Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 170632

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Mon Jan 16 2017

Trade winds will build slowly from east to west across the state
through Wednesday as a ridge of high pressure pushes north away
from the islands. The trades will then ease briefly Thursday and
Friday as a front begins to approach from the northwest. Limited
showers will favor the windward and mountain areas through the
week. A change in weather is then expected over the weekend as a
front pushes southward through the island chain, bringing an
increase in showers and strong trade winds to the Aloha state.


Currently at the surface, a 1025 mb high is centered around 1300
miles east-northeast of Hilo, with the ridge axis extending
westward to a location around 140 miles north of Kauai. This is
resulting in moderate trade winds over the Big Island and Maui
County, with light trades over Oahu and Kauai. Infrared satellite
imagery shows variably cloudy skies in place across the state,
with radar imagery and rain gauge observations showing little if
any shower activity across the area early this evening. Main short
term concern for the overnight revolves around summit winds on the
Big Island and Maui.

The east-west oriented high pressure ridge axis will remain just
to the north of Kauai overnight, keeping moderate trade winds
blowing over the Big Island and Maui County, with light trades and
localized land breezes expected over Oahu and Kauai. 00Z soundings
from PHTO and PHLI showed a very dry and stable airmass, with
precipitable water values ranging from 0.67 inches at PHTO to 1.00
inches at PHLI, and inversion heights at both locations around 4
to 5 kft. With the dry and stable airmass in place, showers will
be few and far between, with the best chance for any measurable
rainfall across windward sections of the Big Island. Summit level
winds on the Big Island and Maui are are expected to remain strong
due to an upper low south of the state and upper level ridging to
the north. As a result, a High Wind Warning remains in effect for
the Big Island summits with a Wind Advisory in place for the
Haleakala summit.

Tuesday through Friday,
The high pressure ridge axis will lift northward slightly through
Wednesday, allowing the trade winds to gradually strengthen across
the island chain. The trades will remain light across Kauai and
Oahu on Tuesday, while moderate trades continue across Maui County
and the Big Island. The trades will then peak at moderate levels
on Kauai and Oahu on Wednesday, with locally breezy trades across
Maui County and the Big Island. A front will then approach from
the northwest Thursday and Friday, with the trades easing back to
light to moderate levels across the state. A dry and stable
weather pattern is expected through the work week, with inversion
heights remaining suppressed due to mid-level ridging just north
of the state and precipitable water values remaining around or
below 1 inch. Any showers that due develop are expected to remain
limited to windward and mauka areas. Winds on the Haleakala summit
are expected to drop below Wind Advisory levels by daybreak
Tuesday, but winds will likely remain at Warning or Advisory
levels across the Big Island summits through the day Tuesday and
possibly as long as Thursday.

Friday night through next Monday,
The weather pattern will change over the weekend as a front pushes
southward through the island chain. There remains some timing
differences between the GFS and ECMWF, so will utilize a blended
forecast approach for forecast details. Overall, it appears that
the front will be closing in on Kauai Friday night, then push
southward through the island chain Saturday through Sunday. The
GFS shows the front hanging up near the Big Island through early
next week, while the ECMWF allows the front to clear the island
chain. With the approach of the front and the break down in mid-
level ridging Friday night, we should see an increase in shower
activity mainly across windward and mauka areas. Showers coverage
will increase in association with the front Saturday through
Sunday, with showers likely reaching leeward areas of the smaller
islands as well. Model solutions differ by early next week, so
will keep more shower activity in place across windward sections
of the Big Island, while showing a drying trend for the remainder
of the island chain. In addition to the increase in showers
associated with the front, the trade winds will become strong over
the weekend and possibly lingering through early next week, with
the trades potentially reaching wind advisory levels across much
of the island chain.


Light background east to east-southeast flow is allowing local
land breeze circulations to develop over Kauai and Oahu early
this evening. The low-level trade wind flow, which is slightly
stronger near Maui County and the Big Island, will be
transporting some low clouds and isolated light showers into the
windward facing slopes and coasts of the central and eastern
islands. Therefore, brief periods of MVFR CIGS are possible in
passing low clouds and -shra over some windward sections.
Otherwise, VFR conditions are expected to prevail.

There are currently no AIRMETS in effect, and none are
anticipated through early Tuesday morning.


A Small Craft Advisory (SCA) remains in effect for the Alenuihaha
Channel and waters south of the Big Island this evening, due to
locally strong trades blowing in that area. A surface ridge some
120 nm north of Kauai remains slow in moving north this evening
that would allow stronger trades to spread westward from the Big
Island. The process is slow but eventually it will come about in
24 hours or by Tuesday night where we may have SCA level winds
blowing over the waters around Maui County, such as the Pailolo
Channel, Maalaea Bay, and Maui windward waters. The SCA will then
be expanded to cover these waters.

If not the wind that will warrant an SCA, the seas will, brought
on by an elevated WNW swell slated to arrive late Tuesday night
through Wednesday. Therefore, the SCA will cover many of the
marine zones. On Thursday, the trades will back off slightly while
the WNW swell starts subsiding to where we may begin scaling down
the SCA.

As for the surf, as noted above, warning level surf is expected
with the WNW swell for most N and W facing shores of the smaller
islands starting as early as Tuesday night and running through
Wednesday night. The swell will peak late Wednesday, then drop to
moderate levels by Thursday night. Due to its west component, the
Kona coast of the Big Island may likely experience surf near or at
the warning level late Wednesday night and early Thursday.

The weakening of the trades is due to the ridge dissipating N
of the islands by an advancing front. The front will move through
the islands Saturday and Saturday night, and will bring in
blustery NE winds, and large, rough, short-period seas over area
for at least the rest of the weekend, prompting SCA conditions to
near gale force winds over all Hawaiian waters, and a High Surf
Advisory for E facing shores.


Wind Advisory until 6 AM HST Tuesday for Haleakala Summit.

High Wind Warning until 6 PM HST Tuesday for Big Island Summits.

Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.



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