771
FXPQ50 PGUM 191906
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
506 AM CHST MON OCT 20 2025
MARIANAS UPDATE
SATELLITE EARLY THIS MORNING SHOWS AN AREA OF NUMEROUS HEAVY SHOWERS
AND ISOLATED TO SCATTERED THUNDERSTORMS PUSHING WESTWARD INTO
MARIANAS COASTAL WATERS, ASSOCIATED WITH CONVERGENCE ALONG A BROAD
SURFACE TROUGH CENTERED NEAR THE ISLANDS. THE TROPICAL UPPER-
TROPOSPHERIC TROUGH (TUTT) REMAINS JUST NORTH OF THE CNMI, AND IS
ENHANCING THUNDERSTORMS ALONG THE TROUGH AT THE SURFACE, WITH THE
BEST SUPPORT FOCUSED JUST EAST AND NORTHEAST OF THE ISLANDS FOR NOW.
THE TUTT WILL CONTINUE TO SHIFT WESTWARD THROUGH TONIGHT, EXITING THE
AREA TUESDAY, WITH DIMINISHING UPPER-LEVEL SUPPORT AND ASSOCIATED
THUNDERSTORM CHANCES INTO MIDWEEK. FOR TODAY, SHOWERS WERE INCREASED
FROM SCATTERED TO NUMEROUS IN THE FORECAST, WITH WORDING FOR LOCALLY
HEAVY SHOWERS INCLUDED. ADDITIONALLY, GUSTS TO 30 MPH WERE ADDED
WITH HEAVIER SHOWERS. SHOWERS WILL DIMINISH OVERNIGHT AS THE TROUGH
SHIFTS WEST OF THE ISLANDS, AND PROBABILITY OF PRECIPITATION (POP)
WAS LEFT AT 40 PERCENT TONIGHT. IN THE MARINE FORECAST, COMBINED SEAS
AND WIND WAVES WERE BUMPED UP BY A FOOT WITH AN EXPECTATION FOR
FREQUENT HIGHER GUSTS AND CHOPPIER SEAS TODAY AS THE TROUGH AND
HEAVIER SHOWERS MOVE THROUGH.
EASTERN MICRONESIA UPDATE
SHOWERS HAVE BEEN ON THE INCREASE THIS MORNING NEAR POHNPEI AND
KOSRAE, ALONG THE SOUTHERN EXTENT OF A BROAD, SHALLOW TROUGH
EXTENDING FAR NORTH OF THE REGION. SHOWERS WERE INCREASED TO
SCATTERED TODAY OVER POHNPEI AND KOSRAE WATERS, WITH OVERNIGHT
SATELLITE OBSERVATIONS AND LATEST MODEL GUIDANCE IN MIND. POPS WERE
ALSO INCREASED FOR POHNPEI TONIGHT, AS SHOWERS LOOK TO INCREASE ALONG
THE SHALLOW TROUGH, CURRENTLY LOCATED BETWEEN KOSRAE AND POHNPEI, AS
IT PROPAGATES WEST THROUGH THE AREA. ADDITIONALLY, SEAS AND SWELLS
WERE REDUCED SLIGHTLY, TO BETTER ALIGN WITH LATEST ALTIMETRY AND BUOY
DATA. COMBINED SEAS ARE AROUND 3 TO 5 FEET FOR POHNPEI AND KOSRAE,
AND AROUND 4 TO 6 FEET NEAR MAJURO.
WESTERN MICRONESIA UPDATE
VERY LITTLE CHANGE WAS MADE IN THE FORECAST. SATELLITE EARLY THIS
MORNING SHOWS SHOWERS AND THUNDERSTORMS HAVE INCREASED WITHIN THE
AREA OF CONVERGENCE TO THE SOUTH AND WEST OF YAP, BUT THESE ARE
EXPECTED TO DIMINISH AND REMAIN OUTSIDE OF COASTAL WATERS TODAY.
SHOWERS WERE INCREASED SLIGHTLY TO SCATTERED FOR PALAU'S FORECAST
TONIGHT, AS THE AFOREMENTIONED CONVERGENCE LOOKS TO SHIFT SLIGHTLY
WEST-SOUTHWEST INTO THE AREA, INCREASING SHOWER COVERAGE NEARBY. FAR
TO THE EAST, SHOWERS ARE INCREASING TO THE SOUTH OF CHUUK WITHIN A
SURFACE TROUGH OVER THE AREA. SHOWERS WERE INCREASED SLIGHTLY IN
CHUUK'S FORECAST FOR TONIGHT AS CONVERGENCE BUILDS OVERHEAD ALONG THE
NEARBY TROUGH, BUT KEPT OVERALL COVERAGE AT SCATTERED.
