Area Forecast Discussion
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FXPQ60 PGUM 200815 AAA

Area Forecast Discussion...Updated
National Weather Service Tiyan GU
318 PM ChST Thu Jul 20 2017

.Marianas Synopsis...
Satellite shows fair skies over the local region with significant
weather farther to the south and southwest. VAD gradient winds are
east about 15 kt. Ipan Buoy shows combined seas near 4 feet and
Saipan and Ritidian Buoys show 2 to 3 ft.


Introduced isolated thunderstorms mainly Sunday through Monday.
For the near term; Relatively dry mid-levels and gentle to
moderate trade winds are expected the next couple days. Mostly
sunny and hot conditions for the Liberation Day Parade tomorrow.
Then; Models show increasing moisture arriving from the southeast
along with lighter surface winds by Sunday through Monday, and
this could trigger a few thunderstorms, especially during the
afternoon hours over Guam.


.Eastern Micronesia and Chuuk...
Eastern half of Micronesia south of 10N is dominated primarily by
trade-wind convergence and upper-level westerly shear with a
couple of north- south orientated waves just west of Chuuk near
151E, also near 168E, between Kosrae and the Marshalls, and a
weak wave near the dateline. In other words, no significant
organized system approaching any of our sites at this time. Expect
all sites to be mostly cloudy with short periods between waves
with breaks in the clouds, especially between waves. As the case
has been all season...the primary models used: GFS, ECMWF and the
NAVGEM keep an active ITCZ pattern but rarely agree on any
particular weather-specific event for any site. Currently, Majuro
looks to be moving from an enhanced area of precipitation to a
temporary lull on Friday and back to scattered showers on Saturday
with the next impulse. Kosrae looks to be most likely under the
greatest influence of the wave just passing Majuro on Friday.
For Pohnpei, models show a more prolonged rain event over the
weekend due to a combination of the next wave and the north-south
position of the ITCZ. Finally, Chuuk`s forecast is less certain
since it is located between the monsoon regime to the west and the
ITCZ to the east. I have kept with the previous forecast although
the models are less certain. For now, expect scattered showers
through Saturday night. Very little confidence here, though.

Seas at all locations look to remain below 4 to 6 feet.


.Western Micronesia...
Forecast scenario for Yap and the Republic of Palau is dominated
by the future positioning of the monsoon trough to the south and
the development, if any, of the weak circulation currently
located south of Yap Island near 6N 139E. All three of the above
mentioned models develop a circulation out to the northwest of Yap
and Koror by the end of the weekend. The forecast problem is
whether this is the same circulation as just mentioned, or a
combination of the westerly surge southwest of Palau and the
integration of a weak circulation trying to develop just to the
east of Luzon and the circulation south of Yap. This matters for
both the timing of rain events and the changes in wind direction
at Palau (not as much of a concern for Yap). I went with the
combined GFS and ECMWF forecast which keeps the circulation south
of Yap and moving towards the northwest...thus the change in the
wind forecast direction from the previous forecast period for
Koror. However...the NAVGEM does offer a solution that would keep
the circulation at the current 6N while drifting to the west and
south of Palau before intensifying and moving to the north and
then east of Luzon. This scenario would change the forecasted wind
direction back to the northeast, and give less winds and
precipitation overall. Hopefully the next run...and movement of
the clarify the situation.

In the mean time, I changed the seas forecast format for the
later time periods for Koror to show the predicted WW3 onset of a
small but significant southwest swell. This would be a change to
the past pattern.


Marianas Waters...None.


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