Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
834 FXPQ50 PGUM 051845 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 445 AM ChST Thu Feb 6 2025 && .Marianas Update... Only minor edits overnight were needed. Most notably the potential for the incoming north-northwest swell may exceed the 9 foot mark. The latest altimetry pass shows 9 to 11 foot seas approximately 680 miles to the west. A shear line is currently over the northern most islands of CNMI and is expected to push southward over the weekend. Depending on how far south the shear line pushes, it could produce much needed rain for the Marianas, but also increase wind strength over the weekend. Showers tonight were maintained due to satellite imagery showing a line of showers to our southeast moving towards the Marianas. Scattered showers are anticipated tonight. && .Eastern Micronesia Update... Little changes to the forecast this morning. The ITCZ is bringing scattered showers to Majuro today and numerous showers and isolated thunderstorms to Kosrae and Pohnpei. The ITCZ is expected to linger for most of the period, but will dip slightly south beginning tonight, with showers becoming isolated for Majuro tonight and scattered for Kosrae and Pohnpei tomorrow. The ITCZ is then expected to drift back northward over the weekend, brining an increase potential for showers and thunderstorms for all locations. Fresh to strong winds and seas 7 to 10 feet (8 to 11 feet for Majuro) expected. A Small Craft Advisory continue through tonight for Pohnpei and Kosrae. The Small Craft Advisory was extended through Friday night for Majuro as the hazardous marine conditions are expected to linger a bit longer. In addition, a High Surf Advisory continues through Saturday night for east facing reefs of Kosrae. Marine conditions improve this weekend as the trade surge and swell subside. However, a larger pulse of northerly swell looks to arrive early next week and could bring an increase in surf and possibly seas. && .Western Micronesia Update... Little changes to the forecast this morning. The NET with an embedded circulation (Invest 92W sub-low) centered near 5N137E southeast of Palau will help bring scattered showers and isolated thunderstorms today for Yap and especially Palau. Invest 92W is not expected to develop into a tropical cyclone over the coming days as it slowly drifts away from Palau and Yap. Showers and potential for thunderstorms look to increase for Chuuk today as a leading disturbance along the ITCZ approaches. The NET will drift south and west, with showers decreasing for Yap and Palau. Over the weekend into next week, multiple trade disturbances will bring rounds of scattered showers and isolated thunderstorms for Yap and Palau. Expired the Small Craft Advisory for Chuuk, however winds and seas look to stay elevated over the coming days. Winds will become fresh to strong today and seas will rise 7 to 10 feet for Palau and 8 to 11 feet for Yap. A Small Craft Advisory is in effect for Yap and Palau through tomorrow. A larger northerly swell arrives today and surf is expected to rise for Yap and Palau. A High Surf Advisory has also been issued for north facing reefs through Friday night. && .Prev discussion... /issued 540 PM ChST Wed Feb 5 2025/ Marianas Synopsis... Drier air associated with the overall trade-wind pattern is moving back into the Marianas, but scattered showers continue to develop near Guam and Rota this evening. The potential for showers will increase again this weekend and into early next week as a shear line drops south towards the Marianas. Seas will be choppy at times, reaching up to 9 feet. Discussion... Satellite imagery shows mostly cloudy skies across Guam and Rota, mainly due to mid to upper-level cloud cover, but radar shows scattered showers developing to the east of the two islands. Up by Tinian and Saipan, cloud cover looks to be thinning as drier air is moving in, keeping showers isolated. Drier weather is expected across all of the Marianas Friday, before a shear line is pushed south into the Marianas this weekend and early next week, as an area of high pressure moves off the coast of Japan. Typically, a shear line is associated with breezy winds and mostly cloudy to cloudy skies with light rain ahead of the shear line. On rare occasions, if there is increased tropical moisture ahead of the shear line, shear lines can produce several rounds of heavy showers. However, moisture is expected to be limited across the Marianas when this shear line moves through the Marianas. Marine/Surf... Satellite altimetry and buoys show combined seas of 5 to 7 feet this evening. Seas are expected to slowly build through Friday as a long-period north swell moves through the region, with seas expected to peak around 9 feet, just below hazardous levels for small craft. Surf, however, is expected to reach up to 9 feet along north facing reefs Thursday night, creating hazardous surf and a high risk of rip currents. The north swell is expected to subside Friday night and allow surf and seas to diminish slightly. Then, an approaching shear line is expected to move into the Marianas this weekend. This will increase the potential for showers, while increasing winds, and building seas and surf, especially along north and east facing reefs. Tropical systems... The Joint Typhoon Warning Center (JTWC) opened Invest 92W yesterday, centered near 5N136E, to monitor the broad cyclonic motion seen within the near-equatorial trough (NET) that extends across western Micronesia near Palau. As of this evening, the latest CIMSS satellite tropical analysis shows the 850-500mb vorticity layer is slightly consolidated within the NET, south-southeast of Palau near 5N136E. However, visible