630
FXPQ50 PGUM 210742
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
542 PM CHST MON JUL 21 2025
MARIANAS SYNOPSIS
SATELLITE AND RADAR OBSERVATIONS SHOW GENERALLY PARTLY CLOUDY SKIES
AND ISOLATED SHOWERS ACROSS THE MARIANAS. RITIDIAN AND IPAN BUOYS
SHOW COMBINED SEAS BETWEEN 3 AND 5 FEET.
DISCUSSION
SHORT TERM (TONIGHT THROUGH WEDNESDAY):
OVERALL, LITTLE CHANGE WAS MADE WITHIN THE SHORT TERM FORECAST. THE
MARIANAS LOOK TO REMAIN IN A RELATIVE LULL WITH ISOLATED TO SCATTERED
SHOWERS THROUGH WEDNESDAY. THE BULK OF THE SHOWERS ON WEDNESDAY LOOK
TO BE DUE TO INCREASED CONVERGENCE AS THE MONSOON TROUGH EXPANDS
WESTWARD JUST SOUTH OF GUAM AND INTERACTS WITH THE EAST TO SOUTHEAST
TRADE-WINDS.
LONG TERM (WEDNESDAY NIGHT THROUGH MONDAY):
THE TROPICAL DISTURBANCE THAT IS EXPECTED TO APPROACH THE MARIANAS
HAS BEEN DESIGNATED INVEST 98W BY THE JTWC. THE SYSTEM IS LOCATED IN
EASTERN CHUUK STATE AND IS EXPECTED TO MOVE WEST-NORTHWEST OVER THE
COMING DAYS BEFORE TURNING NORTHWARD TOWARDS THE MARIANAS. WE'LL
START THE PERIOD WITH EAST-NORTHEAST WINDS GRADUALLY TURNING
NORTHWARD AND THEN WESTWARDS AS THE TROPICAL DISTURBANCE APPROACHES.
MODEL GUIDANCE CONTINUES TO SHOW 5 TO 10 INCHES OF RAIN ACROSS THE
MARIANAS FROM THURSDAY THROUGH SUNDAY NIGHT, WITH LOCALLY HIGHER
AMOUNTS GREATER THAN 12 INCHES POSSIBLE. THE RAINFALL FORECAST WILL
DEPEND ON THE PATCH OF THE STORM AND WHETHER OR NOT IT TAPS INTO THE
MONSOONAL FLOW. BOTH THE GFS AND ECMWF SHOW THIS HAPPENING LEADING TO
COPIOUS OF RAINFALL ACROSS THE MARIANAS. CONFIDENCE IS LOWER IN THE
POTENTIAL STRENGTH FOR 98W. PLEASE SEE THE HYDROLOGY AND TROPICAL
SYSTEMS SECTIONS FOR MORE DETAILS.
MARINE
THE MAIN CHANGE FOR MARINE WAS TO INTRODUCE A PRIMARY WESTERLY SWELL
IN THE SHORT TERM. RITIDIAN BUOY DATA SHOWS A 2 TO 3 FOOT WESTERLY
SWELL WITH AN EASTERLY SWELL AROUND 2 FEET. THIS WESTERLY SWELL IS
EXPECTED TO BE SHORT LIVED AS THE TRADE SWELL INCREASES TOMORROW.
BOTH SWELLS REMAIN LOW ENOUGH TO KEEP THE RIP CURRENT RISK LOW ALONG
ALL REEFS. A MODERATE RISK FOR RIP CURRENTS WILL BE POSSIBLE TUESDAY
NIGHT ALONG EAST FACING REEFS AS THE TRADE-SWELL INCREASES.
HYDROLOGY
A TROPICAL DISTURBANCE, JOINT TYPHOON WARNING CENTER'S (JTWC) INVEST
98W IS CENTERED SOUTHEAST OF GUAM NEAR 9N154E. THIS INVEST IS
EXPECTED TO MOVE TOWARD THE MARIANAS THROUGH THE WEEK AS IT SLOWLY
DEVELOPS. 98W IS EXPECTED TO BRING HEAVY RAINFALL TO THE MARIANAS
THURSDAY EVENING THROUGH SUNDAY. MODELS DIFFER ON LOCATION AND
STRENGTH OF 98W AS IT MOVES THROUGH THE MARIANAS, HOWEVER, THEY DO
AGREE THAT HEAVY RAINFALL IS LIKELY AS EARLY AS THURSDAY, CONTINUING
THROUGH THE WEEKEND.
