Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 160623

Area Forecast Discussion
National Weather Service Honolulu HI
823 PM HST Fri Dec 15 2017

An upper level low and increasing moisture will bring periods of
rainfall with locally heavy downpours possible and a slight
chance of thunderstorms early this weekend. A brief respite from
the active weather pattern is expected from late Sunday into
Monday. However, increasing rainfall is possible again starting
around Monday night as an upper level trough moves over the
islands from the northwest.


A 1035 mb surface high is near 42N 136W, or more than 1900 miles
northeast of Honolulu. A surface ridge extends southwest from
this high through a point approximately 770 miles north of Kauai.
Elsewhere, a nearly north to south oriented surface trough, which
is about 215 miles east of Hilo, is slowly tracking westward. The
tight pressure gradient between these features is maintaining
locally breezy north winds across the extreme western end of the
state early this evening. Winds have already been tapering off
across the eastern end of the state as the trough approaches.
These lighter winds are expected to continue spreading westward
across the remainder of the island chain tonight.

Aloft, middle and upper-tropospheric lows are located around 230
miles north of Hilo. The surface trough described above is a
reflection of these features, which are propagating slowly towards
the west. The close proximity of the lows has already destabilized
the atmosphere over the eastern end of the state. The early
afternoon Hilo balloon sounding indicated an elevated low-level
inversion near 10 thousand feet. Satellite imagery and radar
reflectivity data show broken low- and mid-level clouds with
embedded numerous showers streaming down from the north across
Oahu and Maui County, with fewer clouds and showers across Kauai
and the Big Island.

The lows aloft are expected to reach a position just northeast of
the islands on Saturday, and then slowly pull away toward the
northeast on Sunday. The surface trough will eventually move
westward over the island chain on Saturday. An area of enhanced
moisture will move over the islands as the lows continue to
destabilize the atmosphere. This will continue to produce wet
weather for most of the state through Saturday, and possibly
Saturday night as well. In addition, the unstable atmosphere will
allow for heavier showers and isolated thunderstorms. The light
and variable surface winds will also allow local afternoon sea
breezes to develop on Saturday, and possibly Sunday. The sea
breezes may provide additional support for deep convection over
some of the islands on Saturday.

The cold air aloft associated with the upper lows may bring near
freezing temperatures on Haleakala Summit. The moisture is already
deep enough to reach the summit, so a Winter Weather Advisory is
in effect for Mount Haleakala above 9 thousand feet due to the
potential of freezing rain. See the Winter Weather Message
(WSWHFO) product on our website for more information.

The upper lows will eventually lift away to the north Sunday,
which may provide a brief respite from the active weather from
late Sunday into Monday. However, an upper-level trough is
forecast to dig down toward the islands from the northwest, with
deep tropical moisture pushes up from the southeast Monday night
into Tuesday. This may result in widespread showers across the
islands. In addition, a couple of cold fronts may sweep across the
islands from the northwest later in the week.


.AVIATION...A cut off low 200 miles northeast of Honolulu will
continue to produce enhanced showers and clouds over the islands
tonight and tomorrow. Heavier shower activity and low cloud
ceilings will favor mountain areas and north through east slopes
of Maui, Molokai and Oahu. A dry slot wrapping around the low continues
to linger over Kauai and the Big island this evening limiting
clouds and showers for both islands in the overnight period.

Slight chances for thunderstorms remain in the forecast, however
the best chances for storm development will occur within the
coldest core of the low arriving in the islands from Saturday
morning to afternoon.

AIRMET Sierra remains in effect for Tempo Mountain Obscuration
over north through east sections of Maui, Molokai and Oahu. This
AIRMET will likely continue through the morning hours on
Saturday. Expansion of this AIRMET for Kauai is possible by
Saturday afternoon.


A surface trough located northeast through southeast of the Big
Island will be moving westward across the coastal waters through
the weekend. The trough has already lowered the winds over the
eastern waters. The weakening trend should proceed into the
western waters overnight and Saturday as the trough continues
moving westward. Winds should remain light across the entire area
from Saturday night through early next week.

As the winds and seas gradually subside, conditions are expected
to drop below Small Craft Advisory criteria, likely late tonight
or Saturday morning.

Observed wave heights from the nearshore PacIOOS buoys continue
to slowly decline this evening. The nearshore buoys are also
showing a short period northeast wave component produced by strong
northeasterly winds west of the trough. The northeasterly swell
has been large enough to warrant a High Surf Advisory for east
facing shores. As the winds decrease, the northeasterly swell will
decrease and drop below advisory level, possibly by early
Saturday morning.

A small northwest swell will produce surf well below advisory
levels Saturday night through Monday, then a larger north-
northwest swell may produce surf approaching advisory levels late
Tuesday into Wednesday.

Heavy showers and isolated thunderstorms will be possible across
parts of the coastal and offshore waters through at least Saturday
night due to a nearby upper level trough.

See the Collaborative Nearshore Swell and Wind Forecast for
details on expected swell and resulting surf.


High Surf Advisory until 6 AM HST Saturday for east facing shores
of Kauai, Oahu, Molokai, Maui, and the Big Island.

Winter Weather Advisory until noon HST Saturday for Haleakala

wind Advisory until 6 AM HST Saturday for Big Island Summits.

Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters.



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