Prognostic Meteorological Discussion
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FXSA20 KWBC 171627
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1227 PM EDT WED SEP 17 2014

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 17 AT 00UTC): THE GLOBAL
MODELS AGREE QUITE WELL ON EVOLVING SHORT WAVE PATTERN ACROSS THE
DOMAIN THROUGH 84-90 HRS. THEY THEN DIVERGE DURING THE MEDIUM
RANGE PERIOD...MAKING ADJUSTMENTS TO AMPLITUDE/SPEED OF MID LEVEL
PERTURBATIONS ENTERING THE DOMAIN. IN THIS CYCLE THE GFS AND UKMET
CORRECTED IN FAVOR OF FASTER PERTURBATIONS THAN WHAT THEY
PREVIOUSLY INDICATED. BUT THIS IS A PERIOD OF HIGH
UNCERTAINTY...WITH GLOBAL ENSEMBLE MEMBERS SHOWING A LOT OF
VARIABILITY AMONG THE MEMBERS.

A BROAD TROUGH ON THE SOUTHERN STREAM EXTENDS AT MID LEVELS FROM
THE BELLINGSHAUSEN SEA TO THE SOUTHERN CONE OF SOUTH AMERICA. THIS
TROUGH SUSTAINS A TIGHT BELT OF MID LATITUDE WESTERLIES ACROSS
SOUTHERN CHILE TO PATAGONIA IN ARGENTINA. EMBEDDED IN THIS
FLOW...A PROGRESSIVE SHORT WAVE TROUGH IS TO ENTER SOUTHERN
CHILE/TIERRA DEL FUEGO BY 24 HRS...AND OFF THE COAST TO THE
WESTERN ATLANTIC BY 48 HRS. BUILDING RIDGE TO THE WEST IS TO THEN
DISPLACE THIS TROUGH TO THE SOUTH ATLANTIC. AS IT MEANDERS
EAST...IT IS TO THEN DEEPEN ACROSS THE SOUTH GEORGIA
ISLANDS...WITH AXIS TO BOUND BETWEEN 50W-10W AND TO THE SOUTH OF
40S LATER IN THE CYCLE. AT LOW LEVELS...A PROGRESSIVE FRONT IS TO
ENTER SOUTHERN CHILE EARLY IN THE CYCLE...TO QUICKLY MOVE ACROSS
PATAGONIA TO THE WESTERN ATLANTIC. THIS IS TO FAVOR STRONG
BOUNDARY LAYER WINDS WHILE TRIGGERING MODERATE CONVECTION...WITH
ACCUMULATION OF 15-30MM EXPECTED BETWEEN TIERRA DEL FUEGO AND
PUERTO MONTT IN SOUTHERN CHILE.

