Area Forecast Discussion
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FXPQ60 PGUM 152026
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Area Forecast Discussion
National Weather Service Tiyan GU
626 AM ChST Mon Jul 16 2018

.Marianas Synopsis...
Showers and thunderstorms are evident just southeast of Guam and
continuing to move into the area as a surface trough approaches from
the southeast. A broad monsoon trough extends to the south of Guam
from newly-formed TD 11W just east of Luzon. Another circulation is
found within the monsoon trough near 15N133E. A broad upper-level low
is centered to the north near 24N147E.

&&

.Discussion...
Showers increased yesterday as an upper-level trough extending from
the broad upper low swept over the Marianas from the west to the
east. Once that upper-level influenced moved away, showers decreased.
However, now, a surface trough is barging in from the southeast,
increasing low-level convergence nearby. As a result, scattered
showers have arrived to Guam a little earlier than expected 24 hrs
ago. Showers will spread into the CNMI for tonight and will linger in
the area through tomorrow. Models have joined the party in showing
this increased short-term moisture as compared to model runs
yesterday. Isolated thunderstorms will persist through Thursday.
After a brief respite in activity Thursday night, isolated showers
and thunderstorms are back for the weekend, mainly due to another
surface trough that is expected to be in the vicinity. The question
is whether this trough will be a stand-alone trough near the Marianas,
or an extension of the monsoon trough. Computer models all show
the circulation NW of Yap drifting northward while developing into
a depression in the next day or two. By late week, as the system
heads north, models also show the monsoon trough lifting northward
to the north of the Marianas. If far enough north, drier weather may
return by early next week, but a lot has to happen in the meantime
as the pattern in the region is quite ripe for activity.

&&

.Marine...
Fairly quiet seas are seen across the area with buoys indicating seas
of around 4 feet, comprised of an east and southwest swell. The
southwest swell will increase a couple feet by midweek due to
persistent southwest flow across western Micronesia. The southwest
swell will gradually become westerly as the developing circulation
lifts northward and becomes better organized. If the circulation is
slow moving, that westerly swell could be prolonged into next week.

&&

.Tropical systems...
Tropical Depression 11W is moving westward and will pass the PI near
Luzon over the next day or two. Closer to home, the circulation near
15N133E, JTWC Invest 94W, is expected to gradually develop over the
next day or so, and models show it could possibly become a depression
Tuesday or Wednesday. While not directly affecting the Marianas, it
will affect the southwest to west swell by maintaining southwest
monsoon flow over western Micronesia. As it lifts northward, the
monsoon trough will shift northward, possibly resulting in more rain
for the Marianas down the road.

&&

.Eastern Micronesia and Chuuk...
A moderate to fresh trade-wind convergence zone will sustain showery
weather across the entire region thru at least Tuesday. Then
weakening trade winds should allow a near-equatorial trough to form
over the area as a continuation from the end of a monsoon trough
southwest of Chuuk which will prolong instability thru midweek. Once
the near-equatorial trough has lifted northwestward toward the
weekend, drier trade winds will finally introduce drier weather for
the region.

&&

.Western Micronesia for Yap and Koror...
Latest ASCAT satellite imagery indicates strengthening monsoonal
winds south of a tropical disturbance, JTWC Invest 94W near 15N133E.
Both Koror and Yap are just south of the strongest monsoonal winds.
Nevertheless, a wet monsoonal pattern will linger near Koror and Yap
thru at least thru Tuesday night. Once 94W has moved northward over
the Philippines Sea, improvement should gradually come in the form of
a surface ridge near midweek.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
Marianas Waters...None.
&&

$$

W. Aydlett/Chan



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