714
FXSA20 KWBC 121634
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1233 PM EDT TUE AUG 12 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 12). DIFFERENCES IN THE
LATEST RUN OF THE GFS/EUROPEAN MODELS ARE SUBTLE/NEGLIGIBLE
DURING MOST OF THE CYCLE. THIS IS PARTICULARLY TRUE FOR THE GFS-
UKMET SOLUTIONS. THE ECMWF ONLY DIVERGE ON THE INTENSITY OF A
TROUGH ENTERING THE SOUTHERN CONE BY MID CYCLE...SHOWING MOST OF
THE SHORT WAVE ENERGY TRACKING SOUTH OF CONCEPCION/TEMUCO IN
CENTRAL CHILE. IN CONTRAST...THE GFS/UKMET...WITH THE SUPPORT OF
THE GLOBAL ENSEMBLES...FAVOR A DEEPER TROUGH WITH ENERGY AS FAR
NORTH AS LA SERENA CHILE. THIS IS THE WORST CASE SCENARIO...AND
IT IS STARTING TO LOOK LIKE IT MIGHT VERIFY. SO WE ARE INCREASING
OUR FORECAST OF RAINFALL AMOUNTS OVER THIS AREA. AS BOUNDARY
LAYER MIXING RATIO VALUES HOVER BETWEEN 08-10G/KG...THE RISK OF
HEAVY RAINFALL AMOUNTS ACROSS CENTRAL CHILE PERSISTS.
AT 500 HPA...A PROGRESSIVE SHORT WAVE TROUGH OVER ARGENTINA IS
TO QUICKLY MOVE EAST ALONG 45W AND SOUTH OF 30S BY 24 HRS. BY
48 HRS IT WILL EJECT ACROSS 15W...MEANWHILE DAMPENING AS IT FLOWS
UNDER A BLOCKING RIDGE. THE TROUGH SUPPORTS A FRONT OVER RIO
DE LA PLATA/URUGUAY-MENDOZA IN ARGENTINA. THIS FRONT IS TO
QUICKLY PUSH NORTH INTO SOUTHERN BRASIL-PARAGUAY-EASTERN BOLIVIA
BY 24 HRS....WHERE IT WILL THEN REMAIN THROUGH AT LEAST 72-84
HRS. BY 96 HRS IT WILL START TO RETROGRESS AS A WARM FRONT ACROSS
MESOPOTAMIA VALLEY/CHACO ARGENTINO TO URUGUAY. AS IT PUSHES
NORTH...IT WILL TRIGGER SCATTERED CONVECTION WITH ACCUMULATION OF
10-15MM/DAY. LOCALLY HIGHER AMOUNTS WILL PEAK AT 15-30MM.
MODERATE CONVECTION WILL PERSIST THROUGH 48-60 HRS AS A NORTHERN
STREAM TROUGH INTERACTS WITH THE SURFACE FRONT. THE MID LEVEL
TROUGH ENTERS NORTHWEST ARGENTINA/SOUTHERN BOLIVIA BY 24 HRS...
AND MOVES ACROSS PARAGUAY TO SOUTHERN BRASIL BY 48 HRS...
TRIGGERING THE AFOREMENTIONED CONVECTION.
ALSO AT 500 HPA...A SHORT WAVE TROUGH CLOSELY FOLLOWS...MOVING
ACROSS PATAGONIA TO THE WESTERN ATLANTIC BY 24 HRS. AS IT TRACKS
EAST...THIS TROUGH WILL CONFINE TO SOUTH OF 45S. THROUGH 48 HRS
THE TROUGH WILL MOVE ACROSS 40W/45W. AT LOW LEVELS IT SUPPORTS AN
OCCLUDED LOW OFF THE SOUTHERN COAST OF CHILE. THIS LOW IS
FORECAST TO MOVE JUST EAST OF PATAGONIA BY 24 HRS...WHILE THE
ASSOCIATED FRONT PUSHES NORTH INTO NORTHERN PATAGONIA-TEMUCO
CHILE. IT IS TO THEN QUICKLY EJECT TO THE EAST-SOUTHEAST. AS IT
ENTERS THE SOUTHERN CONE IT WILL FAVOR SCATTERED SNOWFALL ON THE
SOUTHERN ANDES AND TIERRA DEL FUEGO...AND RAINFALL BETWEEN ISLA
DE CHILOE AND TEMUCO CHILE...WITH RAINFALL MAXIMA OF 05-10MM/DAY.
