Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
692 FXPQ50 PGUM 221803 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 403 AM ChST Sat Nov 23 2024 .Marianas Update Synopsis... A few convergence driven showers pop up over Rota and Tinian waters overnight. These showers were fairly short lived. Overall no meaningful changes were needed to the forecast. The main concerns for the Marianas is the incoming north swell and trade-wind swell. These increased swells will bring choppier seas which may build to advisory levels for both Surf and Small Crafts. Winds are expected to be moderate to fresh. && .Eastern Micronesia Update... Only minor edits to the forecast this morning. Did bump up shower coverage slightly through tonight based off current satellite and model trend. The ITCZ is expected to be active through the weekend, with multiple surface troughs traversing through it as well, bringing scattered to at times numerous showers and isolated thunderstorms for Pohnpei, Kosrae and Majuro. Gusty winds and heavy downpours possible near stronger showers and thunderstorms. For next week, the models are hinting at drier conditions as the ITCZ diminishes, but multiple trade troughs and trade convergence look to keep an elevated chance of showers. Elevated winds and seas will continue through the weekend into next week. Seas rise to 6 to 9 feet for Pohnpei and Majuro, 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 possible, especially near heavier showers and thunderstorms. Surf will be elevated along east facing reefs of Kosrae, up to 7 feet, but should stay below advisory criteria. && .Western Micronesia Update... No major changes to the forecast this morning. Scattered showers and isolated thunderstorms expected for most of the period with elevated trade flow and multiple trade troughs pass through western Micronesia. Showers could become numerous at times for Chuuk late Sunday and Monday as the ITCZ and surface creep north and west over the lagoon. Moderate to fresh trade winds with some strong gusts possible, especially near heavier showers and thunderstorms. Seas will rise over the weekend as multiple elevated northerly swells pass through the region and the east-northeast trade flow increases. Seas rise to 5 to 7 feet for Chuuk by late weekend and for Palau by early next week. Yap waters could feel more impacts from the swells, with seas possibly rising to 7 to 9 feet early next week. && .Prev discussion... /issued 542 PM ChST Fri Nov 22 2024/ Marianas Synopsis... Fairly quiet conditions persist across the region with few features to speak of. Satellite and radar this afternoon show mostly clear skies over Saipan and Tinian with just isolated showers far upstream. To the south, widely scattered showers are seen crossing Guam and Rota coastal waters, moving along the back side of a very subtle, shallow surface trough. Scatterometry data this morning shows an area of fresh trade winds over Guam and near Rota, with moderate trades further north. Guam surface observations reveal a few gusts to around 20 knots (~23 mph). Ipan, Ritidian and Tanapag buoy data show combined seas of around 5 to 8 feet, which is in agreement with nearby altimetry data. Discussion... Conditions remain quiet across the Marianas this afternoon other than the increase in showers near Guam and Rota, which is likely a result of locally-enhanced wind speed convergence. These showers look to diminish overnight as the area of trade-wind convergence shifts west of the islands. Trade winds should see a slight increase tonight, looking to remain steady with moderate to fresh speeds this weekend. Winds may increase by another few knots Sunday night through Tuesday night as a trade wind surge builds across the region, due to a strengthening subtropical ridge to the north. Winds may sustain at around 20 knots to the low 20s at times, contributing to higher, choppier seas which may be hazardous to small craft at times early next week. Showers will also likely see a boost from time to time, due to locally stronger surface convergence owing to the surge in trade winds. Overall, conditions remain fairly dry until the middle of next week, when a sharp surface trough lifts northwest across the area. This trough looks to break off from a segment of the Intertropical Convergence Zone (ITCZ) early next week as it extends westward toward Chuuk. Expect wetter conditions for the latter half of next week, lasting possibly into the weekend. Marine... Altimetry data shows 6 to 8 foot seas well east of the Marianas and 5 to 7 foot seas to the west, which is in good agreement with local buoy observations. Seas may increase by another 1 to 2 feet early next week due to an increasing trade swell and strengthening winds, possibly becoming hazardous to small craft at times. Eastern Micronesia... A surface trough just west of Majuro is producing scattered showers and isolated thunderstorms across the Marshall Islands from about Ebon Atoll, up through Majuro, and into Kwajalein and Wotje. Showers and thunderstorms have been increasing to the southeast of Kosrae as the trough moves westward and are starting to lift northwest into Kosrae this evening. At Pohnpei, skies are partly cloudy with isolated showers, but as the trough continues to drift west, expect scattered showers and isolated thunderstorms to move into Pohnpei sometime after midnight, with the best chance of rain looking to be after sunrise on Saturday through Sunday night. Watching for the potential for gusty winds this weekend as an upper-level trough is located just north of the region and is slowly moving eastward. Not seeing a lot of support for widespread gusty winds as the upper- level trough is expected to pull away to the northeast Saturday night, but the GFS is the outlier showing small pockets of 30 mph gusts developing just north of Majuro Sunday morning and then progressing westward towards Pohnpei late Sunday night. Gusty winds are always possible around showers and thunderstorms, but confidence is not high that such winds will be widespread and it is difficult to pinpoint exact locations and timing for such localized winds. As the surface trough continues westward, the GFS and ECMWF show a weak ITCZ building east of the trough and extending westward as it follows the westward progression of the trough. This will keep scattered to at times numerous showers and isolated thunderstorms in the forecast for Majuro, Kosrae, and Pohnpei through the weekend. Around Monday or Tuesday, the GFS and ECMWF show the southeast flow associated with the ITCZ diminishing, decreasing convergence and the overall potential for showers and thunderstorms across the region. The GFS is currently trending slightly drier through Wednesday while ECMWF depicts a trade-wind trough crossing the Date Line around Tuesday, followed by increased trade-wind convergence across the region. The GFS eventually shows increasing trade-wind convergence across the region but this is delayed until Thursday. Winds of 10 to 20 knots will continue eastern Micronesia through at least the middle of the upcoming week. The previous forecast mentioned the potential for 25 kt winds at around Pohnpei Saturday and Saturday night, but the potential was around 20 percent. This potential has not changed much within the GEFS wave guidance, so while gusts to 25 kt are included to note gusts near showers, winds are expected to remain around or below 20 kt through the weekend. For waves, Majuro Coastal Waters will start at 5 to 7 feet and increase to between 7 and 9 feet. Kosrae will start at 4 to 6 feet and increase to between 6 and 8 feet. Pohnpei will start at 4 to 6 feet and increase to between 7 and 9 feet Saturday, then start to gradually diminish as the primary northeast and secondary north swell slowly decrease. Western Micronesia... Satellite shows fair weather over Chuuk and Yap today while patchy showers were moving across The Republic of Palau, mostly just south of Koror. ASCAT Analysis indicates a weak surface trough causing the showers over Palau, and light to occasionally moderate trade winds were seen over most of Western Micronesia. Models indicate patchy showers will eventually arrive from the east, moving to near Chuuk by late in the weekend, and toward Yap and perhaps Koror as next week progresses. Moderate to fresh trade-wind flow is gradually developing across Micronesia, especially northern parts of Micronesia, and this will cause some increase in winds and waves over the next few days. Marine conditions may become choppy at times for Chuuk and Yap by next week. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Bowsher Micronesia: Slagle