401
FXPQ50 PGUM 050740
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
540 PM ChST Wed Feb 5 2025

.Marianas Synopsis...
Drier air associated with the overall trade-wind pattern is moving
back into the Marianas, but scattered showers continue to develop
near Guam and Rota this evening. The potential for showers will
increase again this weekend and into early next week as a shear line
drops south towards the Marianas. Seas will be choppy at times,
reaching up to 9 feet.

&&

.Discussion...
Satellite imagery shows mostly cloudy skies across Guam and Rota,
mainly due to mid to upper-level cloud cover, but radar shows
scattered showers developing to the east of the two islands. Up by
Tinian and Saipan, cloud cover looks to be thinning as drier air is
moving in, keeping showers isolated. Drier weather is expected
across all of the Marianas Friday, before a shear line is pushed
south into the Marianas this weekend and early next week, as an area
of high pressure moves off the coast of Japan. Typically, a shear
line is associated with breezy winds and mostly cloudy to cloudy
skies with light rain ahead of the shear line. On rare occasions, if
there is increased tropical moisture ahead of the shear line, shear
lines can produce several rounds of heavy showers. However, moisture
is expected to be limited across the Marianas when this shear line
moves through the Marianas.

&&

Marine/Surf...
Satellite altimetry and buoys show combined seas of 5 to 7 feet
this evening. Seas are expected to slowly build through Friday as a
long-period north swell moves through the region, with seas expected
to peak around 9 feet, just below hazardous levels for small craft.
Surf, however, is expected to reach up to 9 feet along north facing
reefs Thursday night, creating hazardous surf and a high risk of rip
currents. The north swell is expected to subside Friday night and
allow surf and seas to diminish slightly. Then, an approaching shear
line is expected to move into the Marianas this weekend. This will
increase the potential for showers, while increasing winds, and
building seas and surf, especially along north and east facing reefs.

&&

Tropical systems...
The Joint Typhoon Warning Center (JTWC) opened Invest 92W yesterday,
centered near 5N136E, to monitor the broad cyclonic motion seen
within the near-equatorial trough (NET) that extends across western
Micronesia near Palau. As of this evening, the latest CIMSS satellite
tropical analysis shows the 850-500mb vorticity layer is slightly
consolidated within the NET, south-southeast of Palau near 5N136E.
However, visible and infrared satellite does not show heavy showers
near the center, but instead along the outer edges of the NET, and
satellite scatterometer data suggests that any surface circulation in
this area is still broad. The potential for 92W to develop into a
tropical cyclone within the next 24 hours remains very low. This
broad area of vorticity is expected to slowly drift westward towards
the Philippines over the next few days, with models showing little
support for any notable organization of the circulation as it shifts
south of Palau.

&&

.Eastern Micronesia...
The center of the ITCZ (Inter-Tropical Convergence Zone) enters
Eastern Micronesia near 2N157E, stretching northeast to 4N163E, to
Namorik in the FSM, before continuing eastward past the Dateline.
Majuro remains just outside of the deeper moisture associated with
the ITCZ, while Pohnpei is on the edge of the deepper moisture.


Pohnpei:
Moisture depth and quality should improve overnight, especially after
midnight, increasing your rainfall potential. Nocturnal arrival
typically increases convective development as well. With this in
mind, the forecast was adjusted to reflect these thoughts, with the
increased rainfall potential lasting into Thursday.

However, the ITCZ has been slow to reorientate itself today, so for
now, we only have scattered shower coverage in the forecast, so the
mid shift can see if it (the ITCZ) catches up to model forecasts. If
so, then scattered wording may need to be extended into Thursday
night. Adding more uncertainty to the mix is an outflow boundary
racing northwest-ward from the convection near Kosrae. Does this
mark the leading edge/initial stage of the moisture surge, or will
this feature fizzle out as it outruns it`s source region? This is
another reason why for Thursday night we have lower POPs (Probability
of Precipitation. That said, for Friday into the weekend, rainfall
potential is expected to be more of a certainty, as the east-west
portion of the ITCZ lifts north, due to weak downstream cyclogenesis.

Kosrae:
You`ve really "been in the thick of it" this afternoon, with all
model guidance grossly underdone in their forecasts from the last 24
hours. As such, POPs were ramped up to near 100 percent through
midnight, but, given how long the visibilities were in the 1 to 3
mile range at Kosrae International Airport (mainly 1 mile), it`s
probable a meso-high may be forming as evident by recent cloud top
warming. Still, several convective boundaries likely "litter" the
area, providing localized increased surface convergence. As such, we
took "a middle ground approach" using occasionally locally heavy
shower wording as a "proxy" for what we think will occur during the
next 12 hours. For Thursday through the weekend, you`ll either remain
in the ITCZ or very near the fringe of it, so at least scattered
shower coverage, with a few thunderstorms from time to time, appear
to be a good probability, which the forecast portrays.

Majuro:
So close, yet so far, as you`ve been "teetering on the less than 50
percent cloud cover (partly cloudy), versus the mostly cloudy (50 to
74 percent) sky coverage. This is of no surprise as the edge of the
ITCZ is bisecting your southern and western waters (where greater
than 2 inches of precipitable water values) are located (a good
"threshold" for scattered shower coverage in this region, with 2.5
inch (often an approximation of the ITCZ-like moisture depth/quality
that we look for) only ~60 miles to your south. Thus, the mid shift
will be watching this to see if it "waggles" north, with increasing
shower coverage probable if it does so, aided by nocturnal
influences. This looks to be the theme through the weekend as well
(Majuro being on the edge of the deeper moisture) with the midnight
shift hopefully being able to better pin down when scattered showers
versus isolated coverage.

&&

.Western Micronesia...
Yap and Koror; A Near-Equatorial Trough remains across southern
Palau, and disturbed weather is seen over most of the this region.
JTWC Invest 92W is a broad circulation roughly centered at about
5N136E, and nearly stationary. Headlines were used for both these
forecast locations to emphasize elevated marine conditions that will
develop in the coming days. A moderate but long-period north swell
will be moving across the region, and trade-wind waves and winds are
also expected to pick up during the next day or two. Showery weather
will be much more persistent for Palau, while Yap weather may be a
little drier through the longer term, but also more affected by the
arriving north swell and a fresh to locally strong trade-wind flow.

Chuuk; The Small Craft Advisory remains in effect overnight for
Chuuk; Altimetry shows 9 to 10-foot seas northeast of Weno, and
Pohnpei buoy, farther east, still shows combined seas of 9 to 11
feet. Also, a long-period north swell will gradually arrive and
combined seas could rebuild and become elevated again over the
weekend.

Weatherwise, the Intertropical Convergence Zone (ITCZ) remains over
southern Chuuk State, and a surface trough moving westward from over
Pohnpei State this afternoon will help lift showers back toward Weno
on Thursday and Thursday night through Friday night, then slightly
drier weather is expected to develop for Chuuk over the weekend.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk from Thursday evening through Friday
     afternoon for GUZ001.

     High Surf Advisory from 6 PM Thursday to 5 PM ChST Friday for
     GUZ001.

MP...High Rip Current Risk from Thursday evening through Friday
     afternoon for MPZ001>003.

     High Surf Advisory from 6 PM Thursday to 5 PM ChST Friday for
     MPZ001>003.

Marianas Waters...None.
&&

$$

Marianas: Schank
East Micronesia: Doll
West Micronesia: Simpson