328
FXPQ50 PGUM 010826
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
626 PM CHST THU OCT 1 2026
MARIANAS SYNOPSIS
TROPICAL STORM CHOI-WAN IS APPROACHING THE MARIANAS SOMEWHAT NORTH OF
SAIPAN. NUMEROUS TO WIDESPREAD LOCALLY HEAVY SHOWERS ARE SEEN ACROSS
THE MARIANAS WATERS. COMBINED SEAS ARE ESTIMATED TO BE BETWEEN 6 AND
9 FEET, AND AS CHOI-WAN PASSES NORTH AND CONTINUES TO INTENSIFY, SEAS
COULD RISE TO AS HIGH AS 20 FEET, ESPECIALLY CLOSE TO THE CENTER, IN
NORTHERN SAIPAN WATERS.
DISCUSSION
TROPICAL STORM CHOI-WAN AND THE MONSOON FLOWING INTO THE SOUTH SIDE
OF IT ARE DOMINATING WEATHER IN THE MARIANAS TONIGHT, AND WILL
CONTINUE TO DO SO THROUGH THE WEEKEND AT LEAST. GUAM AND ROTA WILL
ALMOST DEFINITELY BE OUTSIDE THE RADIUS OF DAMAGING WINDS, SO AT THIS
POINT WE'RE HANDLING THEM WITH NON-TROPICAL HAZARDS, MAINLY A WIND
ADVISORY AND A HIGH SURF ADVISORY. THIS COVERS WINDS THAT ARE
SIGNIFICANT, BUT NOT QUITE DAMAGING, AND THE SURF THAT WILL BE
CREATED BY THE FAIRLY HIGH WAVES FROM CHOI-WAN.
THE WEATHER ASSOCIATED WITH CHOI-WAN WILL PEAK ON FRIDAY OR SATURDAY,
BUT WILL BE SLOW TO DIMINISH. THE WEATHER MAY NOT REACH NORMAL LEVELS
UNTIL MONDAY OR TUESDAY.
MARINE
WE ALSO HAVE A NUMBER OF MARINE ADVISORIES OUT, DUE TO THE WINDS OF
TROPICAL STORM CHOI-WAN AND THE WAVES THAT THEY ARE GENERATING. THESE
INCLUDE A SMALL CRAFT ADVISORY, A GALE WATCH FOR THE POSSIBILITY OF
FREQUENT GALE FORCE GUSTS WITHIN THE GUAM AND ROTA WATERS, A
HAZARDOUS SEAS WATCH FOR THE GUAM AND ROTA WATERS, AND A TROPICAL
STORM WARNING FOR THE TINIAN AND SAIPAN WATERS, WHERE MINIMAL
TROPICAL STORM FORCE WINDS ARE EXPECTED.
SEAS COULD REACH AS HIGH AS 20 FEET, ESPECIALLY IN NORTHERN SAIPAN
WATERS, WHICH WILL BE THE CLOSEST MARINE ZONE TO THE CENTER OF CHOI-
WAN. WINDS WILL BEGIN TO DIMINISH BY SATURDAY NIGHT, SEAS WILL MOST
LIKELY NOT FOLLOW UNTIL SUNDAY. THEY WILL BOTH BE SLOW TO DIMINISH,
AND COULD TAKE UNTIL MONDAY OR TUESDAY TO REACH NORMAL LEVELS.
