Area Forecast Discussion
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Area Forecast Discussion
National Weather Service Tiyan GU
710 AM ChST Sun Dec 17 2017

.Marianas Synopsis...
Partly cloudy skies, moderate to fresh trade winds, and isolated
showers are seen across the Marianas this morning. Combined seas
have risen a little, and are now 6-8 feet at the buoys. Well south
of Guam, JTWC invest area 97W is apparently moving quickly NW and
elongating NW-SE.

&&

.Discussion...
Models have been struggling with 97W, with rather large run-to-run
variations in how they handle it. But the 12Z runs of the GFS and
ECMWF are in fairly good agreement in stretching 97W out into a
NW-SE trough over the next couple of days, which looks reasonable.
This reduces the expected winds a little for the Marianas the next
few days, and this is reflected in the forecast. Otherwise, weather
remains the same, with isolated showers through the forecast period,
and breezy conditions Tuesday and Wednesday.

&&

.Marine...
Besides refreshing the winds, have also refreshed the marine grids,
which now have slightly higher seas than before, in line with trends
at the buoys. We could see a high risk of rip currents along east
facing reefs toward mid-week.

&&

.Tropical systems...
On satellite imagery, 97W appears to be separating into two distinct
entities; one east of Yap near 8N142E, and the other well southwest
of Chuuk near 2N148E, with the two connected by a trough. The GFS
and ECMWF both depict a weak circulation under the cloud mass east
of Yap, which is supported by the evening ASCAT surface wind
analysis, while the GFS also has a weak elongated circulation under
the other cloud system. Winds aloft are very strong over the Yap
system, ranging from 30 kt to near 50 kt, mainly from the east. The
ECMWF quickly wipes out the Yap system, and weakens the trough as
well over the next few days. The GFS also opens the Yap system into
a trough by this evening, but it takes the other cloud mass SW of
Chuuk and develops it into a tropical depression Tuesday night and
Wednesday well south of Chuuk near 1N152E. With strong upper winds
and fair model agreement, the demise of the Yap circulation seems
likely, and indeed it appears to be losing organization on the
satellite loop. The development of the other system seems doubtful,
especially as the ECMWF shows nothing developing in that area. In
fact, the ECMWF develops nothing at all anywhere over the next ten
days.

&&

.Eastern Micronesia...
Modest trade-wind convergence near the leading edge of moderate to
fresh trade winds will continue to trigger some showers and
thunderstorms near Chuuk thru Monday and Pohnpei this morning.
Farther east, a trade-wind disturbance ahead of a fresh to strong
trade surge is approaching the Marshall Islands. This disturbance
will bring some clouds and showers to Majuro this afternoon and
evening, and to Kosrae Monday and Monday evening. It might impact
Pohnpei late Monday night but should pass south of Chuuk. Afterward,
much drier trades associated with the surge will introduce a dry
season pattern to Majuro by Monday and the rest of the locations
by Tuesday.

Building trade-wind swell and wind waves caused by the approaching
trade-wind surge will prolong hazardous surf on Kosrae thru midweek
and spread to the Marshall Islands later this afternoon, possibly
Pohnpei and Chuuk by Wednesday. Marine conditions will also reach
Small Craft Advisory levels near Majuro this evening, and the rest
of the places by Tuesday morning. Finally, a new King Tide cycle has
begun today. Therefore, need to monitor for possible inundation on
Kosrae and Majuro starting Monday afternoon.

&&

.Western Micronesia...
The tropical/monsoonal disturbance, JTWC Invest Area 97W is slowly
drifting west-northwestward and is near 8N142E this morning. Latest
ASCAT satellite imagery indicates converging fresh to strong monsoon
winds west and south of 97W. Aided by strong divergent east-southeast
flow of 30 to 45 knots above 500mb, widespread deep convection
continues to flare up near the disturbance and is then being sheared
toward the west. This vertical wind shear should hinder 97W from
developing further thru at least Monday night. The western edge of
this convection is already reaching Yap coastal waters this morning
and will likely spread over Yap this afternoon. The lower atmosphere
near Yap might stabilize itself and cause convection to weaken this
evening. With conditions still conducive for upward motion, deep
convection is expected to reform later tonight and Monday. This
convection can produce 2 to 4 inches of rainfall across western Yap
State by Monday afternoon. Converging fresh west to northwest winds
near Koror will also trigger periodic showers and thunderstorms
there thru Monday. As 97W drifts toward Koror on Monday night and
Tuesday, there will be the potential for heavy rainfall as well.
The GFS is forming a secondary circulation near the southeastern
end of the monsoon trough southwest of Chuuk near midweek. If so,
it will probably sustain an unstable monsoonal regime across far
western Micronesia most of this week.

Building monsoonal swell and wind waves are anticipated to cause
seas and surf to rise across western Yap State and Palau over the
next few days, possibly reaching hazardous levels by Tuesday.

&&

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

&&

$$

Middlebrooke/Chan



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