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Hawaiian Islands Synoptic Discussion and Guidance

599
FXHW60 PHFO 221350
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Fri Feb 22 2019

.SYNOPSIS...
A surface trough near the Big Island will support clouds and showers
over the eastern end of the island chain into Saturday. Meanwhile,
weak high pressure to the northwest will support mostly dry weather
over Kauai and Oahu. Light winds will continue today, but a new high
building northwest of the islands late Saturday will bring gradually
strengthening north to northeast winds on Sunday that may become
breezy next week. These winds will deliver clouds and showers to
north and east facing slopes and coasts, while also bringing cooler
air to the islands.

&&

.DISCUSSION...
Currently, a persistent N-S oriented surface trough just E of the
Big Island is fueling clouds and showers near its axis, while a weak
bubble high to the NW of the islands is supplying mostly dry weather
and light winds to most of the state. SW flow just above the surface
is supplying broken to overcast layered clouds (based near 8000
feet) to parts of Maui county and all of the Big Island, with nearly
clear skies over Kauai and Oahu. A few thunderstorms have been
observed overnight over the Hawaiian Offshore waters about 50-70
miles SE of the Big Island.

The surface trough will drift W today, remain nearly stationary into
Saturday, and then move away to the E by Sunday. Clouds and showers
will be maximized near the trough axis, primarily affecting windward
and SE Big Island, but potentially spreading to portions of Maui.
Elsewhere, a mostly dry light wind regime will continue into the
weekend. Light winds today will allow afternoon sea breezes to drive
cloud formation over interior portions of Kauai, Oahu and parts of
Maui county, with little rainfall expected. On the Big Island, the
layered cloud cover is likely to remain thick enough to preclude
daytime heating from prompting shower formation over the slopes, and
instead the approaching surface trough will do so. With PWAT near 2"
near the trough axis, some locally heavy downpours will be possible,
with showers expected to primarily affect the E half of the Big
Island. As a shortwave approaching from the NW moves over the area
this afternoon and tonight, showers along the trough axis may become
enhanced. A slight chance of thunderstorms has been added to the
forecast for the Big Island and portions of the adjacent waters.

Over the weekend, the surface trough will weaken and shift E as a
new high builds NW of the islands. This high will bring light to
moderate N to NE winds that will push the remnants of a cold front
over the islands from the N, and bring a cooler air mass. This
moisture will likely fuel a few showers along N and E facing slopes
and coasts from late Saturday into Monday. A stronger high is
expected to develop NW of the islands Tuesday and Wednesday, likely
leading to locally breezy N to NE winds that will usher additional
frontal moisture over the islands as low pressure develops (once
again!) NE of the islands. This development would keep NW to N winds
over the islands toward the end of the week, delivering passing
clouds and showers.

&&

.AVIATION...
Patches of mid-level clouds continue to stream across the eastern
end of the island chain early this morning. The background low-
level flow will remain light today as a weak surface trough near the
eastern Big Island tracks slowly westward. An area of enhanced
moisture also remains in the vicinity of the Big Island. This is
expected to cause low clouds and showers to increase over the
windward Big Island this morning. AIRMET Sierra for mountain
obscuration due to these low clouds and showers is in effect for the
windward Big Island. By this afternoon, a slight chance of
thunderstorms is also included in the forecast for most of the Big
Island.

The low clouds and showers are expected to spread over portions of
windward Maui later today and tonight. As a result, AIRMET Sierra
may need to be extended to windward Maui. Elsewhere, mainly VFR
conditions are expected to prevail today.

&&

.MARINE...
Light northeast winds will continue across most Hawaiian waters into
the upcoming weekend, then shift out of the north and increase into
the light to moderate category late Saturday through Sunday. Near
the coasts, overnight and early morning land breezes followed by
late morning and afternoon sea breezes will continue through
Saturday. The increase out of the north to northeast direction late
Saturday through Sunday may be strong enough to hold off the sea
breezes from developing Sunday. The exception will be around the Big
Island due to a trough of low pressure that is forecast to linger in
the area into the weekend. Moderate to fresh breezes (northeast
winds west of the trough and southeast winds to its east) will hold
across the windward and southeast Big Island waters today. The
trough will begin to shift east over the weekend as high pressure
builds to the northwest and a front passes to the north.

Despite the light winds in place, seas are slow to lower due to the
large easterly swell that has been in place. This swell has peaked
and will continue to lower through the weekend. Surf along east
facing shores will respond and gradually ease into the weekend.

After a long stretch of large surf and northerly winds for our north
facing shores through the first half of February, surf will near a
minimum for the month through the weekend. Storms over the far
northwest Pacific have been quickly lifting north-northeast from
Japan to the Bering Sea, ultimately limiting our traditional setup
for northwest swell sources.

A cold front will approach and move into the area from the north
Monday night through midweek, which will bring fresh to strong north
winds and rough seas (small craft advisory conditions) across the
waters Tuesday through midweek.

Surf along north and west facing shores is expected to rise next
week in response to a powerful hurricane-force low projected to
develop off the coast of Japan Saturday, then race northeastward
toward the Aleutians/Date line Sunday. WAVEWATCH III and the ECMWF-
Wave solutions depict a large long-period northwest (320 deg) swell
evolving from this source moving through the Hawaiian waters Tuesday
through midweek. If this materializes, surf could near warning
levels for exposed north and west facing shores Wednesday through
Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Big Island Summits.

&&

$$

DISCUSSION...Birchard
AVIATION...Houston
MARINE...Gibbs

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