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Hawaiian Islands Synoptic Discussion and Guidance

398
FXHW60 PHFO 151357
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 AM HST Mon Oct 15 2018

.SYNOPSIS...
A cloud band over Kauai and Oahu will move southeast across Maui County
today, then stall and dissipate over the Big Island tomorrow. A ridge
will build north of the area tonight and persist through the week.

&&

.DISCUSSION...
Satellite loops show a 90 mile wide band of showery low clouds extending
east northeast to west southwest over the islands. The south edge of
the band is over the north shore of Molokai and the north edge is over
Kauai. This band is associated with the remnant of a front. Clouds
associated with this band cover Niihau, Kauai and Oahu and the coastal
waters around Kauai County and Oahu. Clouds are isolated over most areas
from Lanai to the leeward side of the Big Island of Hawaii. A trough
extends northeast from over the Big Island. Broken to overcast low clouds
associated with the trough cover the coast and lower slopes of the Big
Island from Upolu Point to South Point and the coastal waters east
of the Big Island.

A surface ridge will build eastward north of the islands and moderate
northeast trade winds will spread over the islands over the next couple
of days. The cloud band will shift southeast slowly and weaken. Clouds
and showers in the band will spread over Maui County today and tomorrow
and reach the Big Island tonight. The cloud band is rather shallow
and the winds are rather light, so the showers in the band will affect
mainly windward areas of the smaller islands, with only isolated showers
spreading to leeward areas. A trough aloft is making the atmosphere
less stable over the Big Island with deeper moisture there. Showers
will be more active on the Big Island today with some showers building
over interior and leeward slopes during the afternoon. As the trough
aloft lifts north and weakens tomorrow, the atmosphere over the Big
Island will become more stable.

From Wednesday through next weekend a surface ridge will persist north
of the main Hawaiian Islands and maintain moderate to locally fresh
northeast to east trade winds over the area. Ridging aloft will keep
the atmosphere rather dry and stable, so showers will be focused over
mainly windward areas with no heavy rainfall expected. Leeward areas
will remain mostly dry.

&&

.AVIATION...
A diffuse frontal band remains across the central islands early
this morning. The ragged leading edge of this band has advanced
slowly toward the south-southeast during the past few hours. The
low-level flow behind this feature is beginning to shift to
north-northeasterly as surface high pressure builds northwest and
north of the area. As a result, low clouds and showers are moving
across north and northeast facing slopes of most of the smaller
islands. This is producing brief periods of MVFR conditions over
some windward sections of the islands. Elsewhere, mainly VFR conditions
prevail. AIRMET Sierra is in effect for mountain obscuration on Kauai.
This AIRMET may be lowered by daybreak this morning if conditions begin
to improve there.

Abundant moisture and a weak surface trough continue to generate
low clouds and showers, with periods of MVFR conditions over parts
of the windward Big Island early this morning. AIRMET Sierra remains
in effect for mountain obscuration on that island from Honokaa to
Volcano.

AIRMET Tango for upper-level turbulence remains in effect over
the island chain between FL260 and FL420. This AIRMET will likely
remain in effect over the eastern end of the state this morning.

By later today, the stalled front will continue to weaken across
Maui County. The low-level flow is expected to become
northeasterly during the daytime hours. Low clouds and showers
will continue to move into northeastern sections of the central
and eastern islands, which may result in brief periods of MVFR
conditions through late this afternoon. Conditions may improve
slightly over windward Kauai later today as drier air moves down.

&&

.MARINE...
Winds will gradually fill in from a more typical trade wind
direction and increase into the moderate to locally strong range
as a ridge of high pressure noses eastward to the north of the
islands. Some of the guidance is depicting advisory-level winds
over the windier areas around Maui County and the Big Island
tonight into Tuesday. These winds will remain in place through
midweek, then begin to trend down once again as a front passes to
the north Thursday.

An active surf pattern from the north and south is due this week.
A pulse of shorter period north-northwest swell will push surf to
near the October average through the day today. A larger north-
northwest swell will arrive on Tuesday. Resulting surf will likely
be well above the High Surf Advisory level for north and west
facing shores on Tuesday and Wednesday, and combined seas over
exposed waters will meet the 10 ft threshold for a Small Craft
Advisory a that time. This swell will gradually lower Thursday and
Friday and may be reinforced to around October average for the
upcoming weekend.

The run of late season south-southwest swells will continue
through the week. A new pulse has filled in overnight at the
nearshore PacIOOS buoys out of the south, which will support
moderate surf slightly above the October average along south
facing shores. A gradual downward trend from this source is
anticipated Tuesday through midweek. A more significant south-
southwest swell is expected to fill in on Friday and produce
advisory-level surf through the upcoming weekend.

Surf along east facing shores will remain below October average
through the week, though a slight increase is due as winds pick up
during the next couple of days.

&&

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

&&

$$


DISCUSSION...Donaldson
AVIATION...Houston
MARINE...Gibbs

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



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