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Hawaiian Islands Synoptic Discussion and Guidance

976
FXHW60 PHFO 190145
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Sun Feb 18 2018

.SYNOPSIS...
Moist and unstable conditions will prevail over the state through
Monday, with an upper-level disturbance and a slow-moving surface
trough bringing widespread thunderstorms and flooding rainfall.
While light winds are expected to continue for the next several
days, the threat of heavy rain will diminish Monday night and
Tuesday as the upper-level disturbance moves away. Light to moderate
east to southeast winds will deliver a few windward showers Tuesday
through Thursday, with the weather potentially becoming wet again by
the end of the week.

&&

.DISCUSSION...
A sharpening trough aloft just W of Kauai is combining with abundant
low-level moisture and a nearby surface trough to promote the
development of heavy showers and thunderstorms over a large portion
of the state this afternoon. Current radar and satellite loops (and
lightning data) indicate most areas receiving dangerous cloud to
surface lightning strikes, and heavy rainfall. Additionally, a
cluster of thunderstorms to the S of Oahu has persisted for at least
12 hours, and bears watching, as it is moving on a trajectory that
would bring it over the central part of the island chain over the
next several hours. Gusty winds associated with the strong
thunderstorms are occurring and remain possible through at least
tonight.

A Flash Flood Watch (FFA) is in effect for the entire state due to
the ongoing potential for extremely heavy rain. A Winter Weather
Advisory for the Big Island Summits is posted for periods of wintry
weather, mainly snow, with a little snow having already fallen this
afternoon.

The digging upper trough is expected to become a closed low near
Kauai over the next 24 hours, then gradually track ENE and weaken
Monday night and Tuesday. As this occurs, forecast models are
indicating that the greatest convergence and dynamic lift will be
concentrated to the N and NE of the islands. In the low-levels, a
trough currently near the central portion of the island chain is
supporting light winds statewide. The low aloft is expected to
maintain the surface trough and keep light winds over the islands
through Tuesday. The generally easterly low-level flow will favor a
NE direction to the W of the trough axis, and a SE direction to the
E of the trough axis.

Even with guidance indicating the bulk of the action will be just N
of the area, there will enough moisture to trigger (at least)
scattered heavy showers and thunderstorms. Cold temperatures
associated with the developing low aloft will make the island
atmosphere very unstable as 500 mb temps drop to -16C, maintaining
the threat of thunderstorms through Monday. With this kind of
atmospheric profile, updraft strengths become sufficiently strong to
support the development of small hail and funnel clouds, leading to
locally gusty winds.

The closed mid-level low lifts NE and weakens Monday night and
Tuesday, allowing the atmosphere to become more stable. As an
associated mid-level dry slot moves in from the W, the threat of
heavy rain/thunderstorms will diminish. Light to moderate E to SE
winds are expected through Thursday as a surface high will be
centered to the distant NE, and the trough drifts and lingers to the
NW of the islands. This is expected to lead to mainly windward
shower activity for the midweek period, but winds may be light
enough to allow for a few afternoon leeward showers. Forecast
guidance shows yet another deep mid/upper trough developing W of the
islands later in the week, which could bring more wet and unstable
weather to the islands into next weekend.

&&

.AVIATION...
A N to S surface trough near Oahu will move W slowly as a N to S
trough aloft digs SE slowly.

The trough aloft will keep the atmosphere unstable through Monday,
so thunderstorms are possible across the area. MVFR and isolated IFR
conditions will be possible anywhere with thunderstorms. AIRMET
SIERRA remains in effect for MTN OBSC and IFR.

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

The trough is also producing layered middle and high clouds. Light
icing will be possible in these clouds.

&&

.MARINE...
Scattered strong thunderstorms and locally heavy downpours will
continue over most Hawaiian waters through Monday. Expect reduced
visibilities, locally gusty winds, frequent lightning, and possibly
small hail and waterspouts in the strongest thunderstorms.

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, west-northwest swell
is expected to spread down the island chain beginning late Tuesday,
which may cause surf to approach advisory level along some north and
west facing shores Wednesday, especially Kauai and Niihau. This
swell will gradually subside Thursday, but a reinforcing west-
northwest swell is expected Thursday night, before it fades Friday
into next 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 through Monday. This swell will increase through Monday
night, with surf likely reaching the HSA threshold of 8 feet along
most east facing shores Tuesday. This east swell may subside
slightly Wednesday, but increase again Thursday and remain elevated
into next weekend.

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


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST Monday for all islands.

Winter Weather Advisory until 6 PM HST Monday for Big Island
Summits.

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...TS

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



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