456
FXSA20 KWBC 251644
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1244 PM EDT THU SEP 25 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 25). GFS BREAKS CONTINUITY
AS EARLY AS 54-60 HRS...WITH CYCLE-TO-CYCLE INCONSISTENCIES AS
THE MODEL FLIP-FLOPS BETWEEN SOLUTIONS. THE MODEL SEEMS UTTERLY
BAFFLED/CONFUSED BY THE EVOLVING PATTERN. THE ECMWF AND UKMET ARE
NOT WITHOUT FAULT...BUT THEY TEND TO MAKE MINOR CORRECTIONS TO
THE PATTERN AND SEEMS MORE CONSISTENT. THE ENSEMBLES...WE HAVE
FOUND...ARE OF LITTLE HELP UNDER THESE CONDITIONS. CONSIDERING
THE RADICAL CHANGES THE 00 UTC GFS MADE...AND THAT THE 06 UTC RUN
OF THE MODEL CORRECTED/REVERTED IN FAVOR OF THE ECMWF SOLUTION...
IN OUR FORECAST WE OPTED FOR THE LATTER RATHER THAN THE FORMER.
FORECAST CHARTS WERE GENERATED USING THE 06 UTC GFS. IT GOES
WITHOUT SAYING...BUT CONFIDENCE IN THE FORECAST IS LOW.
AT 500 HPA...A SHORT WAVE TROUGH MOVES ACROSS URUGUAY-MESOPOTAMIA
VALLEY-SOUTHERN BRASIL EARLY THIS CYCLE...TO CONTINUE INTO SAO
PAULO/SOUTHEAST BRASIL THROUGH 36-48 HRS. AT 72-96 HRS THIS
VORTEX EVOLVES INTO A CLOSED LOW OFF THE COAST OF BRASIL NEAR 27S
35W. THIS TROUGH IS TO INTERACT WITH A MEANDERING FRONT OVER
BRASIL...TO INDUCE A FRONTAL LOW/WAVE ACROSS NORTHERN SAO PAULO
BY 24 HRS...WITH LOW TO DRIFT ACROSS SOUTHERN MINAS TO RIO DE
JANEIRO THROUGH 48-54 HRS...THEN OFF THE COAST EAST OF RIO DE
JANEIRO BY 72-84 HRS. AS THE WAVE FORMS...IT WILL TRIGGER
MODERATE CONVECTION ACROSS NORTHERN SAO PAULO/SOUTHERN MINAS
GERAIS...WITH MAXIMA OF 10-20MM/DAY. THIS INCREASES TO 20-
40MM/DAY AS IT BUILDS ACROSS RIO DE JANEIRO-SOUTHERN MINAS
GERAIS/NORTHERN SAO PAULO BY 48-60 HRS. THROUGH 72 HRS THIS THEN
DECREASES ONCE AGAIN TO 10-20MM/DAY...WHILE IT CONCENTRATES
ACROSS EASTERN MINAS GERAIS/ESPIRITO SANTO IN BRASIL.
OVER THE PACIFIC...AT 500 HPA...THE SYSTEM BAFFLING THE MODELS IS
INITIALIZED BETWEEN 85W/95W AND SOUTH OF 15S. AS THE MAIN BODY OF
THE TROUGH MOVES ACROSS 80W BY 24 HRS...IT WILL CLASH WITH A
BLOCKING RIDGE OVER CHILE/ARGENTINA. UNDER INFLUENCE OF THIS
RIDGE...THE TROUGH WILL SPLIT IN TWO. THE SOUTHERN HALF WILL MOVE
ACROSS THE DRAKE PASSAGE/SOUTHERN CHILE-PATAGONIA BY 54-60 HRS...
THEN INTO THE SOUTH ATLANTIC THROUGH 72-84 HRS. THE NORTHERN
SEGMENT...MEANWHILE...INITIALLY STALLS OVER THE CENTRAL ANDES OF
CHILE. BUT...BETWEEN 84-96 HRS IT WILL SPILL ACROSS THE ANDES TO
THE CENTRAL PROVINCES OF ARGENTINA AS ANOTHER NORTHERN STREAM
TROUGH ENTERS THE EASTERN PACIFIC. THIS SHORT WAVE TROUGH SLOWLY
MOVES ACROSS CENTRAL ARGENTINA THROUGH 108 HRS...AND OFF THE
COAST BY 108-120 HRS. AT LOW LEVELS EXPECT A POLAR FRONT TO MOVE
ACROSS PATAGONIA ARGENTINA-CONCEPCION CHILE BY 24 HRS...WHERE IT
IS TO STALL THROUGH 48-60 HRS. BY 72-96 HRS IT STARTS TO WEAKEN
OVER THE EASTERN PACIFIC...WHILE IT PERSISTS OVER THE CONTINENT
AS A WAVE/LOW FORMS OVER CENTRAL CUYO. THIS LOW DEEPENS THROUGH
120 HRS WHILE DRIFTING EAST-SOUTHEAST...MEANWHILE PROPELLING THE
FRONT NORTH ACROSS NORTHERN PROVINCES OF ARGENTINA. AS THE FRONT
ACCELERATES...A PREFRONTAL SQUALL LINE WILL FORM AHEAD OF THIS
BOUNDARY. AS THE FRONT ENTERS SOUTHERN CHILE IT WILL FAVOR MOSTLY
LIGHT CONVECTION WITH MAXIMA TO PEAK AT 05-10MM/DAY. OVER THE
CENTRAL ANDES AND PORTIONS OF CUYO IN ARGENTINA MAXIMA WILL PEAK
AT 10-20MM/DAY ON DAY 02...WHILE ON DAY 03-04 THE MAXIMA WILL
PEAK AT 20-35MM AS THE BOUNDARY MEANDERS OVER CORDOBA-MENDOZA IN
ARGENTINA. SIMILAR AMOUNTS ARE EXPECTED ACROSS NORTHERN
ARGENTINA-PARAGUAY BY THE END OF THE CYCLE...WITH SLIGHT RISK OF
SEVERE CONVECTION IN ASSOCIATION WITH THE SQUALL LINE AS IT MOVES
NORTH OF 28S/30S AND FINDS MORE FAVORABLE ENVIRONMENT.
NORTH OVER THE CONTINENT...AT 200 HPA....THE MODELS ARE MORE
CONSISTENT IN THEIR FORECAST OF THE SUBTROPICAL RIDGE...SHOWING A
BROAD RIDGE PATTEN ESTABLISHING ON CONTINENTAL AREA NORTH OF 25S
AND WEST OF 50W BY 48-60 HRS...AND TO PERSIST THROUGH AT LEAST
96-108 HRS. AS THE RIDGE INTENSIFIES...EXPECT A SHARP INCREASE IN
TROPICAL CONVECTION ACROSS WESTERN/NORTHWEST PORTIONS OF BRASIL-
PERUVIAN JUNGLE-SOUTHERN COLOMBIA AND ACROSS MOST OF ECUADOR...
WHERE DAILY MAXIMA WILL LIKELY EXCEED 20-40MM/DAY. THE MOST
INTENSE IS EXPECTED BETWEEN 36-84 HRS. AN INCREASE IN ACTIVITY IS
ALSO EXPECTED ACROSS CENTRAL/SOUTHERN JUNGLE OF PERU INTO
NORTHERN BOLIVIA/RONDONIA BRASIL BY 84-120 HRS AS PREFRONTAL
CONVECTION DEVELOPS IN THIS AREA AND THE LOW LEVEL THERMAL TROUGH
BECOMES BETTER ORGANIZED ACROSS THE JUNGLE.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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