NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1223 PM EST THU DEC 17 2020
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GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
FORECAST DISCUSSION FROM DEC 17/17UTC: A MID/UPPER LEVEL SHORT
WAVE TROUGH ON THE NORTHERN STREAM IS INITIALIZED OVER THE FAR
EASTERN PACIFIC NEAR THE ARCHIPELAGO DE JUAN FERNANDEZ. UNDER
INFLUENCE OF A RIDGE TO THE WEST...THE SHORT WAVE TROUGH IS TO
LIFT ACROSS CENTRAL REGIONS OF CHILE EARLY ON FRIDAY MORNING.
THROUGH THE DAY THIS WILL SPILL ACROSS THE ANDES INTO CUYO/CENTRAL
ARGENTINA. ON SATURDAY...AS IT TRACKS ACROSS THE BUENOS AIRES
PROVINCE TO THE WESTERN PACIFIC...THE MID/UPPER LEVEL SYSTEM IS
FORECAST TO RAPIDLY INTENSIFY. AT 250 HPA...MODELS CONTINUE TO
FORECAST A SUBTROPICAL/POLAR JET PAIR TO PROPAGATE IN TANDEM WITH
THIS SYSTEM...WITH THE JETS TO COUPLE ON THEIR DIVERGENT REGIONS
AS THEY MEANDER EAST ACROSS CENTRAL ARGENTINA ON FRIDAY-SATURDAY.
AT LOW LEVELS...THE MID LEVEL FORCING AND STRONG UPPER LEVEL
SUPPORT WILL HELP SUSTAIN CYCLOGENESIS/FRONTOGENESIS OVER CENTRAL
ARGENTINA ON FRIDAY. AS THE SURFACE LOW INTENSIFIES...IT WILL
SUSTAIN A SOUTHERLY JET ACROSS CENTRAL ARGENTINA. THIS WILL THEN
DRIVE THE ASSOCIATED FRONT NORTH WHILE TRIGGERING A PREFRONTAL
SQUALL LINE ACROSS SAIN LUIS-CORDOBA IN CENTRAL ARGENTINA. THE
FORECASTED STABILITY INDICES STRONGLY SUGGEST RISK OF LARGE HAIL
PRODUCING THUNDERSTORMS...WITH THE HELICITY FORECAST SHOWING
POTENTIAL FOR TORNADIC PRODUCING SUPERCELLS TO FORM THROUGH MIDDAY
ON FRIDAY. IN ASSOCIATION WITH THIS FEATURE EXPECTING HEAVY
RAINFALL AMOUNTS...WITH MAXIMA RANGING BETWEEN 75-125MM ON FRIDAY.
ON SATURDAY THIS SPREADS NORTH ACROSS PARAGUAY/MESOPOTAMIA VALLEY
IN ARGENTINA TO PARANA/SANTA CATARINA IN SOUTHERN BRASIL...WHERE
IT IS EXPECTED TO TRIGGER ACCUMULATION OF 35-70MM IN POTENTIALLY
SEVERE CONVECTION. MODERATE TO HEAVY CONVECTION IS TO ALSO AFFECT
SOUTHERN BOLIVIA AS ENHANCED BY THE SOUTHERLY FLOW...WITH MAXIMA
OF 25-50MM IN SCATTERED DEEP CONVECTION. OTHER CONVECTION TO THE
SOUTH IS TO CLUSTER ON SOUTHERN PORTIONS OF THE BUENOS AIRES
PROVINCE ON FRIDAY AS ENHANCED BY AN OCCLUDING LOW TO THE
EAST...WITH MOST INTENSE EXPECTED THROUGH SATURDAY MORNING.
ALTHOUGH IN THIS AREA EXPECTING STORM TOTAL AMOUNTS OF
50-100MM...THE SLOW TO MOVE SYSTEM COULD LEAD TO AN ECHO TRAINING
PATTERN...WITH POTENTIALLY HIGHER AMOUNTS ACROSS SIERRA DE LA
VENTANA AND TANDIL TO THE SOUTH.
