495
FXSA20 KWBC 181622
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1221 PM EDT MON AUG 18 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 18). THE GFS GLOBAL MODEL
CONTINUES TO EXPERIENCE PROBLEMS RESOLVING SOUTHERN STREAM
SYSTEMS...AS IT CONTINUES TO MAKE CYCLE-TO-CYCLE CORRECTIONS TO
PERTURBATIONS PROPAGATING SOUTH OF 50S. ON THE NORTHERN
STREAM...THE PERFORMANCE IS SLIGHTLY BETTER...AS LEAST AS IT
PERTAINS THROUGH DAY 04. THE ECMWF AND UKMET ARE MORE CONSISTENT
AND MAINTAIN BETTER CONTINUITY ON BOTH STREAM. THE EUROPEAN
MODELS AGREE ON A STRONGER AND MORE PERSISTENT SOUTHERN STREAM
POLAR SURGE ACROSS THE BELLINGSHAUSEN SEA TO THE DRAKE PASSAGE/
TIERRA DEL FUEGO LATER IN THE CYCLE. THE GLOBAL ENSEMBLES SUPPORT
THIS INCURSION...AND THE 06 UTC GFS IS STARTING TO TREND IN FAVOR
OF THIS SOLUTION. SO FURTHER CORRECTIONS TO THE MODEL ARE
EXPECTED IN FUTURE RUNS.
AT 500 HPA...MODELS INITIALIZE A NORTHERN STREAM TROUGH OFF THE
CENTRAL TO NORTHERN COAST OF CHILE. DURING THE NEXT 72 HRS THE
TROUGH IS TO SLOWLY SPILL ACROSS THE ANDES TO THE CENTRAL
PROVINCES OF ARGENTINA/URUGUAY...AS A SECONDARY VORTEX REACHES
CENTRAL CHILE. THE LATTER IS TO QUICKLY EJECT ACROSS THE ANDES TO
LA PAMPA-BUENOS AIRES PROVINCE IN ARGENTINA BY 96-108 HRS. AT
LOW LEVELS A FRONT EXTENDS ACROSS SOUTHERN BRASIL-PARAGUAY TO
SOUTHEAST/CENTRAL BOLIVIA. THE FRONT REMAINS NEARLY STATIONARY
THROUGH 36 HRS...WITH A WAVE/LOW TO FORM OVER CORRIENTES IN
NORTHERN ARGENTINA BY 48 HRS. THIS LOW IS TO EJECT ACROSS
URUGUAY/SOUTHERN BRASIL BY 72 HRS...AND INTO THE ATLANTIC THROUGH
84-96 HRS...WHERE IT WILL RAPIDLY OCCLUDE. THE ASSOCIATED FRONT
IS TO MEANDER NORTH INTO SAO PAULO-MATO GROSSO DO SUL/PARAGUAY
THROUGH 108 HRS. AT 250 HPA...THE SUBTROPICAL JET WILL TRAVERSE
THE CONTINENT ALONG 24S/26S DURING THE NEXT FIVE DAYS...WITH JET
MAXIMA TO PROPAGATE ALONG THIS AXIS. THIS WILL SUSTAIN PERIODS OF
DIVERGENCE ALOFT THAT ARE TO ENHANCE CONVECTION ALONG THE SURFACE
FRONT. AT LOW LEVELS...AN 850 HPA JET OF 20-35KT WILL EXTEND
SOUTHEAST ACROSS SANTA CRUZ IN EASTERN BOLIVIA TO PARAGUAY/
SOUTHERN BRASIL...SUSTAINING A MOIST/WARM TRANSPORT THAT
CONVERGES INTO THE FRONT. THIS WILL HELP SUSTAIN UNUSUALLY HIGH
BOUNDARY LAYER DEW POINT TEMPERATURES OF 16-18C. AS A
RESULT...SCATTERED RAINS ARE TO CONTINUE AFFECTING SOUTHERN
BRASIL-NORTHERN MESOPOTAMIA VALLEY DURING THE NEXT THREE
DAYS...WITH A SURGE OVER URUGUAY/ENTRE RIOS ARGENTINA BY 60-78
HRS. DURING THE NEXT THREE DAYS MAXIMA WILL PEAK AT 20-35MM...
