Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 300937

Area Forecast Discussion
National Weather Service Tiyan GU
737 PM ChST Fri Jul 30 2021

.Marianas Synopsis...
Andersen Air Force Base Doppler weather radar shows widely scattered
showers moving through the Marianas Waters. The buoys reveal combined
seas of 2 to 3 feet, composed of east swell. However, this is almost
certainly missing a similar size west swell shown in the wave model
data, so the real combined seas are probably more like 4 feet.


The expected trough is moving through, and so far the results have
been underwhelming. The GFS still ramps the PoP`s up, since the QPF
(rainfall amounts) remain a bit... anemic... went ahead and raised
the PoP`s, but instead of frequent or something else that might give
a suggestion of copious amounts of rain. Satellite shows a lot of
clouds to our east, so went ahead and raised the cloud amounts,
especially on Saturday night and Sunday, when activity should be
peaking. Overall, the monsoon trough is simply too far north for us
to get truly abundant rainfall, except for early this afternoon on
Guam, which was due to island convection.

Winds are expected to increase as the trough becomes stronger though.
It could also slip farther south. The increased winds will kick up
some waves, please see the Marine section for a fill discussion.


Combined seas of around 4 feet and light winds were the story today,
however we begin a new chapter late in the weekend. Around Sunday or
Sunday night, winds will begin to increase as the monsoon trough
strengthens, and perhaps moves southward toward us. Seas are expected
to reach 7 to 9 feet, at least in the CNMI, and could just reach
small craft advisory criteria. A high surf advisory is pretty likely,
at least in the CNMI. Mariners should keep up to date on changing
weather conditions if planning a trip next week.


.Pagan Volcano...
We received word from the CNMI Homeland Security/Emergency Management
(HSEM) office that the boat sent to Pagan to evacuate the residents
had successfully picked them all up and was soon going to arrive in
Saipan. That was this morning, so they should all be there now. Pagan
is currently assigned a Yellow color code, indicating increased
activity, however an eruption is not yet considered likely. We have
been monitoring it on satellite imagery and have yet to see an ash
signature or a `hot spot`. We will continue watching for those; it
has been pretty cloudy so they may have been obscured. If volcanic
ash starts to threaten the Marianas, the National Weather Service
will put out a Volcanic Ash Advisory.


.Eastern Micronesia...
Visible satellite imagery shows that scattered showers and isolated
thunderstorms are still ongoing around Pohnpei. Global models still
indicate showers will decrease later tonight as the variable to
southeast winds at Pohnpei and Kosrae become more easterly starting
Saturday as the circulation east of the Marianas lifts northward and
opens into a trough and allows the ridge to build westward. A change
in the timing of the evolution of the circulation and ridge will
affect the timing of the change in the directions of the winds.

The pattern over the weekend into early next week looks relatively
dry with periods of increased showers as the trade-wind trough east
of Majuro continues westward through the region. Global models are
trending towards a wetter pattern later next week as the ITCZ
reinvigorates across the region, though the exact timing of increased
showers and thunderstorms is still uncertain.

Marine conditions are not expected to change much over the weekend
into next week as ASCAT continues to show east winds around 10 knots
across the region with higher winds associated with convection.
Heading into next week, models depict similar winds. The primary
swell continues to be an east swell between 2 and 4 feet.


.Western Micronesia...
Gentle to moderate winds are seen across the region with a surface
ridge situated south of Chuuk. The surface pattern becomes more
convoluted SE of Yap and Palau with low clouds indicating anti-
cyclonic turning near Yap while westerly winds are found along and
south of 5N. Ultimately, though, winds are weak and will be subject
to small disturbances in the flow. With that said, a number of
showers and thunderstorms are evident SE of the two areas with
strong upper-level NE winds helping to spread the cloud cover.

Heading through the weekend and into early next week, models show
the overall pattern simplifying as the ridge in Chuuk State extends
westward to near Palau with showers and thunderstorms in the far west
remaining north of 10N as a reinvigorated southwesterly monsoon
takes shape over the Philippine Sea. This pattern will sustain fairly
benign weather at both Yap and Palau with moderate SW to south winds
into midweek. Chuuk will also see relatively quiet conditions as the
bulk of showers and thunderstorms stays to the north. A circulation
to the NW will keep short-term winds out of the SW, but as that
circulation gradually dissipates near the Marianas, winds will shift
to the southeast as the low-level ridge builds across the area.

Seas in the region will largely stay between 2 and 4 feet,
occasionally increasing to around 5 feet, mainly at Yap where winds
will increase slightly more late weekend into early next week.


Marianas Waters...None.


Marianas: Stanko
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