159
FXPQ50 PGUM 031219
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
1019 PM CHST SAT OCT 3 2026
UPDATE
ZONE, COASTAL AND SURF FORECASTS WERE UPDATED TO CANCEL THE TROPICAL
STORM WARNING FOR TINIAN AND SAIPAN AND TO ISSUE A WIND ADVISORY AND
GALE WARNING.
PREV DISCUSSION
/ISSUED 552 PM CHST SAT OCT 3 2026/
MARIANAS SYNOPSIS...
VERY STRONG TO GALE-FORCE SOUTHWESTERLIES AT TINIAN AND SAIPAN, AND
VERY FRESH TO NEAR-GALE AT GUAM AND ROTA, PREVAIL TONIGHT, LARGELY
INFLUENCED BY THE OUTGOING TROPICAL STORM CHOI-WAN (26W) AND
ENHANCED MONSOON SURGE FROM THE SOUTHWEST. SATELLITE IMAGERY AND
DOPPLER RADAR INDICATE SOME SHOWERS TINIAN AND SAIPAN WATERS. RECENT
ALTIMETRY PASSES AND SPOTTER BUOYS INDICATE COMBINED SEAS OF 13 TO 16
FEET ACROSS THE REGION.
DISCUSSION...
TYPHOON CHOI-WAN CONTINUES TO INTENSIFY, AND IS EXPECTED TO BECOME A
CATEGORY 3 TYPHOON TONIGHT AS IT CONTINUES MOVING AWAY FROM THE
MARIANAS AND PASSING NEAR MAUG TO THE NORTHEAST. A TYPHOON WARNING
REMAINS IN EFFECT FOR ALAMAGAN, PAGAN AND AGRIHAN. DUE TO THE
ENHANCEMENT OF THE MONSOON SURGE ALONG THE SOUTHERN PERIPHERIES OF
CHOI-WAN, TS CONDITIONS ARE STILL EXPECTED AT TINIAN, SAIPAN AND
THEIR ADJACENT WATERS IN THE EARLY EVENING HOURS TONIGHT, BUT THEN
BECOMING MORE LIKELY TO FALL BELOW TS CONDITIONS AS THE NIGHT GOES
ON. TROPICAL STORM FORCE WINDS OF 35 TO 45 MPH WITH GUSTS TO 60 MPH
ARE EXPECTED IN THE EARLY EVENING HOURS AT TINIAN AND SAIPAN, TO
THEN DROP BELOW SUSTAINED TS CONDITIONS LATER TONIGHT AS TYPHOON
CHOI-WAN (26W) CONTINUES TO MOVE AWAY FROM TINIAN AND SAIPAN TO THE
NORTH. HOWEVER, EVEN AS TYPHOON CHOI-WAN MOVES AWAY, TINIAN AND
SAIPAN WILL CONTINUE TO FEEL WINDY CONDITIONS INTO NEXT WEEK AS THE
SURGING MONSOON INFLOW SOUTH OF CHOI-WAN CONTINUES TO AFFECT THE
AREA. MEANWHILE AT GUAM AND ROTA, THE ENHANCED MONSOON IS ALREADY
BRINGING SUSTAINED 25 TO 35 MPH SOUTHWESTERLIES, WITH SHOWER AND
THUNDER COVERAGE BEING CONSIDERABLY LESS TONIGHT, TO THEN TAPER DOWN
FURTHER AS THE ATMOSPHERE STABILIZES OVER THE COMING DAYS.
LOOKING IN THE MID TO LONG TERM, AFTER CHOI-WAN'S EXIT FROM THE AREA,
SECONDARY DISTURBANCES ARE EXPECTED TO FORM WITHIN THE LEFTOVER
MONSOON TROUGH, AS THE LOCAL ENVIRONMENT WILL REMAIN CONDUCIVE FOR
IMPROVED CIRCULATION DEVELOPMENT. EVEN SO, HIGH UNCERTAINTY REMAINS
REGARDING WIND AND RAIN IMPACTS AT THIS TIME, BUT IT IS LIKELY FOR A
WETTER PERIOD TO RETURN ONCE AGAIN AROUND MIDWEEK. LAST BUT NOT
LEAST, A DRIER REGIME MAY DEVELOP TOWARD THE NEXT WEEKEND AS WINDS
DIMINISH AND A COL FORMS OVERHEAD.
MARINE...
