Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 262202

Area Forecast Discussion
National Weather Service Tiyan GU
802 AM ChST Tue Oct 27 2020

.Marianas Synopsis...
Southeast winds prevail over the Marianas this morning. A circulation
is located west of Guam.

A few changes made to the forecast. Earlier forecast had scattered
showers and isolated thunderstorms over Tinian and Saipan today.
Satellite and radar trends show most of the scattered showers over
the western half of Guam and Rota Coastal Waters. Expect these to
linger for a few hours before decreasing to isolated showers and
thunderstorms. The circulation that passed near Guam yesterday is
now west of the islands and moving slowly away.

Low-level convergence in the wake of the circulation will produce the
possibility of isolated showers and thunderstorms through Wednesday
night. Drier weather will prevail Thursday through Friday night.

A circulation that is now over the Marshalls will move westward
through the week. Models are surprisingly consistent and in general
agreement this far out in the forecast with respect to this feature.
They suggest this circulation passing south of Guam by Saturday then
being west of the island by Sunday. It is predicted to bring
scattered showers and isolated thunderstorms to the local area
Saturday and Sunday. At this time models hint that this circulation
may be a little stronger than the one that passed by on Monday. While
it is too early to tell what will eventually happen as there is many
days to go and many more model runs to pass through until that time.
At this time the models show the possibility of breezy conditions
next weekend.


Ritidian...Tanapag and Ipan buoys indicated seas between 4 and 5 feet.
Ipan depicted a easterly trade-wind swell while Ritidian and
Tanapag showed a northeast swell. Models show the east swell
persisting through the forecast while the northeast swell will
continue through Thursday night. These swells will produce a
moderate risk of rip currents on east facing reefs the next few


.Tropical systems...
The circulation west of Guam will remain relatively weak as it moves
westward. Models continue to show this circulation will develop
further and could become a tropical storm by the end of the week. At
this time it will be far to the west of the Marianas.

Another circulation that is currently in the Marshall islands will
move westward and pass south of Guam by Saturday. Models are
currently suggesting that this circulation may have some potential to
develop further. At the present time the circulation is weak and
is even hard to locate. It will be monitored for any changes as it
treks across Micronesia.


.Eastern Micronesia and Chuuk...
A surface trough curves eastward from east of Guam at 14N148E,
passing thru Chuuk Lagoon at 8N152E and two weak circulations near
Pohnpei at 7N158E and northeast of Kosrae near 8N166E to end
northeast of Majuro at 10N174E. An upper-level trough curves east-
southeastward from a low northeast of Chuuk at 11N156E, passing near
Pohnpei and Kosrae to end south of Majuro at 5N173E.

Despite the presence of the surface trough, residual drier air above
500mb should sustain partly cloudy skies near Chuuk today. Moderate
trade winds converging toward the surface trough and associated
circulations are coupling with the upper trough to trigger sporadic
convection near Pohnpei and Kosrae this morning. This trend will
continue today with convection steadily spreading over both Pohnpei
and Kosrae, then reaching Chuuk later tonight. Farther east, the
surface trough axis is already north of Majuro and weak surface
ridging can be seen over the capital. This ridge should bring quieter
conditions to Majuro thru Wednesday afternoon.

The GFS, ECMWF and NavGem are all transforming this surface trough
and related circulations into a semi monsoonal system, linking it
with a tropical disturbance currently over the Philippine Sea by
Wednesday night or Thursday. The two circulations will probably
consolidate into one near Pohnpei and Chuuk during this process, and
introduce convergent southwest to southeast winds across Chuuk,
Pohnpei and Kosrae. Therefore, anticipate periodic heavy showers for
Pohnpei Wednesday and Wednesday night, Kosrae tonight thru Wednesday
with 1 to 3 inches of rain probable for both locations. This same
pattern is also possible for Chuuk from late Wednesday night thru
Friday, depending on the future track and status of the monsoon
circulation. As the monsoon circulation and related trough lift west-
northwestward starting Thursday afternoon, the same ridge near Majuro
will also shift westward and promote improving weather for Kosrae by
late Wednesday night, Pohnpei on Thursday and Chuuk during the
weekend. A near- equatorial trough can be seen east of the Date Line,
stretching eastward from 8N178W to beyond 10N160W. This feature is
going to gradually progress westward and bring back wetness to Majuro
by Wednesday night or Thursday, then possibly to Kosrae by this

Northeast trade-wind swells and a residual longer-period northerly
swell should keep seas at 4 to 6 feet near Pohnpei and Majuro, and 3
to 5 feet near Kosrae into the weekend. Seas near Chuuk should stay
between 3 and 5 feet thru Wednesday, then gradually rise and reach 5
to 7 feet by Thursday due to increasing monsoonal swell and wind
waves. If the potential monsoonal system develop faster and/or stay
near Chuuk longer, seas and surf for both Chuuk and Pohnpei will
build higher.


.Western Micronesia for Yap and Koror...
A surface ridge persists over Koror and Yap this morning. The
tropical disturbance can be seen on both ASCAT and VIS satellite
imagery to be northeast of Yap near 14N142E. An upper-level trough
runs south-southwestward from a low near the tropical disturbance
across Koror to end at EQ135E.

Sounding data from Koror last night reveals a PW value of just over
two inches with a thin layer of drier air near 250mb, and some modest
vorticity at low to mid levels. This conditionally unstable
environment will allow island heating to generate patchy showers near
Koror this afternoon. Further enhanced by the upper trough in the
vicinity, a few thunderstorms are also possible this afternoon.
Otherwise, the surface ridge along with increasing mid-level dry air
from the southwest are going to prolong nice weather near Koror thru
Wednesday night, and near Yap thru Wednesday afternoon. Some gradual
development is expected for the tropical disturbance over the
Philippine Sea as it moves westward for the next few days. This can
introduce modest convergent southwest to south winds across far
western Micronesia which will slightly increase the chance of showers
for Koror by Thursday, and for Yap near midweek. Afterward, another
surface ridge should build from the east and resume pleasant weather
to Yap by Friday and possibly to Koror later this weekend.

Modest trade-wind swells are anticipated to maintain seas at 3 to 4
feet for the next several days. However, if the tropical disturbance
develop more rapidly than forecast; it can generate new sets of
northerly swells toward far western Micronesia. This will cause seas
and surf to build higher near Koror and Yap.


Marianas Waters...None.



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