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South American Forecast Discussion
 
(Latest Discussion - Issued 1852Z Jul 30, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 PM EDT WED JUL 30 2014

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

MODEL DISCUSSION (VALID FROM 30 JULY 00UTC): MINOR SHORT WAVE
PATTERN CORRECTIONS TO PREVIOUS FORECAST PACKAGE...AS MODELS
CONTINUE TO CONVERGE ON A SIMILAR SOLUTION OVER THE NEXT FEW DAYS.
CONFIDENCE IN THE FORECAST IS HIGH.

AT MID LEVELS...CLOSED LOW ON THE SOUTHERN STREAM IS TO ANCHOR A
BROAD TROUGH ACROSS THE SOUTHEAST PACIFIC. THIS PATTERN IS
FORECAST TO HOLD THROUGH 84-96 HRS...THEN RAPIDLY WEAKEN LATER IN
THE CYCLE AS A LONG WAVE RIDGE BUILDS SOUTH ACROSS THE EASTERN
PACIFIC. AS THE TROUGH TO THE SOUTH HOLDS...IT IS TO PRESS AGAINST
A RIDGE TO THE NORTH. THE RIDGE IS FORECAST TO EXTEND WEST TO EAST
ALONG 30S DURING THE NEXT FEW DAYS. THIS...IN-TURN...WILL RESULT
IN A TIGHT BELT OF MID LATITUDE WESTERLIES ACROSS THE EASTERN
PACIFIC-SOUTHERN CONE OF SOUTH AMERICA TO THE WESTERN ATLANTIC.
THIS IS TO SUSTAIN A VERY STRONG JET STREAM ACROSS THE SOUTH
AMERICAN DOMAIN...WITH UPPER LEVEL WINDS TO PEAK AT 170-200KT.
SHORT WAVE PERTURBATIONS EMBED IN THIS FLOW. IN THIS
PATTERN...STRONG PERTURBATIONS ARE TO ENTER THE CONTINENT AT 24
AND 72/84 HRS...TO SUSTAIN HEIGHT FALLS OF 100-150GPM AS THEY
STREAM ACROSS THE CONTINENT. IN BETWEEN...WEAKER PERTURBATIONS ARE
EXPECTED TO ENTER THE SOUTHERN CONE. THIS IS TO ALSO SUSTAIN A
MOIST ADVECTIVE PATTERN...FAVORING TRANSPORT OF SUBTROPICAL
MOISTURE ACROSS THE EASTERN PACIFIC TO CENTRAL/SOUTHERN PORTIONS
OF CHILE. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A BROAD TROUGH
ACROSS THE BELLINGSHAUSEN SEA/SOUTHEAST PACIFIC. SUCCESSIVE POLAR
FRONTS ARE FORECAST TO REVOLVE AROUND THIS AXIS. THESE ARE TO THEN
RAPIDLY STREAM ACROSS THE SOUTHERN CONE OVER THE NEXT FEW
DAYS...ENHANCING LOW LEVEL CONVERGENCE WHILE SUSTAINING BOUNDARY
LAYER WINDS OF 25-40KT. INFLOW OF SUBTROPICAL MOISTURE AND STRONG
TOPOGRAPHICAL FORCING IS TO THEN FAVOR ORGANIZED CONVECTION AND
HEAVY RAINFALL AMOUNTS. MOST INTENSE IS EXPECTED EARLY ON DAY 02
AND LATER ON DAY 03...WITH MAXIMA TO PEAK AT 50-100MM. LOCALLY
HIGHER AMOUNTS ARE POSSIBLE DUE TO STRONG TOPOGRAPHICAL FORCING.
OTHER ACTIVITY TO THE NORTH IS EXPECTED BETWEEN TALCA AND
VALPARAISO IN CENTRAL CHILE...WITH RAINFALL MAXIMA IN THIS AREA TO
PEAK AT 20-30MM. THERE IS ALSO POTENTIAL FOR ORGANIZED SNOWFALL
AMOUNTS ACROSS THE CENTRAL ANDES OF CHILE ON DAY 02...WITH MAXIMA
OF 10-15CM.

