


Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
028 FXPQ50 PGUM 060147 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 1147 AM ChST Sun Jul 6 2025 .Marianas Update... A disturbance sliding south along the east side of the upper-level ridge has drifted over the Marianas this morning, bringing abundant mid to high level clouds. This has limited the threat for island- effect showers and thunderstorms to develop today due to the extent of sunshine and heating over the islands needed for destabilization significantly reduced. Kept the potential for scattered showers and isolated thunderstorms for this afternoon with the upper-level disturbance sliding south today and for the outside shot of island- effect convection late this afternoon if partial clearing can occur. && .Prev discussion... /issued 434 AM ChST Sun Jul 6 2025/ Marianas Update... An unsettled weather pattern induced by a ridge immediately to the north, a lull in local winds, and saturated vertical moisture columns, is still likely to produce heavy island showers and thunderstorms Sunday. Light to gentle southwesterlies in the early morning hours will allow for greater island heating effects, along with winds aloft being just strong enough to push cloudtops offshore and allow for further cycles of island convection throughout the day. Even if island showers do not develop, these conditions are likely to strain equipment and vehicles even further with elevated risks of overheating, so it is advised to monitor said equipment for any potential breakdowns during the peak heating hours Sunday and most likely Monday. Cooling systems may be under higher load than usual today as well, especially in stationary vehicles and homes more exposed to direct sunlight. Meanwhile, the marine forecast remains unchanged. Eastern Micronesia Update... There were a few changes made to the inherited forecast this morning. There is a weak Inter-Tropical Convergence Zone (ITCZ) stretching west-northwest from south of Majuro, across Kosrae and Pohnpei and into western Micronesia. Along with the ITCZ, weak trade-wind troughs are also moving through the area. These troughs will interact with the ITCZ to bring increased convection to the islands through tonight. One trough is just east of Pohnpei, producing scattered showers there this morning, with showers expected to become numerous by noon. By Sunday evening, the trough will be west of the island, allowing showers coverage to decrease to low-end scattered. Isolated thunderstorms will continue through tonight. Another trough is seen east of Kosrae with scattered to numerous showers and isolated thunderstorms approaching the island. Expect isolated showers this morning to become scattered this afternoon. Kosrae is then expected to see numerous showers as the trough moves over the island Sunday evening. Heavy showers are not expected at this time. Majuro will see low-end scattered showers, with POPs (Probability of Precipitation) increasing to 40 percent by afternoon. Scattered showers look to continue through Sunday night. No changes were made for the remainder of the forecast. Marine conditions will remain fairly benign, with mostly light to occasional gentle winds through the middle of the week for Kosrae and Pohnpei, with occasional moderate winds possible at Majuro as early as Tuesday. No changes in seas. Western Micronesia Update... An overall dry pattern will move into far western Micronesia this afternoon. Low-end scattered showers and isolated thunderstorms will continue today for Palau, becoming isolated showers for tonight. Showers at Yap will remain isolated through tonight. For Chuuk, a weak ITCZ extends west from eastern Micronesia to just north of Weno. A very weak circulation, with a trough extending northeast from it, is seen south of Weno as well. The circulation is expected to weaken through the day, however, the trough and ITCZ interaction will maintain scattered showers and isolated thunderstorms across Weno through today and into tonight. The remainder of the forecast remains unchanged. Benign marine conditions continue across the region, with generally 2 to 3 foot combined seas for Palau and Yap, and 3 to 4 foot seas expected for Chuuk. Mostly gentle winds are expected through the week. Prev discussion... /issued 519 PM ChST Sat Jul 5 2025/ Marianas Synopsis... Satellite and radar imagery are showing partly to mostly cloudy skies and isolated to low-end scattered showers. A few island-effect showers are occurring along the leeward side of Guam. Local buoys are indicating seas of 2 to 4 feet. Discussion... Light to gentle southerly flow around a surface ridge, along with ample moisture (precipitable water values of 2.25" and higher), has brought slow-moving and locally heavy showers across the Marianas. Expect mainly isolated showers for tonight. Based off the island- effect showers along the leeward side of Guam this afternoon, confidence is increasing for island-effect convection on Sunday, and possibly Monday, as the moisture profile trends up and winds become lighter and more parallel to the islands from the south-southwest. Have increased coverage to 40 percent and introduced locally heavy downpours for the Marianas for Sunday. As we head into next week, the ridge will shift further