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Hawaiian Islands Synoptic Discussion and Guidance

523
FXHW60 PHFO 141338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Mon Oct 14 2019

.SYNOPSIS...
Breezy trade winds will continue today, then trend down Tuesday
through midweek as a front approaches from the north. This
approaching front combined with an upper-level disturbance moving
into the area will support increasing rainfall chances by
Wednesday, which could linger into Thursday. Drier trade wind
weather will resume Friday.

&&

.DISCUSSION...
The latest surface analysis showed high pressure centered far
northeast of the islands, which is supporting the moderate to
breezy trades in place. This combined with an upper high centered
overhead, dry air and a strong subsidence inversion (5,000 to
6,000 ft) has led to stable conditions. The latest radar loop
reflects this with minimal shower coverage. Peak rainfall totals
over the past six hours ranged from a tenth to a quarter of an
inch.

The short-term guidance (through Wednesday) remains in good
agreement and depicts the dry and stable trade wind pattern holding
into Tuesday. Clouds and showers will focus over windward and
mountain locations, with the best coverage most likely occurring
through the overnight and early morning periods.

Shower coverage is forecast to trend up Tuesday night through
Thursday. High pressure aloft will shift west of the area as an
upper trough dips south and closes off into an upper low over the
islands by Wednesday. Trades will trend down, potentially enough for
a land and sea breeze regime to become established in some areas, as
a dissipating front approaches from the north. Guidance suggests
this boundary weakening into a broad surface trough as it reaches
the islands Wednesday, then drifting west of the islands by Thursday
night.

This surface feature combined with the upper low and plenty of
moisture will support better shower coverage Wednesday through
Thursday, especially through the afternoon and evening periods.
Daytime heating combined with a relatively cool pool aloft may be
enough added instability to trigger a thunderstorm or two.

For the extended period (Friday through the weekend), expect a
return of a more typical trade wind weather pattern as the upper
low moves west of the state and the atmosphere stabilizes. Trades
may briefly shift out of a more east to southeast direction over
the weekend.

&&

.AVIATION...
Expect moderate east-northeasterly trade winds through this
forecast period. The winds have not been strong enough to warrant
a SIGMET for low-level turbulence and may not through the day
today. Will monitor and issue accordingly. Periods of passing MVFR
conditions are possible over windward mauka areas through the day
today and will issue SIGMET for mountain obscuration only if
warranted if MVFR conditions will persist. VFR conditions
elsewhere.

No AIRMETs are currently in effect.

&&

.MARINE...
A ridge of high pressure north of the state will keep moderate to
locally breezy trade winds in place through tonight. As a result,
a Small Craft Advisory is now in effect for the typically windy
waters around Maui and the Big Island through 6 AM Tuesday. The
trades will ease Tuesday through Thursday as a front approaches
and stalls out to the north of the waters. High pressure building
northeast of the islands will then strengthen the trades back to
moderate and locally strong levels Friday.

A series of south and south-southwest swells will continue to
move through the islands, keeping surf from going flat along south
facing shores.

Surf along exposed west facing shores will trend up today as a
small westerly swell from former Tropical Cyclone Hagibis fills
in. A gradual rise is expected through Tuesday before dropping off
on Wednesday. Some of this energy may wrap into some northern
exposures as well.

Surf will remain small along north facing shores through the
first half of the week. A small long-period northwest swell will
build Wednesday night, peak Thursday, then gradually lower through
the end of the work week, keeping some surf in place along north
and west facing shores. Another long-period north swell will be
moving through Wednesday night through Friday as well. A smaller
long-period northwest swell is expected over the weekend, keeping
some surf in place along north and west facing shores.

Rough and choppy surf will continue along east facing shores
through tonight, before trending down Tuesday through Thursday as
the trades ease. East shore surf will then trend of toward the end
of the week and over the weekend as the trades increase once
again.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...Jelsema

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



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