Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 231315
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
315 AM HST Wed Aug 23 2017

.SYNOPSIS...
A dry and stable airmass will remain in place across the island
chain through the remainder of the work week and into the
upcoming weekend. Tropical moisture and muggy conditions may
return Sunday through early next week. Clouds and showers will
favor windward and mauka areas through the period, with a stray
shower spilling into leeward areas from time to time. Moderate to
breezy trade winds will continue through the remainder of the work
week, with the trades then easing over the weekend into early
next week as high pressure weakens to the north of the State.

&&

.DISCUSSION...
Currently at the surface, a 1026 mb high is centered around 1100
miles northeast of Honolulu, with another 1030 mb high around 1300
miles north of Kauai. Meanwhile aloft, the upper level low which
resulted in the unstable conditions over the past few days, is
now around 450 miles west of Kauai and is shifting westward and
further away from the State. Infrared satellite imagery shows
partly to mostly cloudy skies in place across the area. Radar
imagery shows scattered light showers moving into windward areas,
with a stray shower spilling over into leeward areas from time to
time. Main short term concerns revolve around rain chances and the
increasing trades over the next couple of days.

Today through Saturday,
The two areas of high pressure will merge and settle southward
closer to the islands over the next couple of days, increasing the
trade winds across the State. Moderate to breezy trade winds are
expected through the remainder of the work week due to the tighter
pressure gradient across the State, with the peak in the trades
expected on Thursday. We should then see the trades ease to
moderate levels on Saturday, as high pressure weakens north of the
island chain.

As for sensible weather details, a drier airmass will continue to
build over the islands today, with these drier than normal
conditions then expected to remain in place through Saturday.
Clouds and showers will favor windward and mauka areas through the
period, with a stray shower reaching leeward areas from time to
time due to the strength of the trades. Showers are expected to be
most prevalent during the overnight and early morning hours.

Saturday night through next Tuesday,
The high pressure ridge north of the islands will weaken in
response to a front passing by well to the north of the State.
This will weaken the trades into the light to locally moderate
range Sunday through early next week. Both the GFS and ECMWF show
some deeper tropical moisture trying to lift northward into the
area Sunday through early next week, and this may result in an
increase in trade wind showers across the island chain. In
addition to the increase in showers, muggy conditions will likely
make a return as well, with both models showing dewpoints
climbing back into the lower to middle 70s.

&&

.AVIATION...
A strong high pressure center well north of Hawaii will produce
moderate to locally breezy easterly trade winds over the islands.
These wind speeds will produce moderate low level mechanical
turbulence over and down wind of mountain ranges. Scattered shower
activity will continue into the morning hours with prevailing VFR
conditions. Brief periods of MVFR conditions in showers are
possible this morning along northeastern slopes of all islands
potentially affecting Hilo and Lihue terminals. Drier air will
filter into the islands from the northeast later today with a
reduction in shower activity statewide through the rest of the
week.

AIRMET Tango for moderate low level turbulence over and south
through west of all mountains remains in effect.

&&

.MARINE...
The warm muggy air mass continues to exit the region today, and
the upper level low should be out of range by tonight to support
thunderstorm activity. But, for today, a slight chance of
thunderstorms remains for the offshore waters west of Kauai.

A Small Craft Advisory (SCA) remains posted this morning for the
windier zones around Maui and waters south of the Big Island.
Beginning at 2 pm HST this afternoon, a new expanded SCA goes
into effect which includes the Kaiwi Channel, the leeward and
windward waters of Maui County, and leeward waters of Oahu. This
new SCA will last through Thursday afternoon, but there is a
strong possibility that it will be extended into Thursday night.
The trades are expected to diminish thereafter, and the coverage
of the SCA will shrink accordingly.

A slight rise in the surf is expected along the east facing
shoreline for the next couple of days. This is due to the
strengthening trades and the arrival of the long period swell
generated by tropical cyclone activity in the eastern Pacific.
Waters will also turn very choppy and rough. A swell from the
Tasman Sea, generated by a gale about a week ago, is due to
arrive Friday afternoon. This swell and a persistent southeast
swell will maintain some level of small surf along the south
facing shores into the weekend and beyond.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
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 from 2 PM this afternoon to 6 PM HST
Thursday for Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Lau



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