


Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
897 FXPQ50 PGUM 151901 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 501 AM ChST Wed Jul 16 2025 .Marianas Update... The potential for showers and thunderstorms is increasing through the end of the week as a couple of surface troughs pass through the region, along with the TUTT and TUTT cell passing overhead. Periods of unsettled weather expected to continue into the weekend as the monsoonal pattern in western Micronesia and near-equatorial trough near Chuuk move near the Marianas. Gentle to moderate east to southeast winds will continue across the region through the weekend, with seas of 4 to 6 feet increasing to 5 to 7 feet by Thursday night as the west-southwest swell builds slightly. There will be a moderate risk of rip currents along east facing reefs of the Marianas through at least the end of the week. Around Thursday, a moderate risk looks to begin along south and west facing reefs as the west-southwest swell builds. && .Tropical Systems Update... Invest 96W remains a weak and broad circulation located near 14N133E, north of Palau and northwest of Yap Proper. However, convection is flaring closer to the center. Invest 96W remains classified a sub-low by the Joint Typhoon Warning Center and the potential for it to develop into a significant tropical cyclone over the next 24 hours is very low. General model trend is for 96W to slowly develop and become better organized as it creeps west- northwest into the Philippine Sea and out of the region. As 96W develops and strengthens, the monsoon flow could over the Republic of Palau and western Yap State. && .Eastern Micronesia Update... A moderate Inter Tropical Convergence Zone and a few trade-wind troughs running through Eastern Micronesia will keep scattered showers and mostly cloudy skies in play through the weekend. Clouds have stratified out for now so eliminated the chance of thunderstorms for the next period or two. Mainly east winds with a few southeast winds of 5 to 15 knots will be in the Eastern Micronesian Waters for all 3 marine zones this week. For seas, Majuro will have 4 to 7 feet, Kosrae a slightly lower 4 to 6 feet, and Pohnpei the lowest at 3 to 5 feet. However, the seas at Kosrae could start diminishing around Friday and could reach levels similar to those at Pohnpei for the weekend. && .Western Micronesia Update... Western Micronesia has seen quite a bit of stratification also, but not quite to the same extent as Eastern Micronesia, so did not remove the thunderstorms, just cut the numerous showers at Palau to scattered. The models are adamant that Yap is going to get some serious stuff later today, so left Yap at numerous showers. Due to the proximity of a monsoon disturbance, Koror, Palau will have scattered showers the entire week, and Yap will have them until Thursday, then they will come back again that night, so a pretty wet week ahead overall. Chuuk will also have scattered showers all week, except for Thursday and Thursday night, so again fairly wet there as well. Chuuk will have mainly east winds 5 to 10 knots, and much of their shower activity will likely be from East Micronesia features intruding their way into Western Micronesia. Seas at Chuuk will be 3 to 5 feet. Seas at Yap will also be in the 3 to 5 foot range. For winds, Yap will have southeast varying to southwest as the monsoon battles the trade winds for supremacy. Meanwhile at Koror, Palau, that battle is over, and the winds will be mainly southwest as the monsoon makes its presence known. && .Prev discussion... /issued 600 PM ChST Tue Jul 15 2025/ Marianas Synopsis... Satellite and radar imagery this afternoon shows isolated showers across the Marianas with scattered showers off to the east along a subtle trade-wind trough that is expected to move through tonight. Latest buoy and altimetry data show seas are around 4 to 5 feet, increasing to 5 to 7 feet east of 150E. Discussion... A subtle trade-wind trough is approaching the Marianas, with scattered showers visible on the eastern edge of the radar`s range, and should reach the Marianas by midnight tonight. The potential for thunderstorms is increasing as the TUTT and embedded TUTT cell west of Wake Island is nearing the Marianas, with the most favorable environment for thunderstorms Wednesday through at least Friday. There were a couple of short-lived thunderstorms west of Saipan and Guam earlier this afternoon, likely influenced by the localized heating of the air near the islands. Model guidance also continues to support the monsoonal pattern across western Micronesia, along with the near-equatorial trough in near Chuuk and Pohnpei, moving closer to the Marianas Thursday night and Friday, which will promote a wet pattern for the Marianas. However, southwest to west monsoon flow is not expected to extend all the way to the Marianas at this time, as the GFS continues to trend slightly towards the ECMWF, which pulls the monsoon trough more northwest as a broad circulation develops in the