Flash Flood in Guam, August 27, 2020

Area Forecast Discussion
National Weather Service Tiyan GU
1240 PM ChST Thu Aug 27 2020

.Update...
A weak surface trough moving through the Marianas is interacting with
upper-level divergence to generate scattered showers and isolated
thunderstorms across the region. Brief heavy showers are possible
with thunderstorms.

&&

.Prev discussion... /issued 743 AM CHST Thu Aug 27 2020/

Marianas Synopsis...
Calm winds and fair weather persist across the area early this
morning, but a band of showers are seen developing closer to Saipan
and Tinian. A monsoon trough sits just south of Guam with deep
convection found to the south and east of the islands slowly moving
toward the area. A broad circulation well west of the Marianas near
15N132E continues to slowly organize.

Discussion...
Little change was made to the forecast for the short term as pleasant
morning weather gradually deteriorates through the day. As the
monsoon trough gradually lifts into the area this afternoon and
evening, showers and thunderstorms are expected to continue to
increase for the Marianas for tonight and Friday. Drier weather is
expected over the weekend with easterly trade winds returning early
in the week. By next Tuesday, a broad surface trough will bring
additional showers and thunderstorms to the area. The current
forecast brings that round of wetter weather to the area about a day
earlier than the prior forecast.

Marine...
Buoys and altimetry show seas between 2 and 4 feet this morning with
larger seas found farther west in the vicinity of the stronger
southwest monsoon flow. Seas are comprised of a mix of a long-period
northwest swell, an east swell, and a small southwest swell. Each is
small at 1 to 2 feet. Seas will hover between 2 and 4 feet through
the weekend before increasing early next week in response to the
developing circulation over the Philippine Sea. The rip risk will
remain low the next few days.

Tropical systems...
The Philippine Sea circulation, now near 15N132E continues to improve
in organization as it slowly heads west. Models have come into better
agreement in showing further development in the coming days as the
circulation slowly turns WNW. While models continue to indicate this
system will develop into a significant tropical cyclone in the coming
days, its only effect on the Marianas and west Micronesia will be due
to ocean swell generated by the circulation.

Eastern Micronesia...
Convection has diminished in Eastern Micronesia to the point where
only isolated showers are expected the next several days. Therefore,
the forecast is just trying to time the movement of clouds and
whether or not there will be any thunder. There had been an area of
cooling cloud tops approaching Kosrae, so went ahead and put some
thunderstorms in for them today... since then, the whole thing fell
apart and they are likely to remain mostly sunny for the whole day
now. This highlights the uncertainties in forecasting convection.

All winds will remain below 15 knots, and all seas will remain below
4 feet. Enjoy the benign marine conditions.

Western Micronesia...
Weather is more active in Western Micronesia. The monsoon trough is
creating scattered showers and isolated thunderstorms across wide
areas. Chuuk should break free after today, although it could be back
early next week. Yap and Koror will probably not dry out until Friday
night. However, they will last through Monday for drier weather.

Western Micronesia will have slightly higher winds and seas, expected
because of the circulation to their north as it slowly intensifies.
However, at 15 knots and 5 feet, it is still well below hazardous
levels, so enjoy the benign marine conditions here as well.

&&

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

Kleeschulte/W. Aydlett