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Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
969 FXPQ50 PGUM 172023 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 623 AM ChST Thu Jul 18 2024 .Marianas Update... Model data has backed off a bit on rainfall amounts, but only a bit. Looks like the peak will be Friday, so hit that day with numerous showers as opposed to scattered for the rest of the week. The Ritidian buoy briefly spiked to a height of 6 to 8 feet yesterday, prompting the issuance of a high risk of rip currents. It has since settled back down to between 4 and 6 feet this morning, so allowed the high risk of rip currents to expire on schedule. The next week is going to be fairly rainy, so as usual, Liberation Day is probably going to be on the wet side for Guam...hopefully we can avoid a total washout though. && .Eastern Micronesia Update... Kosrae remains under a high surf advisory through today. The ITCZ remains the dominate feature of the region bringing locally heavy showers to all 3 regions. Pohnpei and Kosrae look to be going into a lull in the showers come tonight while Majuro will be later. Seas just west of Majuro are staring to drop, last altimetry pass shows 7 to 8 feet. Winds are expected to diminish over the course of the next 24 hours. The winds are expected to be gentle to fresh to start the day off and by night become light to gentle. The strongest winds are going to be near the heaviest of the showers. && .Western Micronesia Update... Overall little change in the forecast for Western Micronesia. Based on satellite trends, trimmed the numerous showers for Palau and Yap back to scattered. Reduced the winds a little at Chuuk, which ended up lowering the wind waves, so the swells increased a little, and are now at 4 to 6 feet through tonight. Winds are expected to stay 15 knots or less, swells at Chuuk 4 to 6 feet today diminishing to between 3 and 4 feet for the weekend, and swells at Palau and Yap being 3 to 4 feet the entire forecast period, looks like more benign marine conditions are on tap, if you can deal with the showers. && .Prev discussion... /issued 543 PM ChST Wed Jul 17 2024/ Marianas Synopsis... Very unsettled weather persists over Micronesia, and satellite shows a large area of clouds and patchy showers approaching the Marianas from the east and southeast. A gust front with 30 kt winds moved across the islands at about 3PM, otherwise surface winds reported from local airports have been east at 15 to 20 kt with occasional stronger gusts. Ritidian buoy has been reporting combined seas of 6 to 7 feet with maximum waves up to the 9 to 12 foot range. Discussion... An area of surface troughing and patchy showers are arriving this afternoon. Locally heavy rainfall is likely later tonight into Thursday. Latest model guidance indicates a little less rainfall than previously expected and shower coverage could trim back to scattered by Friday through Saturday. Sunday weather is still questionable at this time. It`s usually difficult to get fair weather for the Annual Liberation Day. Changeable and unstable weather will continue over the region. Models differ quite a bit in the details, and as usual, the GFS is more aggressive than the ECMWF and indicates that a developing disturbance will be just to the southwest and west of Guam over the weekend. Also, GFS indicates a showery weather pattern will likely develop by early next week. Marine/Surf... A surface trough approaching from east and southeast will cause showers and isolated thunderstorms at times. Winds will be moderate to fresh with gusts of 25 to 30 kt near showers. Combined sea of 5 to 8 feet will also be choppy as secondary swell arrives from southwest to west during the next few days. The risk of rip currents has become high along east facing reefs, especially with the occasional bigger waves near the approaching showers. Otherwise, there is a moderate risk along the east and north facing reefs through Friday night. There is a low risk along south and west facing reefs, unless a bigger than expected southwest swell arrives. There will also be a moderate risk of lightning through the next few days. Hydrology... The ESF/Hydrologic Outlook was re-issued for the heavy rain threat for tonight through Friday. The trough approaching the Marianas will cause numerous showers tonight and Thursday at least, and scattered showers may linger into the weekend. Computer models differ in the details quite a bit, and have generally backed off in the rainfall amounts for the Marianas, while indicating that very heavy weather could develop just southwest and west of Guam over the weekend. Rainfall amounts of 2 to 4 inches are now expected for the local forecast area tonight thru Friday, and showers could still linger over the weekend. In the longer term, The GFS model in particular indicates more showers for the Marianas next week. Eastern Micronesia... Active weather pattern across eastern Micronesia over the next few days, with an ITCZ meandering and transitioning at times to more of a zone of trade convergence. The convergence zone is enhanced to the southwest of Pohnpei and Kosrae where it runs into the monsoon trough. A surge in trade winds is also pushing into Majuro. Numerous to occasional showers and isolated thunderstorms expected through at least Thursday for Pohnpei, Kosrae and Majuro. Heavy downpours and gusty winds expected. The convergence zone looks to fragment into a series of troughs by the weekend. This pattern will still support scattered showers and isolated thunderstorms across the region, but showers will start to cluster more around troughs passing through the convergence zone. The ITCZ could build back into the region from the east next week, increasing shower and thunderstorm potential. Gentle to moderate winds for Pohnpei and Kosrae, with moderate winds and some fresh gusts to Majuro. Combined seas have lowered to 5 to 7 feet for Pohnpei and Kosrae and 6 to 8 feet for Majuro. Higher seas and stronger winds will be possible with heavy showers and thunderstorms through at least Thursday. The trade swell was high enough to continue the High Surf Advisory for east reefs of Kosrae through Thursday afternoon. As the trade surge weakens, seas look to lower to 3 to 5 feet for Pohnpei and Kosrae and 4 to 6 feet for Majuro, with winds becoming light to gentle for Pohnpei and Kosrae and gentle to moderate for Majuro. Western Micronesia... Scatterometry data shows the monsoon trough entering the region around 10N to a disturbance near Yap Proper and continues east- southeast across eastern Yap State and Chuuk State. Over the past several days, short-lived vorticity features have been embedded in the trough, leading to localized areas of more intense showers and coverage. Today, satellite and model trends do show a loose circulation near Yap, but models does not suggest much development with this particular feature. With the gradual north to northwest shift in the monsoon trough, westerly flow can finally extend to Yap tomorrow, while still supplying showery monsoon weather in far western Micronesia over the next few days. Models show showers decreasing over the weekend and early next week for Yap and Palau, as the trough pushes farther to the north. For Chuuk, there will be a shift in light to gentle west winds, becoming variable overnight as a trade disturbance moves into the area tonight. Then, a fragmented ITCZ pattern is expected for the remainder of the week through early next week, maintaining periods of showery weather. Altimetry and scatterometry data show seas of 3 to 5 feet and light to moderate winds. Palau`s buoy shows seas slightly less than 3 feet since it is sheltered from the westerly swell. Localized effects of heavier showers will still lead to choppy seas at times across Palau and Yap over the next few days, and for Chuuk tonight with a passing disturbance. Southwest to west winds and swell in far western Micronesia will keep seas between 3 and 5 feet, while seas near Chuuk look to stay between 4 and 6 feet. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Stanko Micronesia: Bowsher