Philippine Sea northwest of Yap and swell west of the Marianas, keeping the Marianas in a east to southeast flow. This supports the forecast that showers will be more scattered and episodic in nature, but there is always the potential for locally heavy showers as minor shifts in the pattern could be the difference between heavy showers remaining west of the Marianas or developing over the islands. Looking towards the weekend and early next week, most models continue to support that the monsoon pattern and potential for tropical disturbance development will remain west of the Marianas, but model variability on this high remains quite high. Continue to monitor the forecast and future discussions to keep up with any changes in the overall trends. Marine... Gentle to moderate southeast to east winds will continue across the region through the weekend, with seas of 4 to 6 feet building to 5 to 7 feet Thursday night as the west-southwest swell builds slightly. This will build surf along south and west facing reefs slightly, likely leading to a moderate risk of rip currents. Not expecting seas and surf to build much higher through the weekend, as west to southwest winds upstream from the Marianas and in western Micronesia have been holding close to 15 kt, with only some gusts to 25 kt near heavier showers, limiting how much the west-southwest swell will build. The potential for thunderstorms is increasing as the TUTT and TUTT cell are moving closer from the east. Scattered showers are expected to move into the Marianas tonight as a subtle trade-wind trough moves through tonight, with periods of unsettled weather expected to continue as the monsoonal pattern in western Micronesia and near-equatorial trough near Chuuk move near the Marianas. Tropical systems... The Joint Typhoon Warning Center has opened Invest 96W on the weak and broad circulation located near 14N134E, north of Palau and northwest of Yap Proper. 96W is currently disorganized so the potential for develop into a significant tropical cyclone over the next 24 hours is very low. Models have favored a circulation developing near this location, and predict that 96W will remain northwest of Palau and Yap as it slowly develops in the Philippine Sea, slowly moving west-northwest and out of the region. However, as 96W develops it may strengthen the monsoon flow across Palau and western Yap State, leading to locally heavy showers, gusty winds, and choppy seas at times. Eastern Micronesia... Satellite observations show scattered showers across eastern Micronesia with numerous showers seen east-southeast of the RMI. Buoy and altimetry data shows combined seas between 4 to 6 feet across Pohnpei and Kosrae and 5 to 7 feet across the RMI. There has not been much change in thought from the previous forecast packages. Eastern Micronesia looks to remain active through much of the period with brief pauses in activity. Shower activity for Kosrae and Pohnpei looks to remain 30 to 40% through much of the period with Majuro seeing a 40 to 50% chance of showers. Towards the latter half of this weekend, guidance is starting to show another robust ITCZ/trade-wind convergence, which would allow for an increase in shower and thunderstorm activity. For now, kept "likely" PoPs for only Majuro, but guidance does suggest that higher PoPs could trend further west towards Kosrae and Pohnpei. Combined seas are generally between 4 to 6 feet across Pohnpei and Kosrae and 5 to 7 feet across the RMI. Model guidance shows a weakening in the trade-winds this week, allowing combined seas and swells to fall to 3 to 5 feet across Pohnpei and Kosrae and 4 to 6 feet across the RMI. Choppy seas will remain possible near heavier showers along with occasional gusts to 25 kt. Western Micronesia... A building convergence zone along the southern peripheries of a large developing monsoon gyre will continue to produce convectively active southwesterlies to Palau and Yap the next few days. The deepest convection is expected to form Wednesday, with numerous showers and some thunderstorms carried by fresh to near-gale gusts through the afternoon. Showers are likely to be locally heavy at Yap, as the aforementioned monsoon gyre shifts northward. Showers are expected to diminish considerably at both locations in the second half of the week as a ridge builds along the lee side of said convergence, followed by a return of patchy showers toward the weekend. Meanwhile at Chuuk, Patchy showers will diminish slowly through midweek, with a brief drier cycle expected Thursday and Thursday night, followed by another unsettled pattern toward the weekend. Combined seas of 4 to 6 feet are expected near Palau and Yap the next several days, with occasional upticks of 1 to 2 feet as localized wind waves pick up considerably, to then diminish to between 3 and 5 feet by early next week. Combined seas of 3 to 5 feet are expected in Chuuk waters for much of the forecast period. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Slagle East/West Micronesia: Stanko