Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 212005

Area Forecast Discussion
National Weather Service Honolulu HI
1005 AM HST Fri Oct 21 2016

Strong trade winds will persist through the weekend and will
supply periods of wet weather to the area. Trades will focus most
of the shower activity across windward and mauka areas, but many
of the showers will carry over to leeward sides of the smaller
islands. A drier airmass will move into the region next week,
bringing more typical trade wind weather back into the area.


Currently at the surface, a 1030 mb high is centered around 900
miles north of Kauai, and is driving the strong trade winds
across the island chain. The high is slowly weakening, but
building closer to Hawaii which is essentially maintaining the
pressure gradient and winds over the state. A Wind Advisory is in
effect for portions of the Big Island and Maui County through 9 pm
HST today.

Visible satellite imagery shows mostly cloudy skies windward and
partly cloudy skies leeward across most of the state. Meanwhile,
radar imagery shows numerous showers moving into windward areas
carried by the trade winds, with many the showers reaching leeward
areas of the smaller islands. Expect the relatively wet trade wind
weather and strong winds to continue today.

MIMIC total precipitable water (PW) imagery shows an area of
enhanced moisture with PW values between 1.4 and 1.7 inches over
and upstream of the smaller islands islands, resulting in fairly
widespread shower activity across windward and mauka areas today.
By this afternoon, some slightly drier air will begin to work
into the Big Island, so we should see a decrease in windward
showers here, while daytime heating allows for scattered showers over
the Kona slopes. Across the smaller islands, the deeper moisture
is expected to hang around through the rest of today. Given the
strength of the trades, some of the showers will make it into
leeward sections of the smaller islands. Showers are likely to
fill in again tonight and again on Saturday night, but coverage
should not be as great as today. Winds will carry showers along,
so while a few of the showers could bring a quick bout of moderate
or heavy rain and blustery conditions, we are not anticipating
flooding concerns at this time.

The high 900 miles to the north will dig closer to Hawaii over
the next day or so allowing the trade winds to peak on Saturday
and Saturday night. A statewide Wind Advisory is likely to go into
effect for tomorrow. Sustained winds in exposed areas and through
gaps in the mountains will be the main concern over the next 24 to
48 hours. Downsloping gusts will be possible for leeward areas.

Monday through Thursday,
The high will weaken north of the state during the early next week,
allowing the trade winds to ease to moderate to locally fresh
levels. The trades will then strengthen by late next week as
another strong high builds well north of the Aloha state. As for
sensible weather details, a typical trade wind weather regime is
expected through the period, with a slight increase in showers
possible on Wednesday as a weak frontal boundary pushes southward
through the island chain. Showers will continue to favor windward
and mauka areas, with an occasional shower spilling over into
leeward areas of the smaller islands.


Radar VAD wind profiles and AMDAR soundings out of Honolulu show
30 knots of wind within a few thousand feet of the surface.
Inversions are elevated, but winds this strong should still
support turbulence in the lee of the mountains and an AIRMET is
in effect for moderate low-level turbulence. The turbulence
threat will increase tonight as the inversion lowers and low-
level winds remain just as strong. Moderate turbulence potential
will continue through the weekend, and possibly into early next

A pocket of moisture mainly from Oahu to Maui will begin to
affect Kauai by afternoon. This airmass is quite showery, with
brief IFR visibilities in the heavier showers. MVFR conditions
will be more widespread across windward sections, and an AIRMET is
in effect for mountain obscuration. With the strong trade winds
and elevated inversion, occasional showers will spread leeward and
brief MVFR conditions are also possible. Conditions will begin to
improve this evening.


A Small Craft Advisory (SCA) covers all coastal waters today, and
will continue for most area through the weekend due to strong
trade winds. Winds are forecast to reach gale strength in the
typically windy areas around the Big Island and Maui Saturday and
Saturday night. A Gale Watch has been issued for these typical
windy areas for Saturday and Saturday night. Please see list of
affected areas below and the Marine Weather Message (MWWHFO)
product on our website. The trades will gradually weaken Sunday
through Tuesday with the areas under the SCA likely being scaled
back to the typically windy waters early next week.

There will be a series of mainly small northwest to north
northwest swells into next week, and a small background swell
from the south. The strong trades, along with the upstream fetch,
will build surf for east facing shores beginning today. Surf
heights are likely to reach advisory levels over the weekend, and
a High Surf Advisory will likely be issued for east facing shores
later today or tonight.


Wind Advisory until 9 PM HST this evening for Lanai-Kahoolawe-
Maui Leeward West-Maui Central Valley-South Big Island-Big
Island North and East-Kohala.

Gale Watch from Saturday morning through late Saturday night for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Monday for all other Hawaiian
coastal waters.



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