078
FXCA62 TJSJ 030900
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
500 AM AST MON MAR 3 2025
SYNOPSIS
A PATCH OF MOISTURE WILL PROMOTE LIGHT TO MODERATE SHOWERS UNDER
SOUTHEASTERLY FLOW THIS MORNING, AFTERNOON SHOWERS MAINLY OVER
NORTHWESTERN PR ARE FORECAST. THE ARRIVAL OF A SURFACE TROUGH ON
TUESDAY WILL INCREASE THE POTENTIAL FOR SHOWERS ACROSS THE
ISLANDS. THEN, AN UPPER- LEVEL TROUGH WILL INCREASE INSTABILITY,
AND INTERACTING WITH THE INDUCED PRE- FRONTAL TROUGH WILL RESULT
IN SCATTERED TO NUMEROUS SHOWERS AND EVEN THUNDERSTORMS THROUGH AT
LEAST WEDNESDAY.
SHORT TERM...TODAY THROUGH WEDNESDAY
LOTS OF CLOUDS WERE OBSERVED ACROSS THE VIRGIN ISLANDS DURING THE
NIGHT HOURS, WHILE SKIES WERE MOSTLY CLEAR TO PARTLY CLOUDY ACROSS
PUERTO RICO. EARLY IN THE NIGHT HOURS, SOME SHOWERS MOVED OVER
EASTERN PUERTO RICO, AND NEW SHOWERS DEVELOPED EARLY IN THE MORNING
ACROSS THE CARIBBEAN WATERS. TEMPERATURES COOLED DOWN TO THE LOW AND
MID-70S FOR COASTAL AREA, AND THE LOW AND MID-60S FOR THE
MOUNTAINS.
WETTER CONDITIONS ARE ANTICIPATED THIS WEEK FOR THE ISLANDS. FIRST,
WINDS WILL BE MODULATED BY A SURFACE HIGH PRESSURE OVER THE EASTERN
ATLANTIC AND A MID TO UPPER LEVEL TROUGH (WITH ITS ASSOCIATED COLD
FRONT) TO THE NORTHWEST OF THE REGION. THESE FEATURES ARE WEAKENING
THE PRESSURE GRADIENT, CAUSING LIGHTER WINDS, OUT OF THE SOUTHEAST
AT 10 KNOTS OR LESS. AS THE TROUGH APPROACHES, CONDITIONS ALOFT WILL
BECOME MORE FAVORABLE FOR SHOWER PRODUCTION TOO. A PATCH OF
MOISTURE IS MOVING INTO THE ISLANDS TODAY, BRINGING NEAR NORMAL
PRECIPITABLE WATER CONTENT (1.4-1.6 INCHES). THIS LAYER OF
MOISTURE STRETCHES INTO THE MID-LEVELS (TO AROUND 650 MB).
CONSIDERING ALL THESE FACTORS, AS WELL AS DIURNAL HEATING AND
LOCAL EFFECTS, IT IS LIKELY THAT SHOWERS WILL FIRE UP THIS
AFTERNOON ACROSS THE INTERIOR AND NORTHWESTERN PUERTO RICO,
POTENTIALLY LINGERING INTO THE NIGHT HOURS (50-80% CHANCE). THE
RISK OF URBAN AND SMALL STREAM FLOODING WILL BE ELEVATED, WITH
WATER SURGES AND POTENTIAL MUDSLIDES AS WELL. CONDITIONS ARE NOT
TOO FAVORABLE FOR THUNDERSTORMS YET, AS 500 MB TEMPERATURES ARE
STILL WARMER THAN NORMAL, BUT ISOLATED LIGHTNING CANNOT BE RULED
OUT.
THEN, A SURFACE TROUGH WILL APPROACH EARLY ON TUESDAY, INCREASING
THE FREQUENCY OF SHOWERS FOR THE VIRGIN ISLANDS, VIEQUES, AND
CULEBRA. AGAIN, AFTERNOON CONVECTION IS ANTICIPATED FOR THE
INTERIOR, WEST AND NORTHWEST PUERTO RICO IN THE AFTERNOON HOURS. ONE
KEY DIFFERENCE FOR TUESDAY IS THAT THE MID LEVEL TEMPERATURES WILL
COOL DOWN, SO IT WILL BE MORE LIKELY TO SEE LIGHTNING WITH THE
STRONGER SHOWERS. MOVING INTO WEDNESDAY, INSTABILITY PERSIST, BUT
THERE IS UNCERTAINTY ON WHERE THE AFTERNOON SHOWERS WILL FORM.
