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Hawaiian Islands Synoptic Discussion and Guidance

098
FXHW60 PHFO 240138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Thu Nov 23 2017

.SYNOPSIS...
High pressure north of the State will keep breezy to windy trades
blowing through the Holiday weekend. Passing showers will favor
windward and mauka areas, with a stray shower reaching leeward
areas from time to time due to the strength of the trades. A wet
trade wind pattern is expected to develop on Sunday and continue
through early next week, as deep tropical moisture moves into the
area from the southeast. The wettest conditions are expected over
the eastern end of the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1034 mb high centered around 1350
miles north-northwest of Honolulu, is driving breezy to windy
trades across the island chain this afternoon. Satellite imagery
shows mostly cloudy skies across most windward areas, and
generally partly cloudy conditions in leeward locales. Radar
imagery shows scattered showers moving into windward areas, with
a few showers reaching leeward areas from time to time. Main
short term concerns revolve around the rain chances and the
strong trade winds.

Tonight through Sunday,
Breezy to windy trades will continue right through the Holiday
weekend, as the strong area of high pressure to the north-
northwest of the State gets re-enforced by another strong high
dropping southward from the Aleutian islands. Clouds and showers
will continue to favor windward and mauka areas through the
period, with a stray shower reaching leeward areas from time to
time. Both the GFS and ECMWF show deep tropical moisture lifting
northwestward into the eastern end of the island chain on Sunday,
and this is expected to bring a particularly wet trade wind
pattern to Maui County and the Big Island. Showers are also
expected to increase across Kauai and Oahu, but coverage and
intensity shouldn't be as high as over the eastern end of the
State.

Additionally, with deep moisture moving into the Big Island, snow
may begin to affect the Big Island summits as early as Sunday.

The Wind Advisory for areas over and downwind of the Kohala
mountains on the Big Island, as well as the island of Lanai
remains in effect through 6am Friday. The strong winds are
expected to drop off slightly on Friday, and the thought for now
is that the winds will remain just below Advisory levels.
Nevertheless, we can't entirely rule out the need to extend the
Advisory through the day on Friday for the areas where it
currently is in effect, but will let the night shift watch
observational trends and make the decision on whether to extend
the Advisory or not.

With the re-enforcing high building north of
the State, Advisory level winds may periodically affect the
typically windier locations on Maui and the Big Island right
through the weekend.

Sunday night through next Thursday,
High pressure north of the State will keep a moderate to breezy
trade wind flow in place through the period. Model solutions are
in good agreement showing deep moisture lingering over the eastern
end of the State through the first half of next week, so a wet
trade wind pattern is expected over Maui County and the Big
Island. Across Oahu and Kauai, a wetter than normal trade wind
weather pattern is expected to prevail, with shower coverage
expected to be the highest Sunday night and Monday. This will need
to be monitored closely however, as it could be significantly more
wet if the slug of deeper moisture lifts further northward than
currently anticipated.

Model solutions suggest that snow will continue to affect the Big
Island Summits Sunday night through at least Tuesday. Although
it's too early to determine how much snow may fall, Warning level
amounts appear to be a decent probability, and if these trends
continue a Winter Storm Watch will likely be required later this
weekend.

&&

.AVIATION...
Trade winds will remain strong through the weekend. AIRMET Tango
remains in effect for moderate low level turbulence over and
downwind of the higher terrain due to the strong winds. AIRMET
Tango is also in effect for strong surface wind speeds for
portions of Lanai and the Big Island.

Cloud and showers will be most prevalent over windward areas, and
AIRMET Sierra for mountain obscuration is currently in effect for
windward parts of Molokai, Maui and the Big Island, where the
showers have been particularly persistent. While some of the
showers will reach leeward areas at times due to the strengthening
trades, expect VFR conditions to prevail there.

&&

.MARINE...
The very large north swell continues to come in above the models,
particularly near the Big Island. Throughout the day, the buoy
near Hilo has reported 16+ ft at 15 seconds. The latest model run
has come closer to reported values near Kauai, but still is below
reported values near the Big Island. Have tweaked the existing
forecast to bump numbers up a bit, primarily near the Big Island.
The Hanalei buoy has been picking up 13 second energy over the
last couple of hours, so suspect we have, or are seeing the start
of the decline of the swell.

The High Surf Warning continues for north and east facing shores
due to the swell. The warning goes through 6 am Friday morning,
but surf is still expected to be high, necessitating a High Surf
Advisory for Friday.

This swell is also bringing seas in excess of 10 ft, and when
combined with the strong and gusty northeast winds, necessitates
the Small Craft Advisory (SCA) that remains in effect. With a
combination of winds and seas meeting SCA criteria expected to
continue into the weekend, the SCA has been extended for most
zones. The few areas not in the extension may need to be added
when the next swell arrives Sunday and Monday.

A new north swell is expected to arrive Sunday, peaking Monday
near advisory levels, then subsiding through Tuesday. Additional
swells are expected to arrive Wednesday.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.

Wind Advisory until 6 AM HST Friday for Lanai-Big Island North
and East-Kohala.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Saturday for Big Island
Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Foster
MARINE...M Ballard

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



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