Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 280849

Area Forecast Discussion
National Weather Service Tiyan GU
649 PM ChST Tue Jun 28 2022

.Marianas Synopsis...
The visible satellite imagery shows scattered showers with
isolated thunderstorms diminishing in coverage west of Apra Harbor
and Tumon Bay, with just isolated showers crossing the rest of
the Marianas. The Doppler radar shows this trend as well, with a
few outflow boundaries from earlier convection lurking in the
area. The buoys show combined seas of 3 to 4 feet with water
temperatures of 86 degrees at Ritidian and Ipan, and 87 degrees at


The main challenge in the short term was determining areal extent
of convection as model guidance is not the most aggressive with
shower or thunderstorm coverage tonight, with the best potential
bisecting Guam`s coastal waters from northwest to southeast, with
just isolated showers north of there. The thing is, there`s plenty
of outflow boundaries lurking around throughout all of the
Marianas, and to our east between 148E to 150E. Additionally, a
weak surface trough extends from Guam through Saipan`s eastern
coastal waters, becoming more amplified here as it extends to the
northeast. This feature is a slow-moving once, and this combined
with the outflow boundaries could act as foci for additional
convective development tonight, aided by our nocturnal "bump"
that we typically see.

Then, there`s another trough to our east near 148E. The new hi-res
ARW and GFS FV3 are showing a more pronounced signature with this
feature and both show convection increasing between 15 and 18z.
We`ll continue to monitor the numerical runs this evening to see
if we need to adjust the coverage of scattered showers versus

Otherwise the story remains the same. Look for a fairly cloudy
week, especially towards Guam and Rota based of the time height
series. A series of disturbances will cross the region from time
to time over the next several days, with the best rainfall
potential currently looking to be tonight, Thursday night, and
then again on the weekend, followed by more next week.


Combined seas of 3 to 5 feet will drop by a foot on Wednesday,
before increasing by a foot or two towards the end of the week.
Light to Gentle winds will persist through Thursday, becoming
gentle to moderate for Thursday night and Friday, dropping back to
light to gentle for the weekend.


.Eastern Micronesia...
Morning soundings revealed precipitable water totals below 2 inches,
however, embedded troughs within a fragmented ITCZ bring batches of
showery weather over the next several days. Separate troughs move
through Pohnpei and Majuro tonight, bringing an uptick in showers
tonight. Majuro stays fairly active throughout the forecast period,
with numerous showers expected Wednesday night and Thursday, whereas
Pohnpei/Kosrae may expect spotty showers until a fairly active ITCZ
pattern encompasses the region over the weekend.

Gentle to moderate trades build toward the weekend. A small craft
advisory may be warranted as early as Friday as frequent gusts are
expected to surpass hazardous levels of 25 kts. Trade swell between
3 and 5 feet is expected the next several days, before building an
additional foot or two as the trade-wind surge strengthens into the


.Western Micronesia...
A broad tropical disturbance, known as Invest Area 98W, moves
northwest, now near 10N132E. High-end scattered showers trail behind,
keeping Yap and Palau fairly busy through tonight. Strongest gusts
are seen further north, so allowed the small craft advisory to expire
this afternoon. A ridge filters in behind the disturbance,
lightening conditions Wednesday and Wednesday night. Then, scattered
showers and isolated thunderstorms return with passing troughs, while
the ITCZ builds in from the east over the weekend. A trough
completes its passage through Chuuk, so that conditions may improve
the next couple days, then ITCZ showers start to rebuild by Friday
and through early next week.

Expect the trade swell of 3 to 5 feet to slightly subside by a
foot or two over the next several days. Then, models indicate
swell strengthening near Chuuk by an additional foot by the
weekend from trade-wind surge over eastern Micronesia.


Marianas Waters...None.


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