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Hawaiian Islands Synoptic Discussion and Guidance

181
FXHW60 PHFO 281952
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Mon Sep 28 2020

.SYNOPSIS...
Showers this morning over Kauai and Oahu are expected to taper off
this afternoon. Light to moderate trades this morning will give
way to light and variable winds for the rest of the week. This
will lead to land and sea breezes, with a concentration of clouds
and showers over the interior of the islands during the afternoon
hours.

&&

.DISCUSSION...
The overnight analysis shows a weak surface trough moving over the
islands is conjunction with the band of moisture over Kauai and
Oahu. This has been bringing a steady rain to windward areas over
the last 12 hours or so.

A ridge north of the islands will continue to bring some trade
winds to the region today, but the ridge is expected to weaken as
its parent high far to the northeast moves away from the islands.
The result will be weakening winds, with land and sea breezes
expected by tomorrow, and lingering into the upcoming weekend.
Typically during a land and sea breeze pattern we can expect
clouds and showers to form over the interior of the islands during
the afternoon and linger into the early evening, with at least
some clearing at night. One factor that may disrupt this pattern
is the remnants of Lowell off to the east. This showery cloud mass
is expected to move towards the islands, and disperse some as it
does so. This could help to bring some additional moisture to the
islands Wednesday night to Thursday morning. The global models
also suggest some moisture from the south making its way to the
southern island at the end of the week.

For now the forecast seems to be on track, so no updates are
expected. Suspect there will be some tweaking to the forecast for
the afternoon package, but not expecting any major changes.

&&

.AVIATION...
Deep moisture which supported the morning push of showers into
leeward portions of Oahu and Kauai continues to pull northwest.
Activity is beginning to diminish over Oahu with Kauai still a
couple hours behind. The remainder of the day will be
characterized by light winds and the emergence of sea breezes
over leeward areas. Sea breezes will bring the potential for
ceilings and a few brief showers, especially over Oahu where the
developing sea breeze may capitalize on existing moisture before
it exits this afternoon. Some hints of this potential outcome are
already evident on local satellite and radar imagery. The light
gradient will support a tendency for land breezes tonight and
prevailing VFR with occasional windward showers.

AIRMET Sierra remains in effect for windward Kauai and Oahu. It
is expected to expire for Oahu late this morning or early this
afternoon followed by Kauai.

&&

.MARINE...
Gentle to moderate trade winds will continue today, then trend
down enough for a land and sea breeze pattern to setup tonight
into this weekend. The background flow will remain out of the
east today, then shift out of the north-northeast Tuesday through
Thursday, then easterly again Friday and Saturday. A return of
moderate to fresh easterly trade winds is possible for Sunday.

Surf along south facing shores will remain up today, then
gradually lower Tuesday through midweek as the south swell that
filled in over the weekend eases. A mix of southeast trade wind
swell and background small, southerly energy should be enough to
keep the surf from going flat Wednesday into next weekend.

For the long range, the active trend continues down south around
New Zealand within Hawaii's swell window. The latest surface
analysis and satellite data reflect this and depict a storm-force
low near southern New Zealand with a broad area of gale- to storm-
force winds over our swell window from the Tasman Sea to south-
southeast of New Zealand. Altimeter passes showed seas up to
around 25 ft in the Tasman Sea earlier, which is in line with a
drifting SOFAR buoy that recorded seas up to 20 ft farther north
in the Tasman. Guidance continues to develop this system and has
it bottoming out at 935-940 mb today as it continues eastward.
Although this feature isn't as impressive as guidance had depicted
a few days ago, it will be enough to generate a late season south
swell that should arrive locally early next week (Oct. 5th time
frame). More details will follow later in the week once the swell
moves through the nearshore PacIOOS buoy in American Samoa.

Surf along north facing shores will remain small through Tuesday,
then trend up once again Tuesday night through next weekend as
back-to-back north-northwest swells arrive. The first one is from
a compact storm-force low developing around 1500 nm north-
northwest of the islands as it tracks eastward. This source will
arrive Tuesday night and gradually become more northerly by
Thursday as it peaks near the advisory level (15 ft faces). The
second swell is forecast to arrive Thursday night, which should
keep the surf near the advisory level through the day Friday. This
swell combined with a smaller, medium period northwest arriving
Friday night into Saturday will keep the surf up through next
weekend.

Surf along east facing shores will pick up as a small, easterly
swell from former tropical cyclone Lowell in the eastern Pacific
arrives and moves through the islands. This swell should fill in
across the eastern end of the state today, then peak over the
western end of the island chain Tuesday. A downward trend is
expected through the second half of the week due to the lack of
trades locally and upstream of the islands.

The large daily tide cycles will gradually trend down toward the
monthly minimum each day through the first week October.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Foster

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



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