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Hawaiian Islands Synoptic Discussion and Guidance

571
FXHW60 PHFO 240157
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 PM HST Wed Oct 23 2019

.SYNOPSIS...
Trades will continue to ease through Friday as a front approaches
from the northwest, then strengthen Saturday through early next
week as high pressure rebuilds north of the island chain. Typical
trade wind weather will prevail, with showers favoring windward
and mauka areas.

&&

.DISCUSSION...
A 1031 mb high far northeast of the main Hawaiian Islands is
driving moderate to locally breezy trade winds across local waters
this afternoon. Satellite imagery shows scattered to patchy
broken low clouds across Kona and Kau slopes on the Big Island as
well as across leeward areas of Kauai and Oahu. Scattered low
clouds or clear skies are noted elsewhere. Radar shows isolated
showers at best, with coverage greatest across leeward sections of
Kauai and the Big Island.

While a late morning ASCAT pass showed a few 25 kt wind barbs
within the Alenuihaha Channel, models show a downward trend in
wind speeds statewide. A later ship observation in that channel
confirmed this and we have allowed the Small Craft Advisory to
expire. 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 second 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.
Trades will remain in the moderate range through Friday night,
with localized land and sea breezes possible in the more sheltered
locations. Trades will then strengthen Saturday through early
next week as high pressure rebuilds to our north. Trade wind
weather should prevail, even as wind speeds fluctuate, with
showers favoring windward and mauka areas. However, leeward
showers may increase Thursday and Friday as trades ease just
enough for leeward sea breeze development. Overall however,
rainfall amounts should remain light through early next week.

&&

.AVIATION...
An approaching cold front north of the island chain will continue
to gradually knock our moderate trade winds down a notch or two
into Thursday. Lighter winds could lead to land breezes overnight
for the western half of the state. This would allow the clouds
that have built up on leeward sections of Oahu and Kauai to clear
out. Elsewhere, clouds and showers will continue favor windward
and mauka slopes and coasts, particularly though the overnight and
early morning hours. Brief periods with MVFR ceilings and
visibilities will be possible with these showers. Overall, VFR
conditions will prevail.

Due to the decrease in wind speeds, AIRMET Tango for tempo
moderate turbulence below 8,000ft has been cancelled. There are no
AIRMETS in effect at this time.

&&

.MARINE...
A front passing north of the state is causing the ridge of high
pressure to weaken and shift further east. This has resulted in
the weakening of the pressure gradient, allowing the winds to
decrease. The Small Craft Advisory (SCA) has been cancelled and
winds are expected to remain below SCA levels at least through
Friday. The front is expected to weaken and dissipate north of the
offshore waters over the next few days. The ridge of high pressure
may briefly rebuild north of the state late Friday into Saturday,
but another front approaching the area will likely weaken the
ridge once again Sunday into early next week.

A series of small to moderate northwest swells will fill in along
north and west facing shores over the next few days. A Gale force
low currently about 1000 miles north of the state will produce a
mid- period north-northwest swell for Friday. The latest model
guidance is showing 6 feet 11 seconds with this swell. But with
the typical low bias of the models, this swell will likely be
closer to 7 feet to possibly 8 feet when it peaks Friday night.
Thus, surf along north facing shores may approach High Surf
Advisory (HSA) levels on Friday, but the likely solution would be
surf peaking just below HSA levels.

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 Saturday night and persist through Tuesday.
Peak surf heights will almost certainly warrant a HSA for several
days and possibly warning levels when it peaks sometime on
Monday. Due to the spring tides on the morning of the 28th, this
swell may cause some coastal impacts. Stay tuned for more details.

Elsewhere, short- period wind waves will gradually diminish along
E facing shores as trade winds ease the next couple of days. Tiny
background surf is expected along south facing shores over the
next several days.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Dye
MARINE...Kino

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



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