Area Forecast Discussion
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FXPQ60 PGUM 140931

Area Forecast Discussion
National Weather Service Tiyan GU
659 PM ChST Thu Dec 14 2017

.Marianas Synopsis...East trade-winds and isolated showers prevail
across the Marianas this afternoon.


Made no change made to the forecast. Deep-layer moisture and a thick
plume of upper-level cirrus will keep skies mostly cloudy tonight.
There will be enough deep-layer moisture to keep skies mostly cloudy
with isolated showers through Sunday.

The question in the forecast will center around a circulation now
south of Chuuk this afternoon or near 2N154E. Models show the
circulation moving northwest and being located near Palau or Yap next
Thursday...depending on which model you look at. Models all hint at
the circulation developing into a possible tropical storm. All models
agree that the circulation will not have any direct effect on the
Marianas no matter how strong it gets. The debate is how strong will
it get? Models show the pressure gradient between the circulation and
high pressure to the north causing breezy conditions across the
Marianas through next week. Latest model data showed winds over the
local area stronger than the current forecast has. Forecast already
has breezy conditions for next week and was reluctant to go too high
as later model iterations may weaken the circulation. Want more
confidence in the forecast before increasing the winds. Models
suggest that rainfall will not increase over the area because of the
circulation with only isolated showers expected. Subsidence north of
the circulation could be the possible reason for keeping the showers


Ipan, Tanapag and Ritidian buoys show sea heights around 5 feet.
All three buoys show an east swell. Ritidian and Tanapag
buoys show a northwest swell. Model data does show both swells over
the area through the forecast. Sea height of 4 to 6 feet are expected
through Saturday. Seas are predicted to start to increase Sunday.
As mentioned above, winds will increase next week. Both sea height
and wind speed may be high enough for a small craft advisory
some time next week.

.Eastern Micronesia and Chuuk...
ASCAT Analysis shows a circulation south of Chuuk near 2N153E.
Also, low-level convergence and surging trade winds are producing
showers from the Date Line over the RMI and over northern Kosrae
State, and across Pohnpei State toward Chuuk this afternoon.
Divergent upper- level flow is also over the area. Areas of
Showers will continue across much of the area for a few more days
and a convergent flow may prolong the showery weather. GFS model
indicates fresh to strong winds developing over eastern Micronesia
next week as high pressure moves across the north Pacific and
tightens the pressure gradients.

Large trade-wind swell and wind waves should keep marginal hazardous
surf on Kosrae thru Sat. By early next week, fresh to strong trade-
wind conditions will cause surf to rise higher. This will spread
hazardous surf to the Marshall Islands and possibly to Pohnpei and
Chuuk. Seas from Chuuk State eastward to the Marshalls might also
reach Small Craft Advisory levels. Finally, a new King Tide cycle
will start this weekend. Therefore, need to monitor for possible
inundation on Kosrae and Majuro.


.Western Micronesia...
ASCAT Analysis shows a circulation south of Chuuk near 2N153E,
and it will need monitoring in the coming days. Meanwhile, the
monsoon trough runs southeastward between Koror and Yap to the
circulation south of Chuuk. Monsoon winds across Koror will still
trigger a few showers and thunderstorms thru Friday evening. For
Yap, gentle winds near a neutral area along the trough will allow
fairly dry weather to last thru Friday. The longer range forecast
looks more unstable. The circulation south of Chuuk is expected to
move very slowly west- northwest, and the slow movement could
allow more time for potential development over the weekend. For
now, still have winds capped at 15 to 20 knots and seas just below
Small Craft Advisory levels for Yap and Koror into early next
week, but that will be subject to change with later forecasts.


Marianas Waters...None.


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