Area Forecast Discussion
Issued by NWS
FXPQ60 PGUM 282123
Area Forecast Discussion
National Weather Service Tiyan GU
716 AM ChST Mon Aug 29 2016
.Synopsis...Light and variable winds and a few showers could
be found across the Marianas this morning. A ridge of high
pressure stretched across the local area and will keep winds light
Updated winds with new model data. This newer data suggest that
winds will be lighter for a longer time than originally expected.
Scatterometer data from Sunday night showed the axis of a ridge of
high pressure over Guam. VAD winds from 2kft this morning
indicated winds were mainly variable and less than 5 knots. There
was a hint of southerly winds. Sounding data from 12Z last night
showed higher winds...20 mph or higher from 400 mb and above.
Models keep the ridge near the local area through at least Tuesday
night. The proximity of the ridge will keep surface winds light
across the Marianas. Expect south to southwest winds of 5 to 10
mph through Tuesday. The ridge axis will sink south and this will
allow surface winds to increase slightly Wednesday.
The precipitable water index from the 12Z sounding was 2.09 inches and
the K-index was 32. This sounding data indicates that there is sufficient
moisture for afternoon thunderstorms. Light winds...ample moisture
and daytime heating allowed showers to form across Guam during
Sunday afternoon so the same is possible today. The higher winds
in the upper-levels prevented afternoon thunderstorms Sunday.
While these winds will remain present today could not rule out a
slight chance of afternoon thunderstorms over Guam.
The monsoon trough, which has been present north of 25 N the last
week, is predicted to move southward and be just north of 20N by
Wednesday. It will remain in that position through at least
Saturday. A few circulations are expected to form on and move
along the trough. This may increase the chances for clouds and
showers the second half of the coming week. Have not added
anything to the forecast yet as exact timing of events is not
Earlier model projections indicated an increase in wind speeds
over the Marianas associated with the monsoon trough getting
closer. These models now keep the winds a bit lighter.
Models show the return of the east trade winds by the coming Sunday.
Buoy data showed average seas of 7 to 8 feet this morning. Expect
seas to decrease to 5 to 7 feet tonight. Ritidian buoy showed
swell heights of between 5 and 8 feet with a period of 12 seconds.
This would yield a surf 8 to 11 feet. The swell will decrease by
Tuesday allowing the surf to decrease to 7 to 9 feet. A further
decrease Tuesday night will cause surf to fall below hazardous
A series of troughs in the trade-wind flow is generating pockets
of unsettled weather from Chuuk eastward to the date line. The
first is just east of Chuuk near 154E. This trough will affect the
weather at Chuuk causing isolated thunderstorms along with the
usual trade-wind showers through Tuesday. The second trough is
near Kosrae and will affect the weather near Kosrae through
Wednesday then as it moves westward will impact the weather near
Pohnpei through Friday. The third trough just east of Majuro is
mainly affecting the Marshall Islands north of Majuro.
Nevertheless added a slight chance of thunderstorms today as early
this morning we had lightning hits north, east and south of
Majuro, and also east of Kosrae and northeast of Pohnpei. This
shows that Micronesia is in an active weather pattern right now.
A bigger disturbance affects Majuro starting around Wednesday.
Went ahead and introduced scattered showers at this time, although
the bulk of them would likely be Wednesday evening maybe lingering
into Thursday, went ahead and broadbrushed it for now.
Winds surrounding distant Typhoon Lionrock continue to generate
swell across the region. Models show a long-period northwest swell
of about 3 feet reaching the area over the next several days. This
will cause west and north facing reefs to see an increase in swell,
however surf heights are expected to remain below hazardous levels.
Chuuk is included in the Eastern Micronesia discussion as the same
series of trade-wind troughs is affecting their weather.
A ridge continues to dominate the area around Yap and Palau. This
will keep the bulk of the disturbances from eastern Micronesia to
the south of these islands. However, this morning, the very north
edge of the current trough did set off a thunderstorm at Yap and
also seems like it might brush Palau. The pockets of unstable
weather to the east seem to be more limited in northward extent
though, not reaching above 7N. The ridge does not have to shunt
these very far south and it will be a clean miss. Retained the
isolated thunderstorms through Tuesday night at Yap and through
Wednesday at Koror Palau to cover the northern part of the current
Combined seas will remain elevated by about 2 feet the next couple
of days as long-period swell moves through the area from distant
Typhoon Lionrock. Surf will be increased, mainly on north and west
facing reefs, but is expected to stay below hazardous levels.
GU...High Surf Advisory until 6 PM CHST Tuesday for GUZ001>004.