Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 020823

Area Forecast Discussion
National Weather Service Tiyan GU
623 PM ChST Tue Mar 2 2021

.Marianas Synopsis...
The remains of a weak short wave trough is located in western
Marianas waters. This system was underwhelming this morning and only
produced 0.01 inches of rain today. Downstream of the trough skies
are mostly clear. Surf along north facing reefs remains hazardous
with Chip reporting surf between 10 to 12 feet. Buoys continue to
indicate a large long period swell of 9 feet at 11 seconds.


The disturbance this morning was the best chance the Marianas had
for a wetting rain this week. The time heights for both Guam and
Saipan show a RH values less than 15% from 10 kft to around 40 kft.
While this is a significant dry layer, it is not unusual for dry
season. A typical trade wind pattern is in effect, as ridging north
of 20N interacts with troughing near the equator and drives dry air
across the west Pacific.

Long term guidance, Sunday through Thursday, indicates that the dry
air will start to lessen early to mid next week. The reason behind
this increase in moisture will be a resurgence of the ITCZ. With the
GFS and ECMWF trending towards a well defined convergence zone
centered between 6N and 10N. While the Marianas are unlikely to be
directly impacted from the ITCZ. The system will cause our surface
winds to turn and become more easterly; this will reduce to flow of
dry air into the region. Importantly QPF values over this time frame
top out only around an inch. While a small amount it would match
Guam`s accumulated rainfall for the month of February.


A long period north swell continues to roll through the area,
producing hazardous surf around 10 feet along north facing reefs.
Parts of this swell has been fairly well forecasted by the models.
The GFS has done well with the direction while the ECMWF has done
better with the wave height. Tonight will be the last night of the
hazardous conditions. Wednesday morning marine conditions will
gradually begin to improve, as the north swell fades. Even though
the north swell will drop below the 9 foot hazardous surf threshold.
A moderate risk of rip currents will be present along north and east
facing reefs for the next several days.


.Fire weather...
Dry conditions continue as February only produced 1.01 inches of
rain for Guam and approximately 3 inches for Saipan. These dry
conditions will continue as March is climatological our driest
month. Fire weather conditions will remain a concern for the next
several days as no wetting rain is expected. The only good news is
that winds are decreasing, falling below the 20 mph threshold needed
for a Fire Weather Watch. Even without windy conditions fires will
have very little problem spreading quickly in the mountains of
southern Guam.


.Eastern Micronesia...
A weak surface trough could be found west of Majuro this afternoon.
Satellite imagery showed only a few clouds and showers over that
location and models did depict this as well. The same models also
show low-level convergence bringing in scattered showers and isolated
thunderstorms to Majuro tonight. Galvez-Davison Index for Majuro is
above 25 tonight and this would favor scattered showers and
thunderstorms. Only isolated showers are expected at Majuro Wednesday
though Thursday. At Pohnpei and Kosrae isolated showers are all that
is expected the next few day.

Models show an area of trade-wind convergence and possibly an active
ITCZ pattern developing late in the week over the eastern half of
Micronesia. Model show deep layer moisture and the Galvez-Davison
Index above 25 at all three locations. Scattered showers and isolated
thunderstorms are expected at Majuro Thursday night and persisting
through Sunday. The same conditions will be present at Pohnpei and
Kosrae Friday through Sunday.


.Western Micronesia...
A near-equatorial trough stretched across the western half of
Micronesia south of the three locations this afternoon. Low-level
convergence north of the trough is generating an area of numerous to
scattered showers and isolated thunderstorms south of Palau and Yap.
Satellite imagery did indicate a few showers close to Palau. While
most of the activity will remain further south of that
is close enough to keep skies mostly cloudy with isolated showers. As
the Galvez-Davison Index is predicted tonight to be above 25 and
Precipitable Water Index is high enough at Palau a slight chance of a
thunderstorm is possible there. Only isolated showers are expected at
Palau Wednesday though Saturday. At Yap isolated showers are all that
is expected through Saturday and at Chuuk through Thursday.

Models show a surface trough with associated trade-wind convergence
moving across the area later in the week and weekend. It will first
pass across Chuuk bringing scattered showers and isolated
thunderstorms there Thursday night through Sunday. It will continue
its trek westward and bring similar conditions to Palau and Yap


GU...High Rip Current Risk through late tonight for GUZ001>004.

     High Surf Advisory until 6 AM ChST Wednesday for GUZ001>004.

Marianas Waters...Small Craft Advisory until 6 AM ChST Wednesday for PMZ151>154.



Marianas: Slaughter
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