Prognostic Meteorological Discussion
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FXSA20 KWBC 251542
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1142 AM EDT TUE APR 25 2017

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

MODEL DISCUSSION (VALID FROM APRIL 25 AT 0000 UTC): GLOBAL MODELS
REMAIN FAIRLY CONSISTENT ON HOW MID/UPPER LEVEL PATTERN OVER THE
CONTINENT/WESTERN ATLANTIC IS GOING TO EVOLVE DURING THE NEXT FIVE
DAYS. OVER THE EASTERN PACIFIC THEY AGREE ON GENERAL EVOLUTION
THROUGH 84-96 HRS...THEN DIVERGE ON THE INTENSITY/AMPLITUDE AND
SPEED OF MID LEVEL PERTURBATIONS REACHING THE SOUTHERN CONE. THE
HIGHER UNCERTAINTY IS REFLECTED BY A LARGER VARIABILITY AMONG THE
GLOBAL ENSEMBLE MEMBERS...RESULTING IN LOWER CONFIDENCE IN THE
UPSTREAM FORECAST BEYOND 108 HRS.

A HIGH AMPLITUDE MID/UPPER LEVEL TROUGH OVER THE CONTINENT IS THE
SYSTEM OF CONCERN...WITH 500 HPA TROUGH TO EXTEND NORTH ACROSS
CHILE/ARGENTINA TO MID SECTIONS OF SOUTH AMERICA EARLY IN THE
CYCLE. AS IT PULLS ACROSS RIO DE LA PLATA BASIN LATER TODAY THE
TROUGH WILL INDUCE HEIGHT FALLS OF 150-200 GPM. AT 250 HPA...A
SUBTROPICAL JET WILL ROUND THE BASE OF THE TROUGH WHILE A POLAR
JET IS TO MEANDER TO THE SOUTH. THE SUBTROPICAL JET WILL FAVOR AN
UPPER DIVERGENT PATTERN ON ITS RIGHT ENTRANCE REGION THAT IS TO
VENT CONVECTIVE DEVELOPMENT ACROSS MID SECTIONS OF SOUTH AMERICA.
BUT AS THE SUBTROPICAL AND POLAR JETS EXIT THE CONTINENT...THEY
ARE TO THEN COUPLE ON THEIR DIVERGENT EXIT/ENTRANCE REGIONS. THIS
WILL THEN SUSTAIN A BROAD AREA OF DIVERGENCE OVER THE WESTERN
ATLANTIC. AT LOW LEVELS THE DEEPENING MID LEVEL TROUGH WILL THEN
SUSTAIN EXPLOSIVE CYCLOGENESIS ON TUESDAY TO WEDNESDAY...WITH LOW
TO DEEPEN FROM A 998-1000 HPA LOW THIS EVENING TO A 962 HPA LOW ON
WEDNESDAY...RESULTING IN A METEOROLOGICAL BOMB THAT IS TO SUSTAIN
TROPICAL STORM INTENSITY WINDS OVER THE WESTERN ATLANTIC. THIS
WILL COMBINE WITH BUILDING POLAR RIDGE OVER ARGENTINA AND ENSUING
PAMPERO LOW LEVEL JET TO SUSTAIN A PROGRESSIVE FRONT THAT SURGES
ACROSS SOUTHERN BRASIL-NORTHERN ARGENTINA TO PARAGUAY LATER TODAY.
ENVIRONMENTAL CONDITIONS AHEAD OF THIS BOUNDARY WILL FAVOR
GENERATION OF A MESO SCALE CONVECTIVE SYSTEM ACROSS
PARAGUAY-SOUTHERN BRASIL LATER TODAY-EARLY WEDNESDAY MORNING. IN
THIS AREA WE NOW PROJECT RAINFALL MAXIMA OF 75-125MM...WITH
LOCALIZED MAXIMA OF 200MM HIGHLY PROBABLE. EARLY ON WEDNESDAY THE
SOUTHERLY JET WILL THEN ENHANCE CONVERGENCE ACROSS BOLIVIA TO ACRE
IN BRASIL/SOUTHERN JUNGLE OF PERU...WITH MAXIMA OF 50-100MM. HEAVY
CONVECTION IS TO THEN SPREAD ACROSS CENTRAL-NORTHERN JUNGLE OF
PERU/WESTERN AMAZONAS IN BRASIL ON THURSDAY MORNING...WITH MAXIMA
OF 40-80MM. LATER IN THE DAY IT WILL INCREASE TO 50-100MM AS IT
SPREADS ACROSS SOUTHERN COLOMBIA/NORTHERN PERU TO EASTERN ECUADOR.

