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FXPQ60 PGUM 240732

Area Forecast Discussion
National Weather Service Tiyan GU
532 PM ChST Mon Oct 24 2016

Andersen Air Force Base Doppler weather radar shows isolated
showers moving through the Marianas waters. The VAD wind profile
reveals east-northeast winds of 7 to 12 knots through the lowest 6
thousand feet of the air. The weather balloon from this morning
additionally indicates that the east winds increase to 20 to 30
knots around 14 thousand feet through 40 thousand feet then
increase further above that.


A dry season like trade-wind pattern will dominate the Marianas
through at least Friday. Both the GFS and the ECMWF-HiRes bring a
wind max through Tuesday through Wednesday. The winds don`t get
any higher than 20 mph so no concern there. It could provide areas
of focus for showers to gather at. Ascat does not show this, but
it could right in the underlap, which would be directly north of
the weak circulation near Chuuk and directly south of the
subtropical ridge. This would be the most likely place for a wind
max of this type, so while Ascat does not prove its existence,
which would be nice, it does not disprove it either. Another
possibility is that the wind max is developing. It could also be
both are true. With no reason to doubt the wind max, went ahead
and populated it into the grids.

In the longer term, clouds and showers will increase a bit
Saturday. The GFS40 paints a narrow band across Saipan, while the
ECMWF-HiRes first favors Guam then shifts northward to Saipan.
With the detail still a bit sketchy left the current grids largely
alone with mostly cloudy but still just isolated showers. Looks
like it could clear up around the middle of next week, so when
extending the grids added a period going back toward partly


Seas will remain benign with 3 to 5 foot values dominating. A few
6 foot values in eastern and northern Saipan waters will be
possible. A north swell generated by a strong storm moving
eastward from Japan will be responsible for the increase. The
increase will barely be noticeable on the open oceans, however
surf on north facing reefs will rise from low risk to moderate
risk levels around Wednesday night.


.Eastern Micronesia...
Visible satellite imagery shows a broad area of showers and
thunderstorms over Chuuk and Pohnpei and north of Kosrae. A weak
circulation southeast of Yap near 5N141E anchors a surface trough
that extends east-northeast to north of Pohnpei. An upper-level
trough stretches west to east along 10N to 12N, helping to generate
the deep convection seen across Chuuk and Pohnpei. Quieter weather
and gentle south to southeast winds are seen at Kosrae and Majuro.

GFS and ECMWF models both show convection tapering at Pohnpei
tonight, but lingering into tomorrow at Chuuk as the circulation
drifts west. The upper-level trough will remain in place the next
couple days just north of the islands. Combined with weak surface
forcing, could see a few thunderstorms across the region through
midweek. Farther east, models show an ITCZ-like trough/feature
developing along 9N to 10N...with east trades to the north and
light south to southeast winds to the south. This appears to be
the main focus for showers later this week. Both models show weak
circulations forming within that boundary towards the weekend, but
otherwise no major weather features expected through Saturday.

Seas will remain benign the next several days. A small long-period
north swell will build in at Chuuk and Pohnpei on Tuesday...and
Kosrae and Majuro on Wednesday. It will diminish later in the week.


.Western Micronesia...
Reasoning for the Chuuk forecast can be found under the Eastern
Micronesia section above.

A surface trough passes just south of Koror to a weak circulation
centered southeast of Yap near 5N141E. Showers and isolated
thunderstorms are seen west of the circulation over Koror and just
southwest of Yap. A fairly dry pattern is expected at Yap the next
couple days, but showery weather will persist at Koror through
Wednesday night. Models show the weak circulation drifting westward
with drier weather and light and variable winds for late in the
week. For this reason, added a slight chance of thunderstorms at
Koror for Thursday and Friday. In the long-term, models do show a
monsoon-type pattern materializing Friday and Saturday over the
region. ECMWF is a bit more aggressive with a monsoon circulation
over or north of Yap and Koror for the weekend. Will watch closely
to see how this scenario evolves.


Marianas Waters...None.


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