VERY STRONG TO TROPICAL STORM-FORCE SOUTHWESTERLIES ARE STILL
EXPECTED AT TINIAN AND SAIPAN WATERS IN THE EARLY HOURS TONIGHT, AS
TYPHOON CHOI-WAN (26W) CONTINUES TO MOVE AWAY FROM THE REGION AND AN
ENHANCED MONSOON SURGE TAKES ITS PLACE. COMBINED SEAS OF 13 TO 16
FEET WILL PERSIST THROUGH SATURDAY AND THEN BEGIN TO GRADUALLY
DIMINISH, BUT ARE EXPECTED TO REMAIN ELEVATED AS THE SOUTHWEST
MONSOON SWELL REMAINS VERY SIGNIFICANT. FOR SAIPAN AND TINIAN, A
TROPICAL STORM WARNING REMAINS IN EFFECT. THESE PERSISTENT SWELLS AND
HEAVY MIXING BETWEEN THESE SWELLS, ALONG WITH SIGNIFICANTLY HIGHER
WIND WAVES, ARE LIKELY TO PRODUCE SOME MINOR COASTAL FLOODING ALONG
THE SHORES OF TINIAN AND SAIPAN, SO THE COASTAL FLOOD WATCH WAS
UPGRADED TO A STATEMENT.
FARTHER SOUTH, WINDS HAVE DIMINISHED ENOUGH AT GUAM AND ROTA WATERS
TO ALLOW THE GALE WARNING TO EXPIRE. HOWEVER, AN ELEVATED
SOUTHWESTERLY MONSOON SWELL WILL ALLOW HAZARDOUS SEAS TO PERSIST
THROUGH TONIGHT; AS SUCH, IT WAS REPLACED BY A HAZARDOUS SEAS WARNING
FOR SEAS 15 FEET AND GREATER. A HIGH RISK OF RIP CURRENTS AND
HAZARDOUS SURF IS EXPECTED TO PERSIST ALONG NORTH, SOUTH, AND WEST
FACING REEFS OF GUAM AND ROTA. BY SUNDAY NIGHT, SURF HEIGHTS WILL
DIMINISH ENOUGH FOR NORTH FACING REEFS TO BE EXCLUDED FROM SAID
HAZARDS.
TROPICAL SYSTEMS...
TYPHOON CHOI-WAN (26W) WAS CENTERED NEAR 19N146E NORTHEAST OF
AGRIHAN, MOVING NORTHEAST AT 7 MPH WITH MAXIMUM SUSTAINED WINDS OF
100 MPH. THIS MAKES CHOI-WAN A CATEGORY 2 TYPHOON. CHOI-WAN IS
FORECAST TO INTENSIFY FURTHER THROUGH SUNDAY, BECOMING A CATEGORY 3
TYPHOON TONIGHT. IT IS EXPECTED TO MAKE A SLIGHT TURN TOWARD THE
NORTH-NORTHEAST WITH A SLIGHT INCREASE IN FORWARD SPEED THROUGH
SUNDAY, MOVING AWAY FROM THE MARIANAS.
IN EASTERN MICRONESIA, JTWC'S INVEST 94W IS CENTERED NEAR 8N168E JUST
EAST OF KWAJALEIN. IT CONTINUES TO BE LARGELY DISORGANIZED, WITH
CONVECTION DEVELOPING MAINLY AROUND THE OUTER FRINGES OF THE
DISTURBANCE. CONDITIONS IN THE GENERAL VICINITY REMAIN FAVORABLE FOR
FURTHER ORGANIZATION, BUT THE AREAS OUTSIDE THE MARSHALLS REMAIN
HIGHLY UNFAVORABLE, WHICH MAY HINDER SOME TRAJECTORIES TO BETTER
ORGANIZE OVER THE COMING DAYS. 94W WILL LIKELY CONTINUE TO MEANDER IN
THE AREA BEFORE HEADING NORTHWARD TOWARD WAKE ISLAND BY MIDWEEK, TO
THEN EVENTUALLY FOLLOW HURRICANE NOLO ENTERING FROM THE CENTRAL
PACIFIC.