THE SYSTEM OF CONCERN IS A NORTHERN STREAM LOW/TROUGH OVER THE
EASTERN PACIFIC. THE 500 HPA LOW IS TO MEANDER NEAR 40S 100W
THROUGH 48 HRS. BY 72 HRS THIS LOW IS TO PICK SOME MOMENTUM
AS IT MOVES ACROSS 43S 82W...AND BY 96-108 HRS IT IS TO ENTER
CENTRAL CHILE. THE GFS AND UKMET CONTINUE TO FAVOR A DEEPER/
STRONGER TROUGH THAN WHAT THE ECMWF SUGGESTS. AS A RESULT...THEY
THEN SUSTAIN A CLOSED LOW OFF THE CENTRAL COAST OF CHILE THROUGH
120 HRS. AT LOW LEVELS...SURFACE LOWS WILL MOVE ACROSS THE
EASTERN PACIFIC INTO CENTRAL CHILE AHEAD OF THE MID LEVEL
VORTEX...WITH A SURFACE LOW TO REACH CHILE NORTH OF TEMUCO BY 54-
60 HRS. AT THAT TIME...MOISTURE/ENERGY WILL CONCENTRATE BETWEEN
CONCEPCION-PUERTO MONTT. BY 60-84 HRS MOISTURE WILL RAPIDLY SURGE
NORTHWARD TO SANTIAGO...AND BY 108 HRS IT WILL REACH LA SERENA.
THE GFS IS CONSISTENT ON ITS FORECAST OF HIGH BOUNDARY LAYER
MIXING RATIO VALUES OF 08-10G/KG IN ASSOCIATION WITH THIS SYSTEM.
AT 250 HPA SUBTROPICAL/POLAR JET PAIR IS TO ACCOMPANY THIS
SYSTEM...WITH JETS TO DIVERGE AS THE ENTER THE CONTINENT LATER
THIS PERIOD...TO RESULT IN A BROAD AREA OF DIVERGENCE AT
MID/UPPER LEVELS. THE DETERMINISTIC AND ENSEMBLE FORECASTS SHOW
HIGH PROBABILITY OF HEAVY RAINFALL TO AFFECT CENTRAL PORTIONS OF
CHILE BETWEEN 84-108 HRS. IN DEEP CONVECTION RAINFALL MAXIMA IS
NOW EXPECTED TO PEAK AT 25-50MM. THIS IS CONSIDERABLY HIGHER THAN
WHAT WE PREVIOUSLY FORECASTED. ORGANIZED SNOWFALL IS EXPECTED ON
ELEVATIONS ABOVE 2-2.5KM.
AT 200 HPA...THE UPPER LEVEL ANALYSIS SHOWS THE SUBTROPICAL RIDGE
DIVIDING IN TWO CELLS...WITH ONE OVER THE EASTERN PACIFIC AND THE
OTHER OVER NORTHERN SOUTH AMERICA. THIS PATTERN IS TO GENERALLY
PERSIST OVER THE NEXT FOUR TO FIVE DAYS...WHICH IS TO CONTINUE
FAVORING ISOLATED TO SCATTERED CONVECTION ACROSS NORTHERN PARA-
AMAZONAS IN WESTERN BRASIL WITH DAILY MAXIMA TO PEAK AT 15-30MM.
OTHER CONVECTION WILL BUILD ACROSS THE PERUVIAN JUNGLE...WITH
MOST INTENSE ON DAY 01...WITH MAXIMA OF 05-10MM. OTHERWISE EXPECT
WIDELY ISOLATED CONVECTION. OVER NORTHEAST BRASIL AND THE AMAZON
DELTA REGION...TRADE WIND CONVERGENCE IS TO ALSO FAVOR PERIODS OF
SCATTERED RAINFALL.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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