821  
FXCA62 TJSJ 260131 AAA  
AFDSJU  
 
AREA FORECAST DISCUSSION...UPDATED  
NATIONAL WEATHER SERVICE SAN JUAN PR  
931 PM AST MON OCT 25 2021  
   
UPDATE
 
ISOLATED SHOWERS CONTINUE VERY WIDELY SEPARATED. THE  
BEST SHOWERS THIS EVENING WERE WEST OF AGUADILLA IN THE MONA  
PASSAGE WITH TOPS AROUND 25 KFT, ALTHOUGH MOST TOPS OVER LAND WERE  
LESS THAN 10 KFT OF WHICH THERE ARE VERY FEW. THE SOUNDING CAME  
OUT CLOSE TO THE GOES TOTAL PRECIPITABLE WATER PRODUCT AT AROUND  
1.48 INCHES. A BAND OF 1.8 INCHES IS APPROACHING THE NORTHEAST  
CORNER OF THE FORECAST AREA, BUT CLOUDS AND SHOWERS ARE STILL ONLY  
SCATTERED THERE WITH SOUTHEAST LOW LEVEL FLOW. HENCE SHOWER  
ACTIVITY FOR PUERTO RICO AND THE U.S. VIRGIN ISLANDS IS EXPECTED  
TO INCREASE SOMEWHAT OVERNIGHT AND DURING THE MORNING AS BETTER  
MOISTURE MOVES IN FROM THE SOUTHEAST, BUT A DISTINCT AREA OF MUCH  
DRIER AIR AT 850 MB REACHES SAINT CROIX BY 26/12Z AND THE SOUTH  
COAST OF PUERTO RICO BY 26/15Z. THUS SHOWER ACTIVITY SHOULD END  
FROM SOUTH TO NORTH DURING THE FOLLOWING 6 HOURS WITH THE  
EXCEPTION OF CONVECTIVE ACTIVITY IN THE NORTHWEST CORNER OF THE  
ISLAND WHERE WINDS CONVERGE AROUND THE CORNER OF THE ISLAND DURING  
THE AFTERNOON. ONLY MINOR CHANGES TO THE GRIDS--MAINLY TO  
TEMPERATURES.  
 

 
 
   
AVIATION
 
VFR CONDS TO PREVAIL. LCL MTN OBSCURATIONS IN ISOLATED  
SHRA TIL ARND 26/16Z. SHRA WITH MVFR AND MTN OBSCURATIONS TO DVLP  
IN NW PR AFT 26/17Z AND CONT TILL BYD 26/22Z. SFC WINDS LAND  
BREEZES LESS THAN 10 KT, BMCG SEA BREEZES 10-15KT WITH GUSTS TO 20  
ALONG THE COASTS. MAX WINDS NNW 20-25 KT BTWN FL425-440 AND ESE AT  
20 KT NR FL080.  
 

 
   
MARINE
 
WINDS AND SEAS WILL PROGRESSIVELY DIMINISH DURING THE  
NEXT 48 HOURS. LAND AND SEA BREEZES WILL BEGIN TO PREVAIL AGAINST  
THE PREDOMINANT FLOW OF 10-15 KNOTS FROM THE SOUTHEAST. ISOLATED  
SHOWERS. A POSSIBILITY OF THUNDERSTORMS RETURNS ON WEDNESDAY AS  
MID LEVELS COOL SEVERAL DEGREES.  
 

 
   
PREV DISCUSSION
 
/ISSUED 441 PM AST MON OCT 25 2021/  
 
SYNOPSIS...A RIDGE ALOFT WILL PERSIST INTO TUESDAY, RESULTING IN  
STABLE CONDITIONS ALOFT AND THUS INHIBITING THE DEVELOPMENT OF  
WIDESPREAD, ORGANIZED CONVECTIVE ACTIVITY. THE RIDGE ALOFT IS  
EXPECTED TO WEAKEN WEDNESDAY AND THURSDAY AS A SHORTWAVE TROUGH  
MOVES ACROSS AND DEPENDING ON THE AMOUNT OF MOISTURE AVAILABLE,  
CONVECTIVE ACTIVITY IS POSSIBLE, ESPECIALLY ACROSS NORTHERN AND  
NORTHWESTERN PORTIONS OF PUERTO RICO. MORE SOUTHERLY AND LIGHTER  
WINDS ARE EXPECTED FOR TUESDAY AND WILL PERSIST THROUGH THE END OF  
THE WEEK.  
 
SHORT TERM...TONIGHT THROUGH WEDNESDAY...  
 
