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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 260649

Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Tue Apr 25 2017

Moderate easterly trade winds will persist through Wednesday as a
surface ridge holds north of the state. The atmosphere will
remain stable, with pockets of moisture affecting mainly windward
slopes. Trades will ease and shift out of the southeast on
Thursday as a front approaches the state, leading to spotty
afternoon showers. On Friday, the front will bring showers to
Kauai and possibly Oahu, while prefrontal showers, some heavy,
develop across the rest of the island chain. Wet and unstable
conditions are expected over portions of the state through the
weekend as a late season low forms just north of the islands.


Scattered showers are moving across Kauai early this evening,
while mainly dry weather prevails over the remainder of the state.
With a rather dry and stable airmass remaining in place, rainfall
amounts have remained on the light side, generally less than a
quarter inch in six hours. Moderate to locally breezy trade winds
prevail over the islands, driven by a surface ridge located about
400 miles north of Kauai. Mid level ridging over the area is
providing stable conditions, while a weak upper level trough over
the islands is generating areas of high clouds east of the state
but otherwise is having no impact on the sensible weather. MIMIC-
TPW satellite imagery depicts a small area of enhanced moisture
embedded in the trades moving over the western part of the state,
with a drier airmass over the eastern islands. This area of
moisture fell in between the 00Z soundings, which showed a dry and
stable regime with PW values around 0.9 inches and the inversion
based between 6000 and 8000 feet.

Expect little change to the weather pattern overnight and
Wednesday, with windward slopes experiencing modest rainfall in
passing showers. Easterly trade winds will hold at moderate
strength while the mid level ridge maintains stable conditions.
The pocket of low level moisture currently near Kauai will
continue moving slowly westward and will likely clear west of the
islands on Wednesday morning. The existing forecast appears to be
on track, and no updates are planned for this evening.

Wednesday night through Thursday night, trade winds will ease and
veer around to the southeast, as a late season cold front
approaches from the northwest and the surface ridge to our north
weakens. A deep mid/upper level trough driving the surface front
will also advance eastward during this time, displacing the mid
level ridge initially over the islands. Available moisture will
remain quite limited through Thursday morning, but models show
an area of somewhat enhanced low level moisture moving into the
state from the east Thursday afternoon. This increased moisture
and decreased stability combined with an expected seabreeze regime
could allow for some spotty heavier showers to develop during the
afternoon, especially over leeward and interior areas.

The new GFS and ECWMF runs remain consistent with earlier runs in
bringing the surface front to Kauai by Friday afternoon, while the
mid/upper level trough continues to deepen over the state.
Although moisture along the front itself may be rather shallow,
there will probably be ample moisture pooled ahead of the front,
combined with increasingly unstable conditions, to produce some
locally heavy downpours. Due to increased confidence in
developing instability, combined with afternoon sea breezes which
could provide the triggering mechanism, heavy showers were
introduced to the forecast for Friday afternoon, but thunderstorms
have not been included at this point.

The latest guidance continues to point toward an unstable weather
pattern for the weekend, with the potential for heavy and possibly
flooding rainfall. The GFS and ECMWF still show a compact but
potent late-season mid/upper level low developing north of the
islands on Saturday then dropping southward over or just west of
the state Sunday through Monday. The models are still in good
agreement with the overall pattern, but there remains some run to
run inconsistency with the exact placement of the cutoff low when
it is closest to the state Sunday into Monday. The models are
also suggesting that a convergence band will set up along the old
front and become the focus for heavy to possibly flooding rainfall.
A blend of recent GFS and ECMWF runs points toward Maui County as
the most likely position for this band to set up, but it remains
too early to have confidence in those details. Locally heavy
rainfall is mentioned for much of the state through the weekend,
and may need to be added later at least for eastern sections on
Monday as well. We will also continue to evaluate whether the
mention of thunderstorms will need to be added for the weekend.

Unsettled weather may persist across the state into early next
week, as the latest models keep the mid/upper level low lingering
nearby, along with areas of above normal moisture.


With persistent high pressure ridging over the area, expect
moderate to locally breezy trade winds through Wednesday. Although
overall conditions will be VFR, isolated to TEMPO MVFR (lower
ceilings and visibilities in passing showers) is likely in
windward and mauka sections on the individual isles.

An AIRMET for TEMPO moderate low level turbulence to the lee of
the mountains is in effect due to the moderate to breezy trades.


A high pressure ridge remains nearly stationary about 350 nm north
of the state and is maintaining a moderate to locally fresh east
to east southeast flow over the coastal waters. Winds are
strongest in the typically windier locations near Upolu Point and
South Point, and are strong enough to continue the Small Craft
Advisory (SCA) for those areas. The current SCA goes through
tonight, but will likely be extended through the day on Wednesday.

Winds will begin to weaken Wednesday night and into Thursday as a
late season cold front approaches from the northwest. The front
will begin to erode the ridge as it approaches the state. The
front is forecast to stall over the mid-section of the state with
breezy north winds causing SCA-level seas across the western
coastal waters Friday night and Saturday.

The current northwest swell is expected to peak later tonight or
early Wednesday near 4 feet with periods in the 11 to 13 second
range. This will be followed by a moderate north-northwest swell
arriving late Friday and into Saturday. Small surf is expected
along south facing shores through mid-week, with a new, long
period swell forecast to arrive from the south-southwest Thursday.
This swell could raise surf along south facing shores to advisory
levels, and will persist into the weekend. Surf along the east
facing shores will begin to drop off Thursday and Friday as the
ridge to the north is eroded and winds become light.


Small Craft Advisory until 6 AM HST Wednesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.



MARINE...M Ballard

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