245
FXPQ50 PGUM 201957
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
557 AM CHST TUE OCT 21 2025
MARIANAS UPDATE
SCATTERED SHOWERS CONTINUED OVER TINIAN AND SAIPAN WATERS OVERNIGHT
AS A SURFACE TROUGH MOVED FURTHER WEST-NORTHWEST OF THE ISLANDS.
SATELLITE AND RADAR EARLY THIS MORNING SHOW SCATTERED TO NUMEROUS
SHOWERS AND ISOLATED THUNDERSTORMS NOW FOCUSED TO THE WEST OF
MARIANAS COASTAL WATERS. UPPER-LEVEL DIVERGENCE ASSOCIATED WITH THE
NEARBY TROPICAL UPPER-TROPOSPHERIC TROUGH (TUTT) LOOKS TO BE
MAINTAINING SUPPORT FOR DEEP CONVECTION JUST WEST OF THE AREA,
ACCORDING TO MODEL WINDS AND SATELLITE ANALYSIS. DESPITE MOST OF THE
CONVECTION STAYING FURTHER WEST, HEAVY SHOWERS DID PRODUCE SOME
STRONG GUSTS FOR SAIPAN EARLY THIS MORNING, WITH SURFACE OBSERVATIONS
SHOWING GUSTS AROUND 25 TO 35 MPH. FOR TODAY, INCREASED SHOWERS TO
SCATTERED FOR TINIAN AND SAIPAN, WHILE MAINTAINING ISOLATED SHOWERS
FOR GUAM AND ROTA. FOR TONIGHT, PROBABILITY OF PRECIPITATION (POP)
WAS INCREASED TO 40 PERCENT ACROSS THE MARIANAS AS A SHALLOW TROUGH
LOOKS TO CROSS THE AREA. LITTLE CHANGE WAS MADE TO THE MARINE
FORECAST. NORTH SWELL AND TRADE SWELL LOOK TO GRADUALLY DIMINISH
THROUGH MIDWEEK, ALLOWING COMBINED SEAS TO FALL 1 TO 2 FEET DURING
THE LATTER HALF OF THE WEEK.
EASTERN MICRONESIA UPDATE
THE FORECAST REMAINS ON TRACK AND VERY LITTLE CHANGE WAS MADE.
SHOWERS AND THUNDERSTORMS HAVE BEEN ON THE INCREASE EARLY THIS
MORNING WITHIN A BROAD CONVERGENCE ZONE, MAINLY OVER AND JUST EAST OF
POHNPEI AND NORTH TO NORTHEAST OF KOSRAE. SHOWERS LOOK TO DROP OFF
SLIGHTLY FASTER FOR KOSRAE OVERNIGHT BUT CONTINUE FOR POHNPEI AS
CONVERGENCE AND SHALLOW TROUGHING SHIFT WESTWARD. POPS FOR KOSRAE
WERE REDUCED SLIGHTLY FOR TONIGHT, FROM 30 PERCENT TO 20 PERCENT.
WESTERN MICRONESIA UPDATE
LITTLE CHANGE WAS MADE. SCATTERED SHOWERS HAVE BEEN ON THE INCREASE
NEAR PALAU THIS MORNING, WITHIN THE BROAD NEAR-EQUATORIAL TROUGH
(NET) THAT SPANS MUCH OF WESTERN MICRONESIA. POPS WERE INCREASED
SLIGHTLY FOR PALAU FOR TODAY AND TONIGHT, FROM 30 PERCENT TO 40
PERCENT, AS CONVECTION WITHIN THE NET BUILDS WESTWARD WITH THE
NET ITSELF OVER THE NEXT DAY OR SO.
PREV DISCUSSION
/ISSUED 620 PM CHST MON OCT 20 2025/
MARIANAS SYNOPSIS...
SATELLITE IMAGERY SHOWS CLOUDY SKIES, WHILE RADAR SHOWS ISOLATED
SHOWERS ACROSS THE COASTAL WATERS. BUOY DATA SHOWS SEA HEIGHTS OF 5
TO 7 FEET. WINDS OF HAVE DIMINISHED TO GENTLE.
