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Hawaiian Islands Synoptic Discussion and Guidance

806
FXHW60 PHFO 220210
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
410 PM HST Thu Nov 21 2019

.SYNOPSIS...
Trade winds will strengthen Friday and will bring windy and
showery weather through Saturday. Showers will be most prevalent
over windward and mauka areas, with showers spilling over into
leeward communities from time to time. The trades are expected to
ease a bit next week and veer to the east-southeast as a low
pressure system develops west of the state. Precipitation will
depend on how close the low passes to our west.

&&

.DISCUSSION...
Latest satellite imagery is showing an upper level trough located
just west of Kauai. This upper trough is responsible for the
isolated thunderstorms to the west of Kauai. As this upper trough
shifts south tonight, the thunderstorm threat will shift slightly
south, but will remain in the vicinity of Kauai. Meanwhile, a
batch of showers are located just upstream of the eastern half of
the state. The showers will be moving in this evening and will
bring numerous showers for windward and mauka areas of Maui and
the Big Island. For rest of the state, showery conditions will
likely continue with some passing showers over leeward areas.

Starting Friday the pressure gradient will tighten across the
state and will cause the trade winds to strengthen. A Wind
Advisory has been issued for portions of all Hawaiian Islands
starting Friday morning and continuing through the afternoon. The
strongest winds will be over and downwind of the terrain. Winds
are expected to remain strong through Saturday, but confidence is
lower due to the increase of showers expected Friday night into
Saturday. We will continue to monitor the winds throughout the day
Friday to decide if the wind advisory needs to be extended.
Showery and windy conditions will likely persist on Saturday,
followed by a decrease of winds and showers on Sunday.

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.

Forecast details become more uncertain towards the middle 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, there is
uncertainty on how close the low pressure will develop to the
state. 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...
Strengthening surface high pressure northeast of the state will
bring breezy east northeast trades winds through the remainder of
today. Areas of enhanced moisture caught up in the flow, combined
with weak instability, will continue generate scattered to
numerous showers mainly along the windward coasts and slopes. MVFR
conditions may occur with mountain obscuration from low ceilings
and decreased visibility. A few showers could push over the
mountainous terrain and onto the leeward areas.

AIRMET Tango remains in effect over and immediately south thru
west of mountains on all islands below 8000 feet due to tempo
moderate turbulence.



&&

.MARINE...
Trade winds will continue to strengthen tonight with speeds reaching
near gale force in some of the channels Friday due to a strengthening
surface high far northeast of the area. A Small Craft Advisory (SCA)
is currently in effect for all Hawaiian waters, due to the large
northwest swell and/or locally strong trades and rough, choppy seas
that will continue to increase tonight. This SCA, is now in effect
through 6 pm HST Saturday, but is likely to be extended through the
weekend. We have added a Gale Watch for the channels around Maui
County and the Big Island for the day Friday.

The northwest swell peaked earlier today in the local waters, and
has begun a slow decrease this afternoon. Swell and surf will
continue to decline tonight, but remain elevated, so the High
Surf Warning for Kauai and Molokai has been replaced with a High
Surf Advisory (HSA) until 6 AM Friday. The HSA for north and west
Molokai, north Maui, and west Big Island shores continues through
6 AM Friday as well. A reinforcing, moderate northwest swell will
fill in this weekend for another, but brief, round of advisory
level surf for exposed shores. A smaller northwest swell is
expected around the middle of next week.

As a result of the strong trades, east facing shores of most
islands will see advisory level surf over the weekend. Therefore,
have issued an HSA for those shores, which goes into effect at 6
AM Friday. Lastly, a series of a small background southwest swell
will maintain small surf along south facing shores through the
forecast period.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 6 AM to 6 PM HST Friday for portions of all
Hawaiian Islands.

High Surf Advisory until 6 AM HST Friday for north and west facing
shores of Niihau, Kauai, Oahu, and Molokai and north facing shores
of Maui and west facing shores of Big Island.

High Surf Advisory from 6 AM Friday to 6 PM HST for east facing
shores of Kauai, Oahu, Molokai, Maui and the Big Island.

Gale Watch from Friday morning through Friday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory for all remaining waters until 6 PM HST
Saturday.


&&

$$


DISCUSSION...Kino/Wroe
AVIATION...Thomas
MARINE...TS

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



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