Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 261340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Mon Jun 26 2017

.SYNOPSIS...
High pressure northeast of the State will keep trade winds in
place through the upcoming weekend. The trade winds will
strengthen across the island chain today and tonight, reaching
breezy to locally windy levels Tuesday through Friday. Drier and
more stable conditions are expected today through Thursday, but a
more showery trade wind pattern will return Friday through the
upcoming weekend as an upper level low passes by just to the north
of the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1031 mb high centered around 1600
miles north-northeast of Honolulu, is driving light to moderate
easterly trade winds across the island chain early this morning.
Meanwhile aloft, a mid-level 700 mb trough, is located just to the
west of Kauai, and is tracking slowly westward. Infrared
satellite imagery shows partly cloudy skies in most areas, with
some broken to overcast conditions moving into windward sections
of Maui and the Big Island as well as Kauai. Radar imagery shows
very little shower activity over Maui County and the Big Island,
with scattered showers moving into windward sections of Oahu and
Kauai. Main short term concern revolves around rain chances.

Today through Thursday,
High pressure will build northeast of the State through the
period, increasing the pressure gradient across the islands. This
will result in strengthening trade winds today and tonight, with
the trades reaching breezy to locally windy levels Tuesday through
Thursday. In addition to the strengthening trades, a drier more
stable airmass will move into the area. Precipitable water values
will remain in the 1.0 to 1.2 inch range through the period, below
normal for this time of year, and this will result in a drier
than normal trade wind shower pattern. Showers will continue to
favor windward and mauka areas, with a stray shower reaching
leeward areas from time to time due to the strengthening trades.
Rainfall amounts are expected to remain light.

Thursday night through Sunday,
Both the GFS and ECMWF are in good agreement showing an upper
level low tracking eastward toward the islands Thursday night and
Friday, but then differ on the progressiveness of this feature
tracking westward to the north of the State over the upcoming
weekend. High pressure will continue to hold in place north of the
islands through the period, with breezy trade winds expected to
continue through at least Friday. The trades are then expected to
weaken over the upcoming weekend, as the gradient slackens due to
the upper level low passing by to the north of the islands. As
for sensible weather details, both models show deep layer moisture
increasing beginning Thursday night, with this enhanced moisture
remaining over the islands through Sunday. As a result, we should
see a more showery/wet trade wind pattern through the period,
with showers continuing to favor windward and mauka locales.

&&

.AVIATION...
The high pressure ridge north of the Hawaiian Islands will
strengthen and bring moderate strength northeasterly trade winds
today. Cloud ceilings and TEMPO showers, mainly during morning
and evening hours, will occur mainly over north through east
sections of all islands with isolated showers elsewhere.

No AIRMETs are in effect. AIRMET Tango may be needed after 22Z
Monday for low level moderate turbulence across Maui and the Big
Island.

&&

.MARINE...
Trade winds will gradually strengthen today as a surface high
builds far north-northeast of the state. This will produce
locally strong trade winds over the typically windy waters
adjacent to the islands of Maui County and the Big Island.
Therefore, a Small Craft Advisory (SCA) has been issued starting
this morning, and continuing through Thursday for these waters.
The current forecast indicates trade wind speeds will remain
locally strong through Friday, before gradually weakening next
weekend.

As the trade winds strengthen, expect a slight increase in short-
period wind waves along east facing shores this week. In
addition, a northeast swell with dominant wave periods of 12 to
13 seconds, which was produced by a fetch of gales west of Oregon
and California, will continue spreading across the Hawaiian
waters. The energy from this swell is being observed at all of
the windward near shore buoys early this morning. There is a low
probability that this northeast swell combined with the trade wind
waves may produce surf approaching the High Surf Advisory
criteria along east facing shores by Tuesday. Some of this
northeast swell energy will also likely wrap into exposed north
facing shores across the state early this week.

Pulses of swell energy from the south-southeast through south-
southwest directions will produce near to or just below summer
time average surf heights along south facing shores early this
week. Reinforcing long-period south-southwest swells will
generate small to moderate surf along south facing shores,
especially from Thursday through next weekend.

Extreme tides that have been observed during the past several
days are gradually trending back toward normal heights early this
week. As a result, flooding impacts along the coast will continue
to slowly diminish during the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 AM this morning to 6 PM HST Thursday
for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston



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