285  
FXCA62 TJSJ 210912  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
512 AM AST MON OCT 21 2019  
   
SYNOPSIS
 
A LIGHT TO MODERATE EAST TO NORTHEAST WIND FLOW IS  
FORECAST TO HOLD THROUGH AT LEAST MIDWEEK, VEERING TO THE EAST-  
SOUTHEAST BY THE END OF THE WORKWEEK INTO THE WEEKEND AS A SURFACE  
HIGH PRESSURE BECOMES THE DOMINANT FEATURE. A MOIST AND  
RELATIVELY UNSETTLE WEATHER PATTERN IS EXPECTED TO PREVAIL ACROSS  
THE FORECAST AREA DURING THE NEXT FEW DAYS. OVERALL, SHOWERS ARE  
EXPECTED TO MOVE OVER WINDWARD AREAS DURING THE OVERNIGHT AND  
EARLY MORNING HOURS, FOLLOWED BY SHOWER AND THUNDERSTORM  
DEVELOPMENT EACH AFTERNOON. ADDITIONAL RAINS COULD RESULT IN URBAN  
AND SMALL STREAM FLOODING, WHILE LOCALLY HIGH RAINFALL  
ACCUMULATIONS COULD LEAD TO FLASH FLOODING AND MUDSLIDES MAINLY  
ALONG THE INTERIOR AND WESTERN SECTORS OF PUERTO RICO.  
 

 
   
SHORT TERM...TODAY THROUGH WEDNESDAY
 
 
A TUTT EXTENDS FROM THE CENTRAL ATLANTIC TO THE SOUTHEAST CARIBBEAN  
SEA WITH HIGH PRESSURE AT UPPER LEVELS OVER THE FLORIDA PENINSULA. A  
SHORT-WAVE NORTH OF THE THE LOCAL AREA IS MOVING AROUND THE HIGH AND  
WILL FORM A WEAK CUT-OFF LOW ON TUESDAY JUST NORTH OF OUR NORTHERN  
WATERS. IT WILL CROSS INTO THE EASTERN TIP OF THE DOMINICAN REPUBLIC  
ON TUESDAY NIGHT ESSENTIALLY REVERSING THE NORTHERLY UPPER LEVEL  
FLOW OVER THE AREA. LOW PRESSURE WILL CONTINUE IN THIS AREA THROUGH  
WEDNESDAY.  
 
AT MID LEVELS HIGH PRESSURE CONTINUES OVER OR EAST OF FLORIDA  
THROUGH THE PERIOD. A TROUGH WILL DEVELOP OVER AND SOUTH OF THE AREA  
TUESDAY EVENING AND FORM A REFLECTION OF THE UPPER LEVEL LOW OVER  
THE DOMINICAN REPUBLIC ON WEDNESDAY.  
 
FLOW AT LOWER LEVELS WILL CONTINUE NORTHEAST TO EAST. A TROUGH AT  
850 MB IS AT 45 WEST AND EXTENDS NORTH OF 25 NORTH. THIS WEAKENS  
CONSIDERABLY AS IT MOVES TOWARDS THE AREA. IT ENTERS THE EASTERN  
CARIBBEAN ON TUESDAY NIGHT AND PASSES THROUGH THE FORECAST AREA ON  
WEDNESDAY. ALSO IN PLAY, HOWEVER, IS A WEAK BAND OF MOISTURE AT 700  
MB ALONG A BROAD, OLD AND ILL-DEFINED FRONTAL BOUNDARY WHICH  
CURRENTLY IS ALMOST 350 MILES WIDE. IT IS CURRENTLY INVADING THE  
AREA AND IS PART OF THE SOURCE OF THE SHOWERS AROUND US.  
PRECIPITABLE WATER VALUES FLUCTUATE SOMEWHAT AROUND 1.8 INCHES  
THROUGHOUT THE PERIOD AND THIS COMBINATION OF UPPER AND LOWER  
FEATURES WILL KEEP SCATTERED SHOWERS IN THE FORECAST FOR MOST OF THE  
AREA, ALTHOUGH SHOWERS WILL BE MORE DEFINITE DURING THE AFTERNOONS  
IN WESTERN AND SOUTHWESTERN PUERTO RICO AND EXTENDING EAST INTO THE  
INTERIOR PORTIONS EACH AFTERNOON.  
   
