Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 261555 RRA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Thu Apr 26 2018

.SYNOPSIS...
Trade winds will begin to decrease later today through tonight as
high pressure to our north weakens. Low level flow will then
shift to southeasterlies as a front approaches from the northwest.
This front will bring increased rainfall to the islands, with
showers reaching Kauai late tonight and Friday, before moving down
the island chain Friday night through Sunday morning. Showers may
linger into the new work week over the Big Island.

&&

.DISCUSSION...
A 1028 mb high far north of the main Hawaiian Islands continues
to drive moderate to locally breezy trade winds across the chain.
Surprisingly, ASCAT and radar motion show that winds remain
breezy across the traditionally windy waters around Maui and the
Big island this morning in spite of earlier expected decrease
toward daybreak. We extended the Small Craft Advisory through the
day today to cover this, but this advisory may be cancelled early
if winds drop. Satellite loop shows the bulk of a band of broken
to overcast low clouds embedded within trade flow just east of the
islands, and associated rainfall is beginning to increase across
windward portions of the islands east of Kauai. Trade showers will
likely pick up later this morning as the main portion of the
cloud band arrives. Rainfall will continue to favor windward and
mauka areas. POP/Sky/Wx grids have been adjusted for today to
account for these increasing showers.

Models show trade winds will begin to decrease later today
through tonight as the high to our north weakens and gets pushed
eastward by a front expected to develop to our northwest. Local
winds will then begin to change from weak northeasterly trades to
southeasterlies as this front approaches the islands. The front
will reach Kauai early Friday, spreading increased showers along
and ahead of it. Uncertainty remains concerning the strength of
the front as it moves across the islands Friday into Sunday.
Models show colder temperatures aloft developing by Sunday, likely
making this front a more prolific rain producer as it reaches the
Big island late in the weekend. The probability for heavy
rainfall increases from Friday through Sunday, but it remains too
early to tell which island, if any, is under threat. The frontal
boundary may stall near the Big island, possibly providing that
islands with lots of rainfall into early next week.

We expect drier trade wind flow will return to the smaller islands
after frontal passage, setting up a pattern of windward and mauka
showers, post-frontally, into next week. Moisture from the frontal
boundary near the Big island may enhance these showers from time
to time.

&&

.AVIATION...
Moderate strength trade winds will begin to weaken today as a
trough approaches the islands from the northwest. AIRMET Tango
remains in effect this morning for Tempo Moderate Turbulence over
and downwind of mountain areas. This low level turbulence AIRMET
will likely be removed later today as trade wind speeds decrease.

A band of unstable clouds will drift across the state producing
periods of MVFR ceilings and showers mainly along windward slopes
of all islands through at least the morning hours. AIRMET Sierra
in effect for Tempo Mountain Obscurations over mountains and along
north through east slopes of Molokai through the Big Island. This
AIRMET will likely need to expand to Lanai, Oahu and Kauai later
this morning. A few of these showers will drift leeward, however
mostly VFR conditions are forecast along southern and western
slopes of each island.

&&

.MARINE...
A surface high far to the NNE will move E and gradually weaken
over the next two days as a low develops about 1000 miles N of
the islands. Fresh to locally strong trade winds will later today,
then collapse completely tonight as a weak trough develops over
the area. A Small Craft Advisory (SCA) currently posted for the
windier marine zones around Maui and the Big Island will continue
through the day today before the trades begin to diminish.

A front associated with the developing low is expected to reach
Kauai sometime Friday, then move down the chain through Saturday
before potentially stalling near the Big Island through the rest
of the weekend. Initially, right behind the front, N winds may
briefly reach SCA criteria in some of the marine zones. The low
will send a mix of swells and seas toward the islands this weekend
into next week, with associated seas rising above 10 feet in most
zones, necessitating a SCA from late Saturday into Monday.

A moderate long-period NNW swell is expected to arrive today and
peak Friday, with peak surf heights below advisory levels. The low
that develops N of the islands late in the week will remain in
place until early next week. The amount of swell/surf that arrives
in the islands is somewhat uncertain, and will depend on the
evolution of the low and associated fetch. Current indications are
that a fairly significant (but poorly organized) swell will
produce advisory-level surf along exposed N and W facing shores,
with the peak of the swell around Sunday.

Elsewhere, several pulses of relatively small SW to S swell are
expected over the next week or so, with a peak in swell energy
around Sunday.
Rough and choppy surf along E facing shores will diminish Thursday
and Friday. The Oahu Surf Discussion (SRDHFO) contains further
details on the sources of the swells.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Morrison



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