708
FXPQ50 PGUM 232132
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
732 AM CHST FRI SEP 24 2021
MARIANAS SYNOPSIS
AS TROPICAL STORM MINDULLE CONTINUES TO MOVE AWAY FROM THE MARIANAS,
SHOWERS AND GUSTY WINDS WILL PREVAIL THROUGH THE WEEKEND.
DISCUSSION
AS TROPICAL STORM MINDULLE IS MAKING ITS WAY INTO THE PHILIPPINE
SEA, WINDS WILL STAY NEAR GALE FORCE (32 TO 38 MPH) WITH OCCASIONAL
GALE GUSTS POSSIBLE THROUGH THE AFTERNOON, BEFORE DROPPING TO A FRESH
BREEZE (20 TO 24 MPH) TONIGHT. AS SUCH, IF THE NEED FOR AN EARLY
CANCELLATION OF THE TROPICAL STORM WARNING ARISES, CONSIDERATION FOR
SMALL CRAFT ADVISORY CONDITIONS WILL BE ASSESSED.
OCCASIONAL SHOWERS AND ISOLATED THUNDERSTORMS WILL LAST JUST THROUGH
THIS AFTERNOON, BEFORE SHOWERS AND THUNDERSTORM POTENTIAL WILL START
TO DECREASE SLIGHTLY IN THE LOW-LEVEL CONVERGENCE ZONE FORMED IN
MINDULLE'S WAKE. AFTER, A HIGH PRESSURE RIDGE WILL SET THE MARIANAS
INTO A DRIER REGIME THROUGH THE MIDDLE OF NEXT WEEK.
MARINE
IPAN BUOY SHOWED SEAS OF 6 TO 8 FEET IN ITS VICINITY, WITH MODELS
HOVERING CLOSE IN VALUE, BUT WITH ERRONEOUS TIMING OF THE PEAK. AS
OF NOW, THE MODELS HAVE RETURNED TO PREVIOUS BIAS OF UNDERESTIMATING
JUST A LITTLE OVER 1 FOOT.
DUE TO THE LACK OF TWO OTHER BUOYS, VERIFICATION OF COMBINED SEAS OF
9 TO 13 FEET IS STRONGLY RELIANT ON STANDARD TROPICAL STORM TYPE A
(TS-A) OCEAN CONDITIONS, INDICATING THAT SUSTAINED WINDS OF 30 KNOTS
GUSTING 40 KNOTS PRODUCE OPEN OCEAN WAVES BETWEEN 10 AND 13 FEET.
A SOUTHERLY SWELL IS EXPECTED THROUGH THE REST OF THE WEEKEND,
BEFORE MINDULLE'S INDUCED NORTHWEST SWELL WILL ARRIVE TO THE
MARIANAS EARLY NEXT WEEK. AS MINDULLE CONTINUES TO INTENSIFY, THIS
SWELL WILL GROW THROUGH THE MIDDLE OF NEXT WEEK, BEFORE IT STARTS TO
DROP CONSIDERABLY AS THE EAST WELL DOMINATES.
HYDROLOGY
USGS RAIN GAUGES ON GUAM INDICATE THAT 2.5 TO 5 INCHES OF RAIN HAS
ALREADY FALLEN IN THE LAST 24 HOURS, A INCH HIGHER THAN THE PREVIOUS
PROJECTIONS OF 2 TO 4 INCHES FOR GUAM. WFO GUAM'S RAIN GAUGE
MEASUREMENTS INDICATE JUST A LITTLE OVER 4 INCHES ACCUMULATED IN THE
LAST 30 HOURS. SAIPAN ASOS INDICATES ACCUMULATION OF A LITTLE OVER 3
INCHES WITHIN THE LAST 30 HOURS, WITHIN PREVIOUS PROJECTIONS OF 3 TO
5 INCHES.
A FLASH FLOOD WATCH REMAINS IN EFFECT THROUGH THE AFTERNOON, AS THE
POSSIBILITY FOR ADDITIONAL LOCALLY HEAVY SHOWERS IS EXPECTED.
TROPICAL SYSTEMS
TROPICAL STORM MINDULLE, FORMERLY TROPICAL STORM 20W, CONTINUES TO
MOVE RAPIDLY WEST-NORTHWEST AWAY FROM THE MARIANAS. JOINT TYPHOON
WARNING CENTER (JTWC) FORECAST THAT MINDULLE WILL CONTINUE TO
INTENSITY THROUGH MONDAY, WITH A POSSIBILITY FOR IT DEVELOPING INTO
A TYPHOON SOMETIME TONIGHT.
