799
FXSA20 KWBC 311353
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
953 AM EDT THU JUL 31 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z JULY 31). THE GFS AND EUROPEAN
MODELS MADE SOME CORRECTIONS TO THE SHORT WAVE PATTERN LATER
IN THE CYCLE...NOW SHOWING STRONGER SHORT WAVE PERTURBATIONS
TO MOVE TOWARDS THE SOUTHERN CONE. OVER THE CONTINENT...THEY
CONTINUE TO FORESEE A HIGH AMPLITUDE/LONG WAVE POLAR TROUGH TO
DEVELOP ACROSS THE WESTERN ATLANTIC TO MID SECTIONS OF SOUTH
AMERICA. ALTHOUGH THEY SHOW A STRONG POLAR TROUGH...AT LOW LEVELS
THEY NOW FAVOR A SLIGHTLY WEAKER POLAR RIDGE TO ESTABLISH ACROSS
ARGENTINA. THIS...IN-TURN...FAVORS A WEAKER PRESSURE GRADIENT AND
A WEAKER SOUTHERLY JET ACROSS PARAGUAY/BOLIVIA.
CONSISTENT WITH PREVIOUS FORECASTS...A 500 HPA SHORT WAVE IS
SPILLING ACROSS THE NORTHERN ANDES OF CHILE TO NORTHWEST
PROVINCES OF ARGENTINA/SOUTHERN BOLIVIA. IN THIS PROCESS IT IS
SHEARING SHORT WAVE ENERGY EAST-SOUTHEAST ACROSS PARAGUAY-
SOUTHERN BRASIL. AS IT ENTERS MID SECTIONS OF SOUTH AMERICA...IT
WILL MERGE WITH A HIGH AMPLITUDE TROUGH OVER THE SOUTHERN CONE/
WESTERN ATLANTIC BY 48-72 HRS. THIS TROUGH IS INTERACTING WITH A
SURFACE FRONT THAT STRETCHES ACROSS SOUTHERN BRASIL-PARAGUAY-
BOLIVIA...AS IT CONTINUES TO TRIGGER SCATTERED DEEP CONVECTION
AND FORMATION OF FRONTAL WAVES/LOWS. WE NOW EXPECT THIS BOUNDARY
TO REMAIN QUITE ACTIVE THROUGH AT LEAST 48-60 HRS...WITH DAILY
MAXIMA TO PEAK AT 20-35MM. LOCALLY HIGHER AMOUNTS ARE POSSIBLE
OVER SOUTHERN BRASIL DUE TO STRONG TOPOGRAPHICAL FORCING.
ON THE SOUTHERN STREAM...A BROAD MID LEVEL TROUGH EXTENDS ACROSS
THE DRAKE PASSAGE TO THE SOUTHERN CONE. BY 24-48 HRS THE TROUGH
WILL EXTEND BETWEEN 70W/30W AND SOUTH OF 35S. THIS TROUGH WILL
THEN REMAIN NEARLY STATIONARY WHILE AMPLIFYING TO THE NORTH-
NORTHWEST ACROSS RIO DE LA PLATA BASIN TO NORTHWEST
ARGENTINA/PARAGUAY BY 48-72 HRS. BY 84-96 HRS THE APEX OF THE
TROUGH WILL START TO PULL EAST ACROSS SOUTHERN BRASIL...WHILE THE
MAIN BODY PERSISTS OVER THE WESTERN ATLANTIC. AS THE TROUGH PULLS
AWAY...THE SUBTROPICAL JET OVER CENTRAL SOUTH AMERICA IS EXPECTED
TO WEAKEN THROUGH 96-108 HRS. THE MODELS ALSO INITIALIZE A
BUILDING RIDGE TO THE WEST OF THIS TROUGH...WITH MEAN AXIS
BUILDING SOUTH ALONG 80W/90W. THE RIDGE IS TO SLOWLY MOVE ACROSS
50W/60W BY 72-96 HRS...WITH A HIGH TO THEN CLOSE ON THE SOUTHERN
STREAM FLOW BY 120-144 HRS. THE HIGH AMPLITUDE RIDGE IS TO
MEANWHILE FEED COLD/POLAR ENERGY INTO THE BACKSIDE OF THE
AMPLIFYING POLAR TROUGH...TO RESULT IN THE AFOREMENTIONED HIGH
AMPLITUDE LONG WAVE PATTERN BY MID CYCLE. AT LOW LEVELS THE
MID/UPPER TROUGH SUSTAINS A BROAD TROUGH OVER THE SOUTHERN
CONE/WESTERN ATLANTIC. ON THE SOUTHERN STREAM...PROGRESSIVE
CLIPPER LOWS WILL LIFT FROM THE SOUTH POLE AND THEN ROTATE AROUND
THIS TROUGH. ON THE NORTHERN STREAM...EXPECT A FRONT TO PUSH
NORTH ACROSS RIO DE LA PLATA-CENTRAL PROVINCES OF ARGENTINA BY 24
HRS. THROUGH 48 HRS THIS FRONT IS TO PUSH FARTHER NORTH ACROSS
URUGUAY TO RIO GRANDE DO SUL IN BRASIL. AS IT PUSHES NORTH...IT
WILL START TO INTERACT WITH/REINFORCE THE OLD BOUNDARY OVER
SOUTHERN BRASIL-PARAGUAY. SECONDARY VORTICES WILL ENTER CHILE
BETWEEN CONCEPCION-ISLA DE CHILOE...TO CONTINUE FAVORING
SCATTERED CONVECTION OVER THIS AREA. ALTHOUGH WE GENERALLY EXPECT
DAILY MAXIMA OF 10-15MM/DAY DURING THE NEXT COUPLE OF DAYS...