FOR MORE INFORMATION ON TROPICAL STORM CHOI-WAN, PLEASE REFER TO THE
LOCAL STATEMENT ISSUED BY THE NATIONAL WEATHER SERVICE UNDER WMO
HEADER WTPQ82 PGUM, THE TROPICAL CYCLONE PUBLIC ADVISORY ISSUED BY
THE NATIONAL WEATHER SERVICE UNDER WMO HEADER WTPQ32 PGUM, THE
TROPICAL CYCLONE WARNING ISSUED BY THE JOINT TYPHOON WARNING CENTER
UNDER WMO HEADER WTPN32 PGTW, AND THE SIGNIFICANT TROPICAL WEATHER
ADVISORY UNDER WMO HEADER ABPW10 PGTW.
HYDROLOGY...
OVER 2 INCHES OF RAIN HAS FALLEN AT THE GUAM INTERNATIONAL AIRPORT
SINCE MIDNIGHT TODAY, BRINGING THE STORM TOTAL TO OVER 4 INCHES SO
FAR, AND LIKELY HIGHER AT OTHER STATIONS ACROSS THE ISLANDS. SURGING
MONSOON INFLOW WILL MAINTAIN WET CONDITIONS THROUGH SUNDAY, LIKELY
RESULTING IN A TOTAL OF 3 TO 5 INCHES OF RAIN FOR GUAM AND ROTA, AND
4 TO 6 INCHES FOR TINIAN AND SAIPAN DURING THE WATCH PERIOD.
EASTERN MICRONESIA...
THE MAIN FOCUS CONTINUES TO BE ON THE GRADUAL EVOLUTION OF INVEST
94W, WHICH CAN BEEN SEEN NICELY ON THE KWAJALEIN RTS WEATHER STATION
DOPPLER RADAR LOOP. THE CIRCULATION ON VISIBLE IMAGERY ALSO SHOWS A
"GROWING CYCLONIC INFLUENCE" THAT'S STARTING TO AFFECT MUCH OF THE
RMI (REPUBLIC OF THE MARSHALL ISLANDS) AND SURROUNDING AREAS. IN
GENERAL THE ECMWF IS MORE AGGRESSIVE THAN THE GEFS IN DEVELOPMENT
TIMING AND STRENGTH, BUT IT, ALONG WITH SEVERAL AI-ASSISTED MODELS,
SUGGESTS 94W WILL CONTINUE TO REMAIN QUASI-STATIONARY FOR THE NEXT
36 TO 48 HOURS WHILE CONTINUING TO ORGANIZE. THEN, ABOUT 60 TO 72
HOURS FROM NOW (BY TUESDAY AFTERNOON) IT'LL LIFT OFF TO THE NORTH-
NORTHWEST BEFORE TURNING MORE NORTHWEST WHILE CONTINUING TO
STRENGTHEN. UNTIL IT DOES SO, MAJURO WILL CONTINUE TO SEE HIGH-END
CHANCE PROBABILITIES OF RAINFALL (50 PERCENT) THROUGH MONDAY NIGHT.
ALSO, AS THE CIRCULATION BECOMES BETTER DEFINED/STRONGER, A
SOUTHWEST INFLOW INTO THE DEVELOPING CIRCULATION WILL INCREASE
SURFACE CONVERGENCE, ALLOWING SCATTERED SHOWERS RETURN TO KOSRAE BY
SUNDAY NIGHT, REACHING POHNPEI MONDAY NIGHT. THIS INCREASED RAINFALL
SHOULD THEN HANG AROUND FOR A COUPLE OF DAYS BEFORE DRIER WEATHER
RETURNS.
AS FOR MARINE CONDITIONS, LIGHT TO GENTLE WINDS WILL BECOME GENTLE TO
MODERATE BY MONDAY AT POHNPEI AND MAJURO, WHILE REMAINING GENERALLY
IN THE GENTLE TO MODERATE RANGE AT KOSRAE. SEAS OF 4 TO 6 FEET MAY
INCREASE A FOOT TOWARDS THE MIDDLE OF THE WEEK, MAINLY AT POHNPEI AND
KOSRAE, DEPENDING HOW QUICKLY 94W DEVELOPS.
WESTERN MICRONESIA...
