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Hawaiian Islands Synoptic Discussion and Guidance

492
FXHW60 PHFO 211959
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM 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. Trade wind showers will continue to focus
over windward and mauka areas, with less shower activity reaching
leeward locales.

&&

.DISCUSSION...
Latest radar and satellite imagery is showing numerous showers
this morning over windward and mauka areas with scattered showers
over leeward areas. Cloud tops of up to 40,000 feet have been
observed north and northeast of the state. Due to the elevated
cloud tops and numerous showers upstream of the state, the
forecast has been updated to increase shower coverage and to
include slight chance of thunderstorms over Kauai for today.

For tonight, breezy and showery conditions will likely continue
across the state as another band of clouds moves through the
state.

Starting Friday, models are in good agreement with the pressure
gradient tightening across the state. Trade winds are expected to
strengthen and confidence is increasing for wind speeds to reach
wind advisory criteria across many areas through Saturday. The
weekend should remain windy and showery on Saturday, with a
slight decrease of winds and shower coverage possible 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, and have
come into better agreement with respect to the location where the
trough will set up for the 00Z runs, run to run consistency has
shown some significant deviations which will have a considerable
impact on the weather we see locally. 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 generate scattered 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 make it over the mountainous
terrain and on the leeward areas.

AIRMET Sierra remains in effect for north thru east sides of Maui,
Molokai, and Oahu above 2000 feet due to mountain obscuration from
clouds and showers.

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 today and tonight with
speeds reaching near gale force in some of the channels Friday
through and Saturday 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, in effect until 6 pm
HST Friday, is likely to be extended through the weekend.

The northwest swell has arrived, with buoy 51001 peaking at 14
feet yesterday evening with a 16 second period. It is currently on
the way down, but because this swell is running up to 4 feet
higher than Wave Watch, a High Surf Warning was issued for north
and west shores of Kauai and Oahu, while a High Surf Advisory
(HSA) continues for north and west Molokai and north facing Maui
shores through 6 PM this evening. There is enough of a west
component to this swell where the southern half of the Big Island
coastline will be affected, so an HSA is now in effect through 6
AM, followed by a gradual decline through Friday. A reinforcing,
moderate northwest swell will fill in this weekend, followed by a
smaller northwest swell arriving Wednesday of next week.

As a result of the strong trades, east facing shores of most
islands will see advisory level surf over the weekend. 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...
High Surf Warning until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau.

High Surf Advisory until 6 AM HST Friday for Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala-Kona.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters.


&&

$$


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

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