PREV DISCUSSION
/ISSUED 557 PM CHST SUN OCT 19 2025/
MARIANAS SYNOPSIS...
SATELLITE IMAGERY SHOWS GUAM AND ROTA HAVE PARTLY CLOUDY SKIES WHILE
TINIAN AND SAIPAN HAS MOSTLY CLOUDY SKIES. RADAR IMAGERY SHOWS
ISOLATED TO SHOWERS, HOWEVER, SHOWERS LOOK TO BE INCREASING IN
COVERAGE OFF TO THE EAST. ALTIMETRY SHOWS 6 TO 7 FOOT COMBINED SEAS
AND BUOY DATA SHOWS ABOUT 6 FOOT SEA HEIGHTS.
DISCUSSION...
A TROPICAL UPPER-LEVEL TROPOSPHERIC TROUGH (TUTT) CELL IS PRODUCING
SMALL CELLULAR SHOWERS THAT ARE EXPECTED TO MOVE OVER THE MARIANAS
OVER THE NEXT SEVERAL DAYS. THIS WILL BE FURTHER ENHANCED BY A
TRADE-WIND TROUGH AS THIS FEATURE MOVES OVER THE ISLANDS. TO REFLECT
THIS, THE PROBABILITY OF PRECIPITATION WERE INCREASED TO MID TO HIGH
END FOR SCATTERED SHOWERS (40 TO 50 PERCENT ), WITH ISOLATED
THUNDERSTORMS POSSIBLE THROUGH AT LEAST THURSDAY. THE TUTT AND
TRADE-WIND TROUGH ARE EXPECTED TO MOVE WEST OF THE ISLANDS BY THE END
OF THE WEEK. ONCE THESE FEATURES PASS, A DRIER PATTERN IS
ANTICIPATED TO RETURN THE THE MARIANAS.
MARINE...
THE PRESSURE GRADIENT BETWEEN THE TROPICS AND THE MID-LATITUDES IS
EXPECTED HOLD FOR JUST A LITTLE BIT LONGER. THIS GRADIENT IS
PRODUCING MODERATE TO FRESH TRADE WINDS OVER THE REGION. THIS
GRADIENT IS EXPECT TO WEAKEN AROUND MONDAY NIGHT INTO TUESDAY
MORNING. AS THE GRADIENT WEAKENS, SO TO WILL THE WINDS. COMBINED
SEAS ARE EXPECTED TO BE 5 TO 7 FEET THROUGH MONDAY TONIGHT, THEN
GRADUALLY THE NORTH SWELL IS EXPECTED TO WEAKEN OVER THE WEEK
ALLOWING FOR THE SEAS TO FALL TO 3 TO 5 FEET AROUND THE WEEKEND.
THERE WILL BE A MODERATE RISK OF RIP CURRENTS MOST OF THIS WEEK ALONG
NORTH AND EAST FACING REEFS OF THE MARIANAS. THERE IS A MODERATE
RISK OF LIGHTNING THROUGH FRIDAY. THE NORTH SWELL IS EXPECTED TO
START DIMINISHING AROUND THE MIDDLE OF THE WEEK, WHEN THIS HAPPENS,
SO TO WILL THE SURF HEIGHTS. THE SURF MAY FALL BY A FOOT OR TWO BY
THE WEEKEND.
EASTERN MICRONESIA...
A SURFACE TROUGH CROSSING POHNPEI MAY BRING SOME INCREASED RAINFALL
TO POHNPEI TOMORROW, BUT FOR NOW IT LOOKS LIKE JUST ISOLATED SHOWER
COVERAGE WILL BE THE MOST WE CAN HOPE FOR. OTHERWISE, ATTENTION TURNS
TO A TROUGH EAST OF MAJURO WITH SOME SEMBLANCE OF A SUBTLE TROUGH
DEVELOPING, DENOTED BY AN AREA OF ENHANCED CONVERGENCE, WITH ANOTHER
PAIR OF FEATURES EAST OF THE DATE LINE. THESE FEATURES MAY BRING
INCREASING PERIODS OF RAINFALL FROM EAST TO WEST ACROSS THE REGION,
BUT WE QUESTION IF THE MODELS MAY BE A LITTLE AGGRESSIVE FOR
RAINFALL PROBABILITIES IN THE DAY 3 THROUGH 7 (OR ROUGHLY THE LATTER
HALF OF THIS WEEK). FOR NOW, WE OPTED TO GO WITH THE GUIDANCE, BUT
THIS WILL CONTINUE TO BE MONITORED. OTHERWISE, MAINLY GENTLE TO
MODERATE WINDS WITH SEAS OF 4 TO 6 FEET ARE EXPECTED THIS WEEK.