THERE REMAINS THE POTENTIAL FOR GUAM AND THE CNMI TO SEE RAINFALL
AMOUNTS OF 5 TO 10 INCHES BY SUNDAY. ENSEMBLE MEANS LIE ON THE LOWER
END OF THIS RANGE BETWEEN 5 TO 8 INCHES, BUT 75TH PERCENTILES ARE
CLOSER TO 6 TO 10 INCHES FOR THE MARIANAS AND THE 90TH PERCENTILE
SHOWS THE POTENTIAL FOR OVER A FOOT OF RAIN ACROSS SAIPAN AND TINIAN.
DETERMINISTIC GUIDANCE ALSO SHOWS SOME OF THESE HIGHER RAINFALL
TOTALS, SHOWING THE POTENTIAL FOR A HIGHER END RAINFALL EVENT
DEPENDING ON THE EXACT TRACK AND STRENGTH OF INVEST 98W AND THE
POSITIONING OF THE MONSOON TROUGH.
RESIDENTS ON THE MARIANAS NEED TO CLOSELY MONITOR THIS DEVELOPING
SITUATION AS WATCHES, ADVISORIES AND WARNINGS COULD BE ISSUED LATER.
IF LIVING NEAR STREAMS AND RIVERS, PREPARE TO MOVE ITEMS AWAY FROM
STREAM AND RIVER BANKS. MAKE SURE STORM DRAINS NEARBY ARE NOT
CLOGGED, ESPECIALLY IF LIVING IN LOW-LYING AREAS.
TROPICAL SYSTEMS
A TROPICAL DISTURBANCE, JOINT TYPHOON WARNING CENTER'S (JTWC) INVEST
98W IS CENTERED SOUTHEAST OF GUAM NEAR 9N144E. INVEST 98W IS
CURRENTLY RATED LOW, MEANING DEVELOPMENT INTO A TROPICAL CYCLONE IS
NOT EXPECTED WITHIN THE NEXT 24 HOURS. INVEST 98W IS SHOWING SIGNS
FOR GRADUAL DEVELOPMENT BEYOND 24 HOURS WITH SATELLITE OBSERVATIONS
SHOWING PERSISTING CONVECTION AND SOME FEATURES REMINISCENT OF
BANDING. INVEST 98W WILL SLOWLY MOVE WEST-NORTHWEST OVER THE COMING
DAYS BEFORE TURNING NORTHWARD TOWARDS THE MARIANAS. MODEL GUIDANCE
VARIES ON THE STRENGTH OF INVEST 98W. THE POSITION OF 98W AND AN
UPPER TROUGH COULD DRASTICALLY ALTER THE FORECAST WHERE ONE SOLUTION
COULD LEAD TO THE STORM BEING SHEARED AND NOT STRONGER THAN A
TROPICAL DEPRESSION. THE OTHER SOLUTION IS THAT THE UPPER TROUGH
PROVIDES GOOD OUTFLOW FOR 98W AND THE SYSTEM INTENSIFIES INTO A
STRONGER TROPICAL STORM OR TYPHOON. THIS SYSTEM WILL BE MONITORED
CLOSELY AS IT APPROACHES THE MARIANAS. FOR MORE INFORMATION ON INVEST
98W, SEE BULLETINS ISSUED BY JTWC UNDER WMO HEADER ABPW10 PGTW.
ANOTHER TROPICAL DISTURBANCE, JTWC'S INVEST 97W, IS SEEN ROUGHLY 480
MILES NORTHWEST OF YAP, CENTERED NEAR 14.6N 133.3E AS OF 06Z. INVEST
97W IS CURRENTLY RATED MEDIUM, MEANING THE DISTURBANCE SHOWS
ELEVATED POTENTIAL FOR DEVELOPMENT INTO A SIGNIFICANT TROPICAL
CYCLONE, BUT DEVELOPMENT IS NOT LIKELY TO OCCUR WITHIN THE NEXT 24
HOURS. VISIBLE SATELLITE IMAGERY THIS AFTERNOON SHOWS WIDESPREAD
SHOWERS AND EXTENSIVE CLOUDINESS OVER THE AREA, BUT GAPS IN THE MID-
AND UPPER LEVEL CLOUDS SHOW EVIDENCE OF A BROAD CIRCULATION AT THE
SURFACE, WITH CONVECTION BUILDING ALONG THE WESTERN AND NORTHERN
FLANKS OF THE SYSTEM. SCATTEROMETRY THIS MORNING REVEALED A BROAD,
WEAKLY-ORGANIZED CIRCULATION CENTER, WITH MAXIMUM SUSTAINED WINDS
IN THE RANGE OF 15 TO 20 KNOTS. DETERMINISTIC AND ENSEMBLE MODEL
GUIDANCE FAVORS A NORTH-NORTHWEST TRACK OF THE SYSTEM OVER THE
PHILIPPINE SEA, WITH GRADUAL INTENSIFICATION INTO A BROAD MONSOON
DEPRESSION OVER THE NEXT 48 HOURS. FOR MORE INFORMATION ON INVEST
98W, SEE BULLETINS ISSUED BY JTWC UNDER WMO HEADER ABPW10 PGTW.
