Prognostic Meteorological Discussion
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FXSA20 KWBC 201737
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1237 PM EST TUE FEB 20 2018

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

MODEL DISCUSSION (VALID FROM FEB 20 AT 0000 UTC): THE GFS AND THE
EUROPEAN MODELS FOLLOW A SIMILAR EVOLUTION THROUGH 132-144 HRS...A
PATTERN WELL SUPPORTED BY THE MEAN OF THE GLOBAL ENSEMBLE MEMBERS.
AS A RESULT...OUR CONFIDENCE IN THE FORECAST IS HIGH.

OVER MID SECTIONS OF SOUTH AMERICA...A MID LEVEL SHORT WAVE TROUGH
ON THE NORTHERN STREAM IS TO PULL ACROSS CENTRAL/NORTHERN
ARGENTINA EARLY IN THE CYCLE...MEANWHILE TRIGGERING SCATTERED
CONVECTION WITH MAXIMA OF 15-25MM. THE SHORT WAVE TROUGH MOVES
ACROSS URUGUAY/SOUTHERN BRASIL EARLY ON WEDNESDAY. AS IT ENTERS
THE WESTERN ATLANTIC THIS WILL THEN MERGE WITH A MEANDERING SHORT
WAVE TROUGH OFF THE SOUTH COAST OF BRASIL. UNDER FAVORABLE JET
DYNAMICS THE GLOBAL MODELS THEN FORECAST THIS FEATURE TO GRADUALLY
INTENSIFY THROUGH THURSDAY MORNING...WHEN IT IS TO CENTER ON A
CLOSED LOW NEAR 43S 40W. THE TROUGH IS TO THEN MEANDER TO THE
EAST-SOUTHEAST...CROSSING 20W LATER DURING THE WEEKEND. AT LOW
LEVELS THE DEEPENING MID LEVEL TROUGH WILL SUSTAIN CYCLOGENESIS
ALONG A MEANDERING FRONT OVER SOUTHERN BRASIL-NORTHERN
ARGENTINA...WITH SURFACE LOW TO RAPIDLY INTENSIFY ON WEDNESDAY. AS
IT DEEPENS...THIS WILL THEN SUSTAIN A STRONG SOUTHEASTERLY FLOW
ACROSS NORTHERN ARGENTINA-PARAGUAY TO BOLIVIA.

THE ASSOCIATED SURFACE FRONT WILL THEN MOVE ACROSS SANTA
CATARINA-MATO GROSSO DO SUL-CHACO PARAGUAYO EARLY ON
WEDNESDAY...REACHING NORTHERN SAO PAULO-MATO GROSSO-NORTHERN
BOLIVIA LATER ON WEDNESDAY/EARLY THURSDAY MORNING. ON
THURSDAY-FRIDAY...AS IT LOOSES ITS UPPER LEVEL SUPPORT...THE FRONT
IS TO THEN GRADUALLY WEAKEN TO A TROUGH. ACROSS SOUTHERN
BRASIL-MATO GROSSO/MATO GROSSO DO SUL THIS IS TO FAVOR SCATTERED
CONVECTION WITH MAXIMA OF 20-30MM/DAY ON TUESDAY AND WEDNESDAY. ON
THURSDAY CONVECTION ACROSS NORTHERN SAO
PAULO-TOCANTINS/GOIAS-MINAS GERAIS AND ESPIRITO SANTO IS TO BECOME
BETTER ORGANIZED...WITH MAXIMA OF 35-70MM. PEAK IN CONVECTION IS
EXPECTED ON FRIDAY TO AFFECT ESPIRITO SANTO/EASTERN MINAS GERAIS
AND EASTERN BAHIA TO SUSTAIN MAXIMA OF 40-80MM...WITH LOCALLY
HIGHER AMOUNTS HIGHLY PROBABLE. AS CONVECTION TO THE EAST
STRENGTHENS... TRAILING CONVECTION BETWEEN MINAS GERAIS AND
RONDONIA WILL DECREASE TO 20-45MM/DAY. OTHER CONVECTION IS
EXPECTED TO THE WEST AS THE FRONT SURGES ACROSS BOLIVIA. IN THIS
AREA ENVIRONMENTAL CONDITIONS BECOME FAVORABLE FOR AN MCS TO
DEVELOP ON WEDNESDAY-THURSDAY...TO FAVOR ORGANIZED DEEP CONVECTION
WITH MAXIMA OF 75-125MM. ON THURSDAY-FRIDAY THIS DECREASES TO
30-60MM. ACTIVITY IS TO ALSO SPREAD NORTH ACROSS SOUTHERN-CENTRAL
PERU...WITH MAXIMA OF 20-40MM/DAY ON WEDNESDAY THROUGH
FRIDAY...WITH LOCALLY HIGHER AMOUNTS PROBABLE ACROSS CENTRAL PERU.