FIRST, THE LOW LEVEL WINDS WILL CONTRIBUTE IN THE INITIATION OF
CONVECTION IN THE INTERIOR, AS USUAL, BUT THEN THE MOVEMENT OF THIS
ACTIVITY WILL DEPEND ON HOW MUCH THESE SHOWERS GROW VERTICALLY. IF
THE SHOWERS ARE SHALLOW ENOUGH, A NORTH-NORTHWESTERLY PROGRESSION IS
EXPECTED, BUT IF THE SHOWERS MANAGE TO BE STEERED BY THE MID-LEVEL
WINDS, THEN THEY WILL LIKELY MOVE SOUTHWARD. THE FORECAST IS A
COMPROMISE OF BOTH SOLUTIONS, SHOWING A WIDESPREAD AREA OF 30-50% OF
PROBABILITY OF PRECIPITATION ACROSS MOST OF PUERTO RICO, BUT WITH
THE HIGHER CHANCES (50-70%) FOR THE NORTHWEST CORNER.
ALSO, THE AEROSOL MODELS SHOW LINGERING SAHARAN DUST TODAY,
GRADUALLY CLEARING OUT BY MID-WEEK. LOW CONCENTRATION OF SAHARAN
DUST MAY INCREASE THE FREQUENCY OF LIGHTNING WITHIN THE
THUNDERSTORMS. AND FINALLY, TEMPERATURES...IT WILL BE WARMER THAN
NORMAL FOR AREAS THAT DO NOT EXPERIENCE AS MUCH RAIN, SINCE 925-MB
TEMPERATURES ARE ONE TO TWO STANDARD DEVIATION HIGHER THAN NORMAL.
HIGHS CAN BE EXPECTED IN THE MID TO UPPER 80S FOR COASTAL AREAS, AND
COOLER IN THE MOUNTAIN.
LONG TERM...THURSDAY THROUGH MONDAY
BY THURSDAY, THE UPPER LEVEL TROUGH WILL BE EAST-NORTHEAST OF THE
ISLANDS AND WILL CONTINUE TO MOVE AWAY FROM THE ISLANDS. TO START
THE LONG TERM PERIOD, AT SURFACE LEVEL, A HIGH PRESSURE SYSTEM WILL
BE OVER THE CENTRAL ATLANTIC, WHILE A FRONTAL LOW MOVES INTO THE
EASTERN ATLANTIC. CURRENT MODEL GUIDANCE SUGGESTS THIS AND OTHER
FRONTAL LOWS MOVING INTO THE ATLANTIC DURING THE PERIOD ON THURSDAY,
FRIDAY AND MONDAY BEFORE HIGH PRESSURE BUILDS AND DOMINATES LATE
MONDAY. THIS WILL CAUSE CHANGES IN THE STEERING FLOW DURING THE
PERIOD, EAST-SOUTHEAST ON THURSDAY, EAST-SOUTHEAST TO EAST-NORTHEAST
ON FRIDAY (DUE TO AN APPROACHING COLD FRONT), EAST-NORTHEAST ON
SATURDAY AND THEN GRADUALLY VEERING DURING THE WEEKEND TO EAST-
SOUTHEAST LATE SUNDAY INTO MONDAY. THE PERIOD WILL START OFF WITH
PRECIPITABLE WATER (PWAT) VALUES AROUND 1.5 INCHES, MAINLY
MAINTAINED BY AN APPROACHING FRONTAL BOUNDARY WITH THE LINGERING
TRACES OF SAHARAN DUST DISSIPATING. MODEL DISCREPANCIES ARE PRESENT
REGARDING MOISTURE DURING THE WEEKEND, WITH ONE MORE INCLINED TO
PATCHES OF MOISTURE REACHING THE ISLAND DURING THAT TIME. THIS
INCLUDES SMALL POCKETS OF MOISTURE WITH PWAT VALUES AROUND 1.4 TO
1.5 INCHES REACHING THE ISLANDS. AVAILABLE MOISTURE SHOULD REMAIN
BELOW 750 MB THURSDAY THROUGH SATURDAY, AND BELOW 850 MB BY SUNDAY
AND MONDAY. AS THE FRONT APPROACHES THE REGION TO END THE WORKWEEK,
WINDS WILL DECREASE TO BECOME LIGHT AND VARIABLE, THIS WILL RESULT
IN SLOW AND LINGERING SHOWERS, ELEVATING THE EXCESSIVE RAINFALL RISK.
