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Hawaiian Islands Synoptic Discussion and Guidance

667
FXHW60 PHFO 200637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Sat May 19 2018

.SYNOPSIS...
A surface ridge will persist north of the islands through the
coming week.

&&

.DISCUSSION...
As of 8 PM Saturday, surface observations and satellite loops show
locally breezy easterly trade winds blowing across the islands.
Satellite loops show a line of showery low clouds extending about
150 miles east from Kauai. Radars show scattered light to
moderate showers in this line moving over windward Kauai. There
are broken low clouds over windward areas of Oahu and Maui County,
mostly stable clouds, with scattered clouds over lee areas.
Radars show mostly dry conditions over Oahu and Maui County.
Satellite loops show a 120 mile wide band of showery low clouds
extending about 500 miles east northeast from over the windward
side of the Big Island. Numerous showers from this band have
started moving over the windward side of the Big Island. Clouds
have become scattered to broken over the leeward side. Little
change is expected over the next several hours.

A surface ridge north of the area will continue to produce
locally breezy trade winds. The ridge will weaken slightly over
the next few days as a front approaches from the northwest. The
front will stall well north of the islands on Monday. A strong new
high will build northwest of the islands by the middle of next
week.

The cloud band moving over the Big Island tonight will keep the
windward side of that islands rather cloudy and rainy through
tomorrow. Trade winds will push additional bands of showery low
clouds over the islands through the coming week. Ridging aloft
will keep the atmosphere relatively stable so showers will be
focused over mainly windward areas, and excessive rainfall is not
expected. Lower leeward areas will remain mostly dry except for
afternoon showers over the lee slopes of the Big Island and east
Maui.

Close monitoring of Kilauea on the Big Island continues, as
Hawaii Volcano Observatory personnel indicate that additional
steam-driven explosions are possible at any time. Ashfall
Advisories or Warnings will be issued if necessary.

&&

.AVIATION...
The gusty low-level trade wind flow will persist through Sunday.
Ridging aloft is maintaining a relatively strong low-level
inversion between 5,000 and 7,000 ft. As a result, low-level
mechanical turbulence will continue to the lee of the higher
terrain on all islands through Sunday, and AIRMET TANGO remains
in effect.

Low clouds and showers transported by the robust low-level trade
wind flow are streaming into portions of the windward sides of
some islands, especially the windward Big Island. This may
result in brief periods of MVFR conditions. An AIRMET for
mountain obscuration may be required for the windward Big Island
later tonight, or Sunday. Otherwise, VFR conditions are expected
to prevail.

After a series of eruptions at the Kilauea/Halemaumau crater
since last night, a SIGMET for volcanic ash remains in place,
Hawaii Volcano Observatory points out that additional eruptions
are possible at any time, and with little warning.

&&

.MARINE...
Fresh to strong trades associated with high pressure north of the
waters are forecast to hold through much of the upcoming week.
Strongest winds are expected over the channels east of Oahu, south
of the Big Island, and Maalaea Bay. Seas have responded and will
remain rough, likely remaining near or at the advisory level (10 ft)
south of the Big Island and across the Alenuihaha Channel through
the rest of the weekend. A weakness within the ridge is forecast
to develop by Tuesday as a front passes far north of the area,
which may translate to the strong trades slightly weakening
locally for a brief period.

Two overlapping long-period south-southwest (190-200 deg) swells
from a couple of sources that passed near New Zealand around a week
ago continue to fill in this evening. The latest observations from
the nearshore PacIOOS buoys near Pearl Harbor were depicting peak
energy coming in within the 16-18 and 18-22 sec bands. Surf
heights associated with this energy should stay just below the
surf advisory level (8 ft faces) for south facing shores through
Sunday. A gradual downward trend is expected by Monday. Another
similar pulse out of the south-southwest is expected through
midweek or by Thursday.

Up north, surf associated with the north-northwest (330-350 deg)
swell that filled in and peaked Friday will continue to fade
through the rest of the weekend. Another small north-northwest
pulse could bring the surf back to similar levels that were
reached on Friday by midweek along north facing shores. Otherwise,
surf will continue to focus at select reefs exposed to trade wind
energy wrapping in.

Surf along east facing shores will remain rough through the rest
of the weekend as fresh to strong trades hold locally and
upstream south of high pressure. Heights should remain below
advisory levels for east facing shores. Heights should trend down
slightly through the first half of the week as a front passes to
the north and the large area of upstream fresh breezes weaken.

For the long range, guidance continues to depict plenty of energy
setting up within Hawaii's swell window from the Tasman Sea early
next week. ECMWF and GFS show back-to-back gales nosing northward
through the Tasman focusing energy down the 200-220 deg
directional band Sunday night through Wednesday. WAVEWATCH III
depicts two main long-period pulses from these sources out of 220
deg filling in at Pago Pago with the first source arriving
Wednesday night (5/24) and the second pulse arriving Thursday
night (5/25). If this scenario evolves as predicted, these sources
should make it to the local Hawaiian waters toward the end of the
month (around 5/29).

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM Sunday for waters around the Big
Island and Maui County except Big Island windward waters and
waters around Oahu except Oahu Windward waters.

&&

$$
DISCUSSION...Donaldson
Aviation...Houston
Marine...Gibbs

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



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