298  
FXCA62 TJSJ 130058  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
858 PM AST MON APR 12 2021  
   
UPDATE
 
 
A MIXTURE OF LOW AND UPPER LEVEL CLOUDS CONTINUED TO CROSS THE  
REGION TO MAINTAIN PARTLY CLOUDY TO VARIABLY CLOUDY SKIES.  
HOWEVER, NO PRECIPITATION WAS OBSERVED OR DETECTED IN AND AROUND  
THE ISLANDS. RECENT TJSJ 13/00Z UPPER AIR SOUNDING SUGGESTS  
OVERALL STABLE CONDITIONS WITH STRONG 2-3 DEGREE CAP INVERSION  
NEAR 850 MILLIBARS AND A DOMINANT SOUTHWEST WIND FLOW ABOVE 925  
MILLIBARS, THEN BECOMING WESTERLY AND INCREASING WITH HEIGHT  
ABOVE 700 MILLIBARS. THAT SAID, DO NOT EXPECT SIGNIFICANT AMOUNTS  
OF LOW LEVEL MOISTURE ADVECTION OR PRECIPITATION,OVERNIGHT EXCEPT  
FOR OVER THE OFFSHORE CARIBBEAN WATERS WHERE MOSTLY ISOLATED  
SHOWERS WILL BE POSSIBLE LATER TONIGHT AND BY EARLY MORNING.  
OTHERWISE NO CHANGE TO THE INHERITED SHORT TERM FORECAST PACKAGE  
AND REASONING AT THIS TIME.  
 

 
   
AVIATION...VFR DURG PRD. SCT OCNL BKN LYRS NR FL025...FL050
 
FL080  
OVR REGIONAL WATERS AND EN ROUTE BTW P AND THE NORTHERN LEEWARD  
ISLANDS. L/LVL WNDS FM SE-S 5-10 KTS BLO FL025...BCMG FM SW-W AND  
INCR W/ HT ABV. SFC WNDS LIGHT AND VARIABLE BCMG MAINLY FM SE-S  
5-15 KTS EXCEPT FOR SEA BREEZE VARIATIONS AFTER 13/14Z.  
 

 
   
MARINE UPDATE
 
NO CHANGE TO PREVIOUS MARINE DISCUSSION AT THIS  
TIME. RECENT DATA FROM THE NE PUERTO RICO BUOY 41043 SUGGESTS  
SEAS BETWEEN 6-7 FEET AT 11-12 SECS AS THE NE SWELL CONTINUED TO  
SLOWLY FADE. SMALL CRAFT OPERATORS SHOULD EXERCISE CAUTION OVER  
THE ATLANTIC WATERS. NEARSHORE SEAS AND LOCAL PASSAGES WERE  
BETWEEN 3-5 FEET WITH LIGHT TO MODERATE SSE WINDS.  
 

 
   
PREV DISCUSSION
 
/ISSUED 341 PM AST MON APR 12 2021/  
 
SYNOPSIS...  
 
TODAY IS THE LAST DAY OF VERY DRY CONDITIONS, AS HIGHER MOISTURE  
CONTENT WILL MOVE IN OVERNIGHT AND TOMORROW, AND BE WITH US FOR  
THE NEXT SEVERAL DAYS. THUS ISOLATED TO SCATTERED SHOWERS ARE  
EXPECTED FOR THE REST OF THE WORKWEEK. FAIRLY SLOW LOW-LEVEL WINDS  
COULD LEAD TO SLOW-MOVING SHOWERS AND THE POTENTIAL FOR DECENT  
RAINFALL ACCUMULATION IN THE NEXT FEW AFTERNOONS. MARINE  
CONDITIONS ARE CALMING DOWN.  
 
SHORT TERM...REST OF TODAY THROUGH WEDNESDAY...  
 
