686  
FXCA62 TJSJ 192106  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
506 PM AST WED FEB 19 2020  
   
SYNOPSIS
 
SURFACE HIGH PRESSURE SPREAD ACROSS THE CENTRAL AND  
NORTHEASTERN ATLANTIC WILL CONTINUE TO PRODUCE BRISK EASTERLY  
WINDS THROUGH THURSDAY. THIS EASTERLY WIND FLOW WILL CONTINUE TO  
PUSH PATCHES OF LOW-LEVEL MOISTURE INTO THE REGION, RESULTING IN  
PASSING SHOWERS FROM TIME TO TIME. WINDS ARE EXPECTED TO DIMINISH  
BY FRIDAY AND INTO THE UPCOMING WEEKEND AS THE SURFACE HIGH  
PRESSURE MOVES TOWARDS THE EAST AND A SURFACE LOW PRESSURE MOVES  
OVER THE WESTERN ATLANTIC. MORE UNSTABLE WEATHER CONDITIONS ARE  
POSSIBLE BY LATE THIS UPCOMING WEEKEND INTO EARLY NEXT WEEK AS A  
FRONTAL BOUNDARY MOVES CLOSE TO THE AREA.  
 

 
   
SHORT TERM...TONIGHT THROUGH FRIDAY
 
 
NOT MUCH CHANGE IN THE OVERALL WEATHER PATTERN IS EXPECTED THROUGH  
THE SHORT TERM PERIOD. ALOFT, THE AREA WILL CONTINUE TO REMAIN ON  
THE SUBSIDENT SIDE OF A TROUGH TO THE EAST AND A RIDGE TO THE WEST.  
THIS WILL CONTINUE TO PREVENT THE DEVELOPMENT OF SIGNIFICANT  
SHOWERS.  
 
AT LOWER LEVELS, A SURFACE HIGH PRESSURE SPREAD ACROSS THE CENTRAL  
AND NORTHEASTERN ATLANTIC WILL CONTINUE TO PRODUCE BRISK EASTERLY  
TRADE WINDS OVER THE AREA THROUGH THURSDAY. CURRENTLY, THERE IS A  
PATCH OF LOW- LEVEL MOISTURE MOVING INTO THE AREA. THIS HAS  
PRODUCED SOME LIGHT TO MODERATE SHOWERS ACROSS PORTIONS OF  
INTERIOR AND WESTERN PUERTO RICO THIS AFTERNOON. IT IS EXPECTED  
THAT THIS ACTIVITY WILL DIMINISH ACROSS THESE AREAS THROUGH THE  
EARLY EVENING HOURS. ADDITIONAL LIGHT TO MODERATE BRIEF SHOWERS  
ARE THEN POSSIBLE ACROSS PORTIONS OF EASTERN PUERTO RICO AND THE  
USVI DURING THE OVERNIGHT AND EARLY MORNING HOURS OF THURSDAY.  
 
LATEST MODEL GUIDANCE IS INDICATING DRIER AIR WITH PRECIPITABLE  
WATER VALUES DIMINISHING TO BELOW 1 INCH AT TIMES LATER THURSDAY  
AFTERNOON THROUGH FRIDAY. SAHARAN DUST, ALTHOUGH NOT SIGNIFICANT,  
WILL ALSO BE MOVING OVER THE AREA, RESULTING IN SOMEWHAT HAZY SKIES.  
THEREFORE, A DECREASE IN TRADE WIND SHOWER ACTIVITY IS EXPECTED  
THROUGH THIS TIME FRAME. AS THE AFORMENTIONED SURFACE HIGH  
PRESSURE MOVES FURTHER EASTWARD OVER THE NORTHEASTERN ATLANTIC  
AND A SURFACE LOW-PRESSURE DEVELOPS OFF THE SOUTHEASTERN COAST OF  
THE UNITED STATES, LOW-LEVEL WINDS WILL VEER FROM THE SOUTHEAST AS  
WELL AS WEAKEN BY FRIDAY.  
   
LONG TERM...SATURDAY THROUGH WEDNESDAY
 
 
   
PREV DISCUSSION
 
/ISSUED 453 AM AST WED FEB 19 2020/  
 
THE RIDGE THAT DOMINATED THE AREA DURING THE WEEK AT UPPER LEVELS WILL  
MOVE EAST OF THE AREA SATURDAY AND A LONG WAVE TROUGH WILL SINK  
SOUTH INTO THE ATLANTIC WATERS NORTH OF THE AREA SUNDAY. BY  
MONDAY IT WILL PASS TO THE EAST.  
 