DISCUSSION...
SATELLITE AND RADAR IMAGERY SHOWS ONLY A FEW SHOWERS IN THE AREA,
HOWEVER, SHOWERS ARE EXPECTED TO PICK BACK UP AROUND MIDNIGHT DURING
THE DIURNAL MAXIMA TIME FRAME. SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS ARE EXPECTED. THEN, PLEASANT WEATHER IS EXPECT TO
MAKE A SHORT RETURN FOR ABOUT A DAY OR SO. THE NEXT WEATHER MAKER IS
A TROUGH LOCATED TO THE WEST OF WAKE ISLAND. THIS FEATURE IS EXPECTED
TO ARRIVE AROUND WEDNESDAY. THIS FEATURE IS EXPECTED TO BRING HIGH
END SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS TO THE REGION. THE
TROPICAL UPPER-TROPOSPHERIC TROUGH (TUTT) IS EXPECTED TO STILL BE
CLOSE ENOUGH TO LENDS SOME SUPPORT. ONCE THIS TROUGH PASSES,
THE REGULAR WET-SEASON TRADE-WIND PATTERN IS EXPECTED TO RETURN.
MARINE...
BUOY DATA AND LOCAL OBSERVATIONS SHOW COMBINED SEAS OF 5 TO 7
FEET. WINDS ARE GENTLE TO MODERATE. SHOWERS ARE ANTICIPATED TO
BUILD BACK UP TO SCATTERED WITH ISOLATED THUNDERSTORMS AROUND THE
DIURNAL MAXIMA TONIGHT. OVERALL, THE PRIMARY (EAST) AND
SECONDARY (NORTH) SWELLS ARE BOTH ANTICIPATED IN WEAKENING OVER
THE WEEK. THE EAST SWELL IS ANTICIPATED TO WEAKEN TO AROUND 4 TO 5
FEET AND THE NORTH SWELL SUBSIDING AROUND THE END OF THE WEEK.
THERE IS A MODERATE RISK OF RIP CURRENTS ALONG NORTH AND EAST FACING
REEFS AND THIS RISK IS EXPECTED TO LINGER THROUGH AT LEAST THE
MIDDLE OF THE WEEK. THEN, THE NORTH SWELL, TRADE SWELL, AND
ASSOCIATED SURF ARE EXPECTED TO DIMINISH FOR THE LATTER HALF OF THE
WEEK.
EASTERN MICRONESIA...
A MODEST TRADE-WIND REGIME IS EXPECTED TO BRING PATCHY SHOWERS AND
SOME THUNDERSTORMS TO POHNPEI AND KOSRAE THROUGH TUESDAY, AND TO
MAJURO AFTER MIDNIGHT TONIGHT. CONVECTION WILL TAPER DOWN WITHIN THE
ITCZ THEREAFTER, BRING ISOLATED TO LOW-END SCATTERED SHOWERS UNTIL
THE WEEKEND WHEN ANOTHER PULSE WITHIN THE ITCZ BRINGS BACK ON-AND-OFF
SHOWERS AND SOME THUNDERSTORMS FOR THE WEEKEND TO ALL THREE FORECAST
POINTS.
COMBINED SEAS OF 5 TO 6 FEET ARE EXPECTED TO INCREASE ANOTHER FOOT OR
TWO IN POHNPEI, KOSRAE AND MAJURO COASTAL WATERS IN THE SECOND HALF
OF THE WEEK, AS TRADE SWELL STRENGTHENS. THE LINGERING NORTHERLY
SWELL WILL CONTINUE TO DIMINISH TO ALLOW SURF TO DECREASE ALONG NORTH
FACING REEFS OF THE AFOREMENTIONED ISLANDS FOR MUCH OF THE WEEK.
WESTERN MICRONESIA...
