textproduct: Tiyan

Marianas Update

Satellite and radar imagery show developing showers along a trough near the eastern waters of the Marianas. Thin, upper-level clouds continue to spread across the region from deeper convection over the northern CNMI. Ritidian and Ipan buoys continue to show seas between 2 to 3 feet.

Recent developments within the monsoon trough to our northwest, most notably around Invest 93W near Iwo To, continues to induce a moist, southeasterly flow over the region. Spotty showers over the region this morning is expected to increase this afternoon and tonight due to the nearby trough and convergent southeasterlies in its wake. There looks to be a peak in shower activity Saturday and Saturday night and could have a possibility of heavier showers at times.

Combined seas are expected to build to 4 to 6 feet by Monday due to an increase in trade swell and southwest swell. A low risk of rip currents continues along all reefs today, becoming moderate along east facing reefs tonight due to an increase in trade swell. Surf along west and south facing reefs also increases slightly as a minor southwest swell reaches the Marianas tonight.

Tropical Systems

The Joint Typhoon Warning Center is monitoring a monsoon disturbance, Invest 93W, near Iwo To at 24N141E. 93W development potential has increased somewhat quickly since the Invest was opened yesterday morning. 93W is now the subject of a Tropical Cyclone Formation Alert (TCFA), where significant tropical cyclone development is likely within 24 hours. Deterministic and ensemble models are in good agreement on a northward track, likely exiting the region (north of 25N) over the next day or so. However, the monsoon tail will still impact much of the Marianas and western Micronesia.

Eastern Micronesia Update

A moderately active Intertropical Convergence Zone (ITCZ) is expected to be the primary driver of weather in eastern Micronesia for the rest of the week. Significant showers and thunderstorms will continue to move across Pohnpei and Kosrae tonight, with conditions expected to improve considerably at both islands overnight, before picking up once again at Pohnpei as the trade-wind flow buckles against a trade-wind disturbance downstream. Showers and thunderstorms may diminish slightly after, before another pulse in activity within the ITCZ brings on-and-off showers for at least the first half of next week.

At Majuro, patchy showers are expected for much of the forecast period, followed by the aforementioned pulse in the ITCZ that is expected to bring numerous heavy showers and some thunderstorms by early next week.

Due to a large trade swell emanating across much of eastern Micronesia, hazardous surf is expected to continue along Kosrae's eastern reefs, so a High Surf Advisory is out through at least early Sunday morning. Otherwise, the marine forecast remains largely unchanged for the area.

Western Micronesia Update

A busy but unsettled weather pattern induced by a dual-axis monsoon trough spanning much of the western Pacific, is expected to continue to produce periods of heavier showers and thunderstorms at Palau and Yap throughout the forecast period. As a monsoon gyre attempts to consolidate well northwest of the islands next week, a more organized southwesterly monsoon pattern may return and bring patchy showers and some thunderstorms, but uncertainty remains high regarding the timing of these events.

At Chuuk, shower and thunderstorm coverage is expected to increase over the coming days as the aforementioned ITCZ buckles against the general monsoon flow downstream, with numerous showers and isolated thunderstorms expected during the weekend. Showers are then expected to diminish quickly thereafter, as a ridge builds in the area by early next week.

The marine forecast remains unchanged at all three locations.

Marianas Synopsis

Satellite observations show mainly scattered mid and high clouds with a few smaller convective lower clouds in vicinity of the islands. Larger areas of thunderstorms remain outside of the coastal waters to both the west and the east. Light southeasterly trade winds remain in place with the surface high far away to the northeast of the area.

Discussion

The Marianas are experiencing a relatively quiet weather pattern between converging monsoon flows and trade winds. Expect a mix of isolated to scattered showers and isolated thunderstorms over the next couple days, with a potential for heavier showers over the weekend and into next week.

Eastern Micronesia

Eastern Micronesia will remain under the influence of the Inter- Tropical Convergence Zone and trade-wind troughs, leading to a occasionally wet end of the week. Models indicate a broad sustained increase in trades across eastern Micronesia tonight through the next few days, bringing moderate to fresh winds throughout the region. This will lead to increases in trade swell and sea height, and subsequently an increase in surf, mainly for east-facing reefs. For Kosrae, surf is expected to remain at hazardous levels of 8 ft until around Sunday, and the High Surf Advisory has been continued.

Earlier buoy data off Kosrae's southern shoreline showed combined seas of around 4 to 6 feet, which are expected to increase another foot or so through Friday. Earlier altimetry passes showed combined seas of 5 to 7 feet between Kosrae and Majuro, and 6 to 8 feet east of Majuro. Seas will continue to rise tonight through the next day or two, but then trade winds and associated wind waves and swell are expected to relax early next week.

Western Micronesia

Continued unsettled weather due to a convectively active southern flank of the monsoon trough into early next week. Expect significant pulses of showers and thunderstorms over the next several days, with peak potential over the weekend, especially for Palau. Consistent higher rainfall rates next week could lead to a threat of mudslides, and this potential will need to be reassessed over the coming weekend.

Aviation

VFR conditions. Isolated heavy showers can quickly drop conditions to MVFR or IFR CIGS and/or VIS, which would be confined to less than an hour temporarily.

GUM WATCHES/WARNINGS/ADVISORIES

GU...None. MP...None. Marianas Waters...None.