A BROAD/LOW AMPLITUDE RIDGE IS TO THEN DOMINATE THE MID/UPPER
LEVELS OVER THE FAR EASTERN PACIFIC DURING THE NEXT FEW DAYS. THE
PERSISTENT RIDGE WILL THEN STEER SHORT WAVE PERTURBATIONS ACROSS
SOUTHERN CHILE-DRAKE PASSAGE FROM TIME-TO-TIME. GLOBAL MODELS
GENERALLY AGREE ON THIS PATTERN TO PERSIST DURING THE NEXT 84-96
HRS. THEY THEN DISAGREE ON INTENSITY OF FOLLOW UP PERTURBATIONS.
IN THIS CYCLE THE GFS CORRECTED IN FAVOR OF A WEAKER PERTURBATION
TO ENTER SOUTHERN CHILE LATER IN THE CYCLE...WHILE THE UKMET NOW
FORESEES A DEEPER/HIGHER AMPLITUDE TROUGH. THIS ADDS TO THE
UNCERTAINTY AND LACK OF CONFIDENCE IN THE FORECAST DURING THE
MEDIUM RANGE PERIOD. AT LOW LEVELS...AN ELONGATED FRONT IS TO
STRETCH ACROSS THE SOUTHEAST PACIFIC TO SOUTHERN CHILE/PATAGONIA
IN ARGENTINA. UNDER INFLUENCE OF THE MID LEVEL PERTURBATIONS
FRONTAL LOWS/WAVE ARE TO STREAM ACROSS SOUTHERN CHILE FROM
TIME-TO-TIME. THESE PERTURBATIONS ARE TO SUSTAIN A SUBTROPICAL
CONNECTION AS THEY PROPAGATE ACROSS THE EASTERN
PACIFIC...ENHANCING MOISTURE TRANSPORT/CONVERGENCE OVER SOUTHERN
CHILE. ON WEDNESDAY AND ONWARD THIS IS TO THEN RESULT IN MODERATE
RAINFALL AMOUNTS OF 20-30MM/DAY TO CLUSTER OVER CHILE BETWEEN ISLA
DE CHILOE AND TEMUCO.

NORTHERN SOUTH AMERICA...MEANWHILE...WILL REMAIN UNDER THE
INFLUENCE OF A CELL OF THE SUBTROPICAL RIDGE...WITH AXIS AT 200
HPA TO ENVELOP CONTINENTAL AREA TO THE NORTH OF 20S. A CLOSED HIGH
OVER AMAZONAS IN BRASIL IS TO ANCHOR THIS RIDGE DURING THE NEXT
FEW DAYS. AS IT HOLDS OVER THE CONTINENT...A TROUGH IS TO AMPLIFY
ALONG THE EASTERN FRINGES OF THIS RIDGE ON WEDNESDAY-THURSDAY. THE
AMPLIFYING TROUGH WILL THEN FAVOR SCATTERED CONVECTION ALONG THE
NORTH COAST OF BRASIL...WITH MAXIMA OF 15-20MM/DAY. HIGHER AMOUNTS
ARE EXPECTED ACROSS PARA-AMAZONAS-RORAIMA TO THE WEST...WITH DAILY
MAXIMA IN THIS AREA TO RANGE BETWEEN 20-35MM. ACTIVITY GRADUALLY
SPREADS ACROSS SOUTHERN COLOMBIA TO NORTHERN PERU/EASTERN
ECUADOR...WITH SURGE IN CONVECTION LIKELY ON SATURDAY. ON THE
ANDES/WESTERN ECUADOR...INITIALLY EXPECTING MAXIMA OF 40-80MM AS
ENHANCED BY A WESTERLY LOW-MID LEVEL FLOW. THROUGH THURSDAY THIS
IS TO WEAKEN...DECREASING TO 15-20MM/DAY.

ABDEMUR...FAA (ARGENTINA)
ESPINOLA...DNM (PARAGUAY)
DAVISON...WPC (USA)
$$





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