SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1218 PM EDT THU JUL 02 2009
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
 
LAST NO AMENDS... NEXT BULLETIN ON JULY 06.

MODEL COMPARISON (VALID FROM 00Z JUL 02). MODELS CONTINUE TO SHOW
GRADUAL EVOLUTION FROM A HIGH AMPLITUDE SINUSOIDAL PATTERN TO ONE
DOMINATED BY SHORT WAVE PERTURBATIONS. BUT AFTER 120/132 HRS...
THEY ONCE AGAIN SHOW A HIGH AMPLITUDE PATTERN ESTABLISHING AS A
RIDGE BUILDS SOUTH INTO THE EASTERN PACIFIC. MEANWHILE...AS SHORT
WAVE PERTURBATIONS ESTABLISH/DOMINATE...THE MODELS DEVELOP
TIMING/AMPLITUDE DIFFERENCES. THIS LEADS TO MODEL DIFFERENCES ON
DAYS THREE-FIVE...AND AS A RESULT...A LACK OF CONTINUITY DURING
THE MEDIUM RANGE FORECAST. 

ON THE NORTHERN STREAM...AT LOW LEVELS...AN ELONGATED FRONT
EXTENDS ACROSS THE SOUTH ATLANTIC TO SOUTHEAST BRASIL...THEN
INLAND ACROSS MATO GROSSO DO SUL/PARAGUAY TO CENTRAL BOLIVIA. THE
FRONT WILL REMAIN NEARLY STATIONARY THROUGH 48-60 HRS. BY 48-72
HRS IT WILL RETROGRESS TO MESOPOTAMIA VALLEY-PARAGUAY-SOUTHERN
BOLIVIA AS A MID LEVEL TROUGH INDUCES A WAVE ALONG THIS BOUNDARY.
THE FRONT...MEANWHILE...IN INTERACTION WITH UPPER LEVEL SHORT
WAVE TROUGHS/SUBTROPICAL JET...WILL SUSTAIN SCATTERED CONVECTION
OVER PARANA/SAO PAULO BRASIL TO MATO GROSSO DO SUL/CENTRAL
PARAGUAY. IN THIS CYCLE THE MODELS REVERSED TREND AND NOW SHOW
THE FRONT REMAINING ACTIVE A BIT LONGER...WITH RAINFALL MAXIMA OF
20-40MM/DAY THROUGH 36 HRS...AND 15-30MM/DAY AT 36-48 HRS. AS THE
FRONT RETROGRESSES TO PARAGUAY-SOUTHERN BRASIL BY 60-84 HRS...THE
MAXIMA WILL PEAK AT 20-35MM/DAY. IT IS TO THEN SETTLE AT 10-
20MM/DAY. ANOTHER MAXIMA IS EXPECTED OVER RIO DE JANEIRO/EASTERN
SAO PAULO ON DAYS 03-05...WITH COASTAL CONVECTION TO RESULT IN
RAINFALL AMOUNTS OF 10-20MM/DAY THROUGH 96-108 HRS. THIS
INCREASES TO 15-35MM/DAY BY 108-132 HRS.

FARTHER UPSTREAM...OVER THE PACIFIC...A BROAD MID/UPPER LEVEL
TROUGH IS INITIALIZED WITH AXIS BETWEEN 120W-85W. A BLOCKING
RIDGE LIES TO THE EAST OF THIS SYSTEM. AS THE TROUGH PUSHES
AGAINST THE RIDGE...THE SHORT WAVE ENERGY IS TO SHEAR SOUTHWARD.
IN THIS PROCESS THE RIDGE IS TO NEARLY COLLAPSE WHILE BEING
DISPLACED TO THE EAST ACROSS 55W/60W BY MID CYCLE...WHILE THE
TROUGH IS FORECAST TO GRADUALLY WEAKEN. A SERIES OF SHORT WAVE
TROUGHS ARE THEN TO STREAM ACROSS THE SOUTHERN CONE...WITH ONE BY
54-60 HRS AND ANOTHER BY 78-84 HRS. A THIRD...AND MUCH WEAKER
TROUGH...WILL ARRIVE/MAKE LANDFALL OVER SOUTHERN CHILE BY 108
HRS. AT LOW LEVELS EXPECTED A POLAR FRONT TO ENTER SOUTHERN CHILE
BY 30-36 HRS...AND INTO LA PAMPA-CENTRAL CHILE BY 36-48 HRS.
SIMULTANEOUSLY...ANOTHER REACHES SOUTHERN CHILE BY 54-60 HRS...
AND CROSS PATAGONIA/LA PAMPA THROUGH 72 HRS...INTO BUENOS AIRES
PROVINCE/MENDOZA IN ARGENTINA BY 96 HRS. A THIRD FRONT WILL ENTER
SOUTHERN CHILE BY 96-102 HRS. THESE FRONTS ARE TO FAVOR SCATTERED
CONVECTION OVER CHILE SOUTH OF CONCEPCION/TEMUCO...WITH DAILY
MAXIMA OF 15-30MM ON DAYS 02...03 AND 04. FURTHERMORE...AS A
DEEP/OCCLUDED TROUGH MOVES ACROSS THE SOUTHERN CONE BY 96-132
HRS...SNOW SHOWERS ARE POSSIBLE ON THE SOUTHERN ANDES.

NORTH OVER THE CONTINENT...AT 200 HPA...THE MODELS ARE CONSISTENT
ON THEIR FORECAST OF THE SUBTROPICAL RIDGE PATTERN...WITH A HIGH
TO CLOSE OVER PARA/AMAZONAS BRASIL THROUGH 48 HRS...WITH AN UPPER
LEVEL RIDGE TO THEN REMAIN OVER THE CONTINENT TO THE NORTH OF 10S
THROUGH 120 HRS. THE INTENSIFICATION OF THE RIDGE WILL FOLLOW A
GRADUAL INCREASE IN AIR MASS CONVECTION OVER AMAZONAS-RONDONIA
BRASIL AND THE PERUVIAN JUNGLE...WHERE THE DAILY MAXIMA WILL
RANGE BETWEEN 05-15MM/DAY. HIGHER AMOUNTS ARE EXPECTED ALONG AND
NORTH OF THE EQUATOR INTO SOUTHERN COLOMBIA-NORTHERN AMAZONAS
BRASIL...WHERE MAXIMA WILL PEAK AT 20-35MM/DAY. AS THE UPPER
RIDGE ESTABLISHES OVER WESTERN BRASIL...A TROUGH WILL AMPLIFY
INTO NORTHEAST BRASIL BY 24-48 HRS...TO GRADUALLY MOVE OFF THE
COAST BY 72-84 HRS. COASTAL CONVECTION WILL LIMIT TO NORTHEAST
BRASIL BETWEEN RECIFE AND SALVADOR DE BAHIA...WITH MAXIMA OF 10-
20MM/DAY ON DAY 01 AND 05-10MM/DAY ON DAY 03.

MAZA.....SMN (ARGENTINA)
SEABRA...INMET (BRASIL)
DAVISON...HPC (USA)