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Hawaiian Islands Synoptic Discussion and Guidance

320
FXHW60 PHFO 140632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Sun Oct 13 2019

.SYNOPSIS...
Breezy trade winds will continue through Monday, 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 one to two tenths 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.

&&

.AVIATION...
Expect moderate east-northeasterly trade winds through this
forecast period. The winds have not been enough to warrant a
SIGMET for low-level turbulence and may not through Monday. Will
monitor and issue accordingly. Periods of passing MVFR conditions
are possible over windward mauka areas through Monday 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...
High pressure centered far northeast of the islands will maintain
moderate to locally strong trades for another day before trending
lower to light to moderate on Wednesday while a weakening front
approaches the islands from the north. The Small Craft Advisory
(SCA) remains in effect for the typical windier near-shore waters
of Maui County and waters south of the Big Island through late
Monday afternoon.

The current small south swell will be on a gradual decline through
Monday. There will be a few smaller pulses coming in during the
first half of the week. In the long range, satellite data from
earlier today showed a large storm-force low well southeast of
New Zealand that has generated seas of 30 to 40 ft range on
Saturday. Although the bulk of this energy will pass well east of
the islands, small south swell may reach our southern shores this
coming weekend.

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

Surf will remain small along north facing shores through the first
half of the week. Then there is a potential of a small northwest
swell to reach the islands on Wednesday night through Thursday. A
passing storm system far north of the islands, possibly from once
Typhoon Hagibis, may reach the islands north facing shores on
Wednesday night, peak Thursday night before a gradual lowering
through Friday.

Rough and choppy surf will continue along east facing shores
through Monday, then trend down into midweek as the trades eases

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...H Lau

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



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