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Hawaiian Islands Synoptic Discussion and Guidance

793
FXHW60 PHFO 210631
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Wed Nov 20 2019

.SYNOPSIS...
A showery, wet and windy trade wind weather pattern is expected
through the weekend. Showers will be most prevalent over windward
and mauka areas, with the occasional leeward spill over. The
trades are expected to ease next week. Trade wind showers will
continue to focus over windward and mauka areas, with less leeward
spillover.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high centered 1100 miles
north-northeast of Honolulu, is driving moderate trade winds
across the island chain this evening. Infrared satellite imagery
shows clear to partly cloudy skies in leeward areas, with variably
cloudy conditions in windward locales. Radar imagery shows
scattered showers moving into windward areas, with a few shows
drifting leeward from time to time. Main short term concern
revolves around the strengthening trade winds.

Model solutions are in good agreement showing a tight pressure
gradient setting up across the island chain, between nearly
stationary high pressure northeast of the islands and a trough of
low pressure sliding by to the south. Breezy trade winds are
expected to develop by Thursday, with breezy to windy conditions
Thursday night through Sunday. The trades appear to peak in the
Friday through Saturday time frame, and Wind Advisories may
eventually be needed for some of the typically windier locations
on the Big Island and Maui County. We should see the trades ease
and shift around to more of an east-southeasterly direction during
the first half of next week, as a trough of low pressure sets up
several hundred miles west of the state. Moderate to breezy trades
will likely hold in place across windward sections of the Big
Island and Maui, with light to moderate trades expected elsewhere,
lightest in leeward locales.

As for weather details, we expect a showery wet trade wind
pattern to hold through the weekend, as upper level troughing
remains over the island chain. Showers will favor windward and
mauka areas, but will also more regularly affect leeward locales
due to the strength of the trades. Showers will be most prevalent
during the overnight and early morning hours (between 6 pm and 10
am) each day.

Forecast details become more uncertain for the first half of next
week. Although both the GFS and ECMWF are in good agreement that
a trough of low pressure will develop west of the state, they
disagree on the exact location where this feature will set up.
Additionally, the model solutions have not shown the best run to
run consistency. For now and until details become more clear, the
forecast will reflect a trade wind shower pattern favoring
windward and mauka showers and the occasional leeward spillover
through the extended portions of the forecast.

&&

.AVIATION...
Trade winds will strengthen overnight becoming breezy on
Thursday. These stronger wind speeds will likely trigger issuance
of AIRMET Tango by Thursday morning for moderate low level
turbulence south through west of all island mountains. Otherwise
mostly VFR conditions are forecast with brief periods of MVFR
conditions developing mainly along windward slopes favoring the
evening through early morning hours.

No AIRMETs are currently in effect. AIRMET Tango for moderate low
level turbulence will likely be issued with the Thursday morning
forecast update.

&&

.MARINE...
Trade winds are back, and will continue to strengthen overnight
into Thursday, becoming rather gusty and breezy, as a 1030 mb
surface high lies 920 nm north-northeast of Oahu. Although the
high will be moving east-northeast, further away from the islands
during the next couple of days, it will also be strengthening and
with a tight pressure gradient around the islands. So, the trades
will be rather strong and gusty over the weekend area-wide, with
near Gale-force winds in some of the channels.

An anticipated northwest swell will be reaching the north and west
facing shores of most of the islands later tonight. Deep ocean
buoy 51001 had a reading of 12.5 feet swell at 7 pm HST and a
period of 16 seconds. Buoy 51101, located just to the south of
51001, had a smaller reading of 10.5 feet and 16 seconds. Never-
the-less, advisory level surf, possibly to near warning, is
likely for Thursday. We will be monitoring the situation closely
in the coming hours and make any necessary adjustment to the
forecast. This northwest swell is expected to peak Thursday,
followed by a gradual decline through Friday.

With the trade wind and swell in mind, a Small Craft Advisory
(SCA) is in effect for all near-shore waters, including recently
added Maalaea Bay, where it is experiencing trade winds of 23 kt.

The trade winds will favor the typical windy areas of Maui County
and south of the Big Island. The incoming northwest swell will
cover the rest of the near-shore waters. This SCA is good through
late Friday afternoon, with the likelihood of an extension.

This northwest swell will be reinforced by a moderate size swell
Saturday into Sunday. This swell will then be followed by a
smaller size swell on Wednesday of next week. With the significant
boost in the trades into the weekend, the east facing shores may
face a High Surf Advisory with surf 8 feet or higher.

Lastly, a series of a small background southwest swells will
maintain small surf along the south facing shores through the
forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters except Maalaea Bay-

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Lau

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



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