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Hawaiian Islands Synoptic Discussion and Guidance

951
FXHW60 PHFO 190618
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
818 PM HST Sat Jan 18 2020

.SYNOPSIS...
Locally breezy trade winds will continue through the holiday
weekend as surface high pressure passes north of the state. Stable
conditions will produce only light showers across windward areas
of Kauai and Oahu, while modest showers will persist on windward
Big Island and Maui County. Trades will diminish and shift
southeasterly on Tuesday, followed by a stable land and sea
breeze pattern Wednesday through Friday.

&&

.DISCUSSION...
A stable and breezy northeasterly trade wind flow remains in
place. The trades are being driven by a 1026 mb surface high
centered about 750 miles northwest of the state, and a mid level
ridge building in from the west is producing very stable
conditions with an inversion ranging about 4,000 over western
islands to nearly 8,000 ft near the Big Island. Stable
stratocumulus clouds are piled up along windward slopes and waters
around Kauai and Oahu, where scant rainfall occurred today and is
expected tonight. A small pocket of moisture is moving onto
windward slopes of Maui and the Big Island, where modest rainfall
of generally less than a half inch is due tonight.

A stable, rather dry, and breezy trade wind flow will dominate
over most areas this holiday weekend. Surface high pressure will
passing north of the state will produce locally breezy trades,
and mid level ridging will maintain stable conditions with a
strong low level inversion. Showers will remain slightly active
through tomorrow across windward slopes of the Big Island and
Maui, with a few afternoon showers on Kona slopes. Otherwise, at
times extensive cloud cover over windward areas will only produce
light showers, while leeward weather will remain dry. With dew
points hovering in the upper 50s to lower 60s in the breezy
trades, it will feel cool, even though temperatures will be close
to January normal.

Trades will gradually decline on Monday night and Tuesday as the
surface high moves off to the east. Stable trade wind weather will
persist, with light showers confined to windward slopes and
afternoon sea breezes possible by Tuesday.

Trades will likely be disrupted for the second half of the work
week as an approaching front pushes the surface ridge over the
islands. A stable land and sea breeze pattern is expected. The GFS
and ECMWF are now showing the front stalling near or just north of
Kauai on Friday night and Saturday as trades gradually rebuild.

&&

.AVIATION...
Moderate to strong northeast trade winds flow will continue
through Sunday as a high pressure system passes north of the
islands. Expect AIRMET Tango for low level turb below 7k feet
south through west of mountain ranges of all islands to continue
through 191600 UTC.

An area of clouds and showers with MVFR conditions came on shore
affecting the windward sections of Molokai and Maui, resulting in
AIRMET Sierra for mountain obscuration. We expect this area of
weather to thin out by sunrise Sunday. The tops of these clouds
and showers are no higher than 8k feet, and bases are around 2k
feet as per surface observations. The intensity of these showers
are mainly light but with some brief vis restrictions. This same
area of clouds and showers is currently moving onshore of the Big
Island's Hamakua coast. This area poses a threat of issuing AIRMET
Sierra at any time. As for Oahu and Kauai, they are being
affected by a large broken to overcast deck of stable
stratocumulus clouds with cloud bases around 35 hundred feet. Tops
are around 5 to 6k feet. They contain some isolated to patchy
light showers or drizzle with no apparent vis restrictions. Fair
skies ruled the lee areas of Molokai and Maui and the lee side of
the Big Island. Lanai may get some brief periods of low clouds as
they leak over from windward Maui.

&&

.MARINE...
A high northwest of the area will move east through the coming
week. The high is producing strong to near-gale force northeast
trade winds over the Hawaii coastal waters. A Small Craft Advisory
(SCA) is in effect for all Hawaiian waters through Sunday night
due to a combination of the strong winds and elevated seas. As the
high moves farther east, winds will shift out the east and weaken
below SCA speeds by Tuesday. On Wednesday a ridge will shift
south over the coastal waters. Winds speeds will become gentle to
moderate and wind directions variable.

The strong trade winds are producing moderate rough surf along
north and east facing shores. This surf will gradually subside
next week as the trades weaken.

A new swell building Sunday may boost surf to High Surf Advisory
levels along north and west facing shores Sunday afternoon into
Monday. This swell will gradually lower from late Monday through
Tuesday.

A very large northwest swell building on Wednesday may produce
surf reaching the High Surf Warning threshold. As that swell
subsides, an even larger northwest swell may arrive next weekend.

Surf will remain small along south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Monday for all Hawaiian
waters.

&&

$$

DISCUSSION...Wroe
AVIATION...H Lau
MARINE...Donaldson

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



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