326
FXPQ50 PGUM 180757
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
557 PM CHST THU SEP 18 2025
MARIANAS SYNOPSIS
SATELLITE AND RADAR IMAGERY SHOW CLOUDY SKIES AND SCATTERED SHOWERS
WITHIN THE MARIANAS. ALTIMETRY DATA AND RITIDIAN AND IPAN BUOYS
SHOW COMBINED SEAS OF 3 TO 4 FEET.
DISCUSSION
PASSING TRADE-WIND TROUGHS AND OUTFLOW BOUNDARIES WILL BRING PERIODS
OF SCATTERED SHOWERS TO THE REGION OVER THE NEXT FEW DAYS. DURING
THE LULLS BETWEEN BOUNDARIES AND TROUGHS, WINDS MAY DIMINISH ENOUGH
TO ALLOW ISLAND EFFECT CONVECTION TO OCCUR. OVERALL, A GENERALLY WET
PATTERN IS ANTICIPATED FOR THE MARIANAS THROUGH EARLY NEXT WEEK.
MARINE
COMBINED SEAS ARE EXPECTED TO REMAIN 3 TO 4 FEET THROUGH TUESDAY.
GENTLE TO MODERATE NORTHEAST TO EAST WINDS WILL CONTINUE THROUGH
SATURDAY, THEN WINDS WILL DIMINISH TO LIGHT TO GENTLE THROUGH
TUESDAY. THERE WILL BE A LOW RISK OF RIP CURRENTS ALONG ALL REEFS
OF THE MARIANAS THROUGH WEDNESDAY. THERE WILL ALSO BE A MODERATE RISK
OF LIGHTNING THROUGH AT LEAST FRIDAY NIGHT. A WEST SWELL COULD ARRIVE
MONDAY FROM A DEVELOPING TROPICAL DISTURBANCE (INVEST 90W) THAT WILL
BE FAR TO THE WEST BY THEN.
TROPICAL SYSTEMS
THERE ARE TWO SIGNIFICANT DISTURBANCES ACROSS THE REGION. INVEST 90W,
LOCATED WELL NORTH OF PALAU, AND INVEST 91W, LOCATED NORTHWEST OF
WAKE ISLAND.
INVEST 91W IS NOW LOCATED NEAR 24N164E, MOVING WEST AT 12 MPH. 91W
HAS BECOME MORE ORGANIZED OVER THE PAST FEW HOURS, WITH THE LATEST
ASCAT ANALYSIS SHOWING A CLOSED, WELL-DEFINED LOW-LEVEL CIRCULATION
CENTER (LLCC), WITH WINDS OF UP TO 23 KNOTS TO THE NORTHEAST OF THE
CENTER. 91W IS NOW THE SUBJECT OF A TROPICAL CYCLONE FORMATION ALERT
(TCFA) FROM THE JOINT TYPHOON WARNING CENTER (JTWC). THIS MEANS THAT
THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24
HOURS IS HIGH. 91W IS EXPECTED TO CONTINUE TO MOVE WEST, FARTHER
AWAY FROM WAKE ISLAND. INVEST 91W WILL BE MONITORED CLOSELY OVER THE
NEXT FEW DAYS. FOR MORE INFORMATION ON INVEST 91W, SEE BULLETINS
ISSUED BY THE JTWC UNDER WMO HEADERS WTPN21 PGTW AND ABPW10 PGTW.
INVEST 90W, CENTERED NEAR 15N134E, IS NOW THE SUBJECT OF A TCFA BY
THE JTWC, MEANING INVEST 90W IS LIKELY TO DEVELOP INTO A SIGNIFICANT
TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS. ANIMATED VISIBLE SATELLITE
IMAGERY NOW SHOWS A CLEAR LLCC OVER THE PHILIPPINE SEA, NORTH OF THE
MAIN CONVECTION. THE CENTER REMAINS RELATIVELY FREE OF CONVECTION AT
THIS TIME. WIDESPREAD SHOWERS AND SCATTERED THUNDERSTORMS ARE SEEN
MAINLY IN THE SOUTHERN SEMI-CIRCLE OF THE SYSTEM, EXTENDING SOUTHWARD
TO NEAR YAP PROPER, NORTHEAST OF PALAU. 90W IS LOCATED IN A
FAVORABLE AREA FOR DEVELOPMENT, WITH LOW VERTICAL WIND SHEAR AND
EQUATORWARD OUTFLOW. 90W LOOKS TO CONTINUE MOVING SLOWLY WEST-
NORTHWEST AS IT GRADUALLY CONSOLIDATES INTO AN EVENTUAL TROPICAL
CYCLONE OVER THE 24 HOURS. FOR MORE INFORMATION ON INVEST 90W, SEE
BULLETINS ISSUED BY THE JTWC UNDER WMO HEADER WTPN22 PGTW AND ABPW10
PGTW.