WESTERN MICRONESIA...
SATELLITE IMAGERY THIS EVENING SHOWS ISOLATED SHOWERS AROUND YAP AND
CHUUK, WHILE A WEAK TROUGH MOVES WESTWARD THROUGH THE OUTER ISLANDS
OF YAP STATE. OVER BY PALAU, SCATTERED SHOWER AND ISOLATED
THUNDERSTORMS ARE BEING SUPPORTED BY A TROUGH THAT EXTENDS FROM SOUTH
OF PALAU AND NORTH-NORTHWEST, PASSING WEST OF YAP PROPER, AND ENDING
NEAR 13N131E. AS THE TROUGH NEAR PALAU SHIFTS WESTWARD, SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS WILL DECREASE AROUND MIDNIGHT AS
THE BLENDED TPW SATELLITE PRODUCTS SHOWS PRECIPITABLE WATER (PW)
VALUES AROUND 2.1 TO 2.4 INCHES CURRENTLY ACROSS PALAU AND YAP, BUT
DRIER AIR WITH VALUES AROUND 1.8 TO 2.1 INCHES IS STARTING TO PUSH
NORTHWARD TOWARDS PALAU. THE TROUGH IN YAP STATE WILL START TO
INCREASE SHOWERS AT YAP LATER TONIGHT AS IT INTERACTS WITH THE
MOISTURE CURRENTLY AROUND YAP, BUT DRIER AIR IS ALSO EXPECTED TO MOVE
INTO YAP LATER TONIGHT, WHICH SHOULD HELP TO LIMIT THE POTENTIAL FOR
THUNDERSTORMS. SHOWERS AT CHUUK ARE EXPECTED TO REMAIN ISOLATED
TONIGHT, BUT A SUBTLE TRADE-WIND TROUGH NEAR POHNPEI, WITH PW VALUES
AROUND 2.2 INCHES, WILL APPROACH CHUUK STARTING MONDAY, INCREASING
THE POTENTIAL FOR SHOWERS AND THUNDERSTORMS OVER AT LEAST THE NEXT
COUPLE OF DAYS.
SUBTLE TRADE-WIND TROUGHS WILL KEEP PERIODS OF SCATTERED SHOWERS AT
ALL THREE FORECAST PERIODS INTO THE MIDDLE OF THE WEEK, WITH EAST TO
SOUTHEAST WINDS REMAINING ON THE LIGHT TO GENTLE SIDE. AROUND
WEDNESDAY, MODELS SHOW A SLIGHTLY MORE NOTABLE TRADE-WIND TROUGH
MOVING INTO THE REGION, FIRST PUSHING THROUGH CHUUK AND INCREASING
SHOWERS AND THE WINDS TO MODERATE TO FRESH, AND THEN INTO YAP AND
PALAU NEAR THE END OF THE WEEK, INCREASING SHOWERS AND WINDS AS WELL.
THE GFS SHOWS A WEAK GUSTY WIND SIGNATURE AS THE TROUGH NEAR PALAU,
SO INCREASED WINDS TO MODERATE AT YAP AND PALAU FOR FRIDAY, AND ADDED
GUSTS TO 25 KT FOR AT PALAU.
LATEST ALTIMETRY DATA CONTINUES TO INDICATE COMBINED SEAS 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 AND SEAS AROUND MIDWEEK, AS TRADE WINDS AND ASSOCIATED SWELL
BEGIN TO PICK UP, ALONG WITH A VERY SUBTLE UPTICK IN THE NORTHERLY
SWELL. WINDS WILL REMAIN LIGHT TO GENTLE FOR PALAU AND YAP THROUGH
MOST OF WEEK, BUT ARE EXPECTED TO INCREASE SLIGHTLY AROUND FRIDAY.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/EAST AND WEST MICRONESIA UPDATE: DECOU
MARIANAS: BOWSHER
EAST MICRONESIA: DOLL
WEST MICRONESIA: SCHANK
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