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Hawaiian Islands Synoptic Discussion and Guidance

716
FXHW60 PHFO 191329
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
329 AM HST Sun Jan 19 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. Trades will diminish and shift southeasterly
on Tuesday, followed by a stable land and sea breeze pattern
Wednesday through Friday as a front stalls near Kauai. Trade winds
will rebuild Saturday.

&&

.DISCUSSION...
A stable and breezy northeasterly trade wind flow continues. The
trades are being driven by a 1025 mb surface high centered about
650 miles northwest of the state, and a mid level ridge building
in from the west is maintaining very stable conditions with an
inversion ranging from about 4,000 to 6,000 ft. Stable
stratocumulus clouds are piled up along windward slopes and waters
from Kauai to Molokai, where mostly dry conditions will hold
today. A small pocket of moisture moving onto windward slopes of
east Maui and the Big Island, will produce light to moderate
showers through mid morning, then diminish. Aside from spotty
afternoon showers over South Kona and Kau slopes, leeward areas
will be dry today.

A stable, rather dry, and breezy trade wind flow will dominate
over most areas through the remainder of the 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 the Martin Luther King Jr holiday 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 shift
southeasterly on 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.

Trades will likely be disrupted Wednesday through Friday as an
approaching front pushes the surface ridge over the islands. A
stable land and sea breeze pattern is expected over most of the
state, featuring afternoon clouds and isolated showers over
island interior sections followed by clearing overnight. The GFS
and ECMWF are showing slight differences in the progression of the
weak and shallow front. The GFS has the feature reaching Kauai on
Friday and stalling near Oahu on Saturday, while the ECMWF stalls
the front on Kauai's doorstep Saturday. The current extended
forecast is a blend of these models, keeping higher shower chances
near Kauai on Friday into Saturday and rebuilding trade winds
state-wide on Saturday.

&&

.AVIATION...
The area of clouds and showers affecting windward sections of
the Big Island, Maui, and Molokai, is dissipating. Satellite
imagery now shows stable stratocumulus clouds affecting all
islands. Some light showers and or drizzle are expected with these
clouds. Surface observations indicate a high based stratocu deck
around 4k feet. The inversion ranged from 5 to 6k feet from Kauai
to Molokai, to 7k feet over Maui, to 8k feet over the Big Island.
We are expecting a predominant VFR situation today with even less
showers than yesterday.

AIRMET Tango for low level turb below 7k feet, will continue
through at least 04z, as a 1025 mb surface high passes north of
the islands tonight and Monday. The NE low level wind flow will
veer a little more east on Monday.

&&

.MARINE...
A high northwest of the area will move east through the week.
This high continues to produce strong to near-gale force trade
winds over the Hawaii coastal waters. A Small Craft Advisory (SCA)
is in effect for all Hawaiian waters through Sunday tonight 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 subside later in the week
as the trades weaken.

A new swell building today may boost surf to High Surf Advisory
levels along north and west facing shores this 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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