Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
635 FXPQ50 PGUM 230659 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 459 PM ChST Sat Nov 23 2024 .Marianas Synopsis... Himawari visible satellite shows spotty, light showers across the region, some pushed along by very weak, short-lived outflow boundaries. Skies are partly cloudy across the islands, and surface observations show moderate to fresh easterly flow, with occasional gusts up to around 25 mph. Recent buoy data near Guam and Saipan show combined seas of around 4 to 6 feet. Nearby altimetry shows 5 to 7 foot seas, east and west of the Marianas. && .Discussion... A dry trade regime continues across the Marianas. Wind speeds have increased over the last day or so, and scatterometry data shows the moderate trade-wind surge now extends further west of the Marianas over portions of western Micronesia. Recent observations at Yap and Palau show moderate to fresh trades, along with a few gusts exceeding 20 mph. Drier conditions will continue for the remainder of the weekend, the only showers in the area a result of localized trade- wind convergence. On Monday, a weak trough riding along the northern periphery of the Intertropical Convergence Zone (ITCZ) will cross the area, slightly increasing shower chances for the start of the workweek. On Wednesday, a remnant ITCZ fragment looks to lift northwest across the islands as a stronger surface trough, increasing the potential for some locally heavy showers and associated strong wind gusts for the latter half of the week. Models have pulled back slightly on trade wind speeds, but the expected pattern remains the same. Winds will remain fairly steady for the remainder of the weekend, with a possible slight increase Monday through Wednesday as the subtropical ridge strengths north of the Marianas. The ECMWF continues to run roughly 2-4 knots higher than the GFS for the first half of the week. && .Marine... Combined seas of around 5 to 7 feet will gradually rise to 7 to 9 feet by Monday as the moderate trade surge continues, and trade swells see an increase. Winds will remain moderate to fresh with occasional gusts in the low 20s, possibly increasing a few knots for the first half of the week. Elevated trade winds will contribute to higher, choppier seas, which could be hazardous to small craft at times this coming week. The chance of showers and thunderstorms over coastal waters increases during the middle of next week onward. && .Eastern Micronesia... A very active Inter-Tropical Convergence Zone (ITCZ) runs across the region from southern Chuuk State northeastward over southern Pohnpei State and Kosrae, then eastward over the central Marshall Islands to beyond the Date Line. Heavy showers and a few thunderstorms continue to develop in large areas over Eastern Micronesia and that inclement trend will continue for the couple of days while the ITCZ drifts a little farther west and west-northwest. Models are hinting at drier conditions as the ITCZ diminishes, but multiple trade troughs along with trade-wind convergence will maintain an elevated chance of showers into next week. Elevated winds and seas will continue into next week. Combined seas could rise to 6 to 9 feet for Pohnpei and Majuro, and 5 to 8 feet for Kosrae, comprised of a higher trade swell and longer period northerly swells. Winds will be moderate to fresh with some strong gusts near heavier showers and thunderstorms. && .Western Micronesia... The active ITCZ pattern in eastern Micronesia is pushing into Chuuk this evening, with visible satellite imagery showing showers and thunderstorms producing outflow boundaries that are moving at around 25 kt (~30mph). This has led to gusts of 25 kt added to the coastal forecast for tonight, with breezy conditions around 25 mph for the island forecast of Chuuk. Further west, a weak trade-wind trough near Yap is producing scattered showers and isolated thunderstorms from Yap Proper and across Ulithi, Sorol, Woleai, and other nearby islands. Most of Palau is experiencing partly cloudy skies and isolated showers, but as the trough near Yap moves north of Palau tonight and Sunday, scattered showers and isolated will move into Palau after midnight. As the ITCZ continues westward, a wetter pattern is expected across the region for the first half of the upcoming week. Right now models favor most of the showers will be along and south of 8N, which would favor slightly less rainfall over Yap Proper when compared to Palau and Chuuk, especially for Sunday when the trough at Yap Proper moves off to the west and the ITCZ starts to push into Palau, passing just south of Yap Proper. A moderate trade-wind surge, mainly between 10N to 20N, will continue through the first half of the week keeping winds moderate to fresh across the region. The ITCZ will start to fragment and become ill-defined by the middle of the upcoming week, reducing the overall potential for showers and thunderstorms, but passing trade-wind troughs will still bring periods of scattered showers to the region. As the trade-wind surge weakens later in the week, winds will also start to come down slightly. Moderate to fresh trade winds with some strong gusts will be possible, especially near heavier showers and thunderstorms. Seas will rise over the next few days as multiple elevated northerly swells pass through the region and the east-northeast trade flow increases due to the ongoing moderate trade-wind surge. Seas will peak around 5 to 7 feet for Chuuk around Sunday and Monday and for Palau around Monday and Tuesday. Yap waters could feel more impacts from the swells and slightly stronger trade winds, with seas possibly rising to 7 to 9 around Monday or Tuesday. Models continue to keep marine conditions just below hazardous levels for small craft with seas below 10 feet and winds below 22 kt. Any increase in winds or seas could trigger the issuance of a small craft advisory if it looks like conditions warrant such advisory. After Tuesday, models have winds and seas diminishing slightly as the trade-wind surge weakens. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: DeCou Eastern Micronesia: Simpson Western Micronesia: Schank