350
FXSA20 KWBC 291525
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1124 AM EDT TUE JUL 29 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z JULY 29). THE GFS AND ECMWF
MADE CHANGES TO THE SOUTHERN STREAM FLOW...NOW SUSTAINING A LONG
WAVE RIDGE ACROSS THE SOUTHERN CONE/DRAKE PASSAGE TO THE
ANTARCTIC PENINSULA THROUGH DAY 05. UNDER THE INFLUENCE OF THIS
RIDGE...COLD/POLAR AIR IS TO SURGE NORTH ACROSS THE WESTERN
ATLANTIC INTO MID SECTIONS OF SOUTH AMERICA. THIS SOLUTION COMES
WITH THE SUPPORT OF THE GLOBAL ENSEMBLES...SO CONFIDENCE IN THE
FORECAST IS HIGH.
AT 500 HPA...ON THE NORTHERN STREAM...A SHORT WAVE TROUGH WILL
EXTEND TO THE NORTHWEST FROM RIO DE LA PLATA BASIN TO OFF THE
NORTHERN COAST OF CHILE. THROUGH 36 HRS THE TROUGH IS TO SLOWLY
PUSH EAST ACROSS THE NORTHERN ANDES TO THE NORTHERN PROVINCES
OF ARGENTINA...AND THROUGH 72-84 HRS IT WILL MOVE OFF THE
SOUTHERN COAST OF BRASIL. THE TROUGH IS TO INTERACT WITH A FRONT
OVER SOUTHERN BRASIL-NORTHERN ARGENTINA/PARAGUAY...FAVORING
FORMATION OF LOWS/WAVES ALONG THE FRONT. THESE ARE TO ALSO
TRIGGER SCATTERED DEEP CONVECTION...WITH MOST INTENSE TO
CONCENTRATE OVER SOUTHERN BRASIL-NORTHERN URUGUAY/MESOPOTAMIA TO
SOUTHERN PARAGUAY. DAILY MAXIMA WILL PEAK AT 20-40MM THROUGH 48-
60 HRS...WITH A GRADUAL DECREASE IN ACTIVITY BY MID CYCLE...WITH
MAXIMA TO DECREASE TO 10-15MM/DAY.
ON THE SOUTHERN STREAM...ALSO AT 500 HPA...A BROAD TROUGH IS TO
EXTEND ACROSS THE DRAKE PASSAGE TO THE SOUTHERN CONE...AS IT
BOUNDS BETWEEN 85W-35W AND SOUTH OF 40S. THE MEAN TROUGH AXIS IS
TO SLOWLY MOVE ACROSS 55W/60W BY 48 HRS...AND BY 72-96 HRS IT IS
TO BOUND BETWEEN 70W/30W AND SOUTH OF 30S. THE TROUGH WILL THEN
REMAIN NEARLY STATIONARY WHILE AMPLIFYING TO THE NORTHWEST BY THE
END OF THE CYCLE...WITH AXIS TO EXTEND ALONG 60S 20W...50S 30W...
40S 50W...RIO DE LA PLATA TO SOUTHERN BOLIVIA. THIS IS A CHANGE
FROM THE PREVIOUS RUNS...WHEN THE MODELS WERE FAVORING A LOWER
AMPLITUDE TROUGH PATTERN TO REMAIN OVER THE SOUTH ATLANTIC.
UPSTREAM FROM THIS TROUGH...THE MODELS NOW FAVOR A HIGH AMPLITUDE
RIDGE TO ESTABLISH ACROSS THE SOUTHERN CONE/DRAKE PASSAGE TO THE
ANTARCTIC PENINSULA. THIS...IN-TURN...WILL FEED COLD/POLAR ENERGY
INTO THE BACKSIDE OF THE TROUGH...TO RESULT IN THE AFOREMENTIONED
HIGH AMPLITUDE LONG WAVE PATTERN. AT LOW LEVELS...THE MID/UPPER
TROUGH IS TO ALSO FAVOR A BROAD TROUGH PATTERN ACROSS THE
ANTARCTIC PENINSULA TO THE SOUTHERN CONE... WITH SUCCESSIVE
FRONTS TO REVOLVE AROUND THIS AXIS DURING THE NEXT 48-60 HRS.