LONG TERM...THURSDAY THROUGH MONDAY
 
 
A FAIRLY MOIST AND UNSETTLED WEATHER PATTERN IS EXPECTED THROUGH  
MOST OF THE LONG TERM FORECAST PERIOD WITH GFS MODEL-ESTIMATED  
PRECIPITABLE WATER VAPOR VALUES RANGING BETWEEN 1.5 AND 2.0  
INCHES FROM THURSDAY MORNING THROUGH MONDAY AFTERNOON. ON THURSDAY  
AND FRIDAY, THE COMBINATION OF A WESTWARD MOVING SURFACE INDUCED  
TROUGH AND A MEANDERING TUTT LOW WILL SUPPORT INCREASED MOISTURE  
TRANSPORT, LOW-LEVEL CONVERGENCE AND FAVORABLE CONDITIONS ALOFT.  
AS THESE FEATURES MOVES FARTHER AWAY, A SURFACE HIGH PRESSURE  
MEANDERING OVER THE WESTERN ATLANTIC WILL BECOME THE DOMINANT  
FEATURE, DRIVING THE REMNANTS OF A FRONTAL BOUNDARY OVER THE  
NORTHEASTERN CARIBBEAN UNDER A GENERALLY EASTERLY WIND FLOW. THIS  
PATTERN WILL CONTINUE THROUGH AT LEAST MONDAY AFTERNOON, BEFORE  
A DRIER AIRMASS BEGINS TO FILTER IN FROM THE EAST AND  
PRECIPITABLE WATER VAPOR VALUES DROP TO NEAR AN INCH BY MONDAY  
NIGHT. IN THE MEANTIME, ANOTHER CUTOFF TUTT LOW IS FORECAST TO  
MOVE INTO THE EASTERN CARIBBEAN WITH ITS CONVERGENT/SUBSIDENCE  
SIDE SETTLING OVER THE FORECAST AREA ON SUNDAY THROUGH MONDAY.  
 
OVERALL, OVERNIGHT AND EARLY MORNING SHOWERS ARE FORECAST TO  
STREAM OVER THE LOCAL WATERS INTO WINDWARD AREAS OF PUERTO RICO  
AND THE U.S. VIRGIN ISLANDS. AS EACH DAY PROGRESSES, LOCALLY  
INDUCED SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DEVELOP,  
CLUSTERING ALONG THE INTERIOR AND WESTERN SECTORS OF PUERTO RICO  
EACH AFTERNOON. STREAMER-LIKE SHOWERS AND ISOLATED THUNDERSTORMS  
FROM SIERRA DE LUQUILLO AND THE LOCAL ISLANDS MOVING INTO THE SAN  
JUAN METROPOLITAN AREA AND EASTERN PUERTO RICO CANNOT BE RULED OUT  
EACH AFTERNOON. ON SUNDAY AND MONDAY, AREAL COVERAGE AND  
INTENSITY OF SHOWERS AND THUNDERSTORMS IS EXPECTED TO DECREASE DUE  
TO UNFAVORABLE CONDITIONS ALOFT AND EROSION OF AVAILABLE MOISTURE.  
 

 
   
AVIATION
 
SHRA IN NE FLOW TO CONTINUE OVR LCL WATERS AND INLAND 5-  
10 MILES. SHRA TO DVLP INTERIOR PR AND VCTY TIST/TISX AFT 21/15Z.  
MTN OBSCURATIONS AND BRIEF MVFR PSBL TJBQ, AND IN TJMZ AFT 21/17Z.  
OTHERWISE ALL TAF SITES EXPECTED TO REMAIN VFR. SFC WINDS LESS THAN  
10 KT, BCMG AFT 21/14Z NE 10-20 KT WITH LCL HIR GUSTS. MAX WINDS WNW  
20-25 KT BTWN FL400-465.  
 

 
   
MARINE
 
TRANQUIL MARINE CONDITIONS WITH SEAS UP TO 5 FEET AND  
EAST WINDS UP TO 15 KNOTS ARE EXPECTED TO PREVAIL ACROSS THE  
REGIONAL WATERS DURING THE NEXT FEW DAYS. SHOWERS ARE FORECAST TO  
STREAM OVER THE WATERS DURING THE OVERNIGHT AND MORNING HOURS,  
ESPECIALLY ACROSS THE ATLANTIC WATERS. SHOWER AND THUNDERSTORM  
ACTIVITY IS POSSIBLE OVER THE WATERS WEST OF PUERTO RICO, AS WELL  
AS OVER THE WATERS BETWEEN THE U.S. VIRGIN ISLANDS AND PUERTO RICO  
EACH AFTERNOON. A MODERATE RISK OF RIP CURRENTS WILL PREVAIL FOR  
BEACHES ALONG THE NORTH COAST OF PUERTO RICO, INCLUDING VIEQUES  
AND CULEBRA, AS WELL AS FOR SAINT THOMAS AND SAINT CROIX.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 89 78 87 77 / 40 40 40 40  
STT 88 78 87 77 / 50 40 50 50  
 

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

 
 

 
 
SHORT TERM...WS  
LONG TERM....ICP  
 
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