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Marianas Synopsis

Visible satellite imagery shows partly cloudy skies and convection moving off to the west of the Marianas. Buoy and altimetry data support combined seas of 3 to 5 ft.

Discussion

Due to an unsettled weather pattern associated with an inflow band over the Marianas into Tropical Storm Mekkhala (07W), scattered showers and isolated thunderstorms were kept for tonight's period across the region. The greatest uncertainty with this forecast package relies on how fast TS Mekkhala (07W) pulls away from the Marianas, and the future movement of the weak tropical disturbance, as model guidance shows a col setting up in the vicinity of the Marianas. Therefore, winds may be light to gentle, occasionally becoming variable at times through the first half of next week. In addition, a weak upper-level low may move in within the next several days, providing additional support for convection. Due to this background set-up, island-effect showers and thunderstorms were included in the forecast at least through Tuesday afternoon. This situation will continue to be closely monitored and adjusted if needed due to the high uncertainty.

Marine

Combined seas of 3 to 5 feet will briefly diminish to 3 to 4 feet until midweek, before possibly increasing by a foot or two. The secondly southeast swell will become negligible on Sunday, quickly being replaced by a minor southwest swell associated with the monsoon trough over Western Micronesia. Around early next week, the southwest swell will veer to a westerly swell, so surf along west- facing reefs will increase. There is a moderate risk of rip currents along south- and east- facing reefs, as well as a low risk of lightning tonight and during the afternoon hours at least through Tuesday.

Tropical Systems

Tropical Storm Mekkhala (07W) continues to gradually strengthen as maximum sustained winds have increased to 50 mph. However, the system is asymmetric, as evident by the "comma-shaped" appearance on satellite imagery. The latest forecast from the Joint Typhoon Warning Center (JTWC) shows no tropical storm-force extension of the winds on the southern flank, despite favorable environmental conditions. This is due to the cyclone remaining vertically tilted, as discussed in the JTWC prognostic reasoning. With that said, as the system becomes better aligned (vertically-speaking), intensification will continue with RI (Rapid Intensification) cycles possible in 2 to 3 days. Luckily, this system is over the open waters of the Philippine Sea. This system will continue to move away from the Marianas, Yap, and Palau.

Then, what was formally Invest 93W, located yesterday near 10N163E (north-northeast of Pohnpei) has dissolved. After coordinating with JTWC (as mentioned in the NWS Tropical Discussion earlier today), Invest 93W was closed as both ASCAT analysis and visible imagery shows nothing more than a sharp trough.

For more information please refer to bulletins from the Joint Typhoon Warning Center under WMO header WTPN31 PGTW and from the National Weather Service under WMO header WTPQ31 PGUM.

You may also follow our forecast and tropical weather discussions at www.weather.gov/gum.

Eastern Micronesia

Convection is much less prevalent today, as what was formally Invest 93W has collapsed, with no circulation now present. As such, surface convergence has weakened considerably, which was not well-handled by the models. Still, a NET (Near-Equatorial Trough) extends from Chuuk through Pohnpei, continuing through Kosrae, ending at a col southwest of Majuro. South of the NET, westerly flow continues, so folks at Pohnpei and Kosrae can expect light winds from the west to continue (versus their usual east to northeast direction). Given the downward trend in forecasted precipitation amounts, the previously- issued SPS (Special Weather Statement) was allowed to expire. For now, we opted for low-end scattered shower coverage at all three locations (30 percent probabilities) for tonight and Sunday. Shower coverage then looks to become isolated at Majuro, while remaining scattered at Pohnpei and Kosrae, through the middle of next week.

As for marine conditions, light to gentle winds with seas of 3 to 5 feet will prevail through early next week.

Western Micronesia

An unsettled wet monsoon pattern continues across far western Micronesia with TS Mekkhala situated 350 miles north-northwest of Yap Proper. Satellite imagery this afternoon shows numerous heavy showers and isolated to scattered thunderstorms across the Republic of Palau and much of western Yap State, focused mainly northwest of Koror and south of Yap Proper. The environment continues to support heavy showers and thunderstorms throughout the region with a moist, convergent southwest monsoon flow extending across much of far western Micronesia and into TS Mekkhala. Model and satellite trends depict slightly drier conditions for Yap overnight with shower chances kept at 50 percent, but showers become numerous and locally heavy for Sunday and Sunday night for both Yap and Palau as TS Mekkhala continues trekking to the west-northwest, maintaining a very wet monsoon pattern across the area into the start of next week. Conditions dry out around Tuesday and Wednesday as Mekkhala moves away, but another weak disturbance looks to increase showers once more toward the end of the week.

Far to the east, scattered to numerous showers and isolated thunderstorms continue across Chuuk State within a broad unsettled pattern, south of and along the axis of an extensive WNW-ESE oriented surface trough that stretches from TS Mekkhala to the Marshall Islands. Model guidance depicts a downward trend in showers overnight as a surface ridge develops overhead, but scattered showers will continue across the area through the next few days with the broad surface trough and unsettled pattern in close proximity. Showers look to increase in the latter half of the week for Chuuk as the trough and associated showers shift back to the south and overhead.

Benign sea conditions continue across the region. Latest altimetry and Yap buoy data show combined seas between 3 and 5 feet across the region. Palau and Yap will see a building southwest swell over the next few days associated with the southwest monsoon pattern, with a secondary north to northeast swell. Chuuk continues to see a primary southeast swell with a light secondary swell out of the east- northeast.

GUM WATCHES/WARNINGS/ADVISORIES

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