EASTERN MICRONESIA
AN ACTIVE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) REMAINS OVER
EASTERN MICRONESIA. AS A SERIES OF TRADE-WIND TROUGHS MOVE THROUGH
THE REGION, THEY WILL CONTINUE TO INTERACT WITH THE ITCZ TO MAINTAIN
AN UNSETTLED PATTERN OVER THE MARSHALLS THROUGH TUESDAY NIGHT, AT
POHNPEI THROUGH TUESDAY AND AT KOSRAE TONIGHT. ANOTHER BAND OF
CONVERGENCE IS EVIDENT ON THE LATEST SURFACE ANALYSIS OVER KOSRAE,
EXTENDING SOUTHEAST. AS ONE OF THE TROUGHS APPROACHES KOSRAE, THE
ITCZ AND THE OTHER BAND OF CONVERGENCE WILL INTERACT WITH THE TROUGH
TO BRING WIDESPREAD SHOWERS AND ISOLATED THUNDERSTORMS TO KOSRAE
THROUGH THE NIGHT. BOTH THE TROUGH AND THE SECOND BAND OF CONVERGENCE
LOOK TO MOVE WEST WHILE THE ITCZ WEAKENS EASTWARD AND DRIFTS NORTH,
ALLOWING A MUCH DRIER TRADE-WIND PATTERN TO MOVE INTO KOSRAE BY
TUESDAY MORNING, RESULTING IN ISOLATED SHOWERS AT KOSRAE. FOR
POHNPEI, THE ITCZ AND A WEAK TROUGH WILL MAINTAIN SCATTERED SHOWERS
AND ISOLATED THUNDERSTORMS THERE THROUGH TUESDAY AFTERNOON. THE
TROUGH WILL BE WEST OF POHNPEI BY TUESDAY EVENING WHILE THE ITCZ
WEAKENS EASTWARD. FOR MAJURO, THE ITCZ WILL REMAIN OVER THE ATOLL A
BIT LONGER, MAINTAINING CONVECTION THERE THROUGH TUESDAY NIGHT. A
DRY TRADE-WIND PATTERN THEN LOOKS TO CONTINUE ACROSS THE REGION
THROUGH THE END OF THE WEEK. BY THURSDAY, ANOTHER ITCZ LOOKS TO
DEVELOP OVER THE MARSHALLS, BRINGING INCREASED CONVECTION TO MAJURO
FOR THE END OF THE WEEK INTO THE WEEKEND. THIS NEW ITCZ LOOKS TO
SPREAD WEST INTO KOSRAE AND POHNPEI OVER THE WEEKEND.
NO SIGNIFICANT CHANGES MADE TO THE MARINE FORECAST. MOSTLY LIGHT TO
GENTLE WINDS ARE EXPECTED INTO THE WEEKEND WITH COMBINED SEAS SLOWLY
SUBSIDING. SEAS ARE EXPECTED TO DROP TO BETWEEN 3 AND 4 FEET FOR
POHNPEI AND KOSRAE AND BETWEEN 4 AND 5 FEET FOR MAJURO.
WESTERN MICRONESIA
THE MONSOON TROUGH AND INVEST 97W CONTINUE TO PULL FURTHER NORTH,
WITH PALAU AND YAP LOCATED WITHIN THE MOIST WEST TO SOUTHWEST FLOW
INTO THE SYSTEM. SATELLITE SHOWS WIDESPREAD SHOWERS AND CLOUDINESS
ACROSS THE MONSOON TROUGH OBSCURING 97W, WHICH IS NOW LOCATED NEAR
14.6N 133.3E AS OF 06Z, ROUGHLY 480 MILES NORTHWEST OF YAP.
SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS ARE SEEN DEVELOPING
NEAR PALAU AND YAP ALONG AN AREA OF WEAK CONVERGENCE. SCATTERED
SHOWERS IN THE AREA WILL INCREASE OVERNIGHT, PARTICULARLY OVER PALAU
AS INVEST 97W PULLS FURTHER NORTH WITH A MONSOON TAIL DRAGGING
OVERHEAD, BUT THE LOCATION OF SHOWERS WILL VARY SOMEWHAT DEPENDING
ON THE SYSTEM'S MOTION AND DEVELOPMENT. SHOWERS COULD BECOME NUMEROUS
FOR PALAU LATE OVERNIGHT INTO EARLY TUESDAY MORNING, WITH LOCALLY
HEAVY RAINFALL POSSIBLE. CONDITIONS WILL GRADUALLY DRY OUT BY MIDWEEK
AS THE MONSOON TROUGH AND 97W MOVE FURTHER AWAY FROM THE AREA.
HOWEVER, YAP MAY SEE A RESURGENCE IN SHOWERS DURING THE LATTER HALF
OF THE WEEK AS A POTENTIAL TROPICAL CYCLONE DEVELOPS NEAR THE
MARIANAS, WHICH MAY PLACE IT IN A REGION OF MOIST CONVERGENT INFLOW.
TO THE EAST, CHUUK IS LOCATED ALONG THE WESTERN PERIPHERY OF A NEWLY
DESIGNATED DISTURBANCE, JTWC'S INVEST 98W. INVEST 98W, CURRENTLY
LOCATED NEAR 8.5N 154.0E, IS POORLY ORGANIZED, BUT WILL MAINTAIN
NUMEROUS LOCALLY HEAVY SHOWERS AND ISOLATED THUNDERSTORMS OVER THE
AREA AS IT MEANDERS NORTHWEST OF CHUUK OVER THE NEXT DAY OR TWO.
DETERMINISTIC AND ENSEMBLE MODELS REMAIN IN STRONG AGREEMENT THAT
INVEST 98W WILL CONTINUE TO DEVELOP NEAR THE MARIANAS LATER IN THE
WEEK, AND INFLOW INTO THE SYSTEM WILL MAINTAIN AT LEAST SCATTERED
SHOWERS FOR CHUUK INTO LATE WEEK.
WINDS FOR PALAU AND YAP WILL REMAIN LIGHT TO MODERATE OUT OF THE
WEST TO SOUTHWEST OVER THE NEXT FEW DAYS, BACKING TO THE SOUTH AROUND
MIDWEEK AS INVEST 97W PULLS FURTHER TO THE NORTHWEST, DEVELOPING
INTO A BROAD CYCLONIC CIRCULATION OVER THE PHILIPPINE SEA. ALTIMETRY
DATA SHOWS COMBINED SEAS OF AROUND 3 TO 5 FEET ACROSS THE REGION.
NEAR PALAU AND YAP, SEAS ARE COMPRISED OF A WEST TO SOUTHWEST MONSOON
SWELL AND A WEAKER EASTERLY TRADE SWELL. TRADE SWELL LOOKS TO
DIMINISH SLIGHTLY AROUND MIDWEEK AS A SOUTHWEST MONSOON SWELL
PERSISTS. A SECONDARY WEST TO NORTHWEST SWELL LOOKS TO ENTER THE
REGION LATE IN THE WEEK AS INVEST 97W DEVELOPS FAR NORTH TO
NORTHWEST OF THE REGION. BY THE WEEKEND, AN EAST TO SOUTHEAST TRADE
SWELL IS EXPECTED TO TAKE OVER AS THE MONSOON SWELL DIMINISHES.
FOR CHUUK, WINDS WILL BE LIGHT TO MODERATE AND VARIABLE IN DIRECTION
OVER THE NEXT FEW DAYS AS INVEST 98W CROSSES THE AREA, BRINGING
NUMEROUS SHOWERS AND A MODERATE RISK OF LIGHTNING. WINDS WILL VEER
SOUTHERLY BY MIDWEEK AS 98W DEVELOPS NORTHWEST OF THE AREA, BUT ARE
EXPECTED TO REMAIN LIGHT TO GENTLE. AN EAST TO NORTHEAST TRADE SWELL
WILL REMAIN DOMINANT THROUGHOUT THE FORECAST PERIOD.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS: WILLIAMS
EAST MICRONESIA: KLEESCHULTE
WEST MICRONESIA: DECOU
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