469  
FXCA62 TJSJ 241605  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
1205 PM AST SUN MAR 24 2019  
   
UPDATE
 
DIURNALLY INDUCED SHOWERS ARE DEVELOPING ALONG THE  
INTERIOR AND WESTERN SECTIONS OF PUERTO RICO. THESE SHOWERS WILL  
CONTINUE THROUGH THE REST OF THE AFTERNOON HOURS AND URBAN AND  
SMALL STREAM FLOODING CAN NOT BE RULED OUT WITH THE HEAVIEST  
SHOWERS. BETWEEN 1-2 INCHES OF RAINFALL IS POSSIBLE WITH THIS  
ACTIVITY. ADDITIONAL CLOUDINESS AND SHOWERS WERE APPROACHING THE  
U.S. VIRGIN ISLANDS AND STREAMERS WERE DEVELOPING AS WELL OFF THE  
ISLANDS AND MOVING INTO THE NORTHWESTERN WATERS. BUT MOSTLY LIGHT  
RAINFALL AMOUNTS ARE EXPECTED WITH THESE SHOWERS. FOR TONIGHT,  
SHOWERS CURRENTLY OVER THE LEEWARD ISLANDS ARE EXPECTED TO MOVE  
ACROSS THE USVI AND REACH THE EASTERN AND SOUTHEASTERN SECTIONS OF  
PUERTO RICO. ELSEWHERE, CLEAR TO PARTLY CLOUDY SKIES ARE EXPECTED  
THROUGH THE OVERNIGHT HOURS.  
 

 
 
   
AVIATION
 
MAINLY VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS  
ALL TERMINALS. HOWEVER, SHRA EXPECTED IN AND AROUND TJSJ/TJMZ/TJBQ  
THROUGH 23Z. TEMPO MVFR TO BRIEF IFR POSSIBLE AT TJMZ/TJBQ. THE  
24/12Z TJSJ UPPER AIR SOUNDING INDICATED ESE WINDS AT 10-30 KT  
BLO FL120.  
 

 
   
MARINE
 
SEAS WILL GRADUALLY INCREASE TONIGHT THROUGH MONDAY DUE  
TO A COMBINATION OF A SMALL NORTHEASTERLY SWELL AND FRESH TRADES  
ACROSS THE ATLANTIC WATERS. SEAS UP TO 7 FEET ARE EXPECTED ACROSS  
THE OFFSHORE ATLANTIC WATERS ON MONDAY AND A SMALL CRAFT ADVISORY  
WAS ISSUED. ELSEWHERE, SEAS WILL RANGE BETWEEN 3-6 FEET AND EAST  
TO SOUTHEAST WINDS WILL PREVAIL BETWEEN 15-20 KNOTS. THEREFORE,  
SMALL CRAFTS SHOULD EXERCISE CAUTION.  
 
THERE IS A HIGH RIP CURRENT RISK IN EFFECT FOR THE NORTH CENTRAL  
BEACHES OF PUERTO RICO. A MODERATE RISK CONTINUES FOR MOST OF THE  
NORTH AND EAST FACING BEACHES OF THE ISLANDS.  
 

 
   
PREV DISCUSSION
 
/ISSUED 523 AM AST SUN MAR 24 2019/  
 
SYNOPSIS...  
SURFACE HIGH PRESSURE OVER THE CENTRAL AND NORTHEAST ATLANTIC, AND  
A BROAD SURFACE TROUGH OVER THE EASTERN CARIBBEAN WILL CONTINUE TO  
PROMOTE MODERATE EAST TO SOUTHEAST WINDS ACROSS THE REGION. OCCASIONAL  
BANDS OF TRADE WIND MOISTURE CONTINUED TO TRANSPORT PERIODS OF PASSING  
SHOWERS ACROSS THE REGION FROM TIME TO TIME. MODEL GUIDANCE CONTINUED  
TO SUGGEST A MUCH WETTER PATTERN DURING THE LATTER PART OF NEXT WEEK  
AS THE LONG WAVE TROUGH AND ASSOCIATED FRONTAL BOUNDARY IS FORECAST  
TO APPROACH AND EVENTUALLY SINK SOUTHWARDS ACROSS THE REGION.  
 
SHORT TERM...TODAY THROUGH TUESDAY...  
 
