Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 180215
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
415 PM HST Wed Oct 17 2018

.SYNOPSIS...
Moderate trades will slowly weaken through Friday, then strengthen
again through the weekend. Remnants of a front will linger near
the Big Island and Maui through Saturday night with drier
conditions for the western part of the state. A trough aloft will
bring somewhat unstable conditions around the Big Island before
moving away Saturday night.

&&

.DISCUSSION...
A weak surface ridge north of the state is driving gentle to
moderate trade winds across the island chain. Visible satellite
shows a stationary, remnant frontal band lined upstream of the
Big Island with trade winds carrying the clouds and scattered
showers over windward portions of Oahu to the Big Island this
afternoon. Meanwhile, leeward sides of the islands are seeing
mostly sunny skies this morning. Water vapor imagery shows a
trough aloft over the western portion of the state. The
instability from this feature is lining up with the moisture axis
focused on the Big Island, resulting in some heavier showers and
thunderstorms in the windward and southeast side of the Big Island
this afternoon. Anticipate these showers will taper off in
intensity after sunset and focus on the windward Big Island slopes
tonight. Drier trade wind weather is expected across the smaller
island tonight, with a few windward and mauka showers.

On Thursday and Friday, expect a continuation of the current
weather pattern due to the main features remaining nearly
stationary. High pressure will slowly weaken to our north, so
expected a gradual weakening in the trade wind speeds. Afternoon
sea breezes may set up on leeward sides on Friday. A slight
chance of thunderstorms is again expected during the Thursday and
Friday afternoon.

Latest GFS and ECMWF runs are beginning to show the upper trough
amplifying as it shifts eastward across the state Friday night and
Saturday. This will bring in cold air aloft that will act to
destabilize the atmosphere a bit more. The western half of the
state may not see much from the upper trough, however, as it
reaches Maui and the Big Island where it can tap into the remnant
frontal boundary hung up near the Big Island, showers could become
heavy and produce more widespread thunderstorms across the eastern
part of the state Friday night into Saturday. This has been added
to the forecast and we will work to fine tune timing and location
as the event gets closer.

Both the upper trough and remnant front will move east of the
area Saturday night with drier, moderate trade winds filling in
Sunday to early next week.



&&

.AVIATION...
Light to locally brisk trade wind flow continues. Bands of showers
embedded in the the flow will continue to impact mainly north and
east facing slopes and coasts while leeward coasts and leeward waters
experience mostly clear skies.

A thick band of low clouds continues to shroud the Windward Coast
of the Big Island . AIRMET Sierra remains in effect for north
through east facing slopes of the Big Island. Conditions will
persist through the evening hours and possibly overnight.

Sea breeze driven cumulus over Leeward Big Island will dissipate
after sunset, but will redevelop tomorrow afternoon. Locally
heavy downpours possible over the mountains this evening.

The airmass over the Big Island remains moderately unstable.
Slight chance of thunderstorms over interior Big Island through
late this evening. Conditions are not expected to redevelop
tomorrow.

Over the remainder of the state, the air mass is becoming
increasingly stable. High based stratocumulus tops are not
expected to exceed about 8000 ft west of Molokai. Shower activity
will be light and limited over the smaller islands.


&&

.MARINE...
The current north-northwest swell is on the decline after peaking
earlier today, however surf is likely not going to drop below High
Surf Advisory levels until sometime overnight, so have extended
the HSA through tonight.

Seas have dropped below 10 feet and trades have dropped below 25
knots, so the Small Craft Advisory (SCA) is being cancelled this
afternoon. Winds are expected to continue to drop through the rest
of the week, and remain below SCA levels into next week. Seas are
expected to remain below SCA levels through the weekend.

A new north-northwest swell is expected to arrive this weekend
which will boost surf along north and west facing shores, but will
remain below advisory levels. A new south swell will arrive
Friday, peaking Saturday into Sunday is expected to peak at high
end advisory or low end warning levels.

See the latest collaborative surf discussion (SRDHFO) for more
details.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for north and west
facing shores of Niihau, Kauai, Oahu and Molokai and north facing
shores of Maui


&&

$$


DISCUSSION...Foster
AVIATION...Bedal
MARINE...M Ballard


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