DURING THIS PERIOD...THE FRONTS ARE TO GENERALLY FOCUS OVER
CHILE/PATAGONIA SOUTH OF 40S...WITH QUITE A FEW REACHING AS FAR
NORTH AS 34S. BUT MOST ACTIVE CONVECTION IS TO CONCENTRATE
BETWEEN CONCEPCION AND ISLA DE CHILOE...WITH DAILY MAXIMA TO PEAK
AT 10-15MM. AT 60-84 HRS EXPECT A FRONT TO PUSH ACROSS RIO DE LA
PLATA BASIN-CORDOBA IN ARGENTINA. BY 96-108 HRS...A 1025-1027 HPA
POLAR HIGH IS TO BUILD ACROSS THE SOUTHERN CONE...WHICH WILL
SUPPORT THE NORTHWARD MIGRATION OF THE SURFACE FRONT ACROSS
URUGUAY-SOUTHERN BRASIL TO THE NORTHERN PROVINCES OF ARGENTINA.
BY 108-132 HRS THE NEW FRONT WILL MERGE WITH/REINFORCE THE OLD
BOUNDARY OVER NORTHERN ARGENTINA/PARAGUAY...WITH THE POLAR RIDGE
TO THE SOUTH DISPLACING THIS FRONT FARTHER NORTH ACROSS SOUTHERN
BRASIL-PARAGUAY TO EASTERN BOLIVIA. AS IT PUSHES NORTH ACROSS
ARGENTINA THIS FRONT WILL SUPPORT MOSTLY LIGHT RAINFALL. BEST
CHANCE FOR ORGANIZED RAINFALL WILL BE AT 132-144 HRS...WHEN THE
FRONT IS TO MOVE NORTH INTO THE BOLIVIAN JUNGLE AND TRIGGER
MAXIMA OF 25-50MM.
AT 200 HPA...THE SUBTROPICAL RIDGE EXTENDS FROM A HIGH OVER PARA
IN CENTRAL BRASIL TO THE EASTERN TROPICAL PACIFIC. THE RIDGE
PATTERN PERSISTS NEARLY UNCHANGED THROUGH 24-36 HRS. BUT BY 48-60
HRS THE HIGH IS EXPECTED TO RELOCATE EAST TO 10S 53W...WHILE THE
RIDGE EXPANDS INTO NORTHEAST BRASIL. LATER IN THE CYCLE THE RIDGE
IS TO DOMINATE CONTINENTAL AREA NORTH OF 15S. THE POLAR INCURSION
OVER MID SECTIONS OF SOUTH AMERICA WILL LEAD TO GRADUAL EROSION
OF THIS RIDGE PATTERN BY THE END OF THE CYCLE. LOW LEVEL TRADE
WIND EASTERLY CONVERGENCE WILL SUSTAIN COASTAL RAINFALL OVER
NORTHEAST BRASIL...WITH MOST INTENSE ACROSS NORTHERN ESPIRITO
SANTO AND BAHIA WITH MAXIMA OF 10-20MM/DAY THROUGH 48-60 HRS.
OTHER CONVECTION WILL CONCENTRATE ACROSS THE NORTHERN COAST OF
BRASIL...WITH SIMILAR AMOUNTS. OVER AMAPA/GUIANAS AND ALONG THE
NEAR EQUATORIAL TROUGH TO THE NORTH THE DAILY MAXIMA WILL PEAK AT
20-35MM...WHILE OVER WESTERN ECUADOR AND CAUCA VALLEY IN
SOUTHWEST COLOMBIA THE DAILY MAXIMA WILL PEAK AT 10-15MM.
COLMAN...DMH (PARAGUAY)
GUZMAN...SENAMHI (BOLIVIA)
DAVISON...HPC (USA)
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