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Hawaiian Islands Synoptic Discussion and Guidance

976
FXHW60 PHFO 171311
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
311 AM HST Wed Oct 17 2018

.SYNOPSIS...
High pressure north of the state will keep a light to moderate
trade wind flow in place through Thursday. The trades will ease
slightly on Friday as a front passes by to the north of the
island chain. Moderate trade winds will then return over the
weekend as high pressure builds north of the state once again.
Clouds and showers will favor windward and mauka areas through the
period, with a stray shower reaching leeward areas from time to
time.

&&

.DISCUSSION...
Currently at the surface, a weak ridge of high pressure is
centered around 600 miles north of Kauai, with the resulting
gradient south of the high driving light to moderate trade winds
across the island chain early this morning. Infrared satellite
imagery shows mostly clear to partly cloudy skies in leeward
areas, with partly to mostly cloudy conditions in windward
locales. Radar imagery and rain gages indicate that most of the
shower activity remains confined to windward sections of Maui
County and the Big Island, with a few isolated showers moving into
windward sections of Kauai and Oahu from time to time. Main short
term concern revolves around rain chances.

High pressure will hold in place to the north of the state through
tonight, keeping a light to moderate trade wind flow in place. A
front will then pass by to the north of the island chain Thursday
through Friday, weakening the high pressure ridge, and easing the
trades slightly. Overall light trades should prevail across the
entire island chain by Friday, with some of the more sheltered
areas possibly seeing land and sea breezes develop. High pressure
will become better established to the northwest of the state over
the weekend, bringing a return of moderate trade winds to the
island chain that will last through early next week.

As for sensible weather details, remnant moisture associated with
an old front will continue to hang up over the eastern end of the
state through the remainder of the work week. This is expected to
keep showers rolling in with the trades across Maui and the Big
Island through the work week. The airmass will also remain a bit
unstable over the Big Island, and an isolated thunderstorm can't
be ruled out each afternoon through Friday. Drier air will remain
anchored over the smaller islands, particularly to the west of
Maui, so mainly windward and mauka showers are expected here,
primarily during the overnight and early morning hours.

The return of moderate trades and precipitable water values near
normal, should bring a more climatological trade wind shower
pattern to the islands over the weekend through early next week.
As a result, expect scattered windward and mauka showers through
the period, with a shower or two drifting leeward from time to
time.

&&

.AVIATION...
The low clouds shrouding the windward slopes of NE Big Island,
appear to have retreated to the adjacent waters and coast,
perhaps due to the assistance from the off-land breeze. Although
there has been a few showers passing through Hilo Airport, the
slopes appear to be VFR. We will reevaluate whether or not to
take down AIRMET Sierra at 16z. There is a similar patch of low
clouds hugging the windward coastal areas of Maui, with some
indications of being mainly high based stratocumulus clouds above
4k feet. Most of Molokai and Oahu are in fine shape with CAVOK
conditions (Ceilings and Vis are OK). Kauai is in the path of a
east to west oriented band of clouds, so some passing showers and
brief MVFR ceilings are expected for most of this morning. The
showers are mainly light with no restrictions to vis. A small bit
of instability remains over on the east side of the Big Island,
which can result in an isolated thunderstorm at the higher
elevations this afternoon. Otherwise, VFR conditions prevail.

&&

.MARINE...
The current north-northwest swell will peak today. The High Surf
Advisory (HSA) remains in effect for north and some west facing
shores of the smaller islands as well as for north facing shores
of the Big Island exposed to north-northwest swell. We will
continue to monitor should the swell peak higher than forecast,
requiring a warning.

The swell is producing seas in excess of 10 feet over some
waters and, combined with moderate to locally fresh trades, the
Small Craft Advisory (SCA) remains in place for most local waters.
The SCA and HSA are both in effect until 6 pm today, but they may
need to be extended depending on how the event pans out. Trades
are expected to ease during the second half of the week.

A smaller north-northwest swell is expected to arrive this
weekend, but is expected to remain below advisory levels. Surf
along south facing shores will trend down today. A new south
swell is expected to arrive Friday and peak Saturday into Sunday
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 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Alenuihaha Channel-Big
Island Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Lau
MARINE...Powell

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