THE MAIN FOCUS CONTINUES TO BE THE WEST TO SOUTHWEST MONSOON FLOW AT
KOROR AND YAP. THIS MORNINGS WEATHER BALLOON (RAOB - RADIOSONDE
OBSERVATION) SHOWER 30 KNOTS AROUND 900 MB (~2000 FEET) AT YAP, AND
15 TO 20 KNOTS AT KOROR. HOWEVER, MIMIC WATER VAPOR IMAGERY SHOWS
PWATS (PRECIPITABLE WATER VALUES) AT BOTH LOCATIONS REMAIN UNDER 2.5
INCHES, WHILE SLOWLY DECREASING FROM SOUTH TO NORTH. THIS HELPS
EXPLAIN WHY WE'VE SEEN JUST WIDELY-SCATTERED SHOWERS AND
THUNDERSTORMS DESPITE THE RESPECTABLE (KOROR) TO GOOD (YAP) MONSOON
STRENGTH. THIS TREND LOOKS TO CONTINUE THE NEXT FEW DAYS WITH THE
NUMERICAL FORECASTS SUGGESTING SOME INCREASE IN SHOWER COVERAGE AT
BOTH LOCATIONS.
MEANWHILE AT CHUUK, A STRONG MOISTURE ADVECTION ON THE BACK SIDE OF
AN AREA OF HIGH PRESSURE TO YOUR EAST HAS ALLOWED SCATTERED SHOWERS
AND A FEW THUNDERSTORMS TO DEVELOP FROM NEAR KAPINGAMARANGI AND
NUKUORO NORTH-NORTHWESTWARD. SOME OF THIS ACTIVITY MAY MAKE IT TO
CHUUK, ONOUN, OR FARAULEP. CONFIDENCE IN THIS OCCURRING IS LESS
THOUGH SO OPTED FOR JUST ISOLATED SHOWERS FURTHER NORTH TOWARDS THOSE
LOCATIONS. OTHERWISE, NO REAL INCREASE IN RAINFALL POTENTIAL IS SEEN
HEADING THROUGH THE FIRST HALF OF NEXT WEEK.
AS FOR MARINE CONDITIONS, KOROR WILL SEE GENTLE TO MODERATE WINDS THE
NEXT FEW DAYS, MODERATE TO FRESH WINDS AND YAP, AND LIGHT TO GENTLE
WINDS AT CHUUK (AS THE PRESSURE GRADIENT IS LESS AT CHUUK). A
NORTHERLY SWELL FROM TYPHOON CHOI-WAN LOOKS TO ARRIVE AT YAP EARLY
NEXT WEEK, ARRIVING AT KOROR SLIGHTLY LATER. THE MARIANAS CHAIN WILL
DISRUPT THE FETCH LENGTH OF THIS SWELL TRYING TO MAKE IT TO CHUUK,
ALLOWING IT TO MESH WITH THE SOUTHWEST WIND WAVES, WHICH ULTIMATELY
LOOKS TO "MEET IN THE MIDDLE" AS A DEVELOPING WESTERLY SWELL AT
CHUUK. WE HAVE DOUBTS ABOUT THIS THOUGH AS WE THINK THE SWELL MAY
RETAIN MORE OF A SOUTHWEST COMPONENT FOR THE FIRST HALF OF NEXT WEEK
AT CHUUK, BUT WE'LL CONTINUE TO MONITOR THIS POTENTIAL.
GUM WATCHES/WARNINGS/ADVISORIES
GU...FLOOD WATCH THROUGH LATE SUNDAY NIGHT FOR GUZ001.
HIGH RIP CURRENT RISK THROUGH MONDAY AFTERNOON FOR GUZ001.
HIGH SURF ADVISORY UNTIL 4 PM CHST MONDAY FOR GUZ001.
WIND ADVISORY UNTIL 4 AM CHST SUNDAY FOR GUZ001.
MP...FLOOD WATCH THROUGH LATE SUNDAY NIGHT FOR MPZ001>003.
HIGH RIP CURRENT RISK THROUGH MONDAY AFTERNOON FOR MPZ001>003.
HIGH SURF ADVISORY UNTIL 4 PM CHST MONDAY FOR MPZ001>003.
WIND ADVISORY UNTIL 4 AM CHST SUNDAY FOR MPZ001.
WIND ADVISORY UNTIL 10 PM CHST SUNDAY FOR MPZ002-003.
MARIANAS WATERS...HAZARDOUS SEAS WARNING UNTIL 5 AM CHST SUNDAY FOR PMZ151-152.
GALE WARNING UNTIL 4 PM CHST SUNDAY FOR PMZ153-154.
UPDATE: KLEESCHULTE
MARIANAS: MONTVILA/MESA
EAST/WEST MICRONESIA: DOLL
TROPICAL: MONTVILA
IDSS: MESA
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page