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Hawaiian Islands Synoptic Discussion and Guidance

097
FXHW60 PHFO 231337
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Wed Oct 23 2019

.SYNOPSIS...
High pressure northeast of the state will keep moderate to
locally breezy trade winds in place today. The trades will ease
tonight through Friday as a front approaches from the northwest.
The trades are then expected to strengthen Saturday through early
next week as high pressure strengthens north of the island chain.
Fairly typical trade wind weather will prevail, with showers
favoring windward and mauka areas and a stray shower reaching
leeward locales from time to time.


&&

.DISCUSSION...
Currently at the surface, a 1032 mb high is centered around 1750
miles northeast of Honolulu, while a weakening cold front is
located around 650 miles northwest of Kauai. Meanwhile, Invest
93C is located around 950 miles southeast of Hilo. The local
pressure gradient is producing moderate trade winds across the
island chain early this morning. Infrared satellite imagery shows
mostly cloudy conditions across windward sections of Maui County
and the Big Island, with partly cloudy conditions in most other
locales. Radar imagery shows scattered showers moving into
windward areas, with shower coverage the greatest across windward
sections of Maui County and the Big Island. A few showers are also
reaching leeward communities from time to time. Main short term
concerns over the next several days revolve around trade wind
trends and rainfall chances.

High pressure northeast of the state will keep moderate to
locally breezy trade winds in place today. A weakening cold front
will approach from the northwest tonight and Thursday, then stall
out a few hundred miles north of Kauai through the weekend. A
secondary cold front will approach from the northwest over the
weekend and stall out in roughly the same position, a few hundred
miles north of Kauai early next week. The initial front should
ease the trades into the light to moderate range Thursday through
Friday night, with localized land and sea breezes possible in the
more sheltered locations. Moderate trade winds will try to make a
return Saturday through early next week as the gradient across the
island chain tightens up due to high pressure strengthening north
of the state.

As for forecast details, fairly typical trade wind weather should
prevail during the next 7 days, with showers favoring windward
and mauka areas, particularly at night through mid morning each
day. A stray shower will also reach leeward areas from time to
time. There could be a slight increase in leeward shower activity
Thursday and Friday as the trade winds ease, allowing for some
sea breeze driven shower development. There also could be some
enhancement to rainfall over windward sections of the Big Island
and possibly Maui County as moisture associated with the remnant
trough of 93C slides by to the south of the state. Overall
however, rainfall amounts should remain on the light side through
early next week.

&&

.AVIATION...
Moderate to breezy trade winds will continue through the morning
hours, then gradually weaken this afternoon through tonight.
Clouds and showers will favor windward and mountain locations,
with the best coverage expected through the overnight and early
morning hours as pockets of moisture move through. Overnight
clearing will become a possibility late tonight over Kauai and
Oahu, if the winds become light enough for land breezes to
develop. Brief periods with MVFR ceilings and visibilities will be
possible in and around passing showers. Elsewhere, mostly dry
conditions with VFR conditions will prevail.

AIRMET Tango remains in effect for areas south and west of the
mountains of all islands for low-level turbulence below 8,000ft.
This will be discontinued in the morning package.

AIRMET Sierra for mountain obscuration is now in effect for N
through E sections of the Big Island and Maui.

&&

.MARINE...
A ridge of high pressure several hundred miles N of Kauai continues
to support locally strong trade winds, and a Small Craft Advisory
(SCA) remains posted through today for marine zones around Maui
and the Big Island. An approaching front will force the ridge S
and weaken it over the next day or two, leading to weakening
winds that will veer slightly toward the SE. Like last weekend,
this effect will be most pronounced on the W end of the island
chain, but light E to SE winds will prevail over most areas from
late tonight into Saturday.

The front will stall and dissipate well N of the area Thursday
and Friday, but the ridge will remain suppressed southward near
the islands into Saturday as a second front approaches. High
pressure building NW of this second front will start to tighten
the local pressure gradient by late Saturday, leading to an
increase in trade wind speeds heading into next week. As this high
passes N of the area through early next week, it is expected to
support light to moderate trade winds.

Surf along N and W facing shores will be increasing to
climatological levels over the next several days, with increasing
potential for High Surf Advisory (HSA) level surf this weekend
into early next week. A mid-period NW swell arriving Thursday will
back to the N on Friday while peaking below advisory levels, then
diminish Saturday. A powerful low (with hurricane-force winds)
will develop in the NW Pacific over the next 24 hours, generating
a long-period NW swell that will arrive early Sunday and persist
through Tuesday. Peak surf heights will almost certainly warrant a
HSA for several days. Elsewhere, short-period wind waves will gradually
diminish along E facing shores as trade winds ease the next couple
of days. A slight increase in long-period S swell is expected today,
holding into Thursday before fading Friday.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Birchard

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



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