A MID LEVEL RIDGE WILL CONTINUE TO HOLD OVER THE CENTRAL ATLANTIC  
WATERS THROUGH EARLY NEXT WEEK. PATCHES OF SHALLOW MOISTURE WILL  
CONTINUE TO MOVE OVER THE AREA PERIODICALLY, AND INDUCE ISOLATED TO  
SCATTERED SHOWERS DURING THE AFTERNOON. ADDITIONALLY, THE SHALLOW  
MOISTURE PLUS LOCAL AND DIURNAL EFFECTS WILL PRODUCE LIGHT TO  
MODERATE RAINFALL ACROSS PORTIONS OF INTERIOR AND WESTERN AREAS OF  
PUERTO RICO. SIMILAR TO YESTERDAY LIGHT LOW-LEVEL WINDS WILL CAUSE  
CONTINUOUS LIGHT SHOWERS TO RE-DEVELOP ACROSS THE AREA DURING THE  
AFTERNOON. THE TJSJ SKEW-T SHOWS A MOIST PROFILE AT 1000 TO 750 MB,  
BUT ABOVE THE 700 MB THERE IS A LARGE LAYER OF DRIER AIR THAT WILL  
LIMITED THE VERTICAL DEVELOPMENT OF SHOWERS. THIS MEANS THAT  
MOISTURE WILL BE CONFINED TO THE LOWER-LEVEL OF THE ATMOSPHERE.  
 
YESTERDAY SOME AREAS RECEIVED 1.5 TO 2.0 INCHES OF RAIN EXPECT  
SIMILAR CONDITIONS TODAY. IF RAIN DEVELOPS TODAY IN THE AREAS THAT  
RECEIVED THE 1.5 TO 2.0 INCHES OF RAIN, IT COULD LEAD TO URBAN AND  
SMALL STREAM FLOODING; AS WELL AS PONDING OF WATER ON ROADWAYS. THE  
GOOD NEWS ABOUT YESTERDAY'S RAINFALL IS THAT RAIN FELL IN BASIN  
AREAS THAT NEEDED RAIN. HOWEVER MORE RAIN IS NEEDED FOR MANY AREAS  
ACROSS PUERTO RICO.  
 
MONDAY A TROUGH NORTHEAST OF PUERTO RICO WILL IMPINGE ON THE MID-  
LEVEL RIDGE AND BEGIN TO WEAKEN IT. THE MID-LEVEL MOISTURE FROM THE  
BOUNDARY WILL MOVE NEAR PUERTO RICO LATE MONDAY OR EARLY TUESDAY.  
THIS MID-LEVEL MOISTURE WILL MOISTEN THE MID-LEVELS OF THE  
ATMOSPHERE AND HELP ADD IN SHOWER DEVELOPMENT ACROSS THE  
NORTHWESTERN AREAS OF PUERTO RICO DURING THE AFTERNOON ON LATE  
MONDAY AND TUESDAY. BY TUESDAY SURFACE WINDS ARE FORECAST TO BE FROM  
THE EAST TO SOUTHEAST, THIS WIND FLOW TYPICALLY LEADS TO WARMER  
TEMPERATURES ACROSS THE REGION.  
 
LONG TERM...WEDNESDAY THROUGH SUNDAY  
 
A SLOWLY WEAKENING SHORT-WAVE TROUGH OVER THE EASTERN CARIBBEAN WILL  
CONTINUE TO SUPPORT MOISTURE POOLING ACROSS THE REGION THROUGH THE  
END OF THE WORK WEEK AND INTO THE WEEKEND AS IT LIFT NORTHWARDS. THE  
GRADUAL EROSION OF THE UPPER LEVEL RIDGE IN RESPONSE TO A DEEPENING  
LONG WAVE TROUGH ACROSS THE WEST AND SOUTHWEST ATLANTIC WILL WEAKEN  
THE TRADE WIND CAP INVERSION AND SUPPORT GOOD LOW LEVEL MOISTURE  
CONVERGENCE ACROSS THE FORECAST AREA. IN ADDITION, THE APPROACHING  
FRONTAL BOUNDARY AND ASSOCIATED MOISTURE IS FORECAST TO SINK SOUTHWARDS  
ACROSS THE AREA BY THE END OF THE WEEK. THEREFORE EXPECT INCREASING  
CLOUD COVER AND MORE FREQUENT SHOWER DEVELOPMENT ACROSS THE FORECAST  
AREA THROUGH THE END OF THE PERIOD.  
 
