163
FXCA62 TJSJ 022021
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
421 PM AST THU JAN 2 2025
KEY MESSAGES
* FAIR WEATHER CONDITIONS ARE EXPECTED TO CONTINUE THROUGH EARLY
SATURDAY.
* SHOWER ACTIVITY IS LIKELY TO INCREASE FROM SATURDAY INTO SUNDAY,
WITH ISOLATED THUNDERSTORMS POSSIBLE, POTENTIALLY LEADING TO
LOCALIZED FLOODING IMPACTS.
* NORTHERLY SWELLS AND STRENGTHENING NORTHEASTERLY WINDS WILL CREATE
CHOPPY TO HAZARDOUS SEAS FOR SMALL CRAFT OVER THE WEEKEND.
* A HIGH RISK OF LIFE-THREATENING RIP CURRENTS IS EXPECTED TO BEGIN
FRIDAY NIGHT.
SHORT TERM...TODAY THROUGH SATURDAY
TODAY'S RADAR AND SATELLITE ANALYSIS HIGHLIGHTED PRIMARILY FAIR
WEATHER, GRADUALLY SHIFTING TO A SLIGHTLY WETTER PATTERN AS A PATCH
OF LOW-LEVEL MOISTURE ADVANCED FROM THE EAST. THIS MOISTURE PRODUCED
LIGHT TO MODERATE SHOWERS, INITIALLY IMPACTING THE U.S. VIRGIN
ISLANDS, FOLLOWED BY VIEQUES AND CULEBRA, AND EVENTUALLY REACHING
EASTERN PUERTO RICO BY EARLY AFTERNOON. SURFACE OBSERVATIONS
RECORDED UP TO A QUARTER OF AN INCH OF RAINFALL IN SAINT JOHN, WITH
LESSER AMOUNTS ELSEWHERE. RADAR ESTIMATES REFLECTED SIMILAR TOTALS,
PARTICULARLY OVER NORTHEASTERN PUERTO RICO SINCE 8 AM. TEMPERATURES
RANGED FROM THE MID-70S IN HIGHER ELEVATIONS TO AROUND 87 DEGREES IN
NORTHERN AND SOUTHWESTERN PUERTO RICO. NOTABLY, SURFACE OBSERVATIONS
IN GUANICA (RAWS) AND PONCE (AWOS) REPORTED HIGH TEMPERATURES OF 90-
91 DEGREES. WINDS WERE PREDOMINANTLY EAST-NORTHEASTERLY AT 10-15
MPH, WITH 20-25 MPH GUSTS ALONG COASTAL AREAS, WHILE CONDITIONS WERE
LIGHT AND VARIABLE FURTHER INLAND. STRONGER GUSTS ACCOMPANIED
SHOWERS AS THEY MOVED INLAND FROM COASTAL WATERS.
STABLE WEATHER CONDITIONS WILL DOMINATE THROUGH EARLY SATURDAY,
SUPPORTED BY MID-LEVEL RIDGING AND A STRONG TRADE WIND CAP. WHILE
PRECIPITABLE WATER VALUES MAY BRIEFLY REACH THE TYPICAL EARLY
JANUARY LEVELS, UNFAVORABLE ATMOSPHERIC CONDITIONS WILL LIMIT
VERTICAL DEVELOPMENT AND KEEP RAINFALL MINIMAL. INTERMITTENT DIPS IN
MOISTURE WILL FURTHER SUPPORT BELOW-SEASONAL THRESHOLDS, PROMOTING
FAIR AND DRY WEATHER.
WETTER CONDITIONS ARE EXPECTED TO DEVELOP ON SATURDAY AS A MID-LEVEL
TROUGH TO THE NORTH INTERACTS WITH AN APPROACHING SURFACE FRONT AND
ITS ASSOCIATED PRE-FRONTAL FEATURES. THIS PATTERN WILL DRIVE A RAPID
INCREASE IN MOISTURE, WITH PRECIPITABLE WATER VALUES SURPASSING
NORMAL LEVELS (1.5 INCHES) BY SATURDAY AFTERNOON AND CLIMBING TO
WELL-ABOVE-NORMAL THRESHOLDS (1.8 INCHES), NEARING TWO STANDARD
DEVIATIONS BY SATURDAY NIGHT. STRENGTHENING NORTHEASTERLY WINDS UP
TO 15-25 MPH WILL AMPLIFY THIS SETUP, INCREASING THE FREQUENCY OF
SHOWERS MOVING INLAND FROM SURROUNDING WATERS.
INSTABILITY WILL ALSO INCREASE ON SATURDAY INTO SATURDAY NIGHT, AS
INDICATED BY COOLER 500 MB TEMPERATURES AND ELEVATED GALVEZ-DAVIDSON
INDEX VALUES. THESE FACTORS WILL SUPPORT THE DEVELOPMENT OF SHOWERS
AND ISOLATED THUNDERSTORMS, PRIMARILY ACROSS NORTHERN AND
NORTHEASTERN PUERTO RICO, VIEQUES, CULEBRA, AND THE U.S. VIRGIN
ISLANDS, WITH SECONDARY RAINFALL MAXIMA POSSIBLE ACROSS SOUTHWESTERN
SECTIONS. RAINFALL ACCUMULATIONS MAY REACH UP TO AN INCH IN SOME
LOCATIONS, WITH IMPACTS RANGING FROM PONDING WATER ON ROADWAYS AND
POORLY DRAINED AREAS TO LOCALIZED URBAN AND SMALL STREAM FLOODING.