DRY AIR CONTINUES TO PERSIST OVER THE REGION, THOUGH THAT WILL  
CHANGE DURING THE NEXT FEW DAYS. CURRENTLY, PRECIPITABLE WATER  
VALUES ARE AROUND 1.1, WHICH IS HIGHER THAN THE PAST FEW DAYS BUT  
STILL BELOW AVERAGE. WE HAVE HAD A RIDGE OVER US AT MID TO UPPER  
LEVELS, AND THAT HAS ALSO CREATED HOSTILE CONDITIONS FOR SHOWERS.  
THE SOUNDING FROM THIS MORNING MEASURED A STRONG INVERSION AT AROUND  
800MB. SO EVEN THOUGH THE RIDGING IS BREAKING DOWN, WE ARE STILL  
FEELING ITS EFFECTS. THUS MOSTLY TRANQUIL CONDITIONS ARE EXPECTED  
FOR THE REST OF THE AFTERNOON WITH MAYBE ONLY A FEW SHOWERS OVER THE  
INTERIOR AND NORTHERN PUERTO RICO.  
 
A LARGE POLAR TROUGH THAT EXTENDS THROUGH THE FULL LENGTH OF THE  
ATMOSPHERE IS MOVING IN FROM THE WEST. ITS WESTERLY WINDS WILL  
COUNTERACT THE NORMAL EASTERLY TRADE WINDS, MEANING LIGHT NEAR-  
SURFACE WINDS FOR US THE NEXT FEW DAYS. THERE WILL BE QUITE A BIT OF  
MOISTURE TO OUR SOUTH OVER THE NEXT FEW DAYS, YET THE WESTERLY WINDS  
ALOFT WILL FOR THE MOST PART HOLD THAT MOISTURE AT BAY TILL LATER IN  
THE WEEK. STILL, SOME MOISTURE WILL MAKE IT INTO OUR REGION, THUS  
ENHANCING THE POTENTIAL FOR SHOWERS TOMORROW AND WEDNESDAY. THIS  
MOISTURE INCREASE WILL PRIMARILY BE AT LOW LEVELS, BUT STILL THE MID-  
ATMOSPHERIC LEVELS WILL BE QUITE DRY, THUS KEEPING THE POTENTIAL LOW  
FOR STRONG SHOWERS. GIVEN THE LIGHT WINDS THOUGH, THERE WILL BE THE  
POTENTIAL FOR PERSISTENT SHOWERS OVER A GIVEN AREA THAT COULD LEAD  
TO SOME AREAS OF ISOLATED FLOODING. TRANQUIL CONDITIONS ARE EXPECTED  
TOMORROW MORNING, BUT IN THE AFTERNOON, SHOWERS ARE POSSIBLE ACROSS  
INTERIOR AND EASTERN PUERTO RICO, AS WELL AS OVER THE USVI. SOME  
AREAS OF PONDING WATER AND POSSIBLY EVEN ISOLATED FLOODING ARE  
POSSIBLE FOR INTERIOR AND EASTERN PUERTO RICO. ISOLATED TO SCATTERED  
SHOWERS ARE EXPECTED OVERNIGHT TUESDAY AND INTO WEDNESDAY. THEN  
AGAIN ON WEDNESDAY AFTERNOON UNDER FAIRLY LIGHT LOW-LEVEL WINDS,  
MORE SCATTERED AND PERSISTENT SHOWERS CAN BE EXPECTED.  
 
LONG TERM...THURSDAY THROUGH TUESDAY...  
 