ITS PASSAGE ACROSS THE MARIANA ISLANDS HAS BROUGHT WINDS UPWARDS OF
35 TO 40 KNOTS NEAR SAIPAN, SUPPORTED BY OFFICIAL SURFACE OBS AT THE
AIRPORT AND UNOFFICIAL WEATHER STATIONS. ALSO, SCATTEROMETER PASSES
OVER THE REGION SHOWS SUSTAINED TROPICAL FORCE WINDS IN ROTA, TINIAN
AND SAIPAN WATERS. WHILE GUAM WAS SPARED THE BULK OF THE TROPICAL
FORCE WINDS, THE SYSTEM CONTINUES TO HAVE ITS INFLUENCE ON THE AREA
WITH INCREASED SEAS AND SHOWERS THROUGH THE WEEKEND AND BEYOND. THE
CLOSEST POINT OF APPROACH WAS APPROXIMATELY 20 MILES SOUTHWEST OF
GUAM AT 4 PM.
A TROPICAL STORM WARNING STILL REMAINS IN EFFECT, BUT CONDITIONS
WILL BE MONITORED FOR A POSSIBILITY OF A CANCELLATION FOR LATER THIS
MORNING.
EASTERN MICRONESIA AND CHUUK
A TROUGH EAST OF POHNPEI IS PRODUCING SHOWERS AND THUNDERSTORMS
WHICH WILL IMPACT POHNPEI THROUGH THIS AFTERNOON. FOR THE REST OF THE
REGION, TRADE WINDS AND DRY AIR ARE COMBINING TO KEEP THE AREA
QUIET.
MOST OF THE CHANGES TO THE FORECAST ARE MINOR AND CONSIST OF CHANGING
THE TIMING OF SHOWERS THROUGH THE WEEKEND. SIMILAR TO THE LAST
FORECAST PACKAGE, NONE OF THE MODELS ARE PRODUCING STRONG SIGNALS FOR
SHOWERS OR THUNDERSTORMS. INSTEAD, SHOWING ONLY AN INCREASE IN DEEP
MOISTURE WITH LITTLE CORRELATING OMEGA VALUES. THIS IS TYPICAL OF
WEAKER TRADE WIND TROUGHS WHICH TEND TO BE SHALLOW AND DIFFICULT TO
FORECAST IN THE LONG TERM. AT THE START OF NEXT WEEK, MODELS POINT TO
AN INCREASE IN SHOWERS AND THUNDERSTORMS FOR THE REGION.
COMBINED SEAS REMAIN ON THE LOW SIDE ACROSS THE REGION, BETWEEN 3
AND 5 FEET. THE PRIMARY SWELL IS FROM THE EAST WITH A SECONDARY
NORTH SWELL, THAT WILL FADE AWAY OVER THE WEEKEND.
WESTERN MICRONESIA FOR PALAU AND YAP
TROPICAL STORM MINDULLE (20W) WILL CAUSE IMPACTS IN THE SHORT AND
LONG TERM FOR THE REGION. AS THE STORM CONTINUES TO MOVE NORTHWEST,
IR SATELLITE SHOWS A BAND OF CONVECTION FORMING NORTH OF YAP STATE.
CURRENTLY THIS LINE IS MOVING SOUTH, TOWARDS YAP, AND SHOULD ARRIVE
LATER THIS MORNING. BECAUSE OF THIS, THE FORECAST WAS UPDATED FOR
INCREASE IN SHOWERS AND THUNDERSTORMS FOR YAP, WHILE PALAU WILL BE
FAR ENOUGH SOUTH TO MISS MOST OF THE DIRECT IMPACTS FROM TROPICAL
STORM MINDULLE. THE ATMOSPHERE WILL REMAIN UNSTABLE AND CONDITIONS
ARE EXPECTED TO STAY WET THROUGH THE WEEKEND. AT THE SAME TIME, AS
TROPICAL STORM MINDULLE INTENSIFIES, WINDS WILL INCREASE AND BECOME
MORE SOUTHERLY. BY THE START OF NEXT WEEK, WEATHER IMPACTS FROM
MINDULLE WILL LESSEN AS A RIDGE OF HIGH PRESSURE SETTLES ACROSS YAP
AND PALAU. THIS WILL LEAD TO CALMER WEATHER AND WEAKER WINDS TO START
NEXT WEEK.
COMBINED SEAS WERE INCREASED BY A FOOT FOR THE REGION, AS TROPICAL
STORM MINDULLE INCREASES WINDS ACROSS THE REGION. CURRENT GUIDANCE
SHOWS SURF REACHING HAZARDOUS LEVELS OF 9 FEET ALONG NORTH FACING
REEFS AROUND TUESDAY. MARINERS AND BEACH GOERS IN YAP AND PALAU
SHOULD MONITOR THE FORECAST CLOSELY FOR ANY CHANGES.
GUM WATCHES/WARNINGS/ADVISORIES
GU...FLOOD WATCH UNTIL 1 PM CHST THIS AFTERNOON FOR GUZ001>004.
TROPICAL STORM WARNING FOR GUZ001>004.
MP...NONE.
MARIANAS WATERS...TROPICAL STORM WARNING FOR PMZ151>154.
MARIANAS/TROPICAL: PAVLIUKOIT
MICRONESIA/IDSS: SLAUGHTER
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