TROPICAL SYSTEMS
TROPICAL STORM CHOI-WAN (26W) IS CENTERED NEAR 17N147E, JUST ABOUT 80
MILES SOUTHEAST OF ALAMAGAN IN THE FAR NORTHERN MARIANAS. IT IS
CURRENTLY MOVING WEST-NORTHWEST AT AROUND 13 MPH, WITH MAXIMUM
SUSTAINED WINDS OF 45 MPH NEAR THE CENTER. IT IS EXPECTED TO MAKE A
SLIGHT TURN TOWARD THE WEST WITH A DECREASE IN FORWARD SPEED THROUGH
FRIDAY. CHOI-WAN IS ALSO EXPECTED TO CONTINUE INTENSIFYING, LIKELY
BECOMING A TYPHOON BY FRIDAY AFTERNOON AS IT PIVOTS TO THE NORTH AND
MOVES NEARLY PARALLEL TO THE MARIANAS CHAIN. CHOI-WAN IS FOLLOWING A
VERY TIGHT FUNNEL OF FAVORABLE SHEAR CONDITIONS TO AVOID LOSING
MOMENTUM, SO CERTAINTY IN ITS TRAJECTORY REMAINS HIGH UNTIL IT
CROSSES THE WATERS BETWEEN ANATAHAN AND ALAMAGAN, AFTER WHICH THE
SYSTEM IS LIKELY TO BE ON YET ANOTHER PATH OF LEAST RESISTANCE AND
STEER STRAIGHT NORTH WHILE INTENSIFYING. DURING THIS TIME, THE WIND
FIELD IS EXPECTED TO EXPAND A LITTLE TO THE WEST AND SOUTHWEST, BUT
THIS MAY BE ATTRIBUTED TO THE SUDDEN SHIFT IN STORM CENTER AS THE
WIND FIELD ATTEMPTS TO RE-ORIENT ITSELF, FOLLOWED BY AN
INTENSIFICATION CYCLE THAT MAY ALSO ASSIST IN MUCH MORE STABLE PATH
TOWARD THE MID-LATITUDES AS IT BECOMES MORE EAST-DOMINANT. AT THIS
TIME, A TROPICAL STORM WARNING AND TYPHOON WATCH ARE IN EFFECT FOR
AGRIHAN, PAGAN, AND ALAMAGAN ACROSS THE FAR NORTHERN MARIANAS, AS
WELL AS A TROPICAL STORM WARNING REMAINING IN EFFECT FOR TINIAN AND
SAIPAN.
FOR MORE INFORMATION ON TROPICAL STORM CHOI-WAN, PLEASE REFER TO THE
PRODUCTS 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
COPIOUS RAINFALL FROM TROPICAL STORM CHOI-WAN (26W) HAS ONLY JUST
BEGUN, YET HAS ALREADY PROMPTED A FLOOD ADVISORY FOR SAIPAN AND
TINIAN AS THE METAR REPORTS FROM SAIPAN SHOWED ABOUT 0.8 INCHES FOR
THE DAY. WITH THIS AMOUNT LIKELY BRINGING THE SOILS NEAR SATURATION,
IT REALLY SUPPORTS THE FLASH FLOOD WATCH WHICH BEGINS TONIGHT. ANY
ADDITIONAL RAINFALL COULD BRING ADDITIONAL FLOODING THAT WILL BE BOTH
BIGGER AND FLASHIER. IT'S RECOMMENDED YOU AVOID LOW-LYING AREAS AS
MUCH AS POSSIBLE UNTIL THE WEATHER RETURNS TO NORMAL.
EASTERN MICRONESIA
THE MONSOON TROUGH HAS SETTLED A LITTLE FURTHER SOUTH TODAY, WITH
VISIBLE IMAGERY ALSO SUGGESTING A CIRCULATION MAY BE TRYING TO FORM
JUST TO THE NORTHEAST OF MAJURO. BOTH OF THESE FEATURES HAVE ALLOWED
SCATTERED SHOWERS TO PERSIST AT MAJURO, AND THEY'VE BEEN NUMEROUS IN
COVERAGE AT TIMES. WE EXPECT THIS TREND TO CONTINUE FOR THE NEXT 24
HOURS, POSSIBLY LONGER. AS SUCH, RAINFALL COVERAGE/PROBABILITIES
HAVE BEEN INCREASED THERE.
MEANWHILE, POHNPEI LOOKS TOO FAR REMOVED FROM THE TROUGH, WHERE THE
BETTER MOISTURE QUALITY AND SURFACE CONVERGENCE IS NOTED. KOSRAE IS
ALSO FURTHER REMOVED AS WELL, WITH BOTH LOCATIONS SEEING MORE IN THE
WAY OF CONVECTIVE DEBRIS THAN ACTUAL SHOWERS. THIS TREND LOOKS TO
CONTINUE THE NEXT FEW DAYS, POSSIBLY LONGER, AND THIS TOO WAS
REFLECTED IN THE FORECAST.