EASTERN MICRONESIA
THE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) REMAINS THE PRIMARY PLAYER
FOR EASTERN MICRONESIA OVER THE NEXT SEVERAL DAYS. MOST OF THE ITCZ
IS JUST PRODUCING ISOLATED SHOWERS AND A FEW THUNDERSTORMS. HOWEVER,
WHERE WEAK TRADE-WIND TROUGHS ARE INTERACTING WITH THE ITCZ,
SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS ARE FOUND. THIS INCLUDES
POHNPEI FOR THIS AFTERNOON AND TONIGHT. ADDITIONAL TROUGHS
APPROACHING THE ISLANDS AND ATOLLS WILL PRODUCE CONVECTIVE FLAIR UPS
FROM TIME TO TIME AS THEY PROGRESS THROUGH THE REGION. THE ITCZ IS
ALSO EXPECTED TO CONTINUE TO UNDULATE NORTH AND SOUTH. THIS WILL
RESULT IN PERIODS OF ISOLATED SHOWERS AS WELL. BY THE BEGINNING OF
NEXT WEEK, THE ITCZ WILL BEGIN TO WEAKEN AND FRAGMENT AS IT RETREATS
WEST, WITH A DRY PATTERN MOVING INTO POHNPEI BY TUESDAY NEXT WEEK,
THOUGH POPS (PROBABILITY OF PRECIPITATION) LOOK TO REMAIN IN THE
30-40% RANGE FOR KOSRAE AND MAJURO A BIT LONGER.
NO BIG CHANGES TO THE MARINE FORECAST, WITH LIGHT TO GENTLE WINDS AND
COMBINED SEAS OF 3 TO 4 FEET EXPECTED INTO NEXT WEEK.
WESTERN MICRONESIA
A FAIRLY QUIET RIDGING PATTERN CONTINUES OVER FAR WESTERN
MICRONESIA, BUT THE RIDGE AXIS HAS DIPPED SLIGHTLY FURTHER SOUTH OF
PALAU AND YAP. SATELLITE REVEALS AN AREA OF MODERATE CONVECTION JUST
WEST OF BABELDAOB AND KOROR WITH SOME LIGHTER SHOWERS JUST EAST OF
THE ISLANDS, A COMBINATION OF ISLAND ENHANCEMENT AND WEAKLY
CONVERGENT FLOW INTO INVEST 90W TO THE NORTH. ISOLATED TO SCATTERED
SHOWERS ARE ALSO SEEN NEAR YAP, WHICH SITS ON THE FAR SOUTHERN
PERIPHERY OF CONVERGENT INFLOW INTO 90W. TO THE EAST, CHUUK REMAINS
IN A GENERALLY WET AND DISORGANIZED PATTERN, BUT SHOWERS HAVE
SLIGHTLY DIMINISHED THIS EVENING. ABUNDANT CLOUD COVER REMAINS ACROSS
MUCH OF CHUUK STATE, AND NEW CONVECTIVE TOWERS ARE SEEN NEAR A
DEVELOPING DISTURBANCE TO THE EAST-SOUTHEAST OF WENO. SCATTEROMETRY
AND VISIBLE SATELLITE REVEAL DISORGANIZED ROTATION JUST EAST OF
NAMOLUK AND LUKUNOCH, AND CIMSS SATELLITE ANALYSIS SHOWS A VORTICITY
MAXIMUM HERE FROM 850 TO 500 MB.
PALAU AND YAP WILL SEE DRIER CONDITIONS OVER THE NEXT FEW DAYS AS
INVEST 90W SHIFTS FURTHER AWAY TO THE NORTHWEST AND LOW-LEVEL RIDGING
TAKES OVER. LIGHT, GENERALLY SOUTH WINDS AND MOSTLY SUNNY SKIES
SHOULD SUPPORT ISLAND EFFECT CONVECTION OVER PALAU TOMORROW
AFTERNOON, ASSUMING THE CLOUD DECK REMAINS FAIRLY THIN IN THE MID
AND UPPER LEVELS. A DEVELOPING DISTURBANCE OVER CHUUK STATE WILL
GRADUALLY SHIFT WEST INTO THE AREA THROUGH THE WEEKEND, LEADING TO
INCREASINGLY WET CONDITIONS FOR EARLY NEXT WEEK. CHUUK WILL SEE AT
LEAST SCATTERED SHOWERS THROUGHOUT THE FORECAST PERIOD AS THE
AFOREMENTIONED DISTURBANCE TO THE EAST MEANDERS WEST ACROSS THE
REGION OVER THE NEXT DAY OR TWO, FOLLOWED BY ANOTHER POTENTIAL
DISTURBANCE FORMING NEARBY EARLY NEXT WEEK.
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 TWO THIS WEEKEND THROUGH EARLY
NEXT WEEK, AS A NORTHWEST TO WEST SWELL MOVES INTO THE REGION,
EMANATING FROM 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. LIGHT TO GENTLE WINDS ARE EXPECTED ACROSS
THE REGION FOR MUCH OF THE FORECAST PERIOD, BECOMING MODERATE AT
TIMES FOR CHUUK OVER THE NEXT DAY OR TWO.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS: BOWSHER
EAST MICRONESIA: KLEESCHULTE
WEST MICRONESIA: DECOU
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