779  
FXCA62 TJSJ 041844  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
244 PM AST FRI JUL 4 2025  
   
KEY MESSAGES
 
 
* SAHARAN DUST IS ALREADY AFFECTING THE USVI AND WILL SPREAD ACROSS  
THE REGION, CAUSING HAZE AND REDUCED AIR QUALITY THROUGH THE  
WEEKEND.  
 
* ELEVATED HEAT INDICES ARE LIKELY DUE TO ABOVE NORMAL TEMPERATURES,  
DUST, MOISTURE, AND LIGHT WINDS. DAILY HEAT ADVISORIES EXPECTED  
THROUGH THE WEEKEND.  
 
* DAILY AFTERNOON SHOWERS AND THUNDERSTORMS ARE EXPECTED, MAINLY  
ACROSS INTERIOR AND NW PUERTO RICO.  
 

 
   
SHORT TERM...TONIGHT THROUGH SUNDAY
 
 
THE FORECAST CONTINUES LARGELY ON TRACK FROM EARLIER DISCUSSIONS.  
THE MORNING HOURS WERE FAIRLY TRANQUIL ACROSS THE FORECAST AREA,  
WITH THE MOST ACTIVE PORTION OF THE TROPICAL WAVE TRACKING SOUTH OF  
PUERTO RICO AND THE US VIRGIN ISLANDS. HOWEVER, RADAR DEPICTED A  
LINE OF SHOWERS AND THUNDERSTORMS, LOCATED ALONG THE NORTHERN  
PERIPHERY OF THE WAVE OVER THE CARIBBEAN WATERS, MOVING CLOSE TO THE  
SOUTHERN COAST OF PUERTO RICO AND ST. CROIX BY LATE MORNING AND  
EARLY AFTERNOON. MOST OF THIS ACTIVITY, HOWEVER, REMAINED OFFSHORE,  
BUT COULD MOVE CLOSE IN THE NEXT HOUR OR SO. ADDITIONALLY, SATELLITE  
DATA REVEALED A FAIRLY ACTIVE AREA WELL NORTHEAST OF THE FORECAST  
AREA, ASSOCIATED WITH A STRONG UPPER-LEVEL LOW POSITIONED OVER THE  
NORTHERN CARIBBEAN. THIS UPPER-LEVEL INSTABILITY, DUE TO THE  
PERSISTENT TROUGH, IS EXPECTED TO REMAIN IN PLACE FOR THE REMAINDER  
OF THE FORECAST PERIOD. WHILE THIS INSTABILITY IS PRESENT ALOFT, ANY  
SIGNIFICANT CONVECTIVE DEVELOPMENT WILL LARGELY DEPEND ON MOISTURE  
AVAILABILITY AND STABILITY AT LOWER LEVELS. HEAT HAS BEEN BUILDING  
EARLY TODAY, WITH SEVERAL WEATHER STATIONS ALREADY REPORTING HEAT  
INDEX VALUES ABOVE 100F, AND SOME AREAS IN NORTH-CENTRAL PUERTO RICO  
ALREADY EXCEEDING 108F.  
 
FOLLOWING THE PASSAGE OF THE TROPICAL WAVE, A BROAD AREA OF SAHARAN  
DUST WITH MODERATE CONCENTRATIONS IS EXPECTED TO MOVE OVER THE  
FORECAST AREA THIS AFTERNOON AND PERSIST THROUGH THE WEEKEND. THIS  
INTRUSION OF DUST WILL RESULT IN HAZY SKIES AND REDUCED AIR QUALITY,  
WITH POTENTIAL HEALTH IMPACTS, PARTICULARLY FOR SENSITIVE GROUPS  
SUCH AS INDIVIDUALS WITH RESPIRATORY CONDITIONS.  
 