PREV DISCUSSION... /ISSUED 454 AM AST MON APR 12 2021/  
 
RECENT MODEL GUIDANCE CONTINUES TO SUGGEST THAT A SOMEWHAT WETTER  
AND UNSTABLE WEATHER PATTERN WILL CONTINUE ON THURSDAY, BUT NOW A  
BIT DRIER THAN PREVIOUSLY EXPECTED AND LASTING THROUGH LATE  
FRIDAY NIGHT SINCE THE AREA OF ENHANCED MOISTURE IS EXPECTED TO  
STALL OVER THE REGION. A DELAY IN THE PEAK OF MOISTURE IS ONCE  
AGAIN PRESENTED, SUGGESTING HIGHEST MODEL-ESTIMATED PRECIPITABLE  
WATER AROUND 1.80 INCHES BY FRIDAY EVENING. OVER LAND, THE BULK OF  
THE ACTIVITY WILL FAVOR SOUTHERN AND EASTERN SECTIONS OF PUERTO  
RICO, AS WELL AS THE US VIRGIN ISLANDS DURING THE OVERNIGHT AND  
EARLY MORNING HOURS, SHIFTING TO THE INTERIOR AND NORTHERN  
SECTIONS OF PUERTO RICO IN THE AFTERNOON, WITH THE POTENTIAL FOR  
ISOLATED THUNDERSTORM DEVELOPMENT SINCE 500 MB ARE EXPECTED TO  
FALL AS LOW AS -8 DEGREES CELSIUS. THIS SCENARIO, UNDER A  
GENERALLY WEAK STEERING FLOW, PRESENTS THE POTENTIAL FOR PONDING  
OF WATER TO LOCALIZED URBAN AND SMALL STREAM FLOODING THROUGH  
LATE FRIDAY NIGHT.  
 
THEREAFTER, GLOBAL MODELS ARE NOW IN STRONG AGREEMENT  
WITH A DRYING TENDENCY DOMINATING THE LOCAL WEATHER CONDITIONS  
SATURDAY INTO EARLY NEXT WEEK, WITH PEAK DRYING EXPECTED ON  
MONDAY. SO FAR, THIS WILL RESULT FROM A MID-TO-UPPER RIDGE MOVING  
INTO THE AREA, STRENGTHENING THE TRADE WIND CAP WHILE LIMITING  
ANY CHANCE FOR ORGANIZED SHOWER DEVELOPMENT. NEVERTHELESS, LIMITED  
AFTERNOON CONVECTION CANNOT BE RULED OUT. AS CONDITIONS DRY OUT  
AND THE GENERAL WIND FLOW BECOMES MORE SOUTHEASTERLY, EXPECTED  
WARMER TEMPERATURES THAT COULD REACH THE LOWER TO MID 90S WITH  
POSSIBLE HEAT INDICES INTO THE UPPER 90S TO LOWER 100S BY FRIDAY  
ONWARDS.  
 
AVIATION...  
 
MAINLY VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS ALL  
TERMINALS DURING THE NEXT 24 HOURS. LOW-LEVEL WINDS WILL CONTINUE  
FROM THE SE AT LESS THAN 15 KTS, BECOMING LIGHT AND VARIABLE  
OVERNIGHT.  
 
MARINE...  
 
THE WEAK NORTHERLY SWELL IS DISSIPATING ACROSS THE REGION. SEAS  
WILL REMAIN SOMEWHAT CHOPPY TOMORROW FOR THE OUTER ATLANTIC  
WATERS, SO SMALL CRAFT OPERATORS SHOULD EXERCISE CAUTION. OTHER  
AREAS WILL BE FAIRLY TRANQUIL WITH SEAS BELOW 5 FEET. SEAS WILL  
BE EVEN MORE TRANQUIL WEDNESDAY AND THURSDAY, WITH EXPECTED WAVE  
HEIGHTS FOR ALL LOCAL WATERS AT OR BELOW 4 FEET. ANOTHER WEAK  
NORTHERLY SWELL IS EXPECTED TO IMPACT THE ATLANTIC WATERS AND  
LOCAL PASSAGES FOR THURSDAY NIGHT AND FRIDAY.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 73 88 75 87 / 10 60 40 50  
STT 74 85 74 85 / 20 40 40 40  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...HIGH RIP CURRENT RISK THROUGH TUESDAY AFTERNOON FOR CULEBRA-  
NORTH CENTRAL-NORTHEAST-NORTHWEST-SAN JUAN AND VICINITY-  
SOUTHEAST.  
 
HIGH RIP CURRENT RISK UNTIL 6 AM AST TUESDAY FOR MAYAGUEZ AND  
VICINITY.  
 
VI...HIGH RIP CURRENT RISK UNTIL 6 AM AST TUESDAY FOR ST.THOMAS...ST.  
JOHN...AND ADJACENT ISLANDS.  
 
AM...NONE.  

 
 

 
 
SHORT TERM...RAM  
LONG TERM....MB  
PUBLIC DESK...ERG  
 
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