Area Forecast Discussion
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FXPQ60 PGUM 262013

Area Forecast Discussion
National Weather Service Tiyan GU
529 AM ChST Thu Oct 27 2016

Andersen Air Force Base Doppler weather radar shows scattered
showers and isolated thunderstorms to the west of the Marianas
waters. These are moving westward and therefore departing. Only
isolated showers are seen to the east. The VAD wind profile
reveals east-northeast winds of 7 to 11 knots just off the surface
which then increase to between 12 and 18 knots from 2 thousand to
7 thousand feet. The weather balloon from last night indicates
that the winds then increase to between 20 and 35 knots through
54 thousand feet.


Marianas are in deep layer easterly flow with upper-level lows to
the south and trade winds in the lower levels. While this promotes
fairly low values of shear, often good for tropical cyclones, the
TC`s usually do better when the upper lows are to their north and
act as outflow channels. Lower shear is often also good for island
convection, however in this case the speeds seem to be rather too
high. So, the main forecast problem seems to be timing the trade-
wind disturbances. The other major wrinkle is a circulation
currently just east of Pohnpei. The GFS is a bit more aggressive,
however it can still be seen in the ECMWF- HiRes. This could
promote a period of fresh winds somewhere in the Marianas waters.
Unfortunately, the ensemble output showed that confidence in the
location of these winds is still pretty low. Therefore opted to
leave the grids as is at this time, with one exception. Since
there are still upper-level lows in the area, added in isolated
thunderstorms tonight.

Seas will primarily be between 4 and 6 feet. The one exception is
if that circulation comes through. The increased winds around it
could kick up some 7 or even 8 foot waves. Wind speeds could even
challenge small craft advisory thresholds. Models are currently
favoring northern Saipan waters for the higher winds and waves,
however the ensembles have considerable spread. In the long term,
we slip back into a pattern similar to now.


.Eastern Micronesia...
A near-equatorial trough extends east-northeastward from south of
Chuuk State across Pohnpei and Kosrae States to beyond the Marshall
Islands. Two weak circulations can be seen embedded along this
trough, one east of Pohnpei and the other one northeast of Majuro.
Convection associated with these features has been concentrating
near the two circulations away from the forecast points so far.
However, surging trade winds are merging against the trough axis
near Pohnpei. Therefore expect an increase in showers near Pohnpei
today. Otherwise, modest converging winds south of the trough will
maintain a few showers and thunderstorms near Chuuk, Kosrae and
Majuro thru tonight.

As the trough and circulations gradually progress westward over the
next several days, stronger convergence near the circulations might
increase shower coverage near Chuuk, Pohnpei and Majuro thru Friday
night. For Kosrae, weaker winds should provide fairer weather there
instead. After the passage of these two circulations, a brief period
of quieter weather is expected for Chuuk early next week, and the
rest during this weekend. Then a possible trade-wind surge from the
Southern Hemisphere could enhance the near-equatorial trough and
trigger more wet weather near Pohnpei, Kosrae and Majuro early next


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

Latest ASCAT and IR Satellite imagery continues to indicate a weak
circulation southwest of Koror near 3N133E. Modest converging
southeast winds will maintain a few showers and thunderstorms near
Koror thru Friday. Farther north, an upper-level trough is seen near
11N just north of Yap. Divergent winds south of it will combine with
a trade-wind disturbance to sustain showers and thunderstorms near
Yap thru tonight. An upper-level low embedded within the trough is
passing south of Guam today. Even stronger divergence flow related
to this low should produce more convection near Yap Friday and
Friday night, and might spread over Koror this weekend. A near-
equatorial trough extending westward from the east might link up
with the circulation southwest of Koror and provide variable winds
across far Western Micronesia next week. Nevertheless, the upper-
level features should be the main inducer for unstable conditions.


Marianas Waters...None.


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