FARTHER UPSTREAM...BROAD MID LEVEL TROUGH DOMINATES THE SOUTHEAST
PACIFIC. SHORT WAVE PERTURBATIONS ARE TO REVOLVE AROUND THIS
AXIS...WITH ONE TO ENTER SOUTHERN CHILE EARLY ON WEDNESDAY
MORNING. THIS WILL SUSTAIN A PROGRESSIVE FRONT THAT STREAMS ACROSS
THE SOUTHERN CONE EARLY IN THE PERIOD...WITH BOUNDARY TO THEN MOVE
NORTH ACROSS PATAGONIA TO LA PAMPA/BUENOS AIRES PROVINCE ON
THURSDAY. MOST ACTIVE CONVECTION WITH THIS FEATURE IS EXPECTED
BETWEEN TEMUCO-ISLA DE CHILOE...WITH MAXIMA OF 15-20MM. UNDER THE
INFLUENCE OF ANOMALOUSLY WARM SEA SURFACE TEMPERATURES... THE
FORECASTED STABILITY INDICES FOR THIS PERIOD SHOW AN ENHANCED
POTENTIAL FOR THUNDERSTORMS. ON WEDNESDAY-THURSDAY THIS DECREASES
TO 10-15MM/DAY. LATER IN THE WEEK...AS THE FRONT MEANDERS OVER
CENTRAL ARGENTINA/URUGUAY...EXPECTING POTENTIAL FOR SCATTERED
CONVECTION TO RESULT IN ACCUMULATION OF 15-20MM.

NORTH OVER THE CONTINENT...AT 200 HPA...BROAD CELL OF THE
SUBTROPICAL RIDGE DOMINATES AREA TO THE NORTH OF 20S...WHILE
CENTERING ON A CLOSED HIGH THAT MEANDERS BETWEEN SOUTHERN
BOLIVIA-CHACO PARAGUAYO AND MATO GROSSO IN BRASIL. AN ELONGATED
TROUGH TO THE NORTH IS TO EXTEND EAST-TO-WEST ACROSS EQUATORIAL
SOUTH AMERICA. THIS IS TO FAVOR A DIVERGENT PATTERN ALOFT THAT IS
TO VENT ORGANIZED DEEP CONVECTION ALONG THE NEAR EQUATORIAL TROUGH
(NET) OVER BRASIL/ATLANTIC ITCZ. MOST ACTIVE CONVECTION IS
EXPECTED ACROSS AMAPA-PARA-MARANHAO...WITH MAXIMA OF 75-150MM/DAY
OVER THE NEXT TWO DAYS. ON THURSDAY THIS DECREASES TO 20-30MM/DAY.
OTHER CONVECTION TO THE WEST IS TO ALSO CLUSTER ALONG THE NET
ACROSS AMAZONAS IN BRASIL...WITH MAXIMA OF 25-50MM EARLY IN THE
CYCLE. ON THURSDAY-FRIDAY THIS WILL DECREASES TO 20-30MM/DAY.
ALSO...UNDER INFLUENCE OF THE PANAMANIAN LOW/TROUGH...A MOIST
WESTERLY FLOW IS TO PERSIST ACROSS ECUADOR DURING THE NEXT FEW
DAYS. EARLY IN THE CYCLE THIS IS TO FAVOR ACCUMULATION OF
20-40MM/DAY. ON THURSDAY-FRIDAY...HOWEVER...THE MOIST INFLOW
WEAKENS AND THE DAILY MAXIMA IN ASSOCIATION WITH DIURNAL
CONVECTION WILL DECREASES TO 15-25MM/DAY.

BALBINO...INMET (BRASIL)
POMIER...SENAMHI (BOLIVIA)
GALVEZ...WPC (USA)
$$





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