A COLD FRONT WILL BEGIN TO APPROACH THE LOCAL AREA ON SATURDAY. A  
PRE-FRONTAL TROUGH WILL ALSO DEVELOP IN THE WESTERN TROPICAL  
ATLANTIC WITH THE AID OF A TROUGH MOVING TOWARD THE AREA. THE WEAK  
TROUGH AT 700 MB WILL PUSH INTO THE AREA SUNDAY AND MONDAY WHILE  
THE FRONT APPROACHES THE NORTHWEST CORNER OF THE FORECAST AREA.  
CURRENT MODELS HAVE CONSIDERABLY DRIER AIR TRAPPED BETWEEN THE  
PRE-FRONTAL TROUGH AND THE FRONT AND GREATLY REDUCE THE AMOUNT OF  
RAIN EXPECTED IN THE AREA DUE TO THE FRONT, SINCE A JET OF AIR  
FROM THE SOUTHEAST AT LOWER LEVELS WILL TEND TO PUSH BACK AT THE  
FRONT AND HOLD IT JUST NORTH OF THE AREA. BY WEDNESDAY, HOWEVER,  
THE MOISTURE FROM THE TWO FEATURES MERGE OVER THE AREA AND  
NORTHEASTERLY FLOW AT LOWER LEVELS CARRIES THE DAY AND ALLOWS THE  
FRONT TO PASS WITH CONSIDERABLY DRIER AIR BEHIND IT. NEVERTHELESS  
WOULD EXPECT AT LEAST SCATTERED TO NUMEROUS SHOWERS IN THE AREA  
TUESDAY AND WEDNESDAY DUE TO THE CONVERGENCE OF THESE SYSTEMS.  
BUT, BECAUSE RELATIVE HUMIDITIES ARE GENERALLY LESS THAN 30  
PERCENT ABOVE 12 KFT FOR THE ENTIRE PERIOD RAINFALL AMOUNTS WILL  
BE MODEST AND GENERALLY LESS THAN ONE INCH ON THE MOST FAVORED  
SLOPES.  
 

 
   
AVIATION
 
VFR CONDITIONS WILL CONTINUE BUT QUICK PASSING -SHRA AND  
SHRA SHOULD NOT BE RULED OUT IN/NEAR THE LEEWARD ISLANDS AND E/N-PR  
AT TIMES. SHRA WILL DEVELOP NEAR JPS/JMZ THROUGH AT LEAST 20/00Z,  
LEAVING CLEAR SKIES OVERNIGHT. EASTERLY WINDS WILL BLOW AT 15 TO 20  
KT WITH HIGHER GUSTS, DECREASING AROUND 10 KTS AFTER 19/23Z. SIMILAR  
WEATHER PATTERN WILL CONTINUE THROUGH THE FORECAST PERIOD, BUT  
SAHARAN DUST PARTICLES WILL INVADE THE LOCAL FLYING AREA TOMORROW  
WHICH COULD LIMIT VIS.  
 

 
   
MARINE
 
MODERATE TO FRESH EASTERLY TRADE WINDS BETWEEN 15 AND 20  
KNOTS WILL CONTINUE TO GENERATE CHOPPY AND HAZARDOUS SEAS OF UP TO  
7 FEET ACROSS MOST OF THE REGIONAL WATERS THROUGH LATE THURSDAY  
NIGHT INTO EARLY FRIDAY, WHICH BY THEN SEAS ARE EXPECTED TO  
BE BELOW SMALL CRAFT ADVISORY CRITERIA. FAIRLY TRANQUIL SEAS ARE  
EXPECTED LATER FRIDAY INTO MOST OF THE UPCOMING WEEKEND AS WINDS  
DIMINISH. HOWEVER, BY LATE SUNDAY INTO EARLY NEXT WEEK, A  
NORTHERLY SWELL IS EXPECTED TO INVADE THE ATLANTIC WATERS AND  
LOCAL PASSAGES TO ONCE AGAIN GENERATE HAZARDOUS SEAS.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 74 85 74 87 / 50 30 30 30  
STT 74 85 74 84 / 30 20 20 20  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...HIGH RIP CURRENT RISK THROUGH LATE TONIGHT FOR NORTHEAST-SAN  
JUAN AND VICINITY.  
 
HIGH RIP CURRENT RISK THROUGH LATE THURSDAY NIGHT FOR NORTH  
CENTRAL-NORTHWEST.  
 
HIGH RIP CURRENT RISK UNTIL 6 PM AST THIS EVENING FOR CULEBRA.  
 
VI...HIGH RIP CURRENT RISK UNTIL 6 PM AST THIS EVENING FOR ST CROIX.  
 
AM...SMALL CRAFT ADVISORY UNTIL 8 PM AST THURSDAY FOR ANEGADA PASSAGE  
SOUTHWARD TO 17N-CARIBBEAN WATERS OF PUERTO RICO FROM 10 NM  
TO 17N-COASTAL WATERS OF NORTHERN PUERTO RICO OUT 10 NM-  
MONA PASSAGE SOUTHWARD TO 17N.  
 
SMALL CRAFT ADVISORY UNTIL 2 PM AST FRIDAY FOR ATLANTIC WATERS  
OF PUERTO RICO AND USVI FROM 10 NM TO 19.5N.  
 
SMALL CRAFT ADVISORY UNTIL 2 AM AST FRIDAY FOR COASTAL WATERS OF  
NORTHERN USVI AND CULEBRA OUT 10 NM-COASTAL WATERS OF  
SOUTHERN USVI VIEQUES AND EASTERN PUERTO RICO OUT 10 NM.  
 
SMALL CRAFT ADVISORY UNTIL 8 PM AST THIS EVENING FOR COASTAL  
WATERS OF NORTHWESTERN PUERTO RICO OUT 10 NM.  
 

 
 

 
 
SHORT TERM...RAM  
LONG TERM....GL  
PUBLIC DESK...TW  
 
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