327
FXPQ50 PGUM 171953
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
553 AM CHST THU SEP 18 2025
MARIANAS UPDATE
THE MAIN SHIELD OF RAIN ASSOCIATED WITH INVEST AREA 90W CONTINUES TO
MOVE TOWARD THE WEST, AWAY FROM THE ISLANDS. OVERALL THIS WILL USHER
IN A DRIER PERIOD, EXPECTED TO LAST THROUGH TONIGHT. HOWEVER, THE
LAST HOUR HAS SEEN THE EMERGENCE OF A NEW AREA OF SHOWERS EAST OF
GUAM. COULD BE THE LAST HURRAH OF THE SHOWERY TIME WE HAVE BEEN IN.
THE BUOYS ARE READING AROUND 3 FEET, WITH THE VAST MAJORITY OF THE
ENERGY COMING FROM THE EAST AS TRADE-WIND SWELL. RITIDIAN BUOY SHOWS
THE BEGINNING OF A NASCENT WEST SWELL HOWEVER, WHICH IS EXPECTED TO
SLOWLY GROW OVER THE NEXT SEVERAL DAYS AT THE EXPENSE OF THE TRADE-
WIND SWELL, LEAVING OVERALL SEA HEIGHTS ESSENTIALLY UNCHANGED. THIS
COULD BE SUBJECT TO CHANGE DEPENDING ON WHAT ACTUALLY HAPPENS WITH
INVEST AREA 90W.
TROPICAL SYSTEMS
WE CONTINUE TO WATCH 2 SYSTEMS IN THE REGION, THOUGH NEITHER IS
EXPECTED TO DIRECTLY AFFECT ANY ISLANDS IN THE AOR. INVEST 90W, WEST
OF THE MARIANAS, IS CENTERED NEAR 15N135E, THOUGH THE LOW-LEVEL
CENTER IS DIFFICULT TO PINPOINT DUE TO ABUNDANT CLOUD COVER AND
LIMITED SCATTEROMETER OVERPASSES, ESPECIALLY THE ASCAT SENSORS. JTWC
HAS ELEVATED 90W TO A MED FOR DEVELOPMENT, MEANING THE DEVELOPMENT OF
A TC IS LIKELY BUT BEYOND 24 HR. MODELS CONTINUE TO SUPPORT
DEVELOPMENT WITH DETERMINISTIC AND ENSEMBLE GUIDANCE SHOWING STEADING
INTENSIFICATION AS IT TREKS NW THEN W OVER THE CENTRAL PHILIPPINE
SEA. IT WILL PROBABLY MOVE OUT OF MICRONESIA OVER THE WEEKEND, MOST
LIKELY BY MOVING WEST OF 130E.
INVEST 91W, NOW JUST NORTH OF WAKE ISLAND NEAR 23N167E, IS NEARLY-
STATIONARY, PERHAPS WITH A SLIGHT WEST DRIFT. JTWC HAS RATED 91W A
LOW FOR DEVELOPMENT MEANING THAT TC DEVELOPMENT IS UNLIKELY WITHIN
24 HR. COMPARED TO TYPICAL MODEL GUIDANCE ON POTENTIALLY-DEVELOPING
TCS, THE GFS ACTUALLY TAKES A BACK SEAT TO THE ECMWF, THE VARIOUS
ENSEMBLE GUIDANCE SYSTEMS AND EVEN SOME AI MODELS WHICH ALL CARRY
91W TO THE NW AND OUT OF THE GUAM AOR NORTHERN BOUNDARY OF 25N OVER
THE WEEKEND.
EASTERN MICRONESIA UPDATE
A RATHER INTENSE ITCZ (INTER TROPICAL CONVERGENCE ZONE) CONTINUES TO
AFFECT EASTERN MICRONESIA. AS A RESULT, BOTH POHNPEI AND KOSRAE ARE
LOOKING AT SCATTERED SHOWERS FOR THE ENTIRE FORECAST PERIOD, FROM
TODAY THROUGH MONDAY. A BREAK IN THE CONVECTION IS ALLOWING MAJURO
TO DRY OUT A BIT. THAT'S THE ONLY CHANGE IN THIS UPDATE, TOOK MAJURO
DOWN TO ISOLATED SHOWERS TODAY, WITH SCATTERED COMING BACK TONIGHT
THROUGH MONDAY.
