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Prognostic Meteorological Discussion
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FXSA20 KWBC 171530
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1130 AM EDT FRI MAY 17 2013

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

MODEL COMPARISON (VALID FROM 00 UTC MAY 17): THE GFS HAS MADE SOME
FORECAST CORRECTIONS AND IS IN BETTER AGREEMENT WITH OTHER GLOBAL
MODELS AND ENSEMBLE MEMBERS. FORECAST CONFIDENCE IS HIGH.

TWO REMARKABLE SYSTEMS ARE OF INTEREST. ONE IS A COLD FRONT AND
POTENT SOUTHERLY SURGE ACROSS WESTERN BRASIL/SOUTHERN PERU. THIS
SYSTEM IS SUSTAINED BY A STRONG POLAR TROUGH OVER THE SOUTH
ATLANTIC. AS TROUGH AND UPPER FORCING IS RAPIDLY MOVING AWAY...THE
FRONT IS RAPIDLY STALLING ACROSS SOUTHEASTERN BRASIL INTO MATO
GROSSO DO SUL...LOSING DEFINITION ACROSS NORTHEASTERN BOLIVIA. A
POTENT POLAR HIGH CENTERING OVER NORTHEASTERN ARGENTINA IS
INDUCING A STRONG SOUTHERLY SURGE THAT WILL START WEAKENING DURING
THE DAY TODAY. IN THE MEANTIME...THE SURGE WILL CONTINUE ENHANCING
CONVECTION ACROSS SOUTHERN/CENTRAL PERU AND WESTERN AMAZONAS/ACRE
THROUGH 36 HRS...TO PRODUCE ACCUMULATIONS OT 25-50MM/DAY IN
STRONGEST STORMS. BY 36 HRS THE EDGE OF THE SURGE WILL EXTEND
ACROSS CENTRAL AMAZONAS IN BRASIL-CENTRAL LORETO IN PERU RAPIDLY
LOSING DEFINITION.

OTHER SYSTEM OF INTEREST IS A STRONG NORTHERN STREAM TROUGH OVER
THE EASTERN PACIFIC. THIS TROUGH SUSTAINS AN INDUCED LOW-LEVEL
TROUGH OFF NORTHERN CHILE/PERU...AN THIS HAS FORMED AN UNUSUALLY
DEEP MOISTURE POOL FOR THE REGION. ALSO...AN UPPER JET DIVIDES
THIS TROUGH FROM THE SUBTROPICAL RIDGE OVER WESTERN BRASIL. THIS
WILL CONTINUE ENHANCING UPPER DIVERGENCE OVER CENTRAL/NORTHERN
CHILE...WITH ENHANCEMENT PEAKING TONIGHT INTO EARLY TOMORROW
SATURDAY. EXPECTING HEAVIEST ACCUMULATIONS OVER CENTRAL/NORTHERN
CHILE THROUGH 36-48 HRS...DEVELOPING AS FAR NORTH AS THE DRY
ATACAMA REGION AND COASTAL AREAS OF COQUIMBO/LA SERENA. EXPECTING
ACCUMULATIONS PEAKING AT 15-25MM/DAY...ESPECIALLY AT HIGHER
ELEVATIONS WHERE LOCALLY HEAVY SNOW SHOWERS ARE EXPECTED. LIGHTER
RAIN AND SNOW SHOWERS WILL CONTINUE DOTTING SOUTHERN PERU AND
WESTERN BOLIVIA/HIGHLANDS OF NORTHERN CHILE. ON DAY 02...WANING
LIGHT RAIN SHOWERS AND ISOLATED THUNDERSTORMS WILL STILL AFFECT
PORTIONS OF CENTRAL CHILE EXTENDING AS FAR SOUTH AS THE MOUNTAINS
OF SANTIAGO.

THE UPPER TROUGH WILL SPILL ENERGY INTO ARGENTINA ON DAY 02. THIS
WILL UNDULATE THE BOUNDARY MEANDERING ACROSS MID-SECTIONS OF THE
CONTINENT AND STIMULATE CONVECTION. EXPECTING ACCUMULATIONS ACROSS
NORTHERN URUGUAY INTO RIO GRANDE DO SUL AND SANTA CATARINA WITH
ACCUMULATIONS OF 15-30MM. MODELS ARE THEN SUGGESTING CYCLOGENESIS
DEVELOPING OFF THE COAST OF SOUTHERN BRASIL. ONSHORE FLOW ACROSS
SANTA CATARINA/PARANA SAO PAULO WILL ENHANCE ACCUMULATIONS LOCALLY
ON DAY 04...LEADING TO AMOUNTS OF 15-20MM/DAY.

NORTH OVER THE CONTINENT...AT 200 HPA...MODELS CONTINUE CONFIDENT
ABOUT SUBTROPICAL HIGH BECOMING UNSEASONABLY WELL DEFINED OVER
ACRE/SOUTHERN AMAZONAS IN BRASIL. THIS WILL VENT CONVECTION
PRIMARILY ACROSS SOUTHERN COLOMBIA/ECUADOR/NORTHERN PERU...WHILE
QUIET PATTERN ESTABLISHES ACROSS THE NORTHERN COAST OF BRASIL.
ONCE SOUTHERLY SURGE WANES...EXPECTING ACCUMULATIONS IN THE
15-35MM/DAY RANGE TO PERSIST ACROSS SOUTHERN COLOMBIA/NORTHWESTERN
BRASIL/SOUTHERN VENEZUELA. A REACTIVATION OF THE ATLANTIC ITCZ BY
LATE CYCLE SHOULD LEAD TO AN INCREASE IN ACCUMULATIONS OVER
AMAPA/SOUTHERN GUIANAS BY LATE CYCLE TO 20-35MM/DAY.

MAQUEDA...DINAC (PARAGUAY)
SILVA DE SOUZA...INMET (BRASIL)
DAVISON...WPC (USA)
GALVEZ...WPC (USA)
$$
National Weather Service Weather Forecast Office
