573
FXPQ50 PGUM 230809
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
609 PM CHST WED JUL 23 2025
MARIANAS SYNOPSIS
SATELLITE AND RADAR IMAGERY ARE SHOWING SCATTERED TO NUMEROUS SHOWERS
DEVELOPING ACROSS THE MARIANAS AS TROPICAL DISTURBANCE INVEST 98W
APPROACHES. LOCAL BUOYS AND RECENT ALTIMETRY PASSES ARE INDICATING
SEAS OF 5 TO 7 FEET ACROSS THE WATERS.
DISCUSSION
AS DEVELOPING TROPICAL DISTURBANCE INVEST 98W PUSHES ACROSS THE
MARIANAS AND THEN OFF TO THE NORTH AND WEST OF THE ISLANDS OVER THE
NEXT COUPLE OF DAYS, A STRONG SOUTHWEST MONSOON SURGE IS EXPECTED TO
DEVELOP ACROSS THE MARIANAS IN ITS WAKE. HEAVY RAINFALL AND BREEZY TO
WINDY CONDITIONS ARE EXPECTED ACROSS THE MARIANAS THROUGH SATURDAY
WITH THE PASSAGE OF 98W AND THE ENSUING MONSOON SURGE.
MARINE/SURF
A PASSING TROPICAL DISTURBANCE 98W AND A STRONG MONSOON SURGE
DEVELOPING BEHIND IT WILL BRING WINDS AND SEAS HAZARDOUS TO SMALL
CRAFT. CONDITIONS LOOK TO DETERIORATE ACROSS TINIAN AND SAIPAN
WATERS THIS EVENING AND A SMALL CRAFT ADVISORY IS CURRENTLY IN
EFFECT. CONDITIONS WILL THEN DETERIORATE ACROSS GUAM AND ROTA WATERS
THURSDAY MORNING AND THE SMALL CRAFT ADVISORY WILL BEGIN THEN. THE
SMALL CRAFT ADVISORIES ARE ALL IN EFFECT THROUGH SATURDAY NIGHT.
THERE IS A POTENTIAL FOR GALE FORCE WINDS TO DEVELOP BEGINNING FRIDAY
AS THE DISTURBANCE PASSES OFF SLOWLY TO THE NORTHWEST AND THE
SOUTHWEST FLOW INTENSIFIES, WITH THE BETTER PROBABILITY ACROSS THE
TINIAN AND SAIPAN WATERS.
THERE IS CURRENTLY A MODERATE RISK OF RIP CURRENTS ALONG NORTH, EAST
AND SOUTH FACING REEFS AND A LOW RISK FOR WEST FACING REEFS. THIS IS
EXPECTED TO CHANGE AS INVEST 98W PASSES THROUGH THE REGION. SURF IS
EXPECTED TO RISE ALONG EAST AND SOUTH FACING REEFS FIRST, THEN
TRANSITION TO SOUTH AND WEST FACING REEFS AS 98W PUSHES TO THE WEST,
AND THE SOUTH TO SOUTHWEST MONSOON SWELL INTENSIFIES IN ITS WAKE.
THIS WILL FORCE THE SURF AND RISK OF RIP CURRENTS TO RISE ALONG SOUTH
AND WEST FACING REEFS AS SURF WILL EXCEED THE 9 FOOT THRESHOLD. SURF
AND RIP RISK LOOK TO REMAIN HAZARDOUS THROUGH MUCH OF THE WEEKEND.
HYDROLOGY
AN UNSETTLED PATTERN CAUSED BY A PASSING TROPICAL DISTURBANCE
INVEST 98W AND THE MONSOON SURGE IN ITS WAKE IS LIKELY TO PRODUCE
HEAVY SHOWERS AND THUNDERSTORMS, WITH A FLASH FLOODING RISK TO THE
REGION, FOR AT LEAST THE NEXT SEVERAL DAYS. TOTAL RAINFALL OF 6 TO
12 INCHES WITH LOCALLY HIGHER AMOUNTS POSSIBLE THROUGH SATURDAY
NIGHT. A FLOOD WATCH IS IN EFFECT THROUGH SATURDAY NIGHT FOR THE
MARIANAS.
TROPICAL SYSTEMS
THE TROPICAL DISTURBANCE KNOWN AS JTWC INVEST 98W IS CURRENTLY
APPROACHING THE LOWER MARIANAS. SATELLITE AND RADAR IMAGERY ARE
INDICATING A SLOWLY DEVELOPING LOW-LEVEL CIRCULATION SOUTH AND EAST
OF GUAM, CENTERED NEAR 12.9N 145.0E. THIS FEATURE IS ALSO APPARENT
ON THE MIMIC TPW V2. INVEST 98W IS FORECASTED TO MOVE WEST NEAR OR
OVER GUAM OR ROTA AS A DISORGANIZED SYSTEM, BUT IS FORECASTED TO
SLOWLY DEVELOP AS IT SHIFTS NORTHWARD BY LATER THURSDAY, LIKELY
STAYING TO THE WEST OF TINIAN AND SAIPAN. JTWC RATES INVEST 98W AS
MEDIUM, WHICH MEANS THAT DEVELOPMENT INTO A TROPICAL CYCLONE IS
EXPECTED, BUT BEYOND 24 HOURS.