WESTERN MICRONESIA UPDATE
CHUUK IS BEING AFFECTED BY THE WESTERN EDGE OF THE SAME ITCZ
AFFECTING EASTERN MICRONESIA, AND WILL THEREFORE ALSO HAVE SCATTERED
SHOWERS FROM TODAY THROUGH MONDAY. YAP IS BEING AFFECTED BY THE FAR
SOUTHERN EDGE OF 90W, AND WILL HAVE SCATTERED SHOWERS THROUGH TONIGHT
AS A RESULT. CONVERGING WINDS FLOWING INTO 90W HAVE STARTED TO CREATE
SOME SHOWERS JUST EAST OF KOROR PALAU, SO THAT WAS MY ONLY CHANGE TO
THE FORECAST, TO RAISE THEM FROM ISOLATED TO SCATTERED TODAY. NEAR
THE END OF THE PERIOD, THE MONSOON FLOW WILL START AFFECTING KOROR,
BRINGING IN MORE SHOWERS.
PREV DISCUSSION
/ISSUED 545 PM CHST WED SEP 17 2025/
MARIANAS SYNOPSIS...
AFTER A FAIRLY DRY MORNING FOR THE MARIANAS, SHOWERS TO THE EAST
HAVE STEADILY MOVED IN DURING THE AFTERNOON WITH THE ADVANCE OF
ANOTHER SURFACE TROUGH. CONTRARY TO YESTERDAY, THE HEAVIER RAINFALL
IS OVER THE CNMI. RECENT IR SATELLITE TRENDS SHOW A GRADUAL WARMING
OF CLOUD TOPS SIGNALING STRATIFICATION AND LESS INTENSE SHOWERS GOING
INTO THE EVENING HOURS. INVEST 90W CONTINUES TO SPIN WELL TO THE
WEST OF THE MARIANAS AND A NEW CIRCULATION, INVEST 91W, HAS NOW
LOGGED INTO THE REGION AND IS FOUND WELL NORTHEAST OF THE MARIANAS
NEAR WAKE ISLAND. BUOYS AND ALTIMETRY DATA SHOW SEAS OF 3-4 FT AROUND
THE MARIANAS. SEAS CONTINUE TO BUILD WEST OF 140E IN THE PROXIMITY
OF 90W WITH ALTIMETRY SHOWING 6-7 FT IN THAT AREA.
DISCUSSION...
WETTER WET SEASON WEATHER WILL CONTINUE ALONG TONIGHT AND INTO
THURSDAY BEFORE DIMINISHING SOME. AFTER DRIER CONDITIONS THIS
MORNING, THE ANTICIPATED SECOND WAVE OF MOISTURE ARRIVED THIS
AFTERNOON, FOCUSING MORE HEAVILY ON THE CNMI. ONCE THE TROUGH HAS
MOVED TO THE WEST, SHOWER COVERAGE WILL DECREASE OVER THE MARIANAS,
BUT A MOIST FLOW WILL PERSIST IN THE REGION. BLENDED TPW IMAGERY
SHOWS A RIVER OF MOISTURE FLOWING ALONG THE ITCZ ACROSS CENTRAL AND
EASTERN MICRONESIA THAT CURVES TO THE NW ACROSS THE MARIANAS AND INTO
90W. A SEASONABLY HUMID PATTERN WILL PERSIST TO CLOSE THE WEEK BEFORE
THE REGIONAL PATTERN BECOMES MORE UNSETTLED WITH A NEW DISTURBANCE
STILL MODELED TO EMERGE BETWEEN YAP AND CHUUK LATE WEEKEND AND EARLY
NEXT WEEK. DETERMINISTIC MODEL GUIDANCE SUGGESTS THIS COULD LEAD TO A
BUSIER MONSOON PATTERN FOR THE REGION TO CLOSE OUT THE MONTH OF
SEPTEMBER.
MARINE...
