Prognostic Meteorological Discussion
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FXSA20 KWBC 251521
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1121 AM EDT TUE JUL 25 2017

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

MODEL DISCUSSION (VALID FROM JUL 25 AT 0000 UTC): A HIGHLY
AMPLIFIED SINUSOIDAL LONG WAVE PATTERN IS TO DOMINATE OVER THE FAR
EASTERN PACIFIC/SOUTHERN CONE OF SOUTH AMERICA DURING THE NEXT
FOUR DAYS. IN A SLOW TO EVOLVE PATTERN...THE GLOBAL MODEL GUIDANCE
REMAINS IN VERY GOOD AGREEMENT THROUGH 96-108 HRS. LATER IN THE
CYCLE...AS THE TROUGH/RIDGE WEAKENS AND SHORTER AMPLITUDE
PERTURBATIONS ENTER THE DOMAIN...MODELS THEN START TO DISAGREE ON
THE AMPLITUDE AND SPEED OF PROPAGATION. DIFFERENCES AMONG THE
MODELS BECOME MORE PRONOUNCED THROUGH 144-156 HRS.

A MID/UPPER LEVEL RIDGE IS TO BUILD OVER THE SOUTH PACIFIC ALONG
120W/130W TO THE POLAR ICECAP EARLY IN THE CYCLE.
THIS...IN-TURN...SUSTAINS THE NORTHWARD AMPLIFICATION OF A LONG
WAVE TROUGH TO THE EAST. THE POSITIVELY TILTED TROUGH REACHES
MAXIMUM AMPLITUDE EARLY ON WEDNESDAY...WHEN IT IS TO EXTEND TO THE
NORTHWEST FROM THE ANTARCTIC PENINSULA/DRAKE PASSAGE...ALONG 45S
90W...30S 120W...TO 20S 130W. AS THE TROUGH DEEPENS...IT IS TO
FOCUS SHORT WAVE ENERGY ACROSS THE SOUTHERN REGIONS OF
CHILE-PATAGONIA IN ARGENTINA THROUGH FRIDAY MORNING...MEANWHILE
FAVORING HEIGHT FALLS OF 150-200GPM PER DAY. LATER ON FRIDAY...THE
BOTTOM HALF OF THE TROUGH IS TO ACCELERATE ACROSS THE WEDDELL
SEA/SOUTHWEST ATLANTIC...WHILE THE TRAILING END OVER THE PACIFIC
IS TO SLOWLY PULL ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA.
EARLY ON SATURDAY THE REMAINING SHORT WAVE TROUGH IS TO PULL
ACROSS PATAGONIA TO THE WESTERN ATLANTIC. AT LOW LEVELS THIS
SUSTAINS A BROAD TROUGH OVER THE SOUTHEAST PACIFIC/SOUTHERN
CHILE...WITH AXIS TO SLOWLY PULL ACROSS THE ANTARCTIC PENINSULA
AND ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA ON THURSDAY.
POLAR FRONTS WILL REVOLVE AROUND THIS TROUGH....WITH ONE TO MOVE
ACROSS SOUTHERN CHILE TO PATAGONIA EARLY ON WEDNESDAY...WHERE IT
IS TO MEANDER THROUGH THURSDAY EVENING. ON FRIDAY A POLAR RIDGE
OVER PATAGONIA/LA PAMPA IS TO DISPLACE THE FRONT NORTH ACROSS THE
BUENOS AIRES PROVINCE-MENDOZA IN ARGENTINA-CENTRAL REGIONS OF
CHILE. AS IT MEANDERS OVER THE SOUTHERN CONE THE POLAR TROUGH WILL
SUSTAIN A TIGHT GRADIENT AND STRONG BOUNDARY LAYER WINDS OF
35-45KT. THESE WILL ENHANCE MOISTURE TRANSPORTS AND CONVERGENCE
ACROSS SOUTHERN CHILE. MOST ACTIVE IS EXPECTED BETWEEN TEMUCO-ISLA
DE CHILOE...WITH RAINFALL AMOUNTS OF 15-20MM EXPECTED THROUGH
WEDNESDAY MORNING. ON WEDNESDAY TO THURSDAY THIS INCREASES TO
20-35MM...BUT AS THE TROUGH MEANDERS INLAND ACROSS CHILE TO
ARGENTINA THIS WILL DECREASE TO 15-25MM ON THURSDAY. LESSER
AMOUNTS ARE EXPECTED BETWEEN ISLA DE CHILOE-TIERRA DEL
FUEGO...WITH ACCUMULATION OF 10-15MM/DAY EXPECTED TODAY AND
WEDNESDAY. ON THURSDAY...ENVIRONMENTAL CONDITIONS BECOME FAVORABLE
AND CONDUCIVE FOR SCATTERED SNOW SHOWERS...TO SUSTAIN SNOWFALL
AMOUNTS OF 05-10CM.

