055
FXSA20 KWBC 031735
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1234 PM EST TUE NOV 03 2009
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z NOV 03). THE GFS AND EUROPEAN
MODELS AGREE QUITE WELL THROUGH 96-108 HRS. LATER IN THE
CYCLE..HOWEVER...THEY DIVERGE ON THE INTENSITY OF A SOUTHERN
STREAM SURGE ACROSS THE BELLINGSHAUSEN SEA/DRAKE PASSAGE TO THE
SOUTHERN CONE...WITH THE GFS FAVORING A DEEPER/STRONGER SURGE
THAN THE EUROPEAN MODELS. THE MEAN OF THE NCEP-CANADIAN AND
EUROPEAN ENSEMBLES TAKE THE SIDE OF THE GFS DETERMINISTIC
SOLUTION...BUT THE SPREAD IS TOO HIGH TO ESTABLISH CONFIDENCE. SO
FURTHER CORRECTIONS ARE EXPECTED. EARLY IN THE PERIOD THE MODES
CONTINUE TO SHOW A STRONG POLAR SURGE INTO THE SOUTHERN
CONE...WITH CONDITIONS TO FAVOR/SUSTAIN ORGANIZED SNOWFALL ON THE
SOUTHERN ANDES-TIERRA DEL FUEGO AND SOUTHERN PATAGONIA. HEAVY
RAINS WILL CONCENTRATE FARTHER TO THE NORTH ALONG A
QUASISTATIONARY FRONT OVER NORTHERN ARGENTINA/SOUTHERN
BRASIL...AS IT IS TO CONTINUE FAVORING GENERATION OF MESO SCALE
CONVECTIVE SYSTEMS.
OVER THE PACIFIC...AT MID/UPPER LEVELS...THE MODELS INITIALIZE A
HIGH AMPLITUDE RIDGE...WITH AXIS SOUTH BETWEEN 90W-120W. THE
RIDGE IS TO PERSISTS THROUGH DAY 06...MEANWHILE FAVORING A BROAD
SURFACE RIDGE. THE PERSISTENT RIDGE PATTERN WILL FEED COLD/POLAR
ENERGY INTO THE BACKSIDE OF AN AMPLIFYING TROUGH OVER THE DRAKE
PASSAGE/SOUTHERN CONE...WITH TROUGH FORECAST TO ENVELOP AREA
BETWEEN 80W-50W AND TO THE SOUTH OF 30S BY 36-48 HRS. THIS WILL
PERSIST THROUGH 48-60 HRS...WHEN MOST OF THE SHORT WAVE ENERGY IS
EXPECTED TO SHEAR TO THE SOUTHEAST. A SHORT WAVE TROUGH...
HOWEVER...WILL REMAIN ACROSS PATAGONIA/SOUTHERN CHILE THROUGH
72-84 HRS...WITH PERTURBATION TO THEN EJECT ACROSS THE CENTRAL
ANDES OF CHILE TO CENTRAL ARGENTINA THROUGH 108 HRS. IT WILL MOVE
ACROSS RIO DE LA PLATA BY 120-132 HRS. AT LOW LEVELS IT REFLECTS
AS A BROAD SURFACE TROUGH...SUSTAINING A SERIES OF POLAR FRONTS
THAT STREAM ACROSS SOUTHERN CHILE/PATAGONIA DURING THE NEXT FOUR
DAYS. AT 36-60 HRS THE TROUGH WILL OCCLUDE OVER PATAGONIA-WESTERN
ATLANTIC...WHICH WILL FAVOR A COLD/MOIST SOUTHERLY FLOW ACROSS
TIERRA DEL FUEGO TO CENTRAL PATAGONIA. AS THE CYCLE PROGRESSES...
PRECIPITATION OVER SOUTHERN CHILE WILL BECOME BETTER ORGANIZED...
TO INITIALLY FAVOR ACCUMULATION OF 05-10MM/DAY...TO INCREASE TO
15-25MM/DAY BY 36 HRS...THEN SETTLE AT 10-15MM/DAY THROUGH 72-84
HRS. ORGANIZED SNOWFALL IS EXPECTED ACROSS EXTREME SOUTHERN
CHILE-TIERRA DEL FUEGO AND THE SOUTHERN ANDES...WHILE SCATTERED
SNOW SHOWERS ARE TO AFFECT SOUTHERN PATAGONIA. MOST INTENSE
SNOWFALL...HOWEVER...IS EXPECTED BETWEEN 24-54 HRS.
