SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1146 AM EDT MON MAR 11 2013

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

MODEL COMPARISON (VALID FROM 00 UTC MAR 11): CYCLE-TO-CYCLE
INCONSISTENCIES DEVELOP ON DAY 04...AS THE GFS CORRECTED IN FAVOR
OF A WEAKER PATTERN ON THE SOUTHERN STREAM FLOW. THIS IS A
RADICAL CHANGE FROM PREVIOUS RUNS...WHEN THE MODEL WAS SHOWING A
STRONG PERTURBATION ENTERING THE FORECAST AREA. THE EUROPEAN
MODELS APPEAR SOMEWHAT MORE CONSISTENT ON THE FORECAST OF THIS
FEATURE...AND THIS IS WELL SUPPORTED BY THE MEAN OF THE GLOBAL
ENSEMBLES. AS A RESULT WE OPTED FOR THE MORE AMPLIFIED EUROPEAN
MODELS IN THE ELABORATION OF OUR FORECAST GUIDANCE.

BROAD MID LEVEL TROUGH DOMINATES THE WESTERN ATLANTIC...WITH AXIS
ENVELOPING AREA BETWEEN 70W-20W AND TO THE SOUTH OF 25S. UNDER
INFLUENCE OF A BLOCKING RIDGE TO THE WEST...SHORT WAVE
PERTURBATIONS ARE LIFTING NORTH ACROSS THE WEDDELL SEA INTO
PATAGONIA IN ARGENTINA. THE INFLOW OF SHORT WAVE ENERGY IS TO
HELP SUSTAIN THE LONG WAVE TROUGH PATTERN DURING THE NEXT FEW
DAYS. AT LOW LEVELS THIS SUSTAIN A BROAD TROUGH OVER THE WESTERN
ATLANTIC. THIS IS TO INITIALLY CENTER ON A DEEP 952 HPA LOW NEAR
THE SOUTH GEORGIA ISLANDS. THE LOW RAPIDLY PULLS TO THE EAST-
SOUTHEAST THROUGH 48-60 HRS...WHILE BROAD TROUGH PATTERN IS TO
GENERALLY PERSIST. SURFACE FRONTS ARE TO REVOLVE AROUND THIS
AXIS. ONE IS TO MEANDER WEST INTO THE SOUTHERN STATES OF BRASIL-
NORTHERN PROVINCES IN ARGENTINA/PARAGUAY AS ANOTHER APPROACHES
FROM THE SOUTH. THE LATTER CONTINUES INTO THE BUENOS AIRES
PROVINCE EARLY IN THE CYCLE...AND NORTH ACROSS URUGUAY TO THE
NORTHERN PROVINCES IN ARGENTINA BY 48 HRS. THROUGH 72-84 HRS IT
IS TO THEN MERGE WITH OLD BOUNDARY TO THE NORTH OVER BRASIL-
PARAGUAY. AS THE FRONTAL BOUNDARIES COMBINE TO THE NORTH...THIS
WILL BE THE FOCUS OF DEEP/TROPICAL MOISTURE CONVERGENCE OVER MID
SECTIONS OF SOUTH AMERICA. THIS WILL COMBINE WITH FAVORABLE JET
DYNAMICS ALOFT TO SUSTAIN FAVORABLE ENVIRONMENTAL CONDITIONS FOR
GENERATION OF MESO SCALE CONVECTIVE SYSTEMS. PEAK IN CONVECTIVE
ACTIVITY IS EXPECTED ON DAYS 02-03...WHEN RAINFALL MAXIMA IN THIS
AREA IS TO PEAK AT 50-100MM/DAY. DURING THAT PERIOD CONVECTION IS
TO ALSO SPREAD WEST AND NORTH ACROSS BOLIVIA...WHERE IT IS TO
FAVOR MAXIMA OF 25-50MM/DAY.

