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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 271957

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Fri Nov 27 2020

Moderate to breezy northeast trade winds will prevail through the
weekend, with clouds and showers favoring windward and mountain
locations. Windward shower coverage could increase tonight into
Saturday as an upper disturbance and dissipating front move
through from north to south. Trade winds will diminish early next
week as a cold front passes north of the state.


Surface high pressure is centered far to the north of the area
providing for moderate to breezy northeast trade winds across the
state. Meanwhile, the axis of an upper level trough is located
just to the northeast of the Big Island. The airmass is rather dry
and stable at the present time so just a few brief showers are
falling over windward and mauka areas. Expect this trend to
continue through the day today with daytime heating allowing for
some clouds and showers to develop over the Kona slopes this
afternoon and evening.

Tonight and Saturday, model guidance indicates that an upper level
short wave will be moving around the trough axis causing
instability near the Big Island. Some increase in low level
moisture is also indicated as an old frontal boundary gets caught
up in the northeast trades. Some increase in windward and mauka
showers can be expected as this feature moves on through. The
instability caused by the short wave aloft, may allow for some
heavier showers and a few thunderstorms to form over and around
the Big Island. We will take a look at new guidance before
deciding on placing thunderstorms into the forecast.

With breezy trades in place and low dewpoint temperatures
present, it will likely feel rather chilly during the overnight
and early morning hours on Sunday.

By Monday, the surface high will have moved off to the northeast
of the area while a frontal boundary begins to approach the area
from the northwest. This will cause trade winds to back off with
local land and sea breezes developing. The upper trough is
forecast to linger just to the northeast of the Big Island so its
associated instability may allow for higher chances for afternoon
shower development over the leeward Big Island slopes. The front
is forecast to dissipate to our north around mid week. Trade
winds will remain on the lighter side into the end of the forecast


Moderate to locally breezy trades will support prevailing VFR and
occasional windward showers through today. A front noted on
satellite imagery north of the Islands will bring an increased
coverage of showers and potential for MVFR for Windward Kauai
tonight and Oahu by very early Saturday morning. AIRMET Tango
remains in effect for both lee turbulence due to trades as well as
moderate CAT in the FL270-370 layer.


While no changes are expected with this mornings coastal waters
package, anticipating some changes by the afternoon, mainly with
thunderstorm possibilities over the coastal waters. The current
forecast has the possibility of thunderstorms over the waters
northeast of Maui and the Big Island tonight in response to an
upper level shortwave that is expected to swing through, and a
dissipating front approaching the islands. However the latest
model runs shows the feature strengthen and lingering over the
area through Saturday and possibly into Saturday night. The bulk
of the instability would be to the northeast of the islands,
including some of the offshore waters. Will be refining the area
and time of thunderstorms for the waters shortly.

The next significant marine feature will be the large swell
expected by next Wednesday, which is expected to produce warning
level surf along north and west facing shores. This swell will be
generated from a surface low coming off Japan and moving east-
northeast over the weekend, passing near the Aleutian Islands
early next week. WaveWatch III and the European wave guidance has
a 14 foot swell at around 18 seconds reaching the north and west
shores of Kauai late Tuesday afternoon. The swell will spread down
the island chain, peaking Wednesday followed by gradual decline
through Friday. Tides are currently running about half a foot
above predicted levels, and high tide Wednesday morning is
predicted at 2.3 feet. The high tide and surf arrival will further
increase the possibility of runup on roadways during this event.
A Small Craft Advisory (SCA) will also be needed as seas exceed 10
feet as early as Tuesday afternoon for waters near Kauai, then
quickly spreading to other exposed waters Tuesday night. Seas are
expected to fall below SCA levels Thursday afternoon.

High pressure north of the islands will continue to move to the
northeast through tomorrow, and we are expecting trade winds to
weaken some into early next week. The current SCA is in effect
through tonight due to the winds. Once the winds weaken, they are
expected to remain below SCA levels through the upcoming week.

A moderate northwest swell is expected to build today and peak on
Saturday, with another northwest swell arriving Sunday will
maintain moderate through Monday. A small northeast swell will
maintain moderate surf along east facing shores through the
weekend. Without support of this swell early next week and the
lighter trades, surf will be smaller along the east facing shores.
Surf will be small along south facing shores through out the
period, where a couple of long period pulses from the south-
southwest and of south are expected through next week.


Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest
Waters-Kauai Leeward Waters-Kauai Channel-Oahu Leeward Waters-
Kaiwi Channel-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big
Island Leeward Waters-Big Island Southeast Waters.



MARINE...M Ballard

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