NORTH OVER THE CONTINENT...AT 250 HPA...A CELL OF THE SUBTROPICAL
RIDGE IS TO MEANDER OVER NORTHWEST SOUTH AMERICA WHILE CENTERING
ON A CLOSED LOW OVER THE ALTIPLANO OF BOLIVIA. AS THE RIDGE
HOLDS...IT IS TO STEER A SHORT WAVE TROUGH TO THE NORTH ACROSS
EQUATORIAL SOUTH AMERICA...WITH UPPER LEVEL LOW/TROUGH
RETROGRESSING ACROSS PARA TO AMAZONAS IN NORTHWEST BRASIL ON
FRIDAY-SATURDAY. LATER DURING THE WEEKEND THIS IS EXPECTED TO
DISSIPATE. SIMULTANEOUSLY...A HIGHER AMPLITUDE TROUGH IS TO
AMPLIFY TO THE NORTHWEST ALONG 20S 35W TO A CLOSED LOW OVER
NORTHEAST BRASIL EARLY ON SUNDAY. THIS REACHES MAXIMUM INTENSITY
EARLY NEXT WEEK. MEANWHILE...OVER THE NEXT COUPLE OF DAYS...THE
RETROGRESSING UPPER LEVEL LOW/TROUGH WILL ENHANCE DIURNAL
CONVECTION ACROSS PARA-RORAIMA-AMAZONAS IN BRASIL...WITH ACTIVITY
PEAKING AT 25-50MM/DAY OVER THE NEXT COUPLE OF DAYS. THROUGH
SATURDAY...AS THE UPPER LEVEL LOW WEAKENS...THIS WILL DECREASE TO
MAXIMA OF 20-30MM/DAY. LATER DURING THE WEEKEND/EARLY NEXT
WEEK...THE DEEPENING UPPER LEVEL TROUGH WILL THEN SUSTAIN A SURGE
IN DIURNAL CONVECTION ACROSS MINAS GERAIS-BAHIA IN NORTHEAST
BRASIL. THROUGH TUESDAY THIS IS EXPECTED TO BECOME MORE
ACTIVE/BETTER ORGANIZED WHILE SPREADING WEST ACROSS CENTRAL TO
NORTHWEST BRASIL...WITH MAXIMA OF 25-50MM IN SCATTERED DEEP
CONVECTION.
DAVISON/GALVEZ...WPC (USA)
FORECAST DISCUSSION FROM DEC 17/17UTC: A MID/UPPER LEVEL SHORT
WAVE TROUGH ON THE NORTHERN STREAM IS INITIALIZED OVER THE FAR
EASTERN PACIFIC NEAR THE ARCHIPELAGO DE JUAN FERNANDEZ. UNDER
INFLUENCE OF A RIDGE TO THE WEST...THE SHORT WAVE TROUGH IS TO
LIFT ACROSS CENTRAL REGIONS OF CHILE EARLY ON FRIDAY MORNING.
THROUGH THE DAY THIS WILL SPILL ACROSS THE ANDES INTO CUYO/CENTRAL
ARGENTINA. ON SATURDAY...AS IT TRACKS ACROSS THE BUENOS AIRES
PROVINCE TO THE WESTERN PACIFIC...THE MID/UPPER LEVEL SYSTEM IS
FORECAST TO RAPIDLY INTENSIFY. AT 250 HPA...MODELS CONTINUE TO
FORECAST A SUBTROPICAL/POLAR JET PAIR TO PROPAGATE IN TANDEM WITH
THIS SYSTEM...WITH THE JETS TO COUPLE ON THEIR DIVERGENT REGIONS
AS THEY MEANDER EAST ACROSS CENTRAL ARGENTINA ON FRIDAY-SATURDAY.
AT LOW LEVELS...THE MID LEVEL FORCING AND STRONG UPPER LEVEL
SUPPORT WILL HELP SUSTAIN CYCLOGENESIS/FRONTOGENESIS OVER CENTRAL
ARGENTINA ON FRIDAY. AS THE SURFACE LOW INTENSIFIES...IT WILL
SUSTAIN A SOUTHERLY JET ACROSS CENTRAL ARGENTINA. THIS WILL THEN
DRIVE THE ASSOCIATED FRONT NORTH WHILE TRIGGERING A PREFRONTAL
SQUALL LINE ACROSS SAIN LUIS-CORDOBA IN CENTRAL ARGENTINA. THE
FORECASTED STABILITY INDICES STRONGLY SUGGEST RISK OF LARGE HAIL
PRODUCING THUNDERSTORMS...WITH THE HELICITY FORECAST SHOWING
POTENTIAL FOR TORNADIC PRODUCING SUPERCELLS TO FORM THROUGH MIDDAY
ON FRIDAY. IN ASSOCIATION WITH THIS FEATURE EXPECTING HEAVY
RAINFALL AMOUNTS...WITH MAXIMA RANGING BETWEEN 75-125MM ON FRIDAY.
