...WITH MAXIMA TO PEAK AT 15-30MM. OTHER CONVECTION
IS TO BUILD ON THE TAIL END OF THIS SYSTEM OVER CENTRAL/NORTHERN
BOLIVIA TO SOUTHERN PERU...WITH RAINFALL MAXIMA OF 15-30MM/DAY.
THIS PERSIST THROUGH 60-72 HRS...WITH LESSER AMOUNTS LATER IN THE
CYCLE. CONVECTION IS TO ALSO SURGE ACROSS CENTRAL/NORTHERN PERU
BY 48 HRS...AND PERSIST THROUGH 96 HRS...WITH DAILY MAXIMA TO
PEAK AT 20-40MM/DAY ON DAY 03-04.
OVER THE PACIFIC...A BROAD MID/UPPER LEVEL TROUGH PATTERN IS TO
DOMINATE AREA BETWEEN 130W-70W AND SOUTH OF 30S...WHILE A SKEWED
RIDGE ESTABLISHES ACROSS THE SOUTHERN CONE/WESTERN ATLANTIC.
SHORT WAVE PERTURBATIONS WILL ROTATE AROUND THE TROUGH...THEN
PUSH EAST AGAINST THE RIDGE. THIS WILL THEN FORCE SHORT WAVE
VORTICES TO TRACK SOUTHEAST ACROSS SOUTHERN CHILE/PATAGONIA...
WITH A STRONG VORTEX TO ENTER BY 60-72 HRS. THIS WILL SET THE
STAGE FOR A STRONGER PERTURBATION FORECAST TO MOVE ALONG 95W AND
SOUTH OF 25S BY 96 HRS. THROUGH 120 HRS THIS TROUGH WILL MOVE
ALONG 80W/85W...WITH SHORT WAVE ENERGY FOCUSING ACROSS CENTRAL/
SOUTHERN CHILE AND SOUTHERN ARGENTINA. AT LOW LEVELS EXPECT A
WEAK FRONT TO ENTER EXTREME SOUTHERN CHILE/PATAGONIA BY 60 HRS...
WITH A STRONGER FRONT TO ENTER THIS AREA BETWEEN 96-108 HRS.
UNDER INFLUENCE OF THE FIRST BOUNDARY RAINFALL MAXIMA WILL PEAK
AT 05-10MM...WHILE THE SECOND FRONT WILL SUSTAIN MAXIMA OF 15-
30MM. FURTHERMORE...IN A TIGHT PRESSURE GRADIENT...THE SECOND
FRONT IS TO ALSO FAVOR STRONG BOUNDARY LAYER WINDS ACROSS
SOUTHERN PATAGONIA/TIERRA DEL FUEGO AND EXTREME SOUTHERN CHILE.
NORTH OVER THE CONTINENT...AT 200 HPA...MODELS CONTINUE TO
FORECAST AN UPPER LEVEL TROUGH TO AMPLIFY ACROSS BOLIVIA TO
WESTERN BRASIL EARLY IN THE PERIOD. THROUGH 72 HRS THE TROUGH
MOVES EAST ACROSS PARAGUAY-EL PANTANAL TO SOUTHERN PARA...AND
LATER IN THE CYCLE IT ALIGNS ALONG 50W. THIS FEATURE CONTINUES TO
FAVOR ISOLATED TO SCATTERED CONVECTION ON CONTINENTAL AREA NORTH
OF 05S/06S...WITH ACTIVITY FORMING IN CLUSTERS DURING THE LATE
AFTERNOON HOURS. THESE ARE TO FAVOR RAINFALL AMOUNTS OF 15-
30MM/DAY...WITH LOCALLY HIGHER AMOUNTS QUITE POSSIBLE OVER PARA/
AMAZONAS IN NORTHWEST BRASIL.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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