WHILE INSTABILITY ALOFT AND NEAR-NORMAL LEVELS OF MOISTURE WILL  
REMAIN IN PLACE OVER THE REGION, FACTORS THAT TYPICALLY ENHANCE  
SHOWER AND THUNDERSTORM DEVELOPMENT, THE PRESENCE OF THE SAHARAN AIR  
LAYER MAY PARTLY SUPPRESS DEEP CONVECTION BY INTRODUCING DRIER MID-  
LEVEL AIR AND A MORE STABLE ATMOSPHERIC PROFILE. NEVERTHELESS, DAILY  
AFTERNOON CONVECTION IS STILL ANTICIPATED, PARTICULARLY ACROSS THE  
INTERIOR AND NORTHWESTERN PORTIONS OF PUERTO RICO, FAVORED BY LOCAL  
SEA BREEZE INTERACTIONS AND DIURNAL HEATING. STREAMERS DEVELOPING  
DOWNWIND OF EL YUNQUE ARE ALSO POSSIBLE, ESPECIALLY DURING THE EARLY  
AFTERNOON HOURS.  
 
IN ADDITION TO THE HAZE AND AIR QUALITY CONCERNS, THE COMBINATION OF  
SUSPENDED DUST, NEAR NORMAL LOW-LEVEL MOISTURE, AND GENERALLY LIGHT  
EASTERLY WINDS WILL CONTRIBUTE TO ELEVATED HEAT INDEX VALUES ACROSS  
THE REGION. MAXIMUM HEAT INDICES ARE EXPECTED TO EXCEED 100F IN MANY  
URBAN AND COASTAL AREAS, WITH SOME AREAS EVEN REACHING ABOVE 108F,  
SIGNIFICANTLY INCREASING THE HEAT THREAT. AS A RESULT, HEAT  
ADVISORIES ARE LIKELY TO BE ISSUED ON A DAILY BASIS THROUGH THE  
WEEKEND. RESIDENTS ARE URGED TO STAY HYDRATED, AVOID STRENUOUS  
OUTDOOR ACTIVITIES DURING PEAK HEATING HOURS, AND CHECK ON  
VULNERABLE POPULATIONS.  
   
LONG TERM...MONDAY TROUGH FRIDAY
 
 
FROM MONDAY INTO TUESDAY, AN UPPER-LEVEL CUT-OFF LOW WILL POSITION  
ITSELF OVER HISPANIOLA, PROVIDING ENOUGH INSTABILITY ALOFT TO  
SUPPORT SOME CONVECTIVE DEVELOPMENT ACROSS THE REGION. HOWEVER,  
DESPITE THIS INSTABILITY, MOISTURE LEVELS ACROSS PUERTO RICO AND  
THE U.S. VIRGIN ISLANDS WILL REMAIN LIMITED, PARTICULARLY AT MID  
TO UPPER LEVELS, WHERE RELATIVE HUMIDITY VALUES ARE FORECAST TO  
STAY LOW. THIS LACK OF DEEP MOISTURE WILL PREVENT WIDESPREAD OR  
LONG-LASTING SHOWER ACTIVITY. AT THE SURFACE, CONDITIONS WILL BE  
DRIVEN BY A SURFACE HIGH-PRESSURE SYSTEM TO THE NORTH OF THE  
REGION, PROMOTING EASTERLY WINDS FROM MONDAY INTO EARLY TUESDAY.  
BY LATE TUESDAY, AS THE BROAD SURFACE HIGH EXPANDS AND  
ESTABLISHES ITSELF OVER THE CENTRAL ATLANTIC, SURFACE WINDS WILL  
GRADUALLY SHIFT FROM THE SOUTHEAST. ACCORDING TO THE GALVEZ-  
DAVISON INDEX (GDI), THE BEST CHANCE FOR NOTABLE CONVECTIVE  
DEVELOPMENT WILL OCCUR ON TUESDAY, WITH THE POTENTIAL FOR A FEW  
ISOLATED THUNDERSTORMS DURING THE AFTERNOON HOURS. THE FOCUS OF  
THE SHOWERS WILL BE ON THE NORTHWESTERN QUADRANT LEAVING THE REST  
OF THE ISLANDS WITH LESS POTENTIAL TO OBSERVED SHOWER ACTIVITY.  
 