DURING THE PERIOD, THE DIURNAL PATTERN WILL VARY DEPENDING ON THE
STEERING FLOW. THIS INCLUDES, OVERNIGHT AND EARLY MORNING SHOWERS
STEERED TOWARDS WINDWARD SECTORS OF THE ISLANDS WHILE AFTERNOON
CONVECTION WILL FOCUS OVER THE INTERIOR TO EITHER NORTHWESTERN
(UNDER SOUTHEAST FLOW) OR SOUTHWESTERN (UNDER NORTHEAST FLOW) PR
DEPENDING ON THE STEERING FLOW THAT DAY. 925 MB TEMPERATURES WILL BE
UP TO ABOVE NORMAL VALUES DURING THE PERIOD, ESPECIALLY ON DAYS WITH
EAST-SOUTHEAST TO SOUTHEAST FLOW.
AVIATION
(06Z TAFS)
INCREASING SHRA EXPECTED TODAY, ESPECIALLY AFTER 17Z FOR THE
NORTHWESTERN QUADRANT OF PR. PERIODS OF REDUCES VIS AND LOW
CEILINGS EXPECTED FOR TJBQ AT LEAST UNTIL 22Z. MOUNTAIN
OBSCURATION IS ALSO EXPECTED FOR THE WESTERN CORDILLERA CENTRAL.
WINDS WILL BE FROM THE SE AT 8-12 KTS, WITH STRONGER GUSTS.
MARINE
A FRONTAL BOUNDARY APPROACHING FROM THE WESTERN ATLANTIC WILL SLOWLY
LOOSEN THE PRESSURE GRADIENT ASSOCIATED WITH THE BROAD SURFACE HIGH
PRESSURE EXTENDING FROM THE EASTERN TO CENTRAL ATLANTIC. THIS WILL
PROMOTE EAST-SOUTHEAST WINDS TODAY AND TONIGHT. WINDS WILL BACK
TO BECOME EAST TO NORTHEAST THROUGH TOMORROW AS AN INDUCED SURFACE
TROUGH ASSOCIATED WITH THE FRONTAL BOUNDARY MOVES IN. THIS WILL
ALSO INCREASE THE RISK OF SHOWERS AND EVEN THUNDERSTORMS ACROSS
THE REGIONAL WATERS LATE TONIGHT THROUGH WEDNESDAY.
BEACH FORECAST
FOR TODAY, A MODERATE RIP OF RIP CURRENTS WILL BE PRESENT AT THE
EASTERNMOST BEACHES OF ST. CROIX, LIFE-THREATENING RIP CURRENTS ARE
POSSIBLE IN THE SURF ZONE. OTHER COASTAL AREAS ACROSS THE ISLANDS
WILL HAVE A LOW RISK OF RIP CURRENTS TODAY, HOWEVER, LIFE-
THREATENING RIP CURRENTS OFTEN OCCUR IN THE VICINITY OF GROINS,
JETTIES, REEFS, AND PIERS. DURING THE REST OF THE WORKWEEK, A
MODERATE RISK OF RIP CURRENTS WILL BE IN EFFECT FOR THE NORTH-FACING
BEACHES OF PUERTO RICO, CULEBRA AND THE USVI AT TIMES.
SJU WATCHES/WARNINGS/ADVISORIES
PR...NONE.
VI...NONE.
AM...NONE.
SHORT TERM...ERG
LONG TERM....MRR
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