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Area forecast discussion
National Weather Service Honolulu HI
830 PM HST Sat Apr 22 2017

Synopsis...
moderate to locally breezy trade winds will continue the next couple
of days, supported by high pressure northeast of the islands. A wet
trade wind weather pattern is expected to continue overnight and into
Sunday, with windward showers occasionally spreading leeward. Fewer
showers are expected to ride in on the trade winds from Sunday night
through Tuesday. Increasing showers are possible from Wednesday into
next weekend, with the potential for unsettled weather toward the
end of the week.

&&

Discussion...
the current weather pattern features a NE-SW oriented trough axis
aloft located about 350 miles northwest of Kauai, with the associated SW
flow aloft sending broken to overcast high clouds over the state.
Instability associated with the low is allowing incoming moisture to
become enhanced, with cloud tops near Kauai reaching as high as 15-
20 thousand feet, based on radar and the afternoon sounding from
phli. Towering cumulus over Kauai leeward waters are blossoming into
thunderstorms over Hawaiian offshore waters as they near the core of
the low. Latest rainfall report indicates over 3 inches of rain at
mount Waialeale over the past 12 hours, and over 6 inches in the
past 24 hours. A more stable air mass lies over The Big Island.

A surface high centered about 900 miles NE of the area has an
associated surface ridge located about 550 miles north of Oahu, and is
supplying the islands with a moderate trade wind flow. In the low-
levels, an increasingly diffuse, east-west oriented, band of showery low
clouds Marks the remnants of a front that has stalled near the
islands. While the main moisture axis associated with this boundary
was noted to be sliding north of most islands earlier in the day,
leaving only Kauai in it's sights, latest satellite and radar images
reveal showery low clouds increasing in coverage upstream of the
smaller islands, and to a lesser extent windward Big Island.

Latest model guidance indicates that a relatively wet trade wind
flow will continue from Kauai to Maui overnight, with a gradual
drying trend occurring from east to west across the area Sunday and Sunday
night. This will occur as the main moisture band associated with the
old front shifts to the N, with incoming trade showers enhanced by
the low aloft lurking just west of Kauai. Diminished moisture is
expected to fuel fewer trade wind showers Monday through Tuesday,
despite the continued presence of the low aloft to the west of the
area. Associated high clouds are expected to shift east of the area on
Sunday.

The trade wind supplying high to the NE will drift slowly east through
the upcoming week, but a front impacting the west end of the ridge is
expected to lead to some veering (to the east or ese) and weakening of
the trade wind flow Sunday night and Monday. Latest GFS guidance
indicates that moderate east to east-southeast low-level flow will continue over
the area Tuesday and Wednesday. A more significant trough/front is
depicted as developing west of the islands Thursday, and moving
toward/over the islands Friday and Saturday, potentially bringing a
period of unsettled weather and keeping winds on the light side
toward the end of the week. While a similar pattern is noted in the
latest runs of the European model (ecmwf) and navgem, differences are sufficiently
significant to reduce forecast confidence beyond Wednesday/Thursday.

&&

Aviation...
an old frontal boundary remains stalled over the state. Light to
locally moderate trade wind flow has focused the associated moisture
over windward and mountain areas. Leeward sections of Maui and The
Big Island will remain mostly dry, otherwise tempo MVFR ceilings in
scattered showers will persist across all islands.

Airmet Sierra for mountain obscuration continues for Kauai.

&&

Marine...
scatterometer passes from earlier in the day showed some Small Craft
Advisory (sca) winds around the south end of The Big Island, as well
as at phog, gusting over 30 kt there for several hours. However, as
the flow shifts a little more out of the southeast, Maalaea Bay will
likely be in the wind shadow of Haleakala. At the same time, winds
will likely accelerate around the hamakua coast and around north and east
parts of Maui and Molokai. We have added some windward marine zones
to the Small Craft Advisory for tonight, and will likely need to extend the Small Craft Advisory into
Sunday, with the exception of wind-sheltered Maalaea Bay and
possibly Pailolo Channel.

Otherwise, relatively quiet for the next few days with a number of
mainly small swells affecting NW, S, and east shores. A small northwest swell
will arrive Sun night into Mon, and gradually diminish by Tue.
Another will arrive later Tue and spread down the chain Tue night,
then diminish Wed. A couple of small S-south-southwest swells will bring minor
bumps to S facing beaches for much of this week as well. A surface
high will stall roughly halfway between hi and California for a few days,
which will keep a long fetch of moderate trades aimed at US. This
will maintain choppy and elevated East Shore surf, though remaining
below advisory criteria.

Lingering moisture associated with the old frontal band interacting
with an upper level low about 300 nm west of Kauai could result in a
few thunderstorms over the far west and northwest offshore waters through
Sunday.

&&

Hfo watches/warnings/advisories...
Small Craft Advisory until 6 am HST Sunday for Maui County
windward waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island windward waters-Big Island leeward waters-Big Island
southeast waters.

&&

$$

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