and infrared satellite does not show heavy showers near the center, but instead along the outer edges of the NET, and satellite scatterometer data suggests that any surface circulation in this area is still broad. The potential for 92W to develop into a tropical cyclone within the next 24 hours remains very low. This broad area of vorticity is expected to slowly drift westward towards the Philippines over the next few days, although models showing little support for any notable organization of the circulation as it shifts south of Palau. Eastern Micronesia... The center of the ITCZ (Inter-Tropical Convergence Zone) enters Eastern Micronesia near 2N157E, stretching northeast to 4N163E, to Namorik in the FSM, before continuing eastward past the Dateline. Majuro remains just outside of the deeper moisture associated with the ITCZ, while Pohnpei is on the edge of the deeper moisture. Pohnpei: Moisture depth and quality should improve overnight, especially after midnight, increasing your rainfall potential. Nocturnal arrival typically increases convective development as well. With this in mind, the forecast was adjusted to reflect these thoughts, with the increased rainfall potential lasting into Thursday. However, the ITCZ has been slow to reorientate itself today, so for now, we only have scattered shower coverage in the forecast, so the mid shift can see if it (the ITCZ) catches up to model forecasts. If so, then scattered wording may need to be extended into Thursday night. Adding more uncertainty to the mix is an outflow boundary racing northwest-ward from the convection near Kosrae. Does this mark the leading edge/initial stage of the moisture surge, or will this feature fizzle out as it outruns it`s source region? This is another reason why for Thursday night we have lower POPs (Probability of Precipitation. That said, for Friday into the weekend, rainfall potential is expected to be more of a certainty, as the east-west portion of the ITCZ lifts north, due to weak downstream cyclogenesis. Kosrae: You`ve really "been in the thick of it" this afternoon, with all model guidance grossly underdone in their forecasts from the last 24 hours. As such, POPs were ramped up to near 100 percent through midnight, but, given how long the visibilities were in the 1 to 3 mile range at Kosrae International Airport (mainly 1 mile), it`s probable a meso-high may be forming as evident by recent cloud top warming. Still, several convective boundaries likely "litter" the area, providing localized increased surface convergence. As such, we took "a middle ground approach" using occasionally locally heavy shower wording as a "proxy" for what we think will occur during the next 12 hours. For Thursday through the weekend, you`ll either remain in the ITCZ or very near the fringe of it, so at least scattered shower coverage, with a few thunderstorms from time to time, appear to be a good probability, which the forecast portrays. Majuro: So close, yet so far, as you`ve been "teetering on the less than 50 percent cloud cover (partly cloudy), versus the mostly cloudy (50 to 74 percent) sky coverage. This is of no surprise as the edge of the ITCZ is bisecting your southern and western waters (where greater than 2 inches of precipitable water values) are located (a good "threshold" for scattered shower coverage in this region, with 2.5 inch (often an approximation of the ITCZ-like moisture depth/quality that we look for) only ~60 miles to your south. Thus, the mid shift will be watching this to see if it "waggles" north, with increasing shower coverage probable if it does so, aided by nocturnal influences. This looks to be the theme through the weekend as well (Majuro being on the edge of the deeper moisture) with the midnight shift hopefully being able to better pin down when scattered showers versus isolated coverage. Western Micronesia... Yap and Koror; A Near-Equatorial Trough remains across southern Palau, and disturbed weather is seen over most of the this region. JTWC Invest 92W is a broad circulation roughly centered at about 5N136E, and nearly stationary. Headlines were used for both these forecast locations to emphasize elevated marine conditions that will develop in the coming days. A moderate but long-period north swell will be moving across the region, and trade-wind waves and winds are also expected to pick up during the next day or two. Showery weather will be much more persistent for Palau, while Yap weather may be a little drier through the longer term, but also more affected by the arriving north swell and a fresh to locally strong trade-wind flow. Chuuk; The Small Craft Advisory remains in effect overnight for Chuuk; Altimetry shows 9 to 10-foot seas northeast of Weno, and Pohnpei buoy, farther east, still shows combined seas of 9 to 11 feet. Also, a long-period north swell will gradually arrive and combined seas could rebuild and become elevated again over the weekend. Weatherwise, the Intertropical Convergence Zone (ITCZ) remains over southern Chuuk State, and a surface trough moving westward from over Pohnpei State this afternoon will help lift showers back toward Weno on Thursday and Thursday night through Friday night, then slightly drier weather is expected to develop for Chuuk over the weekend. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...High Rip Current Risk from this evening through Friday afternoon for GUZ001. High Surf Advisory from 6 PM this evening to 5 PM ChST Friday for GUZ001. MP...High Rip Current Risk from this evening through Friday afternoon for MPZ001>003. High Surf Advisory from 6 PM this evening to 5 PM ChST Friday for MPZ001>003. Marianas Waters...None. && $$ Marianas: Bowsher East/West Micronesia: Slagle