WITH MOST INTENSE ON DAYS 01 AND 03. ALSO NOTE THAT THERE IS A
SLIGHT RISK OF SEVERE CONVECTION OVER NORTHERN URUGUAY/SOUTHERN
RIO GRANDE DO SUL BRASIL BY 60-66 HRS...WHEN THE STORM RELATIVE
HELICITY PEAKS AT 1600. ALTHOUGH THE FORECASTED STABILITY INDICES
DURING THAT PERIOD ARE MARGINAL...THE ENSUING VERTICAL WIND SHEAR
WILL TEND TO FAVOR LONG LASTING CONVECTIVE CELLS. AFTER 72/84
HRS...DYNAMICS RAPIDLY WANE...AND WE ONLY FORESEE LIGHT
CONVECTION AS THE FRONT ENTERS SAO PAULO. OVER SOUTHERN SIERRA OF
PERU...WE ALSO EXPECT SCATTERED CONVECTION EARLY IN THE PERIOD
DUE TO THE FRONTAL INCURSION IN BOLIVIA...WITH MAXIMA TO PEAK AT
20-35MM.
ON THE SOUTHERN STREAM...A BROAD MID LEVEL TROUGH WILL MOVE
BETWEEN 140W-75W AND SOUTH OF 55S BY 48 HRS. THROUGH 72 HRS IT
WILL EXPAND TO ENVELOP THE DRAKE PASSAGE/TIERRA DEL FUEGO-
ANTARCTIC PENINSULA. BY 96-108 HRS THE FAVORED UKMET/ECMWF THEN
FORESEE A BROAD TROUGH PATTERN TO ENVELOP THE SOUTHERN
CONE...DOMINATING AREA SOUTH OF 40S. THE GFS...AS PREVIOUSLY
NOTED...FAVORS A MORE PROGRESSIVE PATTERN SHOWING THE TROUGH
YIELDING TO A BUILDING RIDGE FROM THE WEST. THE TROUGH
PATTERN...AS FORESEEN BY THE EUROPEAN MODELS...IS TO PERSIST
THROUGH 120-132 HRS. AT LOW LEVELS...THE ASSOCIATED FRONT ENTERS
TIERRA DEL FUEGO BY 72-78 HRS...THEN NORTH INTO SOUTHERN
PATAGONIA/SOUTHERN CHILE BY 96 HRS...AND REACHES NORTHERN
PATAGONIA BY 120 HRS. THIS IS TO FAVOR A WET PATTERN OVER CHILE
SOUTH OF 47S...TO ALSO INCLUDE MOST OF TIERRA DEL FUEGO...WITH
DAILY RAINFALL AMOUNTS OF 05-10MM. SNOWFALL IS EXPECTED ON THE
SOUTHERN ANDES.
AT 200 HPA...THE SUBTROPICAL RIDGE IS TO PERSIST ACROSS
CONTINENTAL AREA NORTH OF 20S...WITH MEAN AXIS WEST ALONG 10S.
MIGRATORY HIGHS/ANTICYCLONES WILL ANCHOR THIS RIDGE OVER THE
CONTINENT. CONVECTION OVER NORTHERN AMAZONAS INTO COLOMBIA-
ECUADOR WILL REMAIN UNUSUALLY ACTIVE DURING THE NEXT 36-48 HRS...
WITH MAXIMA TO PEAK AT 25-50MM. AFTER 60-72 HRS DAILY MAXIMA IN
THIS AREA WILL PEAK AT 25-35MM. SEA BREEZE CONVERGENCE ALONG THE
NORTHERN COAST...TO AFFECT WEST OF FORTALEZA WILL SUSTAIN MAXIMA
OF 15-30MM. MOST INTENSE IS EXPECTED ON THE NORTHEAST COAST...
NEAR CITY OF RECIFE...WITH MAXIMA OF 20-35MM BY 24-48 HRS...AND
15-25MM AT 48-72 HRS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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