798
FXPQ50 PGUM 160839
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
639 PM ChST Fri Apr 16 2021

.Marianas Synopsis...
Dry and calm conditions are present in the Marianas. Buoys show mixed
seas between 4 to 6 feet. There is a northeast trade-wind component
and a long-period southwest swell from Typhoon Surigae. Typhoon
Surigae is located roughly 800 miles southwest of Guam near 9.4N
133.7E.

&&

.Discussion...
The short term forecast remains calm for the Marianas as a dry air
mass settles over head. This air mass is easiest to see on the
sounding from Guam as dry air extends from 850 mb to the EL. As
Typhoon Surigae continues west then eventually turns north, our
trade-wind flow will break down, this will lead to gentle winds over
the islands by late weekend.

By Tuesday, weak winds and an increase in moisture in the low levels
will combine to create a favorable set up for island-effect showers.
Thunderstorms are not expected since the mid to upper levels of the
atmosphere will stay dry. By the end of next week Surigae will be in
the mid-latitudes, and how this will affect the Marianas concerning
peripheral rains or changes in wind direction, is still an open
question. The main question will be how far to the west Surigae gets.
The ECMWF has the typhoon impacting the Philippines, and in that
case, there would be little change to local weather. The GFS and
NAVGEM, however, bring showers to Saipan and Tinian as Surigae lifts
farther north or northeast. Even though there is a lot of uncertainty
in the forecast, increased showers are expected for Friday. Once
again there is a lot of uncertainty for this event and the forecast
will need to be refined.

&&

.Fire weather...
The KBDI is in the extreme category at 706 due to the continued dry
weather. Thankfully, winds are expected to stay gentle to moderate
making a Fire Watch or Red Flag warning unlikely.

&&

.Marine...
The southwest swell from Typhoon Surigae has reached the Ritidian
and Tanapag buoys. This swell will cause surf to increase along south
and west facing reefs with conditions possibly becoming hazardous at
9 feet Sunday night or Monday. Later in the week, as Surigae recurves
northward, the primary swell will become more westerly. This will
shift the hazardous surf away from south facing reefs, but increase
it along west facing reefs.

&&

.Tropical systems...
The center of Typhoon Surigae is located near 9.4N 133.7E and is
starting to increase in forward speed as it moves west-northwestward.
Surigae will continue to intensify overnight as the environment
remains favorable, with low wind-shear, high SSTs and deep moisture.

The worst effects of TY Surigae are beginning to end for Kayangel
and Koror as the typhoon continues to move away from the region, but
lingering peripheral strong, gusty southwest winds and heavy rains
will continue into Saturday.

&&

.Eastern Micronesia...
Showers and thunderstorms are present from just east of Pohnpei,
across much of Kosrae State and the central RMI as a surface trough
moves westward by Kosrae and a convergent trade-wind pattern farther
east. Brisk upper-level westerlies are seen aloft with an upper
trough extending from near Pohnpei northward beyond Wake Island. In
the short term, convection will be focused near the westward moving
trough, spreading to Pohnpei overnight. By late weekend, unstable
conditions are expected to persist across the region with east winds
shifting to the southeast and south in response to the mid-latitude
ridge getting pushed southward from a low pressure system expected to
develop near Wake Island. This development is reflected in the model
consensus. Furthermore, another low pressure system is hinted at by
models to develop well south of Chuuk that will act as a focusing
mechanism for showers and thunderstorms for the first half of next
week. Worth noting is that there has been some bouncing back and
forth in models from run to run, so next week`s forecast does come
with a fair bit of uncertainty and changes to conditions and timing
will certainly be possible. Fortunately, for the time being, only
gentle to moderate winds are expected with seas generally ranging
from 3 to 6 feet.

&&

.Western Micronesia...
TY Surigae is the dominant feature looming in the region. Although
pulling away from Palau, strong SW flow will maintain gusty winds
and rains for much of Palau and around Yap Island through Saturday
night. Drier weather and gentle winds are expected at Yap by Sunday
as a weak ridge moves overhead. However, although the current
forecast also reflects drying weather at Koror around Sunday, models
show the wetter southwest flow just barely west of Koror even on
Monday. Seas, winds, and surf will still be problematic for marine
interests the next few days. Surf is expected to fall from dangerous
levels exceeding 15 ft tomorrow morning, but will ultimately remain
hazardous through next week.

At Chuuk, dry and pleasant conditions are expected to continue
through early next week, but models show a weak circulation will take
shape well to the south, eventually lifting northward by midweek.
This could bring increasing showers and thunderstorms around mid to
late next week.

&&

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

$$

Marianas: Slaughter
Micronesia: Aydlett