THERE IS A RISK OF HIGHER AMOUNTS DUE TO TOPOGRAPHICAL FORCING.
AS THESE VORTICES STREAM ACROSS CUYO-CENTRAL PROVINCES OF
ARGENTINA THEY WILL FAVOR MOSTLY LIGHT CONVECTION. BY MID
CYCLE...A 1025-1027 HPA POLAR HIGH IS TO BUILD ACROSS LA PAMPA/
CENTRAL ARGENTINA. THE HIGH MOVES FARTHER NORTH THROUGH 84
HRS...MEANWHILE DISPLACING THE SURFACE FRONT FARTHER NORTH ACROSS
BRASIL. AT 96 HRS THE FRONT WILL MOVE FARTHER NORTH ACROSS
SOUTHEAST BRASIL-MATO GROSSO TO RONDONIA-SOUTHERN/CENTRAL PERU.
BY 120 HRS THE FRONT WILL START TO WEAKEN OVER THE CONTINENT...
WHILE PERSISTING OVER THE WESTERN ATLANTIC. AS THE FRONT PUSHES
NORTH ACROSS ARGENTINA...IT WILL SUPPORT MOSTLY LIGHT RAINFALL.
BEST CHANCE FOR ORGANIZED RAINFALL WILL BE AT 84-108 HRS...WHEN
THE FRONT IS TO MOVE NORTH INTO THE BOLIVIAN JUNGLE AND TRIGGER
MAXIMA OF 25-50MM. CONVECTION WILL RAPIDLY BUILD WEST ACROSS
SOUTHERN PERU...WITH SIMILAR AMOUNTS. PREFRONTAL CONVECTION OVER
THE PERUVIAN JUNGLE WILL SUSTAIN DAILY MAXIMA OF 10-15MM THROUGH
36 HRS...AND 20-35MM/DAY AT 36-72 HRS. IN THIS CYCLE MODELS SHOW
BOUNDARY LAYER TEMPERATURES TO GENERALLY REMAIN ABOVE FREEZING
ACROSS MID SECTIONS OF SOUTH AMERICA.
AT 200 HPA...MODELS MAINTAIN GOOD CONTINUITY ON THEIR EVOLUTION
OF THE SUBTROPICAL RIDGE. THIS RIDGE DOMINATES CONTINENTAL AREA
NORTH OF 20S...BUT UNDER THE INFLUENCE OF THE AFOREMENTIONED
POLAR INCURSION IT IS FORECAST TO FLATTEN/NEARLY COLLAPSE BY 96-
120 HRS. 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 10-15MM/DAY
THROUGH 48-54 HRS. OTHER CONVECTION WILL CONCENTRATE ACROSS THE
NORTHERN COAST OF BRASIL...WITH SIMILAR AMOUNTS. HIGHER AMOUNTS
ARE EXPECTED OVER AMAPA/AMAZON RIVER DELTA TO ASSOCIATE WITH
TROPICAL WAVES...WITH MAXIMA TO PEAK AT 20-35MM. OVER WESTERN
ECUADOR AND CAUCA VALLEY IN SOUTHWEST COLOMBIA THE DAILY MAXIMA
WILL PEAK AT 10-15MM. ACTIVITY IS TO ALSO DOT THE PERUVIAN JUNGLE
OVER THE NEXT TWO TO THREE DAYS. A SURGE IN ACTIVITY IS EXPECTED
LATER IN THE CYCLE AS THE POLAR FRONT MOVES NORTH ACROSS MID
SECTIONS OF SOUTH AMERICA.
GUZMAN...SENAMHI (BOLIVIA)
DAVISON...HPC (USA)
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