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Temp-0.2 C
RH39 %
WindNNE 34 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

177
FXHW60 PHFO 151340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Sat Dec 15 2018

.SYNOPSIS...
Mostly dry and pleasant weather can be expected over the weekend
thanks to a stable trade wind air mass. Winds will gradually
diminish over the weekend and on into early next week as the ridge
of high pressure weakens, due to a front approaching the islands.
The front is expected to move down the island chain from late
Tuesday through Wednesday with showers increasing with its
passage. Cooler and breezy weather will fill in behind the front.

&&

.DISCUSSION...
Satellite imagery shows a broken low cloud layer made up of a
stable stratocumulus deck, spaced out across the windward waters.
Showers, if any, are of the light, drizzle variety. They are
moving from east to west with a moderate to locally strong trade
wind flow. Some changes were made to the grids overnight,
primarily drying out the trade wind showers to isolated for all
islands' windward and mountain sectors, through Sunday night.
The morning soundings from Lihue and Hilo showed a strong low
level inversion with a base at around 5k feet. Precipitable water
ranged from 0.77 to 0.93 of an inch, which is quite dry.

A 1024 mb surface high lies 560 miles NW of Kauai, with a ridge
extending to 320 miles N of Oahu this morning. The two combined to
sustained a constant flow of trade winds to the islands at
moderate to locally strong speeds. The center of this high is
expected to pass directly N of the islands on Sunday with a 1022
mb center. It is not a very strong high, but it will be strong
enough to maintain locally strong trades to the sensitive near-
shore areas of Maui County and waters south of the Big Island.
Please refer to marine section below for more details.

The strength of the trades will dip to moderate speed by Monday
night while the front continues on its eastward push toward the
islands. The front is expected to reach Kauai Tuesday afternoon,
then proceed down the island chain through Tuesday night, finally
pushing east of the Big Island by Wednesday evening. The front is
expected to have a some scattered to numerous showers, heavily
favoring the windward and mountain areas. These showers won't
have much depth, only to 8 to 10k feet. But they can have some
very low visibilites. Scattered light trade wind showers continues
behind the front, lasting through most of Wednesday before drying
out. Along with these showers, is a cool air mass, where we expect
highs on Wednesday to be in the mid to upper 70s. Wednesday
night's minimum temps are expected to be only in the 60s. The NE
trade winds will turn more easterly on Thursday and Friday,
resulting in slightly warmer temps.

&&

.AVIATION...
A deep-layer ridge centered N of the islands will change little
through tonight, with stable trade wind weather prevailing. The
moderate to locally fresh ENE trade winds will deliver stable low
clouds that will favor windward areas, and a strong subsidence
inversion based near 5000 feet will keep rainfall chances quite
low. While brief and isolated MVFR ceilings may occur, VFR conditions
will prevail statewide. AIRMET Tango for low-level turbulence to
the S and W of the mountains will remain posted today, but may be
cancelled tonight or Sunday as trade winds ease slightly.

&&

.MARINE...
Trade winds have remained elevated for the typically windier
areas near Maui and the Big Island, so the Small Craft Advisory
(SCA) continues for those zones.

A large, long-period northwest swell will arrive late in the day
and rise rapidly. A High Surf Warning (HSW) will be posted to
account for this episode. It is expected to peak Sunday, and then
subside some on Monday. An even larger northwest swell will build
late Monday, peaking well above warning levels. Exposed coastal
waters will once again rise to over 10 feet as these swells come
rolling through. Expect the current SCA to once again be expanded
to most, if not all, the coastal waters for the first half of the
new week as the new swells arrive.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&


.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from 4 PM this afternoon to 6 AM HST Monday
for the north and west facing shores of Niihau, Kauai, Oahu,
and Molokai, and north facing shores of Maui.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the Big
Island.


&&

$$


H Lau/Birchard/Kinel

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office