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Hawaiian Islands Synoptic Discussion and Guidance

641
FXHW60 PHFO 191328
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
328 AM HST Mon Feb 19 2018

.SYNOPSIS...
The potential for flash flooding and strong thunderstorms will
continue today over the state as an upper disturbance and surface
trough remain in place. The threat will begin to diminish tonight
through Tuesday as the upper disturbance begins to lift north and
away from the state and some drier air begins to fill in from the
west. Light east to southeast winds, likely giving way to daytime
sea breezes and overnight land breezes, and isolated to scattered
showers will remain possible through midweek. A return of
unsettled weather will become a possibility once again Thursday
through the upcoming weekend as another upper low drops south into
the area.

&&

.DISCUSSION...
Although the widespread showers and storms that were impacting the
state yesterday afternoon into the overnight period have
diminished this morning, the potential for redevelopment still
exists today due to plenty of instability and moisture associated
with an upper trough and a broad surface trough located over the
state. Recent satellite and radar imagery showed thunderstorms
developing around the islands across the offshore waters as of
this discussion time, especially south of the Big Island within
100 miles. Provided the saturated soils from the activity
yesterday, additional heavy showers and storms could lead to
flooding in a short period of time. As a result, the flash flood
watch remains in effect through 6 PM HST today. At the summits,
the winter weather advisory also remains in effect through this
time (Big Island and Haleakala), as well as a high wind advisory
(Big Island Summits).

Tonight through midweek, the upper low impacting the local weather
is forecast to lift northeast and away from the state, allowing
the atmosphere to become more stable. Light east to southeast
winds are expected as a surface high will be centered to the
distant northeast, and the trough drifts and lingers to the
northwest of the islands. Daytime sea breezes and overnight land
breezes will result each day. The best chance for clouds and
showers will be through the afternoon periods over the interior
areas.

Thursday through the upcoming weekend, guidance remains in decent
agreement and suggests a return of unsettled weather setting up over
the state as another upper low drops south into the area. Plenty of
moisture is shown pooling northward over the state beginning late
Thursday through Friday (PWs 1.5-1.8"). Provided the grounds already
being saturated from this event and last week, flash flooding
will become a concern through this extended period, as well as
strong thunderstorms. Additional fine tuning of the forecast for
this period will be likely over the next couple of days as
forecast confidence rises.

&&

.AVIATION...
With a slow-moving surface trough in the area, an upper trough
over the western part of the state, and plenty of low-level
moisture; the atmosphere continues to be unstable. Heavy showers
and thunderstorms are possible through the afternoon today,
especially for the eastern isles. MVFR and isolated IFR conditions
may occur, so AIRMET SIERRA remains in effect for IFR, currently
for Maui and the Big Island.

The trough aloft will also make turbulence likely. AIRMET TANGO
remains in effect for turbulence above FL200.

The trough is also producing layered middle and high clouds. Light
icing will be possible in this cloudiness above the freezing
level.

&&

.MARINE...
Scattered showers and isolated thunderstorms will continue over
the Hawaiian coastal waters today. Reduced visibilities, gusty
and shifting winds, frequent lightning, and possibly small hail
and waterspouts can be expected in any stronger thunderstorms.
Thunderstorm activity should shift northeast of the area tonight
and Tuesday, with drier weather and just scattered lighter showers
then expected through Wednesday.

Small to moderate northwest swells will continue through early
Tuesday, with surf remaining below advisory level along north and
west facing shores. However, a longer period northwest swell is
expected to spread down the island chain beginning Tuesday night
and peaking Wednesday. This may cause surf to approach advisory
level along some north and west facing shores, especially Kauai
and Niihau, during the peak of the event. This swell will
gradually subside Thursday, but another west-northwest swell is
expected Thursday night into Friday, then fading into the weekend.

A long fetch due to the flow around a surface high over the
northeastern Pacific will send a short-period east swell into the
local waters during the next several days. This swell is forecast
to increase tonight and Tuesday, with resulting surf likely
reaching the advisory threshold of 8 feet along most east facing
shores Tuesday. This east swell is forecast to increase further
on Thursday and remain elevated into the weekend, with high surf
likely continuing along east facing shores.

Combined seas may increase to around 10 feet over most windward
waters starting late Wednesday due to the combination of west-
northwest and east swells, and local wind waves. A Small Craft
Advisory may be needed from that time through the end of the work
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Weather Advisory until 6 PM HST this evening for
Haleakala Summit-Big Island Summits.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

&&

$$


AVIATION...Kinel
MARINE...Jacobson
REST OF DISCUSSION...Gibbs

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



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