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Hawaiian Islands Synoptic Discussion and Guidance

186
FXHW60 PHFO 240643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Wed Jan 23 2019

.SYNOPSIS...
A ridge will remain nearly stationary near Kauai through Thursday,
with fair weather and light to gentle winds over the area. A front
from the northwest will reach Kauai on Friday, then continue
southeast, past the Big Island by late Saturday, bringing clouds and
showers.

&&

.DISCUSSION...
Fair weather prevails across the state this evening, as airmass
remains dry and somewhat stable under an upper level ridge. There is
a persistent surface ridge near Kauai, bringing gentle easterly
trade winds to the Big Island and Maui, and light southeast winds
for the western islands. Latest satellite imagery revealed some high
clouds over Maui county, with some low clouds and showers carried
by the easterly winds reaching windward Big Island and Maui.
Overall, the rather tranquil weather will continue through rest of
the evening into early Thursday morning.

The islands should experience more fair weather on Thursday, as
little changes are expected to the weather features in the island
vicinity. Therefore, light east to southeast background winds will
persist, with afternoon clouds and possibly some showers over the
islands under the sea breezes and some clearing overnight under the
land breezes. Some low clouds carried by the easterly winds will
reach windward Big Island and Maui, bringing some showers. Rainfall
amounts will be limited as airmass will stay dry and stable.

A front from the northwest will reach Kauai on Friday, then
continuing southeast down the island chain and advancing pass the
Big Island by Saturday night. A band of showery low clouds along the
front will bring widespread showers to windward areas and even
reaching leeward areas of the smaller islands as well, but the
clouds will be too shallow to bring much rain to the leeward areas.

A surface high behind the front will provide breezy northeast winds
and cooler temperatures over the area. Latest forecast solutions do
differ on how long the breezy weather will last, as GFS now has the
winds turning more east and weaker by Sunday while the older ECMWF
has much stronger northerly winds.

&&

.AVIATION...
A surface ridge just north of the Hawaiian Islands will continue
to produce light and variable winds with land and sea breezes in
a stable atmosphere. VFR conditions remain in the forecast.

&&

.MARINE...
Light to moderate east to southeast winds will continue through
Thursday as a cold front approaches. Pockets of moderate to fresh
breezes will be likely through this time in the typically windier
areas with southeast winds (Hamakua coast/windward Maui/South Point
waters). Light winds with overnight land and daytime sea breeze
conditions near the coasts will prevail over the Oahu and Kauai
waters. Winds will quickly shift out of the north and increase into
the fresh to strong category through the day Friday as the cold
front advances down the island chain. A more typical trade wind
pattern will return over the weekend as high pressure builds north
of the state. Small craft advisory conditions due to a combination
of strong winds following the front and a reinforcing large swell
expected Thursday night through Friday will likely continue for
exposed waters through Saturday. Forecast uncertainty increases
Sunday into early next week due to model differences shown. The
ECMWF shows moderate northerly winds filling back in, while the GFS
depicts light to moderate trades.

Surf along north and west facing shores came up through the day due
to a large west-northwest (300 deg) swell that filled in from a
distant storm across the northwest Pacific earlier in the week.
Although the swell ended up coming in later than predicted, it is
converging near the predicted peak this evening at the buoys. This
source will hold into Thursday, then slowly begin to lower late
Thursday through Friday.

A similar long-period reinforcement out of the same direction (300-
310 deg) is expected Thursday night through Friday from a large
swath of gale- to storm-force winds focused at the islands from the
northwest Pacific Tuesday/Tuesday night. Altimetry data showed seas
associated with this source reaching the 30-40 ft range within 1500
nm of the islands Tuesday near the head of the fetch. Guidance is
picking up on this well, but depicting different arrival times
(ECMWF-Wave later than GFS/WAVEWATCH III). Will make small
adjustments to the official forecast to account for this
uncertainty. A gradual downward trend is expected through the
weekend.

The current small craft and high surf advisories will be extended
through Friday night due to a combination of the reinforcing swell
and strong northerly winds expected behind the front Friday. Surf
will near warning levels around the peak once again Thursday night
into Friday along exposed north and west facing shores, especially
for Kauai.

Surf along east facing shores will remain up through Friday despite
the lighter winds locally. This groundswell out of 080-090 deg is
from a recent pocket of fresh to strong winds upstream of the state
that has filled in. This source should gradually begin to fade over
the weekend. Guidance is also depicting a moderate to large short
period north-northeast (020 deg) swell filling over the weekend from
a large swath of nearby northerly winds following the front (highest
likely over the eastern end of the state this weekend).

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Thursday for north and west facing
shores of Niihau, Kauai, Oahu and Molokai, north facing shores of
Maui and west facing shores of the Big Island of Hawaii.

Small Craft Advisory until 6 PM Thursday for waters around Kauai,
and Oahu Windward Waters, Maui County Windward Waters and the Big
Island Windward waters.

&&

$$
DISCUSSION...Hui
AVIATION...Bohlin
MARINE...Gibbs

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



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