ALSO AT MID LEVELS...A SHORT WAVE TROUGH TO THE NORTH IS TO
MEANDER OFF THE CENTRAL/NORTHERN COAST OF CHILE THROUGH 30-36 HRS.
IT IS TO THEN SPILL ACROSS THE ANDES INTO THE NORTHERN PROVINCES
IN ARGENTINA...TO TRACK ACROSS URUGUAY/MESOPOTAMIA VALLEY IN
ARGENTINA BY 48 HRS. BY 72 HRS THE TROUGH MOVES OFF THE SOUTHERN
COAST OF BRASIL. AS IT MEANDERS EAST INTO THE SOUTH ATLANTIC THIS
IS TO THEN INTERACT WITH AMPLIFYING POLAR TROUGH TO THE SOUTH. AS
IT SPILLS ACROSS THE ANDES INTO ARGENTINA...THE SHORT WAVE TROUGH
IS TO TRIGGER HEIGHT FALLS OF 50-75GPM...WITH FOCUS OF THE SHORT
WAVE ENERGY ACROSS URUGUAY-MESOPOTAMIA VALLEY/SOUTHERN BRASIL BY
48-60 HRS. AS IT SPILLS ACROSS THE ANDES...THE TROUGH ALOFT IS TO
INTERACT WITH A SURFACE FRONT OVER SOUTHERN BRASIL/PARAGUAY. BY
24-48 HRS THIS IS TO TRIGGER CYCLOGENESIS OVER CORRIENTES IN
ARGENTINA/RIO GRANDE DO SUL. THROUGH 72-84 HRS THIS LOW IS
FORECAST TO OCCLUDE AS IT MOVES OFF THE COAST OF BRASIL INTO THE
SOUTH ATLANTIC. THE DEEPENING LOW WILL THEN REINFORCE THE
BAROCLINIC BOUNDARY AS IT MEANDERS NORTH INTO SAO PAULO-SANTA CRUZ
IN EASTERN BOLIVIA...WITH FRONT TO MOVE NORTH ACROSS RIO DE
JANEIRO/MATO GROSSO IN BRASIL TO NORTHERN BOLIVIA BY 84-96 HRS. IT
IS TO CONTINUE NORTH ACROSS RIO DE JANEIRO TO ESPIRITO SANTO IN
BRASIL BY 120 HRS. UNDER FAVORABLE JET DYNAMICS THIS IS TO SUSTAIN
ORGANIZED HEAVY CONVECTION AS IT MOVES ACROSS MID SECTIONS OF
SOUTH AMERICA....WITH MODELS SHOWING RISK OF AN MCS FORMING
BETWEEN SOUTHERN BRASIL AND SOUTHEAST PARAGUAY BY 42-84 HRS. IN
THIS AREA THE RAINFALL MAXIMA IS TO PEAK AT 40-80MM. OVER
CENTRAL/NORTHWEST PARAGUAY TO SOUTHERN BOLIVIA EXPECTING
ACCUMULATION OF 10-15MM. AS THE FRONT MEANDERS NORTH ACROSS
SOUTHEAST BRASIL IT IS TO SUSTAIN SCATTERED CONVECTION WITH
ACCUMULATION OF 20-30MM. AS THE FRONT RAILS ACROSS BOLIVIA TO
SOUTHERN PERU EXPECTING MAXIMA OF 20-35MM BY 72-96 HRS...WITH
SIMILAR AMOUNTS ACROSS THE PERUVIAN JUNGLE-WESTERN BRASIL TO
EASTERN ECUADOR LATER IN THE CYCLE.
MODELS NOW FORECAST A DEEPENING TROUGH ON THE NORTHERN STREAM TO
QUICKLY MOVE TO THE EASTERN PACIFIC THROUGH 84-96 HRS. THE TROUGH
IS TO THEN DEEPEN AS IT INTERACTS WITH A SHORT WAVE TROUGH TO THE
SOUTH...WITH GLOBAL MODEL GUIDANCE NOW SHOWING A HIGH AMPLITUDE
LONG WAVE TROUGH ACROSS THE EASTERN PACIFIC BY THE END OF THE
CYCLE.
THE HIGH AMPLITUDE TROUGH IS TO ESTABLISH A SUBTROPICAL CONNECTION
AS IT PULLS ACROSS THE EASTERN PACIFIC...TO SUSTAIN A POLAR FRONT
ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA BY 84-96 HRS THIS
MOVES ACROSS PATAGONIA-LA PAMPA TO CENTRAL CHILE BY 96 HRS. STRONG
BOUNDARY LAYER WINDS ARE TO ACCOMPANY THIS BOUNDARY AS IT MAKES
LANDFALL OVER THE SOUTHERN CONE...AND IT IS TO TRIGGER SCATTERED
CONVECTION WITH RAINFALL MAXIMA OF 15-25MM LATER ON DAY 04.

AS THE NORTHERN STREAM TROUGH MOVES ACROSS CHILE TO ARGENTINA IT
IS TO THEN PRESS AGAINST WEAK CELL OF THE SUBTROPICAL RIDGE TO THE
NORTH. THIS WILL THEN FAVOR THE GRADUAL EROSION OF SUBTROPICAL
RIDGE PATTERN AT UPPER LEVEL OF THE ATMOSPHERE. LATER IN THE
CYCLE...AS THE TROUGH MOVES AWAY...A NEW CELL IS TO GRADUALLY
ESTABLISH ACROSS WESTERN BRASIL-NORTHERN BOLIVIA-SOUTHERN PERU. AS
THE PATTERN EVOLVES...DIURNAL CONVECTION ACROSS NORTHWEST SOUTH
AMERICA IS TO BECOME MORE ACTIVE/BETTER ORGANIZED. EARLY IN THE
CYCLE EXPECTING ACCUMULATION OF 15-20MM...BUT THROUGH 96-120 HRS
THIS INCREASES TO 20-35MM. MOST INTENSE IS EXPECTED ACROSS
CENTRAL-NORTHERN PERU TO SOUTHERN COLOMBIA/EASTERN ECUADOR.

JALFIN...SMN (ARGENTINA)
CAROCA...DMC (CHILE)
DAVISON...WPC (USA)
$$





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