AS THE PATTERN TO THE NORTH EVOLVES...A MID LEVEL RIDGE IS TO
GRADUALLY ESTABLISH A FOOTHOLD ACROSS TIERRA DEL FUEGO/PATAGONIA
IN ARGENTINA. THIS IS TO THEN PERSIST THROUGH 84-96 HRS. LATER IN
THE CYCLE THE RIDGE WILL START TO ERODE/WEAKEN AS A STRONG TROUGH
IN THE SOUTHERN STREAM ENTERS THE DRAKE PASSAGE/SOUTHERN CONE OF
SOUTH AMERICA. THROUGH 108-120 HRS THE TROUGH PULLS ACROSS THE
DRAKE PASSAGE...AND BY 120 HRS IT MOVES TO THE WEDDELL SEA/SOUTH
ATLANTIC. AT LOW LEVELS...THE ASSOCIATED FRONT ENTERS SOUTHERN
CHILE BY 96-108 HRS...TO MEANDER INLAND INTO PATAGONIA THROUGH
120 HRS. AS IT ENTERS THE SOUTHERN CONE THIS IS TO SUSTAIN
SCATTERED CONVECTION TO THE SOUTH OF ISLA DE CHILOE...AND
RAINFALL AMOUNTS OF 05-10MM.

AT 200 HPA...NARROW EAST-TO-WEST RIDGE IS TO TRAVERSE THE
NORTHERN PORTIONS OF SOUTH AMERICA BETWEEN 10S-20S. A CLOSED HIGH
OVER SOUTHERN MINAS GERAIS/NORTHERN SAO PAULO IS TO ANCHOR THIS
RIDGE OVER THE NEXT FEW DAYS. POLAR TROUGH TO THE SOUTH...AS IT
PRESSES AGAINST THIS RIDGE...WILL SUSTAIN A TIGHT GRADIENT AND A
FAIRLY STRONG JET MAXIMA ACROSS SOUTHERN BRASIL TO THE
MESOPOTAMIA VALLEY IN ARGENTINA. THIS...IN-TURN...WILL FAVOR A
DIVERGENT PATTERN ALOFT THAT IS TO VENT DEEP CONVECTION ACROSS
MID SECTIONS OF SOUTH AMERICA. OTHER ACTIVITY IS TO CLUSTER ALONG
THE SIERRA OF PERU...WHERE DIURNAL DEVELOPMENT IS TO SUSTAIN
RAINFALL AMOUNTS OF 10-15MM/DAY. OVER PARA-AMAZONAS IN BRASIL
EXPECTING RAINFALL AMOUNTS OF 15-20MM/DAY...WITH A GRADUAL
INCREASE IN BOTH INTENSITY AND COVERAGE THROUGH DAY 05.

THERE IS ALSO AN UPPER LEVEL JET ACROSS THE EQUATORIAL EASTERN
PACIFIC THAT PROPAGATES ACROSS THE GALAPAGOS ISLANDS EARLY IN THE
CYCLE...THEN EAST INTO ECUADOR DURING THE EVENING ON MONDAY TO
EARLY TUESDAY MORNING. THE JET ALOFT IS TO ENHANCE UPPER DIVERGE
AND IS FORECAST TO FAVOR A SURGE IN ACTIVITY ACROSS WESTERN
ECUADOR TO NORTHERN PERU. ALTHOUGH WE FORECAST RAINFALL MAXIMA OF
20-45MM FOR DAY 01...SOME OF THE MODELS SHOW MAXIMA IN THIS AREA
EXCEEDING 100MM. AFTERWARDS...CONVECTION IS TO GRADUALLY
WANE...WITH MAXIMA DECREASING TO 20-35MM/DAY THROUGH 72-84
HRS...AND 05-10MM/DAY LATER IN THE CYCLE.

ALSO AT 250 HPA...A TROUGH TO THE NORTH IS TO EXTEND ACROSS
BRASIL TO THE NORTH OF 10S WHILE CENTERING ON A CLOSED LOW NEAR
THE AMAZON RIVER DELTA EARLY IN THE CYCLE. THROUGH 60-72 HRS THE
LOW FILLS TO AN OPEN TROUGH THAT IS TO THEN PERSIST ACROSS
NORTHERN SOUTH AMERICA WHILE CENTERING ON A SECONDARY LOW NEAR
10S 30W. THE TROUGH ALOFT IS TO ENHANCE ACTIVITY BETWEEN AMAPA
AND MARANHAO...TO RESULT IN RAINFALL AMOUNTS OF 15-20MM/DAY IN
ISOLATED TO SCATTERED CONVECTION. THROUGH 96-120 HRS...HOWEVER...
THIS IS EXPECTED TO DECREASE TO 10-15MM/DAY.

CARDOZA...FAP (PERU)
MAQUEDA...DINAC (PARAGUAY)
DAVISON...WPC (USA)