TEMPERATURES ARE EXPECTED TO REMAIN NEAR OR SLIGHTLY BELOW SEASONAL
NORMS THROUGHOUT THE PERIOD, ADDING TO THE CONTINUATION OF THE
"FRIíTO NAVIDEñO." FOR UPDATES ON EXCESSIVE RAINFALL, WINDS, AND
ANY OTHER HAZARD RISK IN THE COMING DAYS, PLEASE VISIT THE
EXPERIMENTAL GRAPHICAL HAZARD WEATHER OUTLOOK AT
HTTPS://WWW.WEATHER.GOV/ERH/GHWO?WFO=SJU.
LONG TERM...SUNDAY THROUGH THURSDAY
//...FROM PREVIOUS DISCUSSION...//
AT THIS TIME, BOTH THE GLOBAL FORECAST SYSTEM (GFS) AND EUROPEAN
CENTRE FOR MEDIUM-RANGE WEATHER FORECASTS (ECMWF) MODELS AGREE ON A
COLD FRONT MOVING ACROSS THE NORTHEASTERN CARIBBEAN INDUCING A PRE-
FRONTAL TROUGH NEAR PUERTO RICO AND THE U.S. VIRGIN ISLANDS,
PARTICULARLY ON SUNDAY. THE LATEST GUIDANCE SUGGEST PRECIPITABLE
WATER VALUES ABOVE THE 75TH PERCENTILE OR ABOVE NORMAL VALUES FOR
THIS TIME OF THE YEAR, WITH VALUES RANGING AROUND 1.5 TO 2.0
INCHES. ALSO, THE GALVEZ-DAVISON INDEX (GDI)SUGGEST THUNDERSTORM
ACTIVITY ACROSS THE ISLANDS THROUGH SUNDAY AFTERNOON. THE SURFACE
MOISTURE IS FORECAST TO LIFT AND REACH THE 500 TO 700 MB THROUGH
THE BEGINNING OF THE WORKWEEK. THIS INCREASE IN PRECIPITABLE WATER
VALUES AND INSTABILITY LEANS TOWARDS HAVING AN ACTIVE WEATHER DAY
WITH AN INCREASE IN SHOWER ACTIVITY AND A SLIGHT TO MODERATE
CHANCE FOR ISOLATED TO SCATTERED THUNDERSTORMS BY THE END OF THE
WEEKEND, POSSIBLY EXTENDING TOWARDS THE BEGINNING OF THE
WORKWEEK. THEREFORE, URBAN AND SMALL STREAM FLOODING CAN BE SEEN
WITH THE HEAVIEST RAINFALL ACTIVITY.
WEATHER CONDITIONS SHOULD SLIGHTLY IMPROVE BY TUESDAY INTO
WEDNESDAY WITH COOLER TEMPERATURES AND SOMEWHAT DRIER AIR.
HOWEVER, THE WET PATTERN WILL RETURN BY THURSDAY WHEN ANOTHER COLD
FRONT APPROACHES THE REGION.
AVIATION
(18Z TAFS)
SCATTERED SHRA MAY LEAD TO BRIEF MVFR CONDITIONS AT MOST TERMINALS
OVER THE NEXT 6-10 HOURS, THOUGH OPERATIONAL IMPACTS ARE EXPECTED TO
BE MINIMAL. AFTER 02/22Z, SHOWER ACTIVITY WILL SHIFT TO PRIMARILY
AFFECT NORTHERN PR TAF SITES (TJSJ AND TJBQ), GRADUALLY DECREASING
OVERNIGHT AS DRIER AIR MOVES INTO THE REGION. WINDS WILL REMAIN FROM
THE EAST-NORTHEAST AT 10-15 KNOTS, WITH OCCASIONAL STRONGER GUSTS
NEAR SHRA. AFTER 02/23Z, WINDS WILL BECOME LIGHT TO CALM AND
VARIABLE.
MARINE
MODERATE EAST TO NORTHEASTERLY WINDS WILL CONTINUE TO PREVAIL THIS
WEEK UNDER THE INFLUENCE OF A SURFACE HIGH PRESSURE MOVING INTO THE
CENTRAL ATLANTIC AND SURFACE LOW OVER THE EASTERN ATLANTIC. ANOTHER
FRONT FROM THE EASTERN US COAST WILL APPROACH THE NORTHEAST CARIBBEAN
BY THE WEEKEND. ANOTHER LONG PERIOD NORTHERLY SWELL WILL PROMOTE
HAZARDOUS SEAS BY THE END OF THIS WEEK THROUGH THE WEEKEND.
BEACH FORECAST
A LONG-PERIOD NORTHEASTERLY SWELL WILL GRADUALLY SPREAD ACROSS
THE LOCAL WATERS ON FRIDAY. THIS SWELL WILL INCREASE THE RISK OF
LIFE-THREATENING RIP CURRENTS FOR THE NORTHERN EXPOSED BEACHES OF
PUERTO RICO, CULEBRA AND ST. CROIX FROM FRIDAY AFTERNOON ONWARDS
THROUGH THE WEEKEND. IN ADDITION, ANOTHER LONG- PERIOD
NORTHWESTERLY SWELL WILL ARRIVE LATE SUNDAY INTO EARLY NEXT WEEK.
SJU WATCHES/WARNINGS/ADVISORIES
PR...HIGH RIP CURRENT RISK FROM FRIDAY MORNING THROUGH LATE SUNDAY
NIGHT FOR PRZ001-002-005-008-012.
VI...HIGH RIP CURRENT RISK FROM FRIDAY MORNING THROUGH LATE SUNDAY
NIGHT FOR VIZ002.
AM...NONE.
SHORT TERM...ICP/DSR
LONG TERM....MMC
PUBLIC DESK/UPPER AIR...GRS
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