FEW CHANGES ARE EXPECTED ON THE WATERS WITH SEAS STAYING BETWEEN 3
AND 4 FT INTO THE WEEKEND. WINDS WILL BE LIGHT TO MODERATE WITH
OCCASIONAL CONVECTIVE-DRIVEN GUSTS OF 20-25 KT THROUGH TONIGHT. HAVE
INTRODUCED A SMALL WEST SWELL STARTING OVER THE WEEKEND AS WAVE
MODELS SHOW SMALL WESTERLY SWELLS REACHING THE MARIANAS FROM THE
DEVELOPING 90W.
TROPICAL SYSTEMS...
WE CONTINUE TO WATCH 2 SYSTEMS IN THE REGION, THOUGH NEITHER IS
EXPECTED TO DIRECTLY AFFECT ANY ISLANDS IN THE AOR. INVEST 90W, WEST
OF THE MARIANAS, IS CENTERED NEAR 14N137E, THOUGH THE LOW-LEVEL
CENTER IS DIFFICULT TO PINPOINT DUE TO ABUNDANT CLOUD COVER AND
LIMITED SCATTEROMETER OVERPASSES, ESPECIALLY THE ASCAT SENSORS.
JTWC HAS ELEVATED 90W TO A MED FOR DEVELOPMENT, MEANING THE
DEVELOPMENT OF A TC IS LIKELY BUT BEYOND 24 HR. MODELS CONTINUE TO
SUPPORT DEVELOPMENT WITH DETERMINISTIC AND ENSEMBLE GUIDANCE SHOWING
STEADING INTENSIFICATION AS IT TREKS NW THEN W OVER THE CENTRAL
PHILIPPINE SEA.
INVEST 91W, NOW JUST NORTH OF WAKE ISLAND NEAR 23N167E, IS NEARLY-
STATIONARY, PERHAPS WITH A SLIGHT WEST DRIFT. JTWC HAS RATED 91W A
LOW FOR DEVELOPMENT MEANING THAT TC DEVELOPMENT IS UNLIKELY WITHIN
24 HR. COMPARED TO TYPICAL MODEL GUIDANCE ON POTENTIALLY-DEVELOPING
TCS, THE GFS ACTUALLY TAKES A BACK SEAT TO THE ECMWF, THE VARIOUS
ENSEMBLE GUIDANCE SYSTEMS AND EVEN SOME AI MODELS WHICH ALL CARRY
91W TO THE NW AND OUT OF THE GUAM AOR NORTHERN BOUNDARY OF 25N.
EASTERN MICRONESIA...
A MODERATELY ACTIVE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) IS EVIDENT
ACROSS EASTERN MICRONESIA THIS AFTERNOON, STRETCHING WESTWARD FROM
NEAR 9N180 TO JUST NORTH OF MAJURO AND ACROSS KWAJALEIN TO NORTHWEST
OF POHNPEI. THE ITCZ LOOKS TO REMAIN ACTIVE THROUGH THE WEEKEND AND
INTO THE BEGINNING OF NEXT WEEK AS IT UNDULATES NORTH AND SOUTH OVER
THE NEXT SEVERAL DAYS. ALONG WITH THE ITCZ, WEAK TRADE-WIND TROUGHS
ARE EXPECTED TO TRAVERSE THE REGION. THIS COMBINATION WILL MAINTAIN
AN OVERALL UNSETTLED PATTERN ACROSS THE REGION THROUGH THE BEGINNING
OF NEXT WEEK.
OVERALL, MARINE CONDITIONS WILL REMAIN FAIRLY BENIGN, WITH COMBINED
SEAS MAINLY IN THE 3 TO 4 FOOT RANGE, WITH A BRIEF PERIOD OF AROUND 5
FOOT SEAS AT MAJURO FRIDAY INTO FRIDAY NIGHT. WINDS WILL BE MOSTLY
LIGHT, WITH OCCASIONAL GENTLE WINDS. FOR TONIGHT AND THURSDAY, GUSTS
OF 15 TO 20 KNOTS ARE POSSIBLE WITH SHOWERS AND THUNDERSTORMS AT ALL
THREE LOCATIONS.
WESTERN MICRONESIA...