ALSO OVER THE ATLANTIC...A LOW AMPLITUDE SHORT WAVE TROUGH IS TO
PULL ACROSS 15W-25W AND SOUTH OF 30S LATER THIS EVENING. AT LOW
LEVELS THE ASSOCIATED FRONT TRAILS TO THE NORTHWEST ACROSS
URUGUAY-ENTRE RIOS IN ARGENTINA. THE BOUNDARY IS TO REMAIN NEARLY
STATIONARY DURING THE NEXT TWO DAYS...THEN WEAKEN AS IT MEANDERS
SOUTH ACROSS THE BUENOS AIRES PROVINCE ON THURSDAY. INITIALLY THIS
IS TO FAVOR SCATTERED CONVECTION WITH ACCUMULATION OF 10-15MM/DAY.
ON WEDNESDAY...AS THE FRONT STARTS TO WEAKEN...THIS DECREASE TO
00-05MM/DAY.

NORTH OVER THE CONTINENT...AT 200 HPA...AN ILL ORGANIZED LOW IS
INITIALIZED OVER CENTRAL BRASIL...WHERE IT IS TO REMAIN WHILE
GRADUALLY WEAKENING TO AN OPEN TROUGH LATER ON THURSDAY. BUT AS IT
IS TO LIMIT TO UPPER ATMOSPHERE THIS IS TO HAVE LITTLE TO NO
IMPACT ON DIURNAL CONVECTION ACROSS BRASIL. TRADE WIND CONVERGENCE
BETWEEN ESPIRITO SANTO-PERNAMBUCO IN NORTHEAST BRASIL IS TO
GENERALLY FAVOR RAINFALL AMOUNTS OF 10-15MM/DAY...WITH SIMILAR
AMOUNTS ACROSS AMAPA-NORTHERN PARA/SOUTHERN GUIANAS. OVER
RORAIMA-NORTHERN AMAZONAS TO SOUTHERN COLOMBIA/NORTHERN JUNGLE OF
PERU INITIALLY EXPECTING RAINFALL AMOUNTS OF 10-15MM/DAY. THROUGH
THURSDAY THIS INCREASES TO 15-25MM/DAY. LATER IN THE CYCLE AN
UPPER LEVEL HIGH CLOSED OVER THE EASTERN PACIFIC NEAR THE
GALAPAGOS ISLANDS. THIS IS TO ANCHOR A BUILDING RIDGE OFF THE
COAST OF PERU. THE RIDGE...IN-TURN...WILL SUSTAIN THE NORTHWARD
AMPLIFICATION OF A TROUGH ACROSS THE PERUVIAN JUNGLE TO ECUADOR
LATER ON FRIDAY. THIS WILL THEN ENHANCE CONVECTIVE DEVELOPMENT
ACROSS NORTHERN PERU-SOUTHERN COLOMBIA.

ESPINOLA...DNM (PARAGUAY)
GONZALEZ...DMC (CHILE)
DAVISON...WPC (USA)
$$





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