A RIDGE ALOFT PERSISTS OVER THE AREA, RESULTING IN THE  
CONTINUATION OF STABLE WEATHER CONDITIONS. PATCHES OF LOW-LEVEL  
MOISTURE PUSHING MOISTURE LEVELS TO NEAR NORMAL WILL CONTINUE TO  
RESULT IN ISOLATED TO LOCALLY SCATTERED SHOWERS ACROSS THE AREA  
FROM TIME TO TIME. HOWEVER, GIVEN THAT THESE SHOWERS WILL BE  
BRIEF, ONLY MINOR ACCUMULATIONS ARE EXPECTED.  
 
WEATHER CONDITIONS BEGIN TO CHANGE SOMEWHAT ON TUESDAY. A SURFACE LOW  
PRESSURE SYSTEM DEVELOPING AND EXITING OFF THE MID-ATLANTIC AND  
NORTHEASTERN COAST OF THE UNITED STATES WILL FORCE A SURFACE HIGH  
PRESSURE CURRENTLY OVER THE WESTERN ATLANTIC FURTHER EAST INTO THE  
CENTRAL ATLANTIC. THIS WILL CAUSE THE LOW-LEVEL FLOW TO BECOME  
SOUTHEASTERLY AS WELL AS LIGHTER AT LESS THAN 10 KNOTS. HOWEVER,  
ALOFT, THE MID-LEVEL RIDGE WILL CONTINUE TO PERSIST AND THUS  
CONDITIONS WILL REMAIN UNFAVORABLE FOR ORGANIZED AND WIDESPREAD  
CONVECTIVE ACTIVITY. THUS, THE ONLY AREAS THAT HAVE THE HIGHEST  
PROBABILITY OF SEEING MEASURABLE RAINFALL ACTIVITY WILL BE ACROSS  
NORTHWESTERN PORTIONS OF PUERTO RICO DURING THE AFTERNOON HOURS  
AS SEA BREEZE CONVERGENCE WILL HELP ENHANCE ACTIVITY. BY LATE  
TUESDAY INTO WEDNESDAY, A MID AND UPPER-LEVEL SHORTWAVE TROUGH IS  
EXPECTED TO WEAKEN THE RIDGE. HOWEVER, DESPITE CONDITIONS BECOMING  
MARGINALLY MORE UNSTABLE, MOISTURE CONTENT IS EXPECTED TO BE AT  
BELOW NORMAL LEVELS AND THUS CONVECTIVE ACTIVITY WILL BE LIMITED.  
GIVEN THAT THE WINDS ARE EXPECTED TO BECOME MORE SOUTHERLY  
TUESDAY AND WEDNESDAY, HIGH TEMPERATURES ALONG THE NORTH COAST OF  
PUERTO RICO ARE EXPECTED TO BE AT ABOVE NORMAL LEVELS, IN THE  
LOWER 90S.  
 
LONG TERM...THURSDAY THROUGH TUESDAY...  
 
PREV DISCUSSION... /ISSUED 459 AM AST MON OCT 25 2021/  
 
MODEL GUIDANCE CONTINUES TO BE CONSISTENT ON A MARGINALLY UNSTABLE  
WEATHER PATTERN FOR MOST OF THE LONG TERM PERIOD. THURSDAY STARTS  
WITH AN UPPER-LEVEL SHORT WAVE TROUGH CROSSING THE AREA AND LOW-  
LEVEL WINDS SHIFTING TO SOUTHEAST THROUGH FRIDAY. DISCREPANCIES  
STILL EXIST REGARDING THE AMOUNT OF MOISTURE CONVERGENCE THROUGH THE  
END OF THE WORKWEEK AS MENTIONED IN PREVIOUS DISCUSSIONS. GFS  
CONTINUES TO HAVE BETTER MOISTURE ADVECTION DUE TO A SOUTHERLY FLOW  
THAN ECMWF. HOWEVER, WINDS ARE STILL EXPECTED TO WEAKEN AS PRESSURE  
GRADIENT DECREASES OVER THE AREA. THEREFORE, UNCERTAINTY STILL  
EXISTS ON HOW MUCH MOISTURE WILL CONVERGE OVER THE FORECAST AREA.  
 