ON A SIDE NOTE, THE GFS MODEL HAS SHOWN SOME SUGGESTIONS OF
CONVECTIVE WIND BURSTS OF 25 TO 30 KNOTS NEAR MAJURO. CONVERSELY,
VISIBLE AND INFRA-RED SATELLITE IMAGERY WAS LESS THAN IMPRESSIVE. AS
SUCH, NO SIGNIFICANT GUST WORDING WAS INCLUDED IN THE FORECAST, BUT
THESE TRENDS WILL CONTINUE TO BE MONITORED.
MARINE WISE, LIGHT TO GENTLE WINDS MAY BECOME MODERATE FROM TIME TO
TIME THROUGH THE FORECAST PERIOD. SEAS LOOK TO HOLD IN THE 4 TO 6
FOOT RANGE.
WESTERN MICRONESIA
PALAU AND YAP:
THE SPECIAL WEATHER STATEMENT REMAINS IN EFFECT FOR PALAU AND YAP,
WITH A FOCUS ON INCREASING POTENTIAL FOR HIGHER SEAS, SWELL, SURF,
AND LOCALLY HEAVY RAINFALL. IN GENERAL THE MODELS REMAIN CONSISTENT
WITH THE PREVIOUS FORECAST PACKAGE. THE MAIN ADDITIONS TO THE
FORECAST WERE TO INCREASE RAINFALL POTENTIAL AND WINDS SOMEWHAT AT
YAP AND PALAU (MAINLY NEAR KOROR) TO ACCOUNT FOR THE STRENGTHENING
MONSOON FLOW. ADDITIONALLY, AS TROPICAL STORM CHOI-WAN CONTINUES TO
STRENGTHEN, A BUILDING NORTHEASTERLY SWELL WILL ARRIVE FOR THE FIRST
HALF OF NEXT WEEK. GUIDANCE HAS BECOME MORE AGGRESSIVE SOMEWHAT WITH
REGARD TO SWELL HEIGHTS, AND THIS WAS REFLECTED IN THE FORECAST.
CHUUK:
THE FORECAST REMAINS ON TRACK WITH ONLY MINIMAL REFINEMENTS NEEDED.
THE BIGGEST ADDITION WAS TO INCLUDE A SECONDARY SWELL AS YOU TOO WILL
SEE THE INCREASE IN SEA HEIGHTS AS WAVE ENERGY FROM CHOI-WAN
ARRIVES. CURRENTLY, BOTH SEAS AND SURF LOOK TO REMAIN BELOW ADVISORY
LEVELS, BUT, THIS WILL CONTINUE TO BE MONITORED.
GUM WATCHES/WARNINGS/ADVISORIES
GU...HIGH RIP CURRENT RISK FROM 4 AM CHST FRIDAY THROUGH LATE SUNDAY
NIGHT FOR GUZ001.
HIGH SURF ADVISORY FROM 4 AM FRIDAY TO 4 AM CHST MONDAY FOR
GUZ001.
WIND ADVISORY FROM 4 AM FRIDAY TO 4 AM CHST SUNDAY FOR GUZ001.
MP...HIGH RIP CURRENT RISK FROM 4 AM CHST FRIDAY THROUGH LATE SUNDAY
NIGHT FOR MPZ001>003.
HIGH SURF ADVISORY FROM 4 AM FRIDAY TO 4 AM CHST MONDAY FOR
MPZ001>003.
WIND ADVISORY FROM 4 AM FRIDAY TO 4 AM CHST SUNDAY FOR MPZ001.
COASTAL FLOOD WATCH THROUGH MONDAY EVENING FOR MPZ002-003.
TROPICAL STORM WARNING FOR MPZ002-003.
FLOOD WATCH FROM 10 PM CHST THIS EVENING THROUGH LATE SUNDAY
NIGHT FOR MPZ002-003.
MARIANAS WATERS...TROPICAL STORM WARNING FOR PMZ153-154.
SMALL CRAFT ADVISORY UNTIL 5 AM CHST FRIDAY FOR PMZ151-152.
GALE WATCH FROM LATE TONIGHT THROUGH SATURDAY AFTERNOON FOR
PMZ151-152.
HAZARDOUS SEAS WATCH FROM SATURDAY AFTERNOON THROUGH SUNDAY
AFTERNOON FOR PMZ151-152.
MARIANAS/HYDROLOGY: STANKO
EAST/WEST MICRONESIA: DOLL
IDSS: MESA
TROPICAL: MONTVILA
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