Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 231340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Sun Apr 23 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 today, with
windward showers occasionally spreading leeward, and some briefly
heavy downpours on Kauai. Fewer showers are expected to ride in on
the trade winds from tonight 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 nearly stationary NE-SW
oriented trough axis aloft located about 350 miles NW of Kauai, with
the associated SW flow aloft sending broken to overcast high clouds
over all islands. Instability associated with the low is allowing
moisture approaching Kauai to become enhanced, with cloud tops near
Kauai reaching as high as 15-20 kft, based on radar and the early
morning sounding from PHLI. Towering cumulus and isolated
thunderstorms are noted over Hawaiian Offshore Waters near the
trough axis W of Kauai. A more stable air mass lies over the Big
Island, with a subsidence inversion based near 8 kft.

A 1027 mb surface high centered about 1000 miles NE of the area has
an associated surface ridge located about 600 miles N of Oahu, and
is supplying the islands with a moderate to locally breezy trade
wind flow. In the low-levels, low-level moisture associated with a
dissipated front is becoming difficult to identify, partly due to
the thick high cloud coverage, and also due to the boundary becoming
increasingly diffuse. Nonetheless, latest radar imagery depicts
fairly widespread shower coverage over windward areas from Kauai to
Maui.

This relatively wet trade wind flow is expected to continue from
Kauai to Maui this morning, with especially heavy showers expected
to occasionally move across Kauai. A gradual drying trend is
expected from E to W across the area later today. Diminished
moisture is expected to fuel fewer trade wind showers from tonight
through Tuesday, despite the continued presence of the low aloft to
the W of the area. Associated high clouds are expected to shift E of
the area by tonight.

The trade wind supplying high to the NE will drift slowly E through
the upcoming week, but a front impacting the W end of the ridge is
expected to lead to some veering (to the E or ESE) and weakening of
the trade wind flow tonight and Monday. Latest guidance indicates
that moderate E to ESE low-level flow will deliver passing windward
showers Tuesday and Wednesday, which are expected to be most active
nights and mornings. Increasing trade showers are possible Wednesday
night and Thursday, with a trend toward an unsettled weather pattern
toward week`s end.

A significant surface trough/front is forecast to be near/over the
islands toward the end of the week, supported by an amplifying
trough aloft and associated cold air which could significantly
destabilize the island atmosphere. Models are currently honing in on
the Thursday night through Saturday time frame for the greatest
instability, with the ECMWF now more aggressive in destabilizing the
atmosphere. Forecast confidence is low due to poor run-to-run and
model-to-model consistency.

&&

.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 Molokai
and the Big Island will remain mostly dry, otherwise TEMPO MVFR
ceilings in scattered showers will persist across all islands
through midday.

AIRMET SIERRA for mountain obscuration continues for Kauai.
Conditions are expected to persist through midday.

&&

.MARINE...
The latest scatterometer pass missed the windiest areas, but showed
20 kt just downstream of those areas. The station at Makapulapai, on
the north shore of Molokai, has been gusting 25 to 30 kt for the
last several hours, indicating likely Small Craft Advisory (SCA)
winds over the windward waters of Molokai and Maui. This is due to
the flow shifting a little more out of the southeast, which
subsequently puts Maalaea Bay in the wind shadow of Haleakala. We
have extended the SCA through the day today, with the exception of
wind-sheltered Maalaea Bay.

Otherwise, relatively quiet for the next few days with a number of
mainly small swells affecting NW, S, and E shores. A small NW swell
will arrive tonight 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-SSW 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 CA for a few days, which
will keep a long fetch of moderate trades aimed at us. This will
maintain choppy and elevated E 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 W and NW offshore waters today.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maui County
Windward Waters-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Bedal
MARINE...TS



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