RECENT GFS MODEL RUNS ARE CONSISTENT THAT THE BULK OF THE  
MOISTURE PLUME WILL STAY SOUTH OF THE REGION BUT CLOSE ENOUGH FOR  
ABOVE NORMAL MOISTURE CONTENT TO REACH THE AREA. IF THIS PLAYS  
OUT AND WITH ENOUGH INSTABILITY IN THE ATMOSPHERE, WIDESPREAD  
CONVECTIVE DEVELOPMENT WOULD BE LIKELY WITH THE POTENTIAL FOR  
ISOLATED TO SCATTERED THUNDERSTORMS ACROSS THE AREA. THE BULK OF  
THE ACTIVITY COULD BE OBSERVED BY THURSDAY NIGHT WITH SHOWERS AND  
THUNDERSTORMS DEVELOPING OVER THE CARIBBEAN WATERS AND BEING  
ADVECTED TOWARDS THE SOUTHERN AND SOUTHEASTERN COASTAL  
MUNICIPALITIES OF PUERTO RICO AND ST. CROIX. DURING THE DAY,  
SURFACE HEATING AND SEA BREEZE CONVERGENCE COULD PRODUCE SHOWERS  
AND SCATTERED THUNDERSTORMS OVER INTERIOR AND NORTHERN HALF OF  
PUERTO RICO AND OCCASIONALLY AFFECTING THE LOCAL ISLANDS. LIGHT  
STEERING WINDS WILL INCREASE THE POTENTIAL OF FLOODING THREAT  
ESPECIALLY IN URBANIZED REGIONS SUCH AS THE GREATER BAYAMON AND  
SAN JUAN METROPOLITAN AREA. IF THE ECMWF RUN PLAYS OUT INSTEAD,  
THEN A MORE LESS WIDESPREAD AND MORE LOCALIZED ACTIVITY COULD BE  
OBSERVED AS MOISTURE INTRUSION OVER THE FORECAST AREA WILL BE  
GREATLY REDUCED.  
 
THE WEEKEND ALSO LOOKS MARGINALLY UNSTABLE WITH THE NORTHEAST  
CARIBBEAN UNDER THE BOUNDARY OF AN UPPER-LEVEL BUILDING RIDGE AND  
TROUGH. BY SUNDAY THROUGH THE END OF THE FORECAST PERIOD, BOTH  
GLOBAL MODELS ARE CONSISTENT IN THE DEVELOPMENT OF AN UPPER CUT-OFF  
LOW LINGERING OVER PUERTO RICO AND U.S. VIRGIN ISLANDS BUT STRENGTH  
AND POSITIONING STILL NOT COMPLETELY AGREEING WITHIN EACH MODEL  
SOLUTION. MOISTURE STILL DOES NOT LOOK IMPRESSIVE, AT TIMES BOTH  
MODELS SHOWING BELOW NORMAL VALUES. HENCE, CONTINUE TO EXPECT A MORE  
SEASONAL WEATHER PATTERN FOR THE WEEKEND THROUGH TUESDAY WITH DEEP  
CONVECTION DEVELOPING DURING EACH AFTERNOON DEPENDING ON THE  
AVAILABLE MOISTURE AND THE EVOLUTION OF UNSTABLE WEATHER CONDITIONS  
ALOFT. ALTHOUGH MODELS ARE CONSISTENT ON THE PROGRESS OF WEATHER  
FEATURES FOR THIS WEEK AND EARLY NEXT WEEK, LOCATION DIFFERENCES OF  
THESE FEATURES CONTINUE TO BRING FORECAST UNCERTAINTY. THEREFORE,  
CONFIDENCE CONTINUES TO BE LOW TO MODERATE.  
 
AVIATION...PREVAILING VFR CONDITIONS EXPECTED. ISOLATED TO SCT  
SHRA ACTIVITY MAY RESULT IN VCSH AT TIMES ACROSS  
TNCM/TKPK/TIST/TISX/TJSJ THROUGH THE OVERNIGHT PERIOD, HOWEVER, NO  
IMPACTS TO OPERATIONS ARE EXPECTED. WINDS WILL CONTINUE FROM THE E  
BETWEEN 10 AND 15 KTS WITH SEA BREEZE VARIATIONS THROUGH 25/22Z,  
BECOMING LIGHT AND VARIABLE OVERNIGHT.  
 
MARINE...SEAS ARE DIMINISHING ACROSS THE LOCAL WATERS AS THE  
WINDS AND SWELL ARE SUBSIDING. MAINLY TRANQUIL MARINE CONDITIONS  
WITH SEAS OF LESS THAN 5 FEET WILL PREVAIL THROUGH THURSDAY. FOR  
BEACHGOERS, A MODERATE RISK OF RIP CURRENTS IS EXPECTED FOR MOST  
OF THE NORTH AND EAST-FACING BEACHES OF THE ISLANDS TONIGHT AND  
TUESDAY.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 77 93 76 93 / 30 30 30 30  
STT 75 90 78 90 / 30 30 30 20  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...NONE.  
VI...NONE.  
AM...NONE.  

 
 

 
 
SHORT TERM...WS  
LONG TERM....GL  
PUBLIC DESK...LIS  
 
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