A SOMEWHAT DRIER PATTERN PERSISTS OVER FAR WESTERN MICRONESIA WITH
A WEST-EAST ORIENTED RIDGE AXIS SITTING OVER PALAU, WHICH CURVES
NORTHEASTWARD TO NEAR YAP. HOWEVER, YAP SITS WITHIN AN AREA OF
WEAKLY CONVERGENT INFLOW INTO INVEST 90W, LOCATED TO THE NORTH-
NORTHWEST AT AROUND 14.2N 135.7E. SCATTERED SHOWERS ARE SEEN ACROSS
THE AREA, BECOMING NUMEROUS TO THE NORTH OF YAP STATE AND THE
REPUBLIC OF PALAU. MODEL GUIDANCE POINTS TO CONDITIONS DRYING OUT FOR
YAP LATE OVERNIGHT AS 90W MEANDERS A LITTLE FURTHER AWAY. PERIODS OF
SCATTERED SHOWERS WILL REMAIN POSSIBLE FOR YAP AND PALAU FOR THE NEXT
DAY OR SO ALONG THE FAR OUTER PERIPHERY OF INVEST 90W. BY THE END OF
THE WEEK, THE DRY RIDGING ENVIRONMENT SHOULD BECOME DOMINANT ONCE
MORE AS 90W TRACKS NORTH-NORTHWEST ACROSS THE PHILIPPINE SEA. PALAU
LOOKS TO SEE SOME AFTERNOON ISLAND SHOWERS ON FRIDAY AS WINDS ARE
ANTICIPATED TO BE LIGHT AND GENERALLY SOUTHERLY, WITH MOSTLY SUNNY
SKIES EXPECTED BY AROUND MIDDAY. ANY PERSISTENT CLOUD COVER WOULD
DIMINISH OR DELAY THE CHANCE FOR ISLAND CONVECTION, HOWEVER. SHOWER
CHANCES WILL INCREASE ONCE MORE LATE IN THE WEEKEND THROUGH EARLY
NEXT WEEK AS A DISTURBANCE BUILDS WEST INTO THE AREA FROM OVER YAP
AND CHUUK STATES.
TO THE EAST, CHUUK SITS WITHIN A WETTER PATTERN JUST OUTSIDE THE FAR
WESTERN PERIPHERY OF THE ITCZ. SCATTERED SHOWERS ARE EXPECTED
THROUGHOUT THE FORECAST PERIOD AS A SERIES OF EMBEDDED TROUGHS MOVE
WEST THROUGH THE AREA. ONE OR TWO MORE DISTURBANCES LOOK TO BUILD
ACROSS CHUUK STATE AROUND SUNDAY THROUGH EARLY NEXT WEEK, BUT THE
PATTERN IS MESSY FOR NOW WITHOUT MUCH MODEL AGREEMENT IN EXACT TIMING
OR PLACEMENT. NEVERTHELESS, SCATTERED SHOWERS ARE EXPECTED THROUGH
AT LEAST EARLY NEXT WEEK AS CHUUK REMAINS IN THIS REGION OF TROPICAL
DEVELOPMENT.
BENIGN MARINE CONDITIONS CONTINUE, WITH ALTIMETRY DATA INDICATING
COMBINED SEAS OF AROUND 1 TO 3 FEET NEAR PALAU AND 2 TO 4 FEET FOR
YAP AND CHUUK, COMPRISED OF A PRIMARY TRADE SWELL. FOR YAP AND
PALAU, SEAS MAY INCREASE BY A FOOT OR SO THIS WEEKEND THROUGH EARLY
NEXT WEEK, AS A NORTHWEST TO WEST SWELL MOVES INTO THE REGION,
EMANATING FROM THE DISTANT TROPICAL SYSTEM THAT IS CURRENTLY
DESIGNATED INVEST 90W, AND STEERED BY THE DEVELOPING MONSOON PATTERN.
CHUUK WILL CONTINUE TO FEEL A 3 TO 4 FOOT EASTERLY TRADE SWELL
THROUGH THE PERIOD.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/EAST AND WEST MICRONESIA/TROPICAL UPDATE: STANKO
MARIANAS: AYDLETT
EAST MICRONESIA: KLEESCHULTE
WEST MICRONESIA: DECOU
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