Scientific Forecaster Discussion
fxhw60 phfo 252015 cca
Area forecast discussion...corrected
National Weather Service Honolulu HI
1015 am HST Tue Apr 25 2017
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. Somewhat wet and
unstable conditions will develop on Friday and will continue over
portions of the state through the weekend.
a stable easterly trade wind flow is in place over the state this
morning, bringing an area of modest showers to the eastern half of
the state. 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 is generating some high clouds mainly to the east of the
islands. The overnight soundings showed a well defined inversion
around 4500 to 6000 ft with precipitable water values below an
inch. However, the soundings did not capture a pocket of low
level moisture moving in on the trade wind flow. This feature is
producing modest showers from the hamakua coast of The Big Island
to windward slopes of Maui County. Expect these showers to affect
mainly windward areas of the smaller islands today.
The weather pattern will change little tonight and Wednesday.
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 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
night as the approaching upper level trough destabilizes the
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. 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 by this time, and the
inversion will likely be absent. We have not added heavy shower
potential to the forecast, though we will take a close look at it
Unsettled weather 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.
Details remain unclear this far out, but wet and unstable
conditions will likely set up over portions of the island chain.
high pressure northeast of the state will provide for moderate to
locally breezy trade winds over the area. Airmet Tango for low
level turbulence was issued this morning for areas over and
immediately south through west of the mountains. VAD wind
profiles are reporting 20-25 kt winds through much of layer at
phkm and phwa, and near 20 kt with isol 25 kt at phmo and phli.
Morning soundings are also showing a strong inversion between 4-5
The relatively dry conditions this morning and yesterday for the
windward areas will start to fade as a slug of moisture comes in
from the east later today and tonight. Expect increasing chances
for prolonged periods of MVFR conditions.
a high pressure ridge is nearly stationary north of the state
which is maintaining moderate to 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
In terms of surf, a small northwest swell currently affecting the
state is forecast to be reinforced by another northwesterly pulse
later today. 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 should 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
Small Craft Advisory until 6 am HST Wednesday for Alenuihaha
Channel-Big Island windward waters-Big Island leeward waters-Big
Island southeast waters.