Flash Flood Warning 1, NWS Guam

Area Forecast Discussion
National Weather Service Tiyan GU
840 AM ChST Wed Jul 21 2021

.Marianas Synopsis...
Satellite shows mostly cloudy skies and isolated showers moving
through the Marianas waters. Numerous showers are just to the west,
and have been propagating to the southeast. The Ipan buoy reveals
combined seas of 3 to 4 feet. Since there is a slight uptrend, have
reported this in the CWF as 3 to 5 feet.

&&

.Discussion...
The band of rain is now just about 10 miles west of Apra Harbor, so
much for numerous showers in the afternoon, looks more like 9 AM now,
maybe even 8:30 AM. The GFS MOS shows a downtrend for the evening,
hopefully this works out, as it might make it easier to squeeze in
the drone show. Then, after midnight, we go ahead and bring in the
main event, occasional locally heavy showers, and keep it in all day
Thursday. Winds will mostly be in the 15 to 20 mph range, although
gusts up to 35 mph have been reported by Saipan's wireless ridge.

&&

.Marine...
The southwest monsoon winds are expected to continue perhaps even
into August, although the showers seem to subside a bit starting on
Friday. This will continue to bring in monsoon swell, and could
result in a small craft advisory. Mariners should carefully follow
the progress of this monsoon trough.

&&

.Hydrology...
A flash flood watch remains in effect. The rainfall yesterday did not
quite justify issuance of flash flood warnings, however it saturated
the soils, such that if we got another similar day either today or
Thursday, we will definitely need a flash flood warning at that time.
On the bright side, the Keetch-Byram Drought Index is down to 322
today and will likely go down further the next day or two.

&&

.Eastern Micronesia...
No changes were made to the previous forecast. The ITCZ has
flattened out and a few spotty convective showers are seen around
eastern Micronesia. No thunderstorms were seen and no rain was
observed overnight. Pohnpei and Kosrae in the short term are partly
cloudy, but should soon start increasing in cloud cover as
convergent trade winds and a little bit of upper divergence develops
near Pohnpei, but still looks to be more like isolated showers.

For Majuro, it looks like a minor trough passes through on Thursday
to bring a little bit of showers. Other than that, isolated showers
are expected for the rest of the week for all islands.

We will continue to see 3 to 4 foot combined seas for Kosrae and
Pohnpei, and a bit higher for Majuro at 4 to 6 feet.

&&

.Western Micronesia...
A weak circulation south of Chuuk will bring scattered showers to
Chuuk today and tonight. By tomorrow a southerly flow sets in with
the boundary of monsoon winds staying north and west of Chuuk over
the next few days to keep showers isolated. By Friday ridging sets
in to bring partly cloudy skies.

More active weather is seen to the north of Koror and Yap with a few
thunderstorms seen in the area of upper divergence near 09W. A weak
ridging pattern within the monsoon looks to keep showers at a
minimum for the short term. Still moisture advection from the south
and strong upper level winds will remain through the weekend, so the
situation may change. GFS puts winds at about 15 to 20 kts from the
southwest for the weekend, ECMWF is showing 10 to 15 kts, which will
keep winds, seas, and surf below advisory criteria.

Combined seas are 3 to 5 ft for Koror and Yap for the next few days.
By this weekend, expecting higher conditions at 5 to 7 feet. Chuuk
is expected to stay in the 3 to 5 ft range through the weekend.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Flash Flood Watch through Thursday afternoon for GUZ001.
Marianas Waters...None.
&&

Marianas: Stanko
Micronesia: Hong