Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 050916

Area Forecast Discussion
National Weather Service Tiyan GU
716 PM ChST Fri Jun 5 2020

.Marianas Synopsis...
Satellite and radar show cloudiness and light rain across the region
as a surface trough approaches from the east. Heavier showers from
the morning have moved west and additional showers immediately east
of the islands have diminished, but satellite imagery shows more
showers and thunderstorms farther east. Ipan buoy shows that seas
have climbed to between 5 and 6 feet, the result of increased wind
waves from stronger winds associated with the trough. Elsewhere
across the Marianas, seas range between 3 and 5 feet.


Clouds have spread northward up toward Tinian and Saipan this
afternoon and the two locations should expect some evening showers
before convection shifts west and slightly drier weather settles back
in. For Guam and Rota, showers and thunderstorms are expected to
build back in overnight and continue into Saturday before diminishing
in the afternoon hours.

A quieter pattern is expected much of next week, but a trade-wind
trough will bring a few more clouds around Monday night or Tuesday.


Seas of 3 to 5 feet will persist well into next week, however, seas
will occasionally reach closer to 6 feet in Guam waters where
stronger winds will lead to larger wind waves. Surf will maintain a
moderate risk of rip currents along east facing reefs the next
several days.


.Eastern Micronesia...
Gentle to moderate trade winds remain across the region with a trade-
wind disturbance near the western Marshall Islands at 166E. An upper-
level trough is extending southwestward into the region from a low
east of the Date Line at 19N176W. Initially, it will exert subsidence
for the region thru Saturday night. Afterward, divergent flow to its
southeast should cover the entire area.

Surface ridging along with subsidence aloft should prolong fair
weather near Pohnpei thru Saturday and Kosrae early this evening.
Then the trade disturbance will cause an uptick on showers and
thunderstorms near Kosrae by late tonight, and Pohnpei by Saturday
evening. After the passage of this disturbance, a brief period of
quieter weather should return to Kosrae on Saturday evening, and
Pohnpei on Monday. For Majuro, a weak surface impulse embedded within
the trades will generate some clouds and a few showers late this
evening and Saturday. Otherwise, drier trades are going to dominate
the capital until Tuesday. A developing ITCZ can be seen between 3N
and 9N, stretches eastward from the Date Line to beyond 160W. This
feature is going to steadily shift west-northwestward and couple with
increasing divergence related to the upper trough to introduce
wetness to Majuro and Pohnpei, and bring showery conditions to Kosrae
by Tuesday. The combination of moderate trade-wind and southeasterly
swells should maintain seas at 4 to 6 feet across the entire area
thru Wednesday.


.Western Micronesia...
A near-equatorial trough reaches eastward from south of Mindanao thru
a broad but weak circulation southwest of Koror at 5N132E to another
circulation south-southeast of Yap near 4N140E, then turns
northeastward as a surface trough to end just south of Guam at
13N145E. An elongated upper-level low west of the Mariana Islands at
16N140E is drifting very slowly westward into the Philippine Sea. It
will sustain modest upper-level divergence across Koror and Yap thru
at least Monday.

Much weaker winds between the circulations are expected to provide
benign weather near Koror thru Saturday morning. Convergent winds
near the surface trough will prolong unstable weather for Yap thru
Saturday night. As the near-equatorial and surface troughs gradually
lift northwestward over Yap and Koror late this weekend and early
next week, it will return inclement weather to Koror and sustain it
near Yap thru Monday. Some of these showers, especially near Yap
could be heavy; future shifts might need to consider heavy wordings
in the mid-term forecast. Afterward, the near-equatorial trough
should pass north of Koror and Yap into the Philippine Sea.
Therefore, conditions should gradually improve starting Tuesday.
Farther east, showers and thunderstorms will continue near Chuuk
tonight as convergent southeast winds east of the surface trough
remain in the area. A little more than an inch of rain has fallen
upon Chuuk Lagoon today and an additional half an inch to an inch is
probable by Saturday afternoon. Then surface ridging over Pohnpei
should slide westward and promote partly cloudy skies for Chuuk thru
Sunday afternoon. Afterward, a wetting trend will return as a trade-
wind disturbance along with an ITCZ arrive from the east.

A modest trade-wind swell will sustain seas at 3 to 5 feet near Yap
and Chuuk, and seas at 2 to 4 feet near Koror thru Wednesday.


Marianas Waters...None.



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