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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 221333
Area Forecast Discussion
National Weather Service Honolulu HI
333 AM HST Tue Sep 22 2020
Showers will remain focused over mainly windward areas through
tonight. Leeward areas will be mostly dry except for afternoon
and evening showers over the lee slopes of the Big Island. A cloud
area moving west over the islands tomorrow may bring an increase
in showers. Moderate easterly trade winds will strengthen slightly
as a high far northwest of the area shifts southeast. Winds are
expected to diminish again over the weekend.
A surface ridge north of the area is producing moderate easterly
trade winds across the islands. The atmosphere is rather dry and
stable thanks to a ridge aloft. The trade winds will push a few
showers over windward areas, mainly early this morning and again
tonight. Leeward areas will remain mostly dry except for afternoon
showers over the lee slopes of the Big Island. The high clouds
that were dimming the sun mostly cleared away overnight.
Satellite loops show an area of showery low clouds about 300
miles east of the islands moving west in the trade-wind flow. This
cloud area is the remnant of Tropical Storm Karina. As the cloud
area moves west across the islands tomorrow, our weather will
turn wetter, especially over the Big Island. The ridge aloft will
remain over the islands limiting the amount of rainfall. Drier
weather will return as the cloud area moves away to the west
The trade winds will strengthen a bit over the next couple of days
as a surface high far northwest of the islands moves southeast.
Winds will weaken again late in the week as the high moves off to
the east and a front approaches from the northwest.
Autumn arrived at 3:31 am HST this morning.
Light to locally moderate trade wind flow will persist through
twenty-four hours and beyond. The air mass will remain mostly dry
and stable. Low topped cumulus and scattered showers will favor
windward slopes and coasts. Brief MVFR conditions can be expected
over windward slopes and summits as moisture embedded in the trade
wind flow interacts with island terrain. Otherwise, VFR conditions
will prevail all areas.
Light sea breezes will redevelop this afternoon only to once
again give way to weak land breezes after sunset. Expect increased
cloudiness over island interiors in the late afternoon and
evening hours followed by gradual clearing along leeward coasts
and slopes in the late night and early morning hours.
No AIRMETs are in effect or anticipated at this time.
Moderate to fresh trades will remain across the coastal waters for
much of the week. The overnight ASCAT pass showed winds just below
Small Craft Advisory (SCA) levels, but expect the winds to pick
up some today. An SCA is in effect for the typical windier waters
near Maui County and the Big Island through Wednesday night, but
will likely be extended through the week. Winds will be hovering
near SCA levels today, with an uptick expected Wednesday through
Friday when the high to the north builds. Some weakening is
expected this weekend, and some of the global models suggest more
weakening of the winds for next week. Surf along east facing
shores will increase some later this week with the winds
increasing, but is expected to peak below advisory levels.
A long period north northwest swell will build late Tuesday night,
peak Wednesday night/Thursday at around 6 feet and 15 seconds,
followed by gradual lowering through Friday. Expect surf to peak
just below advisory levels for the north shores.
A small south-southwest swell will maintain surf along the south
facing shores near the summer time average of 3 to 5 feet through
midweek. Another small south southwest swell is possible Friday
.Small Craft Advisory until 6 AM Thursday for Maalaea Bay, the
Pailolo and Alenuihaha Channels and waters south of the Big
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office