519
FXPQ50 PGUM 201855
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
455 AM ChST Sun Jul 21 2024

.Marianas Update...
The Special Weather Statement was updated for position of Invest 93W
this morning, and Small Craft Advisory also updated. Satellite loops
indicate clouds and showers should gradually increase today thru
this evening. Previous forecast trends continue, so gusty winds to
30 knots and choppy seas are expected as Invest 93W crosses the
Marianas waters.

&&

.Tropical systems Update...
Invest 93W is causing heavy showers and thunderstorms in an area
centered at about 12N148E, which is 300 miles southeast of Guam.
Winds near 93W are mostly 15 to 25 mph but stronger wind gusts are
likely near heavy showers, and combined seas are likely in the 6 to
8 foot range. Invest 93W is expected to move northwest toward the
Mariana Islands today and move across the area tonight, causing
showers with locally heavy rainfall and winds gusts to 30 mph or
more.

&&

.Eastern Micronesia Update...
The models trended downward over the night last night. There are a
few trade-wind troughs in the region, namely over Pohnpei and near
Kosrae. These troughs are stirring up some convection. Some of the
showers may cause downpours, but luckily, these showers are expected
to be short lived. The probability of precipitation was dropped to
reflect this current trend. Showers have cleared out of Majuro. Winds
are expected to be gentle to moderate, and becoming gentle later for
tonight. Seas are expected to be fairly benign at 4 to 6 feet.

&&

.Western Micronesia Update...
Satellite loops showed that heavy showers developed across Yap last
night and are currently just north and northeast of Koror early this
morning. Chuuk weather has improved for today but scattered showers
will likely move back to Chuuk tonight. Otherwise, continued with
previous forecast trends.

&&

.Prev discussion... /issued 605 PM ChST Sat Jul 20 2024/

Marianas Synopsis...
Isolated showers are seen across the Marianas with Invest 93W
approaching, which will bring scattered showers late tonight. Gentle
to moderate winds will become increasingly gusty as 93W approaches.

Discussion...
Currently showers associated with Invest 93W have decreased, but
potential for showers is most likely to increase as both ECMWF and
GFS show increasing divergence aloft Sunday. An upper-level low near
22N145E looks to shift slightly south and interact with 93W as it
crosses the Marianas waters. A Small Craft Advisory was issued for
a short period Sunday morning through the afternoon. Latest model
run did shift the showers slightly north, with stronger gusts more
likely to occur over the Northern Marianas Waters.

Marine...
Seas are expected to remain in the 4 to 6 foot range, but may
possibly reach 5 to 7 feet when 93W moves overhead the Marianas
waters. Expect choppy seas tomorrow. Larger swells are unlikely as
this system is moving across quickly with a limited fetch for swell
development. Moderate rip risk is expected through Sunday and through
the upcoming week. A slight increase in westerly swell is expected
through the week as westerly monsoon winds continue to develop over
Western Micronesia. This may produce a moderate rip risk for west
facing reefs.

Tropical systems...
A weak circulation, Invest 93W is seen 450 nm east-southeast of Guam
with a few convective showers in its vicinity. Over the past 6 hours
convective activity tied to the circulation has significantly
decreased, exposing its low-level circulation center. It is expected
to become convective again as it nears the Marianas Coastal Waters.
At this time, the chance of this system developing into a tropical
system is unlikely. Latest JTWC assessment of this system is sub-
low. Morning scatterometry at around 2130Z indicated winds 15 to 20
kt near and east of the trough, and winds of 25 kts over a local area
near the trough axis.

Eastern Micronesia...
The ITCZ has dissipated across eastern Micronesia, with areas of
scattered showers and isolated thunderstorms focused more around
passing troughs within the region. One disturbance pushing out of
Kosrae will arrive at Pohnpei late tonight into tomorrow, with
locally heavy downpours and gusty winds possible. Expect the pattern
to continue into early next week as the ITCZ tries to redevelop over
the region and surface troughs passing through, with generally
scattered showers and periods of isolated thunderstorms. For the
latter half of next week, models are indicating the ITCZ
strengthening, with shower and thunderstorm potential increasing.

Combined seas of 4 to 6 feet and gentle to moderate winds over the
weekend will lower early next week, with seas lowering to between 2
to 4 feet and winds becoming generally gentle. A surge in trade winds
and trade swell looks to arrive by late in the work week, with winds
and seas bouncing back up.

Western Micronesia...
Satellite observations show scattered showers across Yap with
isolated showers across Palau and Chuuk.

Scattered shower and isolated thunderstorm coverage continues for
Yap and will persist tonight. Both the GFS and ECMWF shift the
region of convergence westward, bringing scattered showers and
isolated thunderstorsm into Palau tonight and going into Sunday
morning. Further east, scattered showers are seen near and south of
invest 93W to the north of Chuuk. Scattered showers and isolated
thunderstorms will be possible along the trough axis that 93W is
embedded in, which extends south to just east of Chuuk. As we head
into the week, the monsoon trough will re-establish itself over
western Micronesia and will bring scattered to likely showers and
isolated thunderstorms to the region.

Latest altimetry data shows seas generally between 3 and 5 feet for
all of western Micronesia. Seas will generally recede by foot or so
throughout the week and hold steady around 2 to 4 feet.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...Small Craft Advisory from 10 AM this morning to 6 PM ChST this
     evening for MPZ003.

Marianas Waters...Small Craft Advisory from 10 AM this morning
     to 6 PM ChST this evening for PMZ151>154.

&&

$$

Marianas: Simpson
Micronesia: Bowsher