A NEAR-EQUEATOR TROUGH (NET) EXTENDS FROM JUST SOUTH OF PALAU AND
INTO CHUUK STATE, GENERATING SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS ACROSS MOST OF YAP STATE AND INTO CHUUK. YAP PROPER IS
CURRENTLY FAR ENOUGH NORTH OF THE NET AXIS THAT SHOWERS ARE ISOLATED
THIS EVENING, WHILE PALAU IS ON THE WESTERN EDGE OF THE MAIN AREA OF
SCATTERED SHOWERS SO SHOWERS ARE ALSO CURRENTLY ISOLATED, BUT SHOWERS
ARE EXPECTED TO INCREASE TO SCATTERED LATER TONIGHT. WINDS ARE LIGHT
TO GENTLE ACROSS PALAU AND YAP DUE TO THE NET, AND BECOMING GENTLE
TO MODERATE AT CHUUK AS THE TRADE-WIND PATTERN ACROSS EASTERN
MICRONESIA PUSHES INTO THE REGION.
AS THE NET SHIFTS WESTWARD, SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS ARE EXPECTED TO PUSH INTO YAP PROPER TUESDAY AND
CONTINUE ACROSS PALAU, WHILE THE TRADE-WIND PATTERN NEAR CHUUK
EXTENDS WESTWARD, WITH SUBTLE TROUGHS IN THE FLOW, CONTINUING THE
OVERALL TREND OF SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS AT
CHUUK AND THEN AT PALAU AND YAP FOR AT LEAST THE NEXT FEW DAYS AND
POTENTIALLY THROUGH THE WEEKEND. MODELS, ESPECIALLY THE GFS, STILL
DEPICTING A NOTABLE TROUGH, FIRST MOVING THROUGH CHUUK AROUND
WEDNESDAY OR WEDNESDAY NIGHT. THIS WILL INCREASING SHOWERS TO
SCATTERED TO NUMEROUS SHOWERS, WITH THE CHANCE OF SHOWERS AROUND 50
TO 60 PERCENT, AND INCREASING WINDS TO 15 TO 20 KT. THE GFS IS
SHOWING A CONVECTIVE SIGNATURE SUGGESTING WIND GUSTS MAY REACH UP TO
30 KT IN HEAVIER SHOWERS, BUT WHAT LOCATIONS MAY EXPERIENCE THESE
GUSTS WILL DEPEND ON WHERE AND WHEN SHOWERS DEVELOP WITHIN THE TROUGH
AS IT MOVES THROUGH CHUUK STATE. THIS TROUGH WILL THEN CONTINUE
WESTWARD, INCREASES SHOWERS ACROSS YAP STATE BEFORE REACHING YAP AND
PALAU AROUND FRIDAY OR FRIDAY NIGHT AND PUSHING THE POTENTIAL FOR AT
LEAST SCATTERED SHOWERS UP TO 50 PERCENT. THE GFS STILL SHOWS THE
POTENTIAL GUSTY WINDS, MAINLY AROUND PALAU AND SOUTH OF YAP, BUT THE
SIGNATURE IS SLIGHTLY WEAKER AND AGAIN THE EXACT LOCATION WILL DEPEND
ON WHEN AND WHERE SHOWERS DEVELOP AS THE TROUGH PASSES THROUGH.
SEAS ARE STILL AROUND 3 TO 5 FEET NEAR PALAU AND YAP AND AROUND 4 TO
6 FEET NEAR CHUUK, COMPRISED OF A PRIMARY NORTHEAST SWELL. CHUUK
WILL SEE AN INCREASE IN WINDS AROUND WEDNESDAY OR WEDNESDAY NIGHT,
AS A TROUGH MOVES THROUGH, ALONG WITH A VERY SUBTLE UPTICK IN THE
NORTHERLY SWELL. THIS SHOULD INCREASE SEAS ABOUT A FOOT ACROSS CHUUK.
WINDS WILL REMAIN LIGHT TO GENTLE FOR PALAU AND YAP THROUGH MOST OF
WEEK, BUT ARE EXPECTED TO INCREASE AROUND FRIDAY AS A TROUGH MOVES
THROUGH THE AREA.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/EAST AND WEST MICRONESIA UPDATE: DECOU
MARIANAS: BOWSHER
EAST MICRONESIA: MONTVILA
WEST MICRONESIA: SCHANK
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