southeast, with the south to southwest monsoon flow increasing and bringing an elevated potential for showers and thunderstorms. Marine/Surf... Mainly light winds from the south tonight, then veering to southwest Sunday and to westerly Sunday night. Winds will slowly pick up next week as the high pressure ridge shifts east and the southerly monsoon flow increases, with winds becoming gentle to moderate by around Tuesday. Combined seas of 3 to 4 feet are expected through Thursday, possibly increasing by around next weekend as a west- northwest swell emanating from a distant tropical disturbance to our northwest arrives. A moderate risk of rip currents will continue along east facing reefs through Sunday, becoming low Sunday night. A low risk will persist along all other reefs. Surf and rip risk could rise by around midweek along north and especially west facing reefs with the arrival of the aforementioned west-northwest swell. Eastern Micronesia... Short Term (Tonight through Sunday night)... Kosrae: The main items of interest are a pair of troughs, one moving through Kosrae tonight, and a fairly robust trough approaching Majuro from the east. The initial Kosrae trough is less amplified and more compact than its counterpart to the east, so we opted for just scattered shower wording (50 percent probabilities), and with PWATs near 2.4 inches (near seasonal values), any heavy rainfall should be brief and isolated. Then, heading into Sunday and Sunday night, showers increase to numerous coverage as the Majuro trough moves overhead. This feature will "pack more of a wallop", as PWATs on the latest MIMIC water vapor loop are in the 3-3.25 inch range. Majuro caught a glancing blow from this today, where 40 percent probabilities are in place. As this feature moves into Kosrae by Sunday night, if later satellite runs and water vapor imagery continue to show a similar pattern as today, locally heavy rainfall may need to be added to the forecast with rainfall probabilities increased further. Majuro: The afore-mentioned trough will continue to approach from the east, passing overhead later tonight through the first half of Sunday. This afternoon`s scatterometer pass did show convectively-augmented winds up to 30 knots, as measured by the 2358Z observation of gusts to 28 knots. This occurred as convection collapsed upon itself and looks to have been an isolated event. The analysis also shows the best surface convergence bypassing you to the south, generally between 4N and 6N. As such, scattered showers (30 to 40 percent probabilities) looks to be all we can hope for, mainly late tonight or during the first half of Sunday, as current imagery continues to show warming cloud top temperatures. Pohnpei: You`ve been on the edge of high-end isolated to low-end scattered shower coverage thus far, but this should improve tonight and continue through Monday, as the effects of the approaching Kosrae trough increases. Extended (Monday through Thursday)... The models indicate a series of troughs will continue to move westward in the trade flow, with rainfall probabilities highest near any embedded troughs. This is fairly typical for this time of year, and any rain-cooled temperatures or heavy rainfall is too difficult to pinpoint this far out. Western Micronesia... Satellite imagery this afternoon reveals relatively quieter conditions across far western Micronesia, with just isolated to scattered showers and patchy cloud cover seen over Yap Proper and Koror, with more extensive mid-to upper level cloud cover across the southern Republic of Palau and southern Yap State. This is leftover from prior convection associated with a broad, disorganized convergence zone which has weakened and shifted to the southeast of Yap and Palau since yesterday. A very weak monsoon-like trough extends northeast over Koror and Yap Proper but there are few showers associated with it, and model guidance shows this trough weakening further as it shifts north of the area through the weekend. Quieter conditions are expected for Yap and Palau through early next week as a surface ridging pattern moves in behind the departing trough and convergence area. To the east, scattered showers are seen across Chuuk State, associated with a broad north-south oriented trough extending into a weak circulation developing to the south of Weno, just west of Nukuoro Atoll in Pohnpei State. Latest model guidance points to this disturbance shifting west into eastern Yap State and dissipating over the next day or two. This disturbance will support scattered showers for Chuuk through the weekend before quieter conditions set up early next week. Benign marine conditions continue across the region, with generally 2 to 3 foot combined seas for Palau and Yap, and 3 to 4 foot seas expected for Chuuk. A pulse of slightly elevated southwest to west swell looks to move into the Republic of Palau around midweek and increase seas by a foot or so, mainly Wednesday through Thursday night. This will accompany an increase to moderate to fresh southwest monsoon flow, but otherwise, mostly gentle winds are expected through the forecast period, out of the south to west for Palau and Yap, and out of the northeast to southeast for Chuuk. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Slagle