330
FXCA62 TJSJ 151908
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
308 PM AST MON SEP 15 2025
KEY MESSAGES
* A TROPICAL WAVE COMBINED WITH A TUTT LOW EXPECTED AROUND MIDWEEK,
WILL INCREASE THE FREQUENCY OF SHOWERS AND THUNDERSTORMS
THROUGHOUT MUCH OF THE WORKWEEK. AS A RESULT, THE RISK OF FLOODING
WILL REMAIN ELEVATED. ADDITIONAL HAZARDS INCLUDE RAPID RIVER
RISES, LANDSLIDES, AND FREQUENT LIGHTNING.
* THE VIRGIN ISLANDS WILL ALSO EXPERIENCE AN INCREASE IN SHOWERS AND
THUNDERSTORMS, POTENTIALLY LEADING TO WATER SURGES IN DRAINAGE
GUTS, PONDING ON ROADWAYS, OCCASIONAL LIGHTNING, AND ISOLATED
LANDSLIDES AS THE TROPICAL WAVE APPROACHES.
* INCREASING HEAT RISK IS FORECAST AGAIN TOWARD THE END OF THE WEEK
AND INTO THE WEEKEND. A LIMITED HEAT RISK IS EXPECTED TODAY AND
OVER THE NEXT FEW DAYS.
* A MODERATE RISK OF RIP CURRENTS IS IN EFFECT FOR THE NORTHERN
AND EASTERN BEACHES OF PUERTO RICO, AS WELL AS CULEBRA AND ST.
CROIX THROUGH FRIDAY. A LOW RISK IS EXPECTED IN OTHER AREAS.
SIMILAR CONDITIONS ARE LIKELY TO CONTINUE THROUGH MOST OF THE
WORKWEEK.
SHORT TERM...REST OF THE AFTERNOON THROUGH WEDNESDAY
VARIABLE WEATHER CONDITIONS PREVAILED THROUGH THE MORNING HOURS AS
EXPECTED DUE TO THE ARRIVAL OF AN EASTERLY PERTURBATION OVER THE
REGION. SHOWER AND THUNDERSTORM ACTIVITY INITIALLY CLUSTERED MAINLY
OVER THE SURROUNDING WATERS. HOWEVER, AS THE MORNING PROGRESSED,
ACTIVITY EXPANDED IN COVERAGE, WITH SEVERAL STREAMERS DEVELOPING
DOWNWIND OF THE ISLANDS AND MOUNTAINOUS TERRAIN. DESPITE THE
INCREASED COVERAGE, SHOWERS AND THUNDERSTORMS HAVE BEEN MOVING
FAIRLY QUICKLY. SOME THUNDERSTORMS HAVE PRODUCED FREQUENT LIGHTNING
AND GUSTY WINDS. THIS ACTIVITY WILL PERSIST THROUGH THE EVENING
HOURS OVER LAND AREAS. DURING THE NIGHT HOURS, SHOWERS AND
THUNDERSTORMS COULD DEVELOP AGAIN OVER THE WATERS, PARTICULARLY AS
TROUGHINESS ALOFT PERSISTS, WITH SOME OF THIS ACTIVITY BRUSHING
WINDWARD COASTAL AREAS. HOWEVER, ANY ACTIVITY OVERNIGHT SHOULD MOVE
FAIRLY QUICKLY.
A SIMILAR PATTERN IS EXPECTED ON TUESDAY, WITH AFTERNOON CONVECTION
ENHANCED BY DIURNAL HEATING, SEA BREEZE CONVERGENCE, AND LOCAL
EFFECTS. HOWEVER, SHOWERS AND THUNDERSTORMS WILL TEND TO MOVE MORE
EAST-SOUTHWESTWARD, SHIFTING THE HIGHEST ACTIVITY OVER THE INTERIOR
AND WESTERN TO SOUTHWESTERN PUERTO RICO AS WINDS BECOME MORE EAST-
NORTHEASTERLY.
BY WEDNESDAY, WEATHER CONDITIONS ARE EXPECTED TO BECOME
SIGNIFICANTLY MORE ACTIVE. A DEEP-LAYERED TROUGH NORTHEAST OF THE
REGION WILL GRADUALLY MOVE SOUTHWEST AND MEANDER ACROSS THE AREA.
MODEL GUIDANCE SUGGESTS MID-LEVEL VORT MAX WILL BE NEAR OR OVER THE
REGION BY WEDNESDAY NIGHT. IN ADDITION, COOL AIR ADVECTION AT MID-
LEVELS WILL LOWER THE 1000-500 MB THICKNESSES AND STEEPEN LAPSE
RATES BETWEEN 850-700 MB AND 700-500 MB. THIS FEATURE WILL INTERACT
WITH A TROPICAL WAVE (WITH AXIS CURRENTLY NEAR 52.6W) TO CREATE
HIGHLY FAVORABLE CONDITIONS FOR HEAVY SHOWERS AND SCATTERED
THUNDERSTORMS. THEREFORE, EXPECT INCREASING AREAL COVERAGE OF
RAINFALL ON WEDNESDAY WITH AN ELEVATED FLOODING THREAT. URBAN AND
RIVER FLOODING ARE POSSIBLE, WITH LANDSLIDES AND ROCKFALLS EXPECTED
IN AREAS OF STEEP TERRAIN. FREQUENT LIGHTNING AND GUSTY WINDS COULD
ACCOMPANY STRONGER THUNDERSTORMS, POTENTIALLY DOWNING TREE LIMBS AND
BLOWING AROUND UNSECURED OBJECTS.
