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Hawaiian Islands Synoptic Discussion and Guidance

703
FXHW60 PHFO 141953 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
950 AM HST Sat Dec 14 2019

.SYNOPSIS...
High pressure northeast of the islands will keep breezy to locally
windy trades blowing today, with the trades easing somewhat Sunday
as the high moves away. A new high passing to the north will bring
increased trade winds again Monday and Tuesday before easing
Wednesday. An area of enhanced showers over Oahu and parts of Maui
will diminish this afternoon, while variable high cloudiness will
continue today before diminishing Sunday. The trade winds will
deliver low clouds and showers to windward areas through the week,
but their coverage and intensity is expected to be limited Monday
and Tuesday as the island atmosphere becomes very stable.

&&

.DISCUSSION...
Trade winds (at times strong) dominate the island forecast through
the next week, supported by a trio of fast-moving high pressure
cells passing far N of the area. The first high is centered far NE
of the islands and moving steadily E, with some easing of today's
breezy trade winds expected Sunday as a weakness in the ridge passes
N of the islands. A second high will build N of the islands Sunday
night/Monday, leading to increased trade winds until another
weakness in the ridge passes to the N around Wednesday. A third high
is expected to pass N of the islands Thursday/Friday, likely
bringing another period of locally strong trade winds heading into
next weekend.

A low aloft centered several hundred miles W of the islands is
embedded within a NE-SW oriented trough, with the trough axis
located several hundred miles N of Kauai. SW flow aloft is bringing
high clouds of varying thickness over the islands, and they are
currently thickest over Oahu and adjacent waters. As the low weakens
and the trough moves S over the next 24 hours, high clouds will
diminish in coverage from N to S, lingering over the Big Island
Sunday before finally moving E of the area as a ridge aloft builds
in from the W.

The nearby low has led to some destabilization of the island
atmosphere, with radar returns indicating the mixed layer extending
to 10-12 thousand feet. While Kauai and the Big Island have received
just a few windward showers this morning, a slug of moisture has
been fueling showers over Oahu and parts of Maui county, with the
showers easily spreading leeward on the increased trades. Although
mostly dry, low clouds over the lower Kona slopes are somewhat
unusual to see this early in the morning when trade winds are
blowing.

Although the upstream trade wind flow contains open-celled showery
cumulus, expect shower coverage to diminish on Oahu and Maui county
later this morning, while potentially increasing on Kauai as the
moisture slug moves W. Forecast models indciate showery low clouds
will continue to move over the islands with the trade flow through
the forecast period, but they are expected to diminish in coverage
and intensity as the ridge aloft builds over the area Monday and
Tuesday. Current forecast grids indicate light showers for this time
period due to the increased stability, reverting back to the more
typical moderate showers thereafter as the strong ridge aloft
weakens somewhat. A rather wet solution for next weekend is noted
with the 12Z GFS as a shortwave trough aloft passes overhead, but
the dryer ECMWF guidance keeps the trough W of the islands.

&&

.AVIATION...
A 1027 mb surface high far northeast of the state will allow for
moderate to occasionally breezy easterly trades to persist through
the remainder of the day. An area of enhanced low level moisture
caught up in the flow will bring scattered to numerous showers to
Oahu this morning. Elsewhere, isolated to scattered showers are
expected to affect mainly windward areas of all other islands.
Breezy easterly flow could cause turbulence on the lee side of the
mountains. Clouds and showers will bring periodic MVFR conditions to
the windward sides of the islands, especially for Oahu.

AIRMET SIERRA remains in effect for areas north thru east sections
of Oahu due to tempo mountain obscurations from clouds and showers.

AIRMET TANGO remains in effect over and immediately south thru west
of mountains on all islands below 9000 feet due to tempo moderate
turbulence.

&&

.MARINE...
Current northwest and northeast swells will diminish through
tonight. A long period northwest swell is expected to build late
Sunday, peaking Monday and Monday night bringing surf around warning
levels along exposed north and west facing shores. This swell should
slowly decline Tuesday into Wednesday. Another northwest swell will
build Wednesday night into Thursday, then decline Friday into the
weekend.

High pressure north of the state will keep strong trade winds over
the area today with a gradual taper off expected into Sunday. A
Small Craft Advisory is posted for all coastal waters through today
and through tonight for the typical windy zones around Maui County
and the Big Island due the strong trades. Trades will boost back to
strong breezes by Monday through at least midweek. Seas will also
increase due to the combination of swell and winds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

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-Maui County Leeward Waters-
Big Island Windward Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Thomas
MARINE...Foster

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



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