978
FXSA20 KWBC 221701
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
100 PM EDT MON SEP 22 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 22). OVER THE ATLANTIC THE
GLOBAL MODELS AGREE QUITE WELL THROUGH 120 HRS. OVER THE EASTERN
PACIFIC/SOUTHERN CONE...HOWEVER...THE DIFFERENCES BECOME EVIDENT
BY 72-84 HRS...AS THEY DIVERGE ON AMPLITUDE OF A MID LATITUDE
TROUGH ENTERING THE CONTINENT. THE EUROPEAN MODELS FAVOR A
STRONGER SOUTHERN STREAM INCURSION THAN WHAT THE GFS SUGGESTS...
AND THIS COMES WITH THE STRONG SUPPORT OF THE GLOBAL ENSEMBLES...
AND THIS IS A LIKELY TREND FOR THE GFS MODEL TO FOLLOW ON FUTURE
RUNS.
AT 500 HPA...A BROAD TROUGH IS INITIALIZED OVER SOUTHEAST
BRASIL...CENTERING ON A MEANDERING LOW NEAR 30S 43W. THROUGH 48
HRS THE LOW WILL FILL TO AN OPEN TROUGH...WHILE THE TROUGH PUSHES
EAST. AT 72-96 HRS...HOWEVER...ANOTHER LOW FORMS ALONG THIS AXIS
NEAR 40S 30W AS IT RECEIVES AN INFLOW OF SOUTHERN STREAM ENERGY.
AT LOW LEVELS THIS TROUGH SUPPORTS A BROAD/OCCLUDED TROUGH OFF
THE SOUTHEAST COAST OF BRASIL...AND A FRONT WITH AXIS NORTHWEST
ACROSS RIO DE JANEIRO/NORTHERN SAO PAULO TO MATO GROSSO DO SUL/
NORTHERN BOLIVIA. THE LOW/TROUGH SLOWLY PULLS TO THE EAST-
SOUTHEAST OVER THE NEXT THREE DAYS...WHILE THE FRONT DRIFTS NORTH
TO ESPIRITO SANTO...THEN INLAND INTO MATO GROSSO DO SUL/MATO
GROSSO-BOLIVIAN JUNGLE. THIS IS TO MEANWHILE CONTINUE FAVORING
SCATTERED CLOUD COVER WITH EMBEDDED CONVECTION...WITH A GRADUAL
DECREASE IN CLOUD COVER/ACTIVITY OVER CENTRAL BRASIL/NORTHERN
BOLIVIA THROUGH MID CYCLE. BUT COASTAL CONVECTION WILL PERSIST
ALONG THE SERRA DO MAR...TO CONCENTRATE OVER SAO PAULO/SANTA
CATARINA WITH MAXIMA OF 05-10MM/DAY. THIS IS TO GRADUALLY
INCREASE TO 10-20MM BY 96 HRS. OTHER CONVECTION WILL BUILD OVER
ESPIRITO SANTO/SOUTHERN BAHIA...WITH RAINFALL AMOUNTS OF 05-
10MM/DAY.
OVER THE EASTERN PACIFIC...MEANWHILE...A 500 PHA TROUGH ALONG 80W
IS PUSHING EAST AGAINST A BLOCKING RIDGE ALONG 60W/65W. UNDER
INFLUENCE OF THIS AXIS THE TROUGH WILL SHEAR EASTWARD ACROSS
PATAGONIA BY 36-48 HRS...WHILE A NORTHERN STREAM VORTEX
SEGREGATES OVER NORTHWEST ARGENTINA. THIS VORTEX WILL FAVOR
ISOLATED CONVECTION ON THE NORTHERN ANDES OF ARGENTINA BY MID
CYCLE. THE MID LEVEL TROUGH WILL MOVE ACROSS SOUTHERN TO
SOUTHEAST BRASIL BY 72-96 HRS...WHERE IT IS TO THEN STALL THROUGH
120 HRS. IT WILL CONCENTRATE ITS SHORT WAVE ENERGY OVER SAO
PAULO-MATO GROSSO DO SUL. THIS IS TO FAVOR A SURGE IN CONVECTION
ACROSS MINAS GERAIS/NORTHERN SAO PAULO IN BRASIL AND PORTIONS OF
BAHIA/ESPIRITO SANTO...WITH MAXIMA OF 30-60MM BY 96-120 HRS.
THE MODELS THEN FORECAST ANOTHER TROUGH TO MOVE ALONG 90W BY 72
HRS. THE ECMWF/UKMET THEN FORECAST THE MID LEVEL TROUGH TO MOVE
ALONG 80W BY 96 HRS...TO STALL OFF THE COAST OF CHILE AS IT THEN
CLASHES WITH A BLOCKING RIDGE OVER THE CONTINENT/SOUTHERN CONE.
THE GFS INITIALLY FOLLOWS A SIMILAR EVOLUTION...BUT BY 96 HRS IT
STARTS TO DIVERGE AS IT FORECASTS THE TROUGH TO SPLIT IN TWO
AXES...WITH THE SOUTHERN HALF OF THE TROUGH TO QUICKLY PUSH UNDER
THE CONTINENTAL RIDGE AND ACROSS THE DRAKE PASSAGE BY 96-108 HRS.
THE GFS THEN FORECASTS THE NORTHERN PORTION OF THE TROUGH TO
MEANDER OFF THE CENTRAL COAST OF CHILE. ALTHOUGH BOTH SOLUTIONS
ARE VIABLE OPTIONS... THE ECMWF/UKMET SOLUTION COMES WITH THE
STRONG SUPPORT OF THE GLOBAL ENSEMBLES...WITH INDIVIDUAL MEMBERS
CLUSTERING QUITE WELL IN FAVOR OF THIS EVOLUTION. SO WE ARE GOING
TO DOWNPLAY THE GFS SOLUTION/FORECAST...AND GIVE MORE EMPHASIS TO
WHAT THE EUROPEAN PROJECT.
NORTH OVER THE CONTINENT...AT 200 HPA....A SHORT WAVE TROUGH
ALONG 60W/65W SEPARATES TWO RIDGES...WITH ONE OVER THE EASTERN
PACIFIC AND THE OTHER OVER THE WESTERN TROPICAL ATLANTIC/
NORTHEAST BRASIL. THE SHORT WAVE TROUGH WILL GRADUALLY MOVE EAST
ACROSS MINAS GERAIS IN BRASIL BY 60-72 HRS. BY THAT TIME...THE
WESTERN CELL OF THE SUBTROPICAL RIDGE RELOCATES TO AMAZONAS IN
BRASIL...WHERE IT BUILDS/INTENSIFIES AS THE CYCLE PROGRESSES. THE
INTENSIFICATION OF THIS RIDGE WILL ACCOMPANY AN 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. OTHER CONVECTION
IS EXPECTED OVER MINAS GERAIS IN BRASIL...WHERE THE
AFOREMENTIONED SHORT WAVE TROUGH...AND A SECONDARY VORTEX ARE TO
TRIGGER SCATTERED DEEP CONVECTION BY THE END OF THE CYCLE. IN
THIS REGION THE MODELS ARE PROJECTING MODERATE TO HEAVY RAINFALL
AMOUNTS BY 96-120 HRS...AND THEY HAVE BEEN FAIRLY CONSISTENT ON
THEIR FORECAST OF THIS EVENT.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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