FRONTS STREAMING ACROSS CHILE ARE TO THEN MEANDER NORTH ACROSS RIO
DE LA PLATA TO URUGUAY/ENTRE RIOS IN ARGENTINA. THIS BOUNDARY IS
TO GENERALLY PERSIST THROUGH 96-120 HRS. ALSO AT LOW LEVELS...A
NORTHERLY JET ACROSS EASTERN BOLIVIA/PARAGUAY WILL SUSTAIN A
WARM/MOIST ADVECTIVE PATTERN. THE TRANSPORT OF MOISTURE WILL HELP
SUSTAIN BOUNDARY LAYER DEW POINT TEMPERATURES OF 18-20C ACROSS THE
MESOPOTAMIA VALLEY IN ARGENTINA TO URUGUAY. THIS WILL COMBINE WITH
AFOREMENTIONED SHORT WAVE PERTURBATIONS STREAMING ACROSS CHILE TO
ARGENTINA TO SUSTAIN SCATTERED CONVECTION. IN THIS AREA EXPECTING
RAINFALL AMOUNTS OF 15-25MM/DAY THROUGH 60 HRS...AND 20-30MM AT
72-96 HRS. IN THIS AREA...FORECASTED STABILITY INDICES SHOW RISK
OF SEVERE CONVECTION ON DAYS 03-04 AS DEEP MID LEVEL PERTURBATION
MOVES ACROSS ARGENTINA. ALTHOUGH MODELS FORESEE A VERY UNSTABLE
AIR MASS...JET DYNAMICS APPEAR MARGINAL AT BEST...REDUCING
POTENTIAL FOR SEVERE CONVECTION TO LOW.

NORTH OVER THE ATLANTIC...AN ELONGATED FRONT EXTENDS WEST TO
NORTHERN ESPIRITO SANTO/BAHIA IN BRASIL. THE FRONTAL BOUNDARY IS
TO PERSIST DURING THE NEXT FEW DAYS. MEANWHILE...THIS IS TO
ENHANCE LOW LEVEL MOISTURE CONVERGENCE...TO SUSTAIN MODERATE
RAINFALL AMOUNTS. THROUGH 48-60 HRS THIS IS TO PEAK AT
15-25MM/DAY...AND THROUGH 84-96 HRS IT IS TO DECREASE TO 05-10MM.

NORTH OVER THE CONTINENT...AT 200 HPA...A HIGH OVER EASTERN
ECUADOR  ANCHORS A RIDGE OVER THE EASTERN TROPICAL PACIFIC. THIS
WILL PERSIST OVER THE NEXT 48 HRS...WITH RIDGE TO THEN RAPIDLY
WEAKEN. A SHORT WAVE TROUGH LIES TO THE EAST...WITH AXIS NORTHWEST
ACROSS MATO GROSSO IN BRASIL TO AMAZONAS/SOUTHERN COLOMBIA.
THROUGH 72 HRS THE TROUGH IS FORECAST TO DAMPEN AS THE RIDGE TO
THE WEST WEAKENS. MEANWHILE...THE SHORT WAVE TROUGH IS TO ENHANCE
CONVECTIVE INSTABILITY ACROSS NORTHWEST BRASIL-NORTHERN JUNGLE OF
PERU/EASTERN ECUADOR...WITH DIURNAL CONVECTION IN THIS AREA TO
PEAK AT 05-10MM. A SLIGHT INCREASE IS THEN EXPECTED LATER ON DAY
04 AS TROPICAL WAVE MOVES ACROSS NORTHERN SOUTH AMERICA.

FERNANDES...INMET (BRASIL)
JALFIN...SMN (ARGENTINA)
DAVISON...WPC (USA)