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fxhw60 phfo 260157 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
357 PM HST Tue Apr 25 2017

Synopsis...
moderate easterly trade winds 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.

&&

Discussion...
a stable easterly trade wind flow remains in place over the state
this afternoon, bringing an area of modest showers to Oahu and
Maui County. A surface ridge parked roughly 400 miles north of
Kauai is driving the trades. An associated mid level ridge is
maintaining stable conditions, while a weak upper level trough
overhead continues to generate some high clouds mainly to the
east of the islands. Afternoon soundings and recent aircraft data
showed a well defined inversion around 5000 to 8000 ft with
precipitable water values below an inch. Like last night, the
soundings did not capture The Pocket of low level moisture moving
in on the trade wind flow. This feature is producing modest
showers from Molokai to Oahu. Expect these showers to affect
mainly windward areas of Oahu late this afternoon.

The weather pattern will change little tonight and Wednesday, with
windward slopes experiencing modest rainfall. Easterly trade
winds will hold at moderate strength while mid level ridging
maintains stable conditions. The current pocket of low level
moisture will move slowly westward to Kauai and likely clear the
islands on Wednesday.

On Thursday and Thursday night, trade winds will ease and shift
out of the southeast as an approaching front weakens the surface
ridge to the north. The upper level trough driving the front will
erode the mid level ridge over the islands, causing the inversion
to lift and weaken. The European model (ecmwf) and GFS are showing another area of
enhanced low level moisture moving over the state from the east
during this time. Expect an increase in shower activity, including
over leeward areas during the afternoon hours as sea breezes
develop. A few showers could become briefly heavy by Thursday
as the approaching upper level trough destabilizes the atmosphere,
but we opted to leave out heavy showers in the forecast for now.

The front is expected to reach Kauai on Friday afternoon. The GFS
and European model (ecmwf) are depicting a somewhat shallow front, suggesting that
its associated rainfall would be focused across north facing
slopes of Kauai initially as northerly winds increase along the
front. The more active area of rainfall could end up being spotty
heavy showers within the low level moisture already pooled over
the islands. The rather strong upper level trough will be centered
over the state, and the inversion will likely be absent. Due to
increased confidence in developing instability combined with
afternoon sea breezes, heavy showers have been introduced to the
forecast at this time, but we are still not ready to mention
chances for thunderstorms.

Unstable weather with the potential for heavy, possibly flooding,
rainfall is expected over portions of the state this weekend. The
GFS and European model (ecmwf) are showing a compact, but potent, late-season cut
off low developing north of the islands on Saturday then dropping
southward toward the state on Sunday. The models are in good
agreement with the overall pattern, but there is still some run to
run inconsistency with the placement of the cutoff low as it
nears the state on Sunday. The models are also suggesting that a
convergence band will set up along the old front and become the
focus for heavy, possibly flooding, rainfall. A blend of recent
GFS and European model (ecmwf) runs points toward the Maui County as the most
likely position of the band, but it's too early for details.

&&

Aviation...
high pressure northeast of the area will continue to deliver
moderate to locally breezy trade winds into Wednesday. Airmet
Tango for low level turbulence over and immediately south through
west of the mountains will continue.

Enhanced moisture will also continue to push through the smaller
islands tonight. The majority of clouds and showers will favor
windward and Mauka areas. Expect some showers to introduce isol to
tempo MVFR ceilings/vis, especially tonight and early morning. Airmet
Sierra for mountain obscuration along those windward areas may
become a possibility later tonight.

&&

Marine...
a high pressure ridge is nearly stationary north of the state
which is maintaining moderate to locally fresh east winds over
the coastal waters. The highest wind speeds were over the normally
windy areas near Upolu Point and South Point on The Big Island
and were strong enough keep a Small Craft Advisory (sca) up for
those areas. Models are in good agreement in keeping the ridge in
place through Wednesday. A late season cold front is expected to
erode the ridge as it approaches the state from the northwest
Wednesday night and into Thursday. The front is forecast to stall
over the mid-section of the state with breezy north winds causing
Small Craft Advisory-level seas across the western coastal waters Friday night and
Saturday.

In terms of surf, a reinforcing northwest swell appears to have
arrived based on the coastal buoy data. Open ocean swell heights
are forecast 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.

South-facing shores should see small surf through mid-week.
However, a new, long period swell is forecast to arrive from the
south-southwest on Thursday with surf possibly reaching high surf
advisory criteria (8 ft faces for south-facing shores). This
South-Shore surf episode is expected to persist into the weekend.

Small surf along eastern facing shores will hold steady into
Wednesday before dropping off Thursday and Friday as the local winds
become light.

&&

Hfo watches/warnings/advisories...
Small Craft Advisory until 6 am HST Wednesday for Alenuihaha
Channel-Big Island windward waters-Big Island leeward waters-Big
Island southeast waters.

&&

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