NORTH OVER THE CONTINENT...AT MID LEVELS...A BLOCKING RIDGE
PATTERN REMAINS ACROSS BOLIVIA-PARAGUAY/NORTHERN ARGENTINA TO
SOUTHERN BRASIL. THE BLOCKING PATTERN WILL PERSIST THROUGH 60-72
HRS...THEN WEAKEN/NEARLY COLLAPSE TOWARDS THE END OF THE
FORECAST CYCLE. AS THE AFOREMENTIONED POLAR TROUGH DEEPENS AND
AMPLIFIES ACROSS THE SOUTHERN CONE...A TIGHT GRADIENT IS EXPECTED
ACROSS THE CENTRAL ANDES OF CHILE AND INTO ARGENTINA...TO FAVOR A
STRONG SUBTROPICAL/POLAR JET PAIR ACROSS CENTRAL CHILE-ARGENTINA.
THE INCIDENT FLOW TO THE ANDES WILL FAVOR GENERATION OF LEE SIDE
PERTURBATIONS ACROSS THE CENTRAL PROVINCES OF ARGENTINA. AT LOW
LEVELS...MEANWHILE...AN ELONGATED FRONT WILL MEANDER ACROSS RIO
GRANDE DO SUL IN SOUTHERN BRASIL/URUGUAY-CHACO ARGENTINO TO
NORTHWEST PROVINCES IN ARGENTINA...WHERE IT IS TO REMAIN THROUGH
60-72 HRS. AT 84-108 HRS IT IS TO MIGRATE ACROSS SANTA CATARINA
IN SOUTHERN BRASIL TO PARAGUAY/SOUTHERN BOLIVIA UNDER INFLUENCE
OF THE AFOREMENTIONED SHORT WAVE TROUGH MOVING ACROSS CHILE.
MEANWHILE...WAVE/LOWS WILL FORM ALONG THE MEANDERING FRONT AND
THEY ARE TO OCCLUDE WHILE PROPAGATING INTO THE SOUTH ATLANTIC. A
NORTHERLY JET ACROSS PARAGUAY WILL SUSTAIN A HIGH SHEAR
ENVIRONMENT ALONG THE SURFACE FRONT...AND THIS IS LIKELY TO
SUSTAIN ORGANIZED/LONG LASTING CONVECTION. THE FAVORABLE JET
DYNAMICS ALOFT WILL SUSTAIN THE BAROCLINICITY OF THIS BOUNDARY
DURING THE FORECAST CYCLE. AS THE DYNAMICS COMBINE...EXPECT
MODERATE TO HEAVY CONVECTION ACROSS URUGUAY-BUENOS AIRES PROVINCE
TO CENTRAL ARGENTINA THROUGH DAY 02...TO RESULT IN DAILY MAXIMA
OF 20-45MM. AT 72-96 HRS THE MAXIMA WILL INCREASE TO 35-70MM/DAY
WHILE EXPANDING NORTH INTO RIO GRANDE DO SUL/SANTA CATARINA IN
BRASIL-MESOPOTAMIA VALLEY-PARAGUAY AND EASTERN PORTIONS OF
BOLIVIA. ON THE NORTHWEST PROVINCES OF ARGENTINA...INITIALLY
EXPECT DAILY MAXIMA OF 15-25MM/DAY. THERE IS A RISK OF SEVERE
CONVECTION AS MESO SCALE CONVECTIVE SYSTEMS DEVELOP ALONG THE
MEANDERING FRONT.
AT 200 HPA...THE SUBTROPICAL RIDGE WILL SPLIT IN TWO CELLS...ONE
OVER AMAZONAS IN WESTERN BRASIL AND THE OTHER OVER SAO PAULO/
WESTERN ATLANTIC. BY THE END OF THE CYCLE THE ONE OVER THE
CONTINENT WILL BECOME THE DOMINANT CIRCULATION...FAVORING THE
NORTHWARD AMPLIFICATION OF A TROUGH ALONG THE EAST COAST OF
BRASIL. THE TROUGH WILL CENTER ON A CLOSED LOW OVER ESPIRITO
SANTO. THE EVOLVING PATTERN WILL FAVOR A SUBSIDENCE CAP ACROSS
NORTHEAST BRASIL...INHIBITING ORGANIZED DEEP CONVECTION TO THE
EAST OF 55W AND NORTH OF 20S. MOST ACTIVE CONVECTION DURING THE
CYCLE IS EXPECTED TO THE WEST...TO CONCENTRATE ON THE PERUVIAN/
BOLIVIAN JUNGLE-AMAZONAS IN WESTERN BRASIL WITH DAILY MAXIMA O
F20-45MM. THIS WILL WEAKEN TOWARDS THE END OF THE CYCLE AS THE
ACTIVITY ACROSS PARAGUAY/SOUTHERN BRASIL INTENSIFIES (SEE ABOVE).
FERNANDES...CHM (BRASIL)
GARCIA...SENAMHI (PERU)
DAVISON...HPC (USA)
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