LONG TERM...THURSDAY THROUGH MONDAY
PREV DISCUSSION
/ISSUED 505 AM AST MON SEP 15 2025/
BY THURSDAY, UNSETTLED WEATHER CONDITIONS ARE EXPECTED TO PREVAIL
DUE TO THE COMBINED INFLUENCE OF A TROPICAL WAVE AND A TUTT LOW
MOVING ACROSS PUERTO RICO AND THE VIRGIN ISLANDS. LATEST MODEL
GUIDANCE SUGGESTS PRECIPITABLE WATER (PWAT) VALUES WILL REMAIN
ABOVE NORMAL FOR THIS TIME OF YEAR—RANGING BETWEEN 2.0 AND 2.25
INCHES. THESE ATMOSPHERIC FEATURES WILL MAINTAIN HIGH RAIN CHANCES
AND ENHANCE THUNDERSTORM DEVELOPMENT, SUPPORTED BY 500 MB
TEMPERATURES NEAR AVERAGE FOR MID-SEPTEMBER. AS A RESULT, URBAN
AND SMALL STREAM FLOODING IS LIKELY TO PERSIST DAILY, AND SOME
RIVERS MAY REACH FLOOD STAGE. IN THE VIRGIN ISLANDS, PERIODS OF
SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CAUSE WATER SURGES ALONG
GUTS AND DRAINAGE CHANNELS, PONDING ON ROADWAYS, AND ISOLATED
URBAN FLOODING. ADDITIONAL HAZARDS INCLUDE LANDSLIDES, FREQUENT
LIGHTNING, AND GUSTY WINDS.
BY FRIDAY, DRIER AIR IS FORECAST TO MOVE INTO THE REGION, DECREASING
PWAT VALUES TO NEAR NORMAL LEVELS, AND DROPPING FURTHER TO BELOW
NORMAL OVER THE WEEKEND AND INTO EARLY NEXT WEEK. RAINFALL
ACTIVITY WILL DIMINISH, BUT LOCALIZED AFTERNOON CONVECTION,
PARTICULARLY IN NORTHWESTERN PUERTO RICO, WILL STILL BE POSSIBLE
DUE TO LOCAL EFFECTS AND DIURNAL HEATING. AS CLOUD COVER DECREASES
AND RAIN SUBSIDES, WINDS WILL SHIFT FROM THE SOUTHEAST, AND 925 MB
TEMPERATURES ARE FORECAST TO PEAK AT NEARLY TWO STANDARD DEVIATIONS
ABOVE NORMAL, ESPECIALLY ON FRIDAY AND SATURDAY. THIS PATTERN
WILL ELEVATE THE HEAT RISK ACROSS THE REGION. HEAT ADVISORIES OR
EVEN EXTREME HEAT WARNINGS MAY BE ISSUED. RESIDENTS AND VISITORS
ARE STRONGLY ADVISED TO STAY HYDRATED, AVOID PROLONGED SUN
EXPOSURE DURING PEAK HEAT HOURS, AND TAKE EXTRA PRECAUTIONS,
PARTICULARLY THOSE WITHOUT ACCESS TO ADEQUATE COOLING OR HYDRATION.
THIS FORECAST SCENARIO ASSUMES THE CURRENT GUIDANCE REMAINS
CONSISTENT FOR THE TROPICAL WAVE BEING MONITORED BY THE NATIONAL
HURRICANE CENTER, WHICH CURRENTLY HAS A 30% CHANCE OF FORMATION IN
THE NEXT 48 HOURS AND AN 80% CHANCE OVER THE NEXT 7 DAYS. IF
DEVELOPMENT CONTINUES AS EXPECTED, THE SYSTEM MAY TRACK
NORTHWESTWARD TOWARD THE NORTHEASTERN CARIBBEAN. WE WILL CONTINUE
TO MONITOR THIS CLOSELY AND PROVIDE UPDATES THROUGHOUT THE
WORKWEEK.