EASTERN MICRONESIA
SATELLITE OBSERVATIONS SHOW PARTLY CLOUDY TO MOSTLY CLEAR CONDITIONS
ACROSS EASTERN MICRONESIA. ALTIMETRY AND BUOY DATA SHOW COMBINED
SEAS BETWEEN 3 AND 5 FEET ACROSS THE REGION.
A DRY PATTERN HAS SET UP ACROSS EASTERN MICRONESIA WITH ALL
FORECAST ZONES FORECASTED TO HAVE ISOLATED SHOWERS THROUGH TOMORROW.
BY TOMORROW NIGHT, WE START TO SEE A RETURN IN THE TRADE WINDS WITH
TRADE-WIND CONVERGENCE LEADING TO SCATTERED SHOWERS FOR MAJURO. A
MORE TRANSIENT PATTERN LOOKS TO TAKE PLACE LATE THIS WEEK AND INTO
THE WEEKEND WITH SCATTERED SHOWERS OF 30 TO 40 POPS. EARLY NEXT WEEK
GUIDANCE IS STARTING TO HINT TOWARDS THE ITCZ RAMPING UP AGAIN
BRINGING INCREASED CONVECTION TO THE REGION.
LITTLE CHANGE WAS MADE TO THE MARINE FORECAST. COMBINES SEA REMAIN
BENIGN BETWEEN 3 AND 5 FEET REGION WIDE. SEAS MAY BRUSH UP TO 6 FEET
ACROSS THE RMI SPORADICALLY THROUGH THE WEEK DUE TO FLUCTUATIONS IN
WINDS.
WESTERN MICRONESIA
TWO MAJOR FEATURES REMAIN AFFECTING THE REGION. THE FIRST IS INVEST
98W AND THE SECOND IS TS FRANCISCO (10W). INVEST 98W IS STILL
STIRRING UP SHOWERS NEAR CHUUK. THIS IS EXPECTED TO DIMINISH OVER THE
NEXT 24 HOURS. THIS MAY LEAVE BEHIND A SMALL NORTHERLY SWELL. OVER
NEAR PALAU, ISOLATED SHOWERS ARE EXPECTED TO DIMINISH FURTHER AS
FRANCISCO, WELL TO THE NORTH, MOVES FURTHER AWAY. YAP IS THE ONLY
OUTLIER HERE WITH A INCREASED CHANCE OF SHOWERS FOR FRIDAY AS THE
MONSOON TAIL OF 98W MAY COME CLOSE ENOUGH TO INCREASE THE SHOWER
POTENTIAL. OTHERWISE A RETURN TO PLEASANT WEATHER FOR ALL LOCATIONS
IS EXPECTED ONCE THESE TWO FEATURES MOVE AWAY.
COMBINED SEAS ARE ESTIMATED TO BE 3 TO 5 FEET OFF OF THE LATEST
MODEL GUIDANCE. WINDS ARE EXPECTED TO BE GENTLE TO MODERATE DROPPING
TO LIGHT TO GENTLE ONCE THESE SHOWERS DISPERSE. OVERALL, BENIGN SEAS
ARE EXPECTED THROUGHOUT WEST MICRONESIA.
GUM WATCHES/WARNINGS/ADVISORIES
GU...FLOOD WATCH THROUGH LATE SATURDAY NIGHT FOR GUZ001.
HIGH RIP CURRENT RISK FROM THURSDAY MORNING THROUGH LATE SATURDAY
NIGHT FOR GUZ001.
HIGH SURF ADVISORY FROM 6 PM THURSDAY TO 6 AM CHST SUNDAY FOR
GUZ001.
MP...FLOOD WATCH THROUGH LATE SATURDAY NIGHT FOR MPZ001>003.
HIGH RIP CURRENT RISK FROM THURSDAY MORNING THROUGH LATE SATURDAY
NIGHT FOR MPZ001>003.
HIGH SURF ADVISORY FROM 6 PM THURSDAY TO 6 AM CHST SUNDAY FOR
MPZ001>003.
MARIANAS WATERS...SMALL CRAFT ADVISORY FROM 6 AM THURSDAY TO 6 AM CHST
SUNDAY FOR PMZ151-152.
SMALL CRAFT ADVISORY UNTIL 6 AM CHST SUNDAY FOR PMZ153-154.
MARIANAS: SLAGLE
EAST MICRONESIA: WILLIAMS
WEST MICRONESIA: BOWSHER
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