ON SATURDAY THIS SPREADS NORTH ACROSS PARAGUAY/MESOPOTAMIA VALLEY
IN ARGENTINA TO PARANA/SANTA CATARINA IN SOUTHERN BRASIL...WHERE
IT IS EXPECTED TO TRIGGER ACCUMULATION OF 35-70MM IN POTENTIALLY
SEVERE CONVECTION. MODERATE TO HEAVY CONVECTION IS TO ALSO AFFECT
SOUTHERN BOLIVIA AS ENHANCED BY THE SOUTHERLY FLOW...WITH MAXIMA
OF 25-50MM IN SCATTERED DEEP CONVECTION. OTHER CONVECTION TO THE
SOUTH IS TO CLUSTER ON SOUTHERN PORTIONS OF THE BUENOS AIRES
PROVINCE ON FRIDAY AS ENHANCED BY AN OCCLUDING LOW TO THE
EAST...WITH MOST INTENSE EXPECTED THROUGH SATURDAY MORNING.
ALTHOUGH IN THIS AREA EXPECTING STORM TOTAL AMOUNTS OF
50-100MM...THE SLOW TO MOVE SYSTEM COULD LEAD TO AN ECHO TRAINING
PATTERN...WITH POTENTIALLY HIGHER AMOUNTS ACROSS SIERRA DE LA
VENTANA AND TANDIL TO THE SOUTH.
NORTH OVER THE CONTINENT...AT 250 HPA...A CELL OF THE SUBTROPICAL
RIDGE IS TO MEANDER OVER NORTHWEST SOUTH AMERICA WHILE CENTERING
ON A CLOSED LOW OVER THE ALTIPLANO OF BOLIVIA. AS THE RIDGE
HOLDS...IT IS TO STEER A SHORT WAVE TROUGH TO THE NORTH ACROSS
EQUATORIAL SOUTH AMERICA...WITH UPPER LEVEL LOW/TROUGH
RETROGRESSING ACROSS PARA TO AMAZONAS IN NORTHWEST BRASIL ON
FRIDAY-SATURDAY. LATER DURING THE WEEKEND THIS IS EXPECTED TO
DISSIPATE. SIMULTANEOUSLY...A HIGHER AMPLITUDE TROUGH IS TO
AMPLIFY TO THE NORTHWEST ALONG 20S 35W TO A CLOSED LOW OVER
NORTHEAST BRASIL EARLY ON SUNDAY. THIS REACHES MAXIMUM INTENSITY
EARLY NEXT WEEK. MEANWHILE...OVER THE NEXT COUPLE OF DAYS...THE
RETROGRESSING UPPER LEVEL LOW/TROUGH WILL ENHANCE DIURNAL
CONVECTION ACROSS PARA-RORAIMA-AMAZONAS IN BRASIL...WITH ACTIVITY
PEAKING AT 25-50MM/DAY OVER THE NEXT COUPLE OF DAYS. THROUGH
SATURDAY...AS THE UPPER LEVEL LOW WEAKENS...THIS WILL DECREASE TO
MAXIMA OF 20-30MM/DAY. LATER DURING THE WEEKEND/EARLY NEXT
WEEK...THE DEEPENING UPPER LEVEL TROUGH WILL THEN SUSTAIN A SURGE
IN DIURNAL CONVECTION ACROSS MINAS GERAIS-BAHIA IN NORTHEAST
BRASIL. THROUGH TUESDAY THIS IS EXPECTED TO BECOME MORE
ACTIVE/BETTER ORGANIZED WHILE SPREADING WEST ACROSS CENTRAL TO
NORTHWEST BRASIL...WITH MAXIMA OF 25-50MM IN SCATTERED DEEP
CONVECTION.
DAVISON/GALVEZ...WPC (USA)