BY WEDNESDAY, A NEW PLUME OF SAHARAN DUST WILL BEGIN TO MOVE INTO  
THE REGION, LIMITING AFTERNOON SHOWER DEVELOPMENT DESPITE  
LINGERING INSTABILITY. THESE DUST PARTICLES ARE FORECAST TO  
PERSIST THROUGH FRIDAY, CONTRIBUTING TO HAZY SKIES, REDUCED AIR  
QUALITY, AND LIMITED RAINFALL ACTIVITY ACROSS THE ISLANDS.  
ALTHOUGH SOME LOW-LEVEL MOISTURE MAY BECOME TRAPPED BENEATH THE  
DUST LAYER, ANY RESULTING SHOWERS WILL BE BRIEF AND ISOLATED.  
SURFACE WINDS WILL REMAIN FROM THE SOUTHEAST THROUGH MIDWEEK AS  
THE HIGH MAINTAINS ITS POSITION OVER THE CENTRAL ATLANTIC, HELPING  
TO SUSTAIN VARIABLE BUT GENERALLY SUPPRESSED AFTERNOON CONVECTION.  
OVERALL, THE WEATHER PATTERN FOR THE LATTER HALF OF THE WEEK WILL  
FEATURE PARTLY CLOUDY SKIES, HAZY CONDITIONS, AND LIMITED RAINFALL  
POTENTIAL.  
 

 
   
AVIATION
 
 
(18Z TAFS)  
 
VFR CONDITIONS ARE EXPECTED THROUGH THE FORECAST PERIOD. SAHARAN  
DUST IS CAUSING HZ, BUT VIS SHOULD BE ABOVE 6SM. BRIEF SHRA/TSRA  
POSSIBLE NEAR TJBQ THROUGHOUT ABOUT 04/23Z. WINDS WILL BE OUT OF THE  
ESE AT 10 TO 15 KNOTS WITH STRONGER GUSTS AND SEA BREEZE VARIATIONS,  
BECOMING AT 10 KNOTS OR LESS OVERNIGHT.  
 

 
   
MARINE
 
 
A BROAD SURFACE HIGH PRESSURE OVER THE CENTRAL ATLANTIC WILL  
CONTINUE TO RESULT IN MODERATE TO FRESH EASTERLY WINDS ACROSS THE  
REGION. THEREFORE, WIND-DRIVEN SEAS WILL RESULT IN CHOPPY MARINE  
CONDITIONS ACROSS THE ATLANTIC WATERS AND THE CARIBBEAN WATERS.  
SAHARAN DUST PARTICLES WILL BE IN PLACE, RESULTING IN HAZY SKIES  
THROUGHOUT THE WEEKEND. A WEAK TROPICAL WAVE WILL MOVE INTO THE  
REGION TODAY AND SATURDAY, BRINGING A FEW SHOWERS AND  
THUNDERSTORMS.  
 

 
   
BEACH FORECAST
 
 
ALL THE COASTAL AREAS OF PUERTO RICO AND THE US VIRGIN ISLANDS  
WILL REMAIN LOW THROUGH SUNDAY WHEN ST. CROIX BECOMES MODERATE.  
THE RISK OF RIP CURRENTS WILL BECOME MODERATE FOR THE  
NORTHWESTERN SECTIONS OF PUERTO RICO BY LATE MONDAY.  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...HEAT ADVISORY UNTIL 4 PM AST THIS AFTERNOON FOR PRZ001>005-007-  
008-010-011.  
 
VI...NONE.  
AM...NONE.  

 
 

 
 
SHORT TERM/AVIATION...CVB  
LONG TERM...LIS  
MARINE/BEACH FORECAST...MMC  
 
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