Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 261943

Area Forecast Discussion
National Weather Service Honolulu HI
943 AM HST Wed Jul 26 2017

Trade wind weather will prevail for the next week, thanks to high
pressure persisting far to the northeast of the area. Clouds and
passing showers will favor windward and mauka areas with just a
few brief showers expected over leeward locations. The islands may
see an uptick in shower activity by the weekend as an area of
moisture moves across the area.


Water vapor imagery shows an upper level low northwest of the
state, with a weak ridge over the islands. At the surface, high
pressure is located far to the northeast with a wide belt of trade
winds over the area. The 12z sounding from Hilo showed an
inversion around 6kft. Precipitable waters are around 1.4 inches.
Early morning MIMIC total precipitable water imagery shows an area
of drier air with PWs of 1.1-1.3 inches within 400 miles east of
the Big Island, with much higher values farther east near what is
currently Tropical Depression Greg.

Not much change is expected in sensible weather through Thursday.
The surface high far to the northeast shifts southeast and weakens.
However, little change in the pressure gradient is expected
locally as what will be the remnant low from Greg passes by to our
south. Locally breezy trades and a stable inversion will lead to
a typical trade wind distribution for clouds and showers.

Conditions begin to change by Thursday night. The upper low to our
northwest opens into a trough and shifts southeastward over the
state. With these height falls, inversions weaken and climb
leading to a greater moisture depth. Additional deep moisture
spreads in from the east as well, sheared off/picked up by the
trades from the bulge of higher values located near Greg. From
Thursday night into Saturday, we transition to more of a wet trade
weather pattern. The greatest rainfall will still be focused
across windward sections, but with higher amounts and additional
showers spreading leeward as well. By Saturday night/Sunday, drier
and more stable air moves in from the east.


VFR conditions prevail at all TAF sites and these conditions are
expected through the day and on into the evening hours. There may
be some brief MVFR conditions over windward slopes as moisture
riding in on the trades interacts with mountainous terrain.

AIRMET Tango is in effect for moderate low level turbulence over
and south through west of all mountains below 8000 feet.


Locally breezy trade winds will persist through most of the week.
A Small Craft Advisory (SCA) is posted for the typically windier
waters around the Big Island and Maui through Thursday morning,
with an extension not out of the question for the following days.

A tropically active East and West Pacific will support most of
the surf related discussion. A distant typhoon in the West
Pacific is expected to send a mid to long period west swell
towards the islands Friday into the weekend. However, Niihau and
Kauai will help block most of the remaining smaller islands from
this swell. Another round of distant East Pacific tropical
cyclones could send a series of mid to long period east swells to
the islands late Saturday into midweek. This will need to be
monitored over the next several days, as current guidance
suggests reaching near advisory levels late Sunday into Monday.

Otherwise, the locally strong trade winds will keep rough choppy
surf along the east facing shores. A series of small south,
southeast, and southwest swells will produce small background surf
through the period.


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



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