RECENT GUIDANCE WAS UNCHANGED AND CONTINUED TO SUGGEST A FAIRLY  
MOIST SOUTHEAST WIND FLOW THROUGH MOST OF THE PERIOD. THIS ALONG  
WITH THE PROXIMITY OF THE AMPLIFYING LONG WAVE TROUGH AND FRONTAL  
BOUNDARY WILL SUPPORT FAVORABLE CONDITIONS FOR CONVECTIVE DEVELOPMENT  
WITH ENHANCED SHOWER ACTIVITY ACROSS PORTIONS THE ISLANDS AND COASTAL  
WATERS AT LEAST THROUGH SATURDAY. BY SUNDAY THE RETURN FLOW OF MOISTURE  
FROM THE FRONTAL BOUNDARY SHOULD BE SUFFICIENT TO SUPPORT EARLY MORNING  
AND AFTERNOON CONVECTION ACROSS THE COASTAL WATERS AND LOCAL ISLANDS.  
HOWEVER, A GRADUAL IMPROVEMENT IN CONDITIONS IS EXPECTED AS THE UPPER  
TROUGH IS EXPECTED TO LIFT NORTHWARDS AND WEAKEN BRINGING STABLE CONDITIONS  
ALOFT.  
 
AS PREVIOUSLY MENTIONED, GOOD UPPER LEVEL SUPPORT AND THE FAIRLY  
MOIST ENVIRONMENTAL CONDITIONS SHOULD ALL FAVOR ORGANIZED AND  
ENHANCED CONVECTION ACROSS PORTIONS OF THE ISLANDS, ESPECIALLY  
OVER THE INTERIOR AND NORTHERN SECTIONS OF PUERTO RICO THROUGH  
SATURDAY, WITH A GRADUAL IMPROVEMENT EXPECTED BY SUNDAY.  
 
AVIATION...MAINLY VFR/MVFR CONDITIONS ARE EXPECTED ACROSS ALL  
TERMINALS DURING THE NEXT 24 HOURS. MVRF CONDS ARE OCCURRING DUE TO  
LIGHT -SHRA MAINLY ACROSS PR EASTERN AND THE U.S. VI TERMINALS. BY  
24/15Z SHRA WILL DEVELOP ACROSS TJBQ AND TJMZ TERMINALS AND PERSIST  
THROUGH 25/01Z. WINDS WILL BE OUT OF THE EAST AT 10 TO 15 KTS WITH  
SEA BREEZE VARIATIONS.  
 
MARINE....A NORTHERLY SWELL HAS ARRIVED AND WILL SPREAD ACROSS  
THE LOCAL ATLANTIC WATERS AND PASSAGE TODAY THROUGH MONDAY. A  
SLIGHT INCREASE IN THE EAST TO SOUTHEAST TRADE WINDS WILL ALSO  
BRING MODERATE TO LOCALLY STRONG WINDS BETWEEN 15-20 KNOTS IN SOME  
AREAS WITH SEAS UP TO 6 FEET OFFSHORE AND IN THE LOCAL PASSAGES.  
SMALL CRAFT OPERATORS SHOULD THEREFORE EXERCISE CAUTION THOUGH  
MONDAY.  
 
THERE IS A HIGH RISK OF RIP CURRENT FOR SOME OF THE NORTH CENTRAL  
BEACHES OF PUERTO RICO. A MODERATE RISK OF RIP CURRENTS WILL CONTINUE  
ACROSS THE REMAINING NORTH FACING BEACHES OF PUERTO RICO, CULEBRA,  
EAST VIEQUES, NORTHWESTERN SAINT THOMAS AND NORTH AND EASTERN SAINT  
CROIX TODAY. LOW RISK OF RIP CURRENTS ELSEWHERE.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 87 75 88 74 / 40 40 40 40  
STT 85 75 85 76 / 30 30 30 40  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...HIGH RIP CURRENT RISK THROUGH LATE TONIGHT FOR NORTH CENTRAL.  
 
VI...NONE.  
AM...SMALL CRAFT ADVISORY FROM 8 AM MONDAY TO 4 AM AST TUESDAY FOR  
ATLANTIC WATERS OF PUERTO RICO AND USVI FROM 10 NM TO 19.5N.  
 

 
 

 
 
SHORT TERM...DS  
LONG TERM....99  
 
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