AVIATION
(18Z TAFS)
SHRA/TSRA WILL CONTINUE ACROSS PR TERMINALS THROUGH 15/23Z. PERIODS
OF MVFR, BRIEFLY IFR, WITH MTN OBSC ACROSS THE CORDILLERA CENTRAL
ARE POSSIBLE. USVI TERMINALS WILL SEE VCSH/VCTS THROUGH THE PERIOD,
MAINLY DOWNWIND OF ISLANDS. FREQUENT LTG AND GUSTY WINDS POSSIBLE
WITH STRONGER TSRA. OVERNIGHT, SHRA/TSRA DEVELOPING OVER WATERS MAY
BRUSH WINDWARD COASTAL TERMINALS, BUT MOVING QUICKLY. ON TUE, AFTN
SHRA/TSRA EXPECTED MAINLY ACROSS W/SW PR WITH MVFR CONDS POSSIBLE.
WINDS ESE AT 10-15 KTS WITH SEA BREEZE VARIATIONS AND HIGHER GUSTS,
BCMG LIGHT/VRB AFT 23Z, INCREASING AGAIN 10-15 KTS FROM ENE BY
16/14Z WITH STRONGER GUSTS NEAR SHRA/TSRA.
MARINE
A SURFACE HIGH PRESSURE SYSTEM LOCATED OVER THE CENTRAL AND EASTERN
ATLANTIC, IN COMBINATION WITH A LOW-LEVEL PERTURBATION, WILL PROMOTE
MODERATE TO LOCALLY FRESH EAST TO SOUTHEASTERLY WINDS. THIS SURFACE
PERTURBATION WILL ALSO INCREASE THE LIKELIHOOD OF SHOWERS AND
THUNDERSTORMS ACROSS THE WATERS AND LOCAL CARIBBEAN PASSAGES. SMALL
CRAFT ARE URGED TO EXERCISE CAUTION ACROSS THE REGIONAL WATERS. A
STRONGER TROPICAL WAVE IS EXPECTED TO APPROACH BY MIDWEEK, ENHANCING
SHOWER COVERAGE.
BEACH FORECAST
THE MODERATE RIP CURRENT RISK ACROSS THE NORTH AND EAST-FACING
BEACHES OF PUERTO RICO, CULEBRA, AND ST. CROIX WILL PREVAIL
THROUGH FRIDAY. SIMILAR CONDITIONS ARE EXPECTED TO PREVAIL MOST OF
THE WORKWEEK SO BEACHGOERS ARE ADVISED TO EXERCISE CAUTION AT ALL
TIMES AS A MODERATE RISK MEANS LIFE-THREATHERNING RIP CURRENT ARE
POSSIBLE IN THE SURF ZONE. A LOW RISK SHOULD PREVAIL ELSEWHERE.
ADDITIONAL HAZARDS INCLUDE AFTERNOON THUNDERSTORMS. REMEMBER: IF
YOU HEAR THUNDER, SEEK SHELTER INSIDE OF A SAFER PLACE
IMMEDIATELY.
HYDROLOGY
THE DISTURBANCE THAT WILL MOVE IN TODAY, AND THE TROPICAL WAVE
COMING IN ON WEDNESDAY WILL MAINTAIN THE RISK OF FLOODING ELEVATED
FOR MOST OF THE AREA IN PUERTO RICO AND THE VIRGIN ISLANDS. THE
LATEST ROOT SOIL SATURATION PRODUCT FROM PRAGWATER, AND ALSO FROM
THE NATIONAL WATER MODEL SHOWS SATURATED SOILS ALONG THE WESTERN
HALF OF PUERTO RICO, AND ALSO FOR THE EXTREME EAST AND SOUTHEAST.
ALSO, THE 7-DAY AVERAGE STREAMFLOW FROM THE USGS SHOW THAT MANY
RIVERS ARE RUNNING NEAR NORMAL OR WELL ABOVE NORMAL. THEREFORE, IT
IS LIKELY THAT ANY UPCOMING RAIN WILL QUICKLY TURN INTO RUNOFF,
ENHANCING THE RISK OF FLOODING, RAPID RIVER RISES, AND LANDSLIDES.
FOR THE VIRGIN ISLANDS, PERIODS OF HEAVY RAIN WILL CAUSE WATER
SURGES ALONG DRAINAGE GUTS, AND PONDING OF WATER ON ROADWAYS.
URBAN FLOODING COULD DEVELOP TOO, WHILE LANDSLIDES IN AREAS OF STEEP
TERRAIN CANNOT BE RULED OUT.
ALTHOUGH AN ELEVATED FLOOD RISK IS SHOWN EVERYDAY OF THE WEEK, THE
HIGHER RISK IS ON WEDNESDAY AND THURSDAY DUE TO THE ABOVE NORMAL
MOISTURE FROM THE TROPICAL WAVE AND INCREASED INSTABILITY ALOFT FROM
AN UPPER LEVEL LOW. CONDITIONS WILL BEGIN TO IMPROVE BY FRIDAY.
SJU WATCHES/WARNINGS/ADVISORIES
PR...NONE.
VI...NONE.
AM...NONE.
SHORT TERM/AVIATION...CVB
HYDROLOGY...ERG
LONG TERM...YZR
MARINE/BEACH FORECAST...MMC
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