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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 182000
Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sun Feb 18 2018
Moist and unstable conditions will prevail over the state through
Monday, with an upper-level disturbance and a slow-moving surface
trough bringing the threat of 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.
A sharpening trough aloft just W of the islands is combining with
abundant low-level moisture and a nearby surface trough to promote
the development of heavy showers and thunderstorms over the area
this morning. Current radar and satellite loops indicate that the
strongest storms are located over and just N of the coastal waters
from Oahu to Maui, over waters 30-60 miles S and SW of Oahu and
Kauai, and about 150 to 200 miles to the S. The cluster of
thunderstorms to the S has persisted for several hours, and bears
watching, as it is moving on a trajectory that would bring it over
the central part of the island chain later today into tonight.
Meanwhile, strong to severe thunderstorms over Hawaiian Offshore
Waters N of Maui are intersecting portions of the Maui windward
coastal waters. Otherwise, it appears that the best chance for
showers and thunderstorms over land will come this afternoon, as
light winds allow sea breezes to converge.
Conditions are changing fairly rapidly, with storms quickly
developing and weakening in response to passing gravity waves and
outflow boundaries. Spotty moderate to heavy showers are over parts
of the islands from Kauai to Molokai, and 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 in anticipation of deep-layer moisture developing,
potentially producing snow later today through Monday.
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. Although heavy showers
could develop anywhere at anytime in this regime, the light wind
regime means they are most likely over the coastal waters during
nights and mornings, and over the islands during afternoons and
evenings through Monday. 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, potentially 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.
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, so thunderstorms
are possible across the area. There will be gentle SE winds over the
E half of the area and gentle NE winds over the W half. The
generally easterly flow will keep clouds and showers focused over
mainly E sections, but MVFR and isolated IFR conditions will be
possible anywhere with thunderstorms. AIRMET SIERRA remains in
effect for MTN OBSC over E sections.
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 .
Scattered strong thunderstorms and locally heavy downpours will
continue over most Hawaiian waters through Monday. Expect reduced
visibilities, locally gusty winds, frequent lightning, and the
possibility of small hail and/or waterspouts in the strongest
Small northwest swells will continue through early Tuesday, with
surf remaining well below advisory level along north and west facing
shores. However, a longer period west-northwest swell is expected to
spread down the island chain starting 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, too, fades Friday into next
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.
Flash Flood Watch until 6 PM HST Monday for all islands.
Winter Weather Advisory until 6 PM HST Monday for Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office
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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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