Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 260635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sat Feb 25 2017

.SYNOPSIS...
A surface ridge north of the area will maintain gentle to moderate
trade winds over the islands through Monday. Over the next few days,
a trough will deepen northwest of the islands and humid air will
spread north over the area. There will be chance of heavy showers
starting Monday afternoon. Thunderstorms and flooding rains are
possible Tuesday through Wednesday as the trough slowly moves across
the area. A ridge will build over the islands late next week
bringing light winds and dry, hazy weather.

&&

.DISCUSSION...
A surface ridge north of the area will maintain gentle to moderate
trade winds over the islands through Monday. An east to west band of
low clouds lies over Maui County and Oahu and the trades are pushing
some showers from this band over windward areas. The atmosphere is
rather stable, so the showers have been light. The low-cloud band is
shifting north slowly, and should clear Maui County late tonight and
reach Kauai County tomorrow. A band of thick high clouds associated
with a strong jet stream covers the islands. These high clouds are
not producing any rainfall but will keep skies mostly cloudy through
Monday.

A trough aloft will deepen northwest of Kauai over the next few days
and make the atmosphere unstable. The trough will also help pull
moist air up over the islands from the south. This enhanced moisture
may reach the Big Island and windward Maui by Monday afternoon,
where heavy showers are in the forecast along with increasing
chances for snow. Some of this moisture may get caught up in the
easterly trade winds and fuel heavy shower development along
windward slopes across the rest of the islands Monday night.

The ECMWF and GFS models show a strong jet stream diving into the
trough aloft. The trough will deepen further, with a low possibly
developing Tuesday night and moving over the islands Wednesday.
Tropical moisture over the islands from the south producing
thunderstorms and heavy rains which could bring flash flooding. The
freezing level is not expected to be especially low, but winter
storm conditions with strong winds and significant snowfall are
still quite likely on the summits of Mauna Kea and Mauna Loa. The
weather will turn drier and more stable starting Wednesday night as
the trough aloft moves off to the northeast.

A surface ridge building over the islands during the second half of
next week will keep winds light. The atmosphere will stabilize as
the trough northwest of the area lifts north. Afternoon clouds may
continue, but rainfall is expected to become light. The light winds
may allow vog to spread over the smaller islands through the second
half of the week.

&&

.AVIATION...
A strong jet stream will produce moderate turbulence aloft over
Kauai and Oahu tonight. AIRMET TANGO is in effect for these areas.
The jet and associated turbulence are forecast to shift east. A
broad band of high, cirrus, clouds associated with the jet blankets
the area. These clouds may thicken enough over the next couple of
days to start producing moderate icing.

An east to west band of showery low clouds over Maui County and Oahu
will shift north slowly overnight. Isolated MVFR ceilings and
mountain obscuration is possible over windward areas of Maui, Lanai,
Molokai and Oahu this evening. Low clouds and showers will decrease
over Maui County, and increase over Kauai County late tonight.

&&

.MARINE...
The current northwest swell will hold (around 6 ft at 13-14 seconds)
into Sunday evening before dropping further into the early portion
of the week. Surf from this swell will remain below High Surf
Advisory (HSA) levels. A small reinforcement out of the west-
northwest (300 deg) associated with a recent gale over the far
northwest Pacific is forecast to fill in Tuesday through mid week.

The latest surface analysis showed two compact gales centered
northeast and north-northwest of the islands. A tight pressure
gradient between these lows and a 1048 mb high centered to their
north just south of Kodiak was producing strong to gale-force winds
located on the north and northwest sides of the lows, per the last
couple of ASCAT passes. The latest wave model guidance is
initializing well with these synoptic features and depicts a small
to moderate north-northeast swell (030 deg) in the local area from
the gale to the northeast of the islands starting Sunday night,
peaking late Sunday night into Monday then subsiding through Tuesday
night. This swell will produce surf near the HSA threshold  of 8
feet for exposed east facing shores at and around the peak of the
swell.

The latest deterministic and ensemble guidance all depict a gale to
storm-force low developing a few hundred miles west-northwest of
Midway Wednesday through Thursday, then tracking north-northeast
across the Date Line Thursday. A large west-northwest swell
associated with this low is possible across the local waters next
weekend.

A ridge north of the area will maintain light to fresh breezes out
of the east-southeast through Monday night with winds and seas below
Small Craft Advisory (SCA) levels. The wind forecast becomes more
uncertain Tuesday through mid week. The global models show a low or
trough forming over the area then lifting northward. Plenty of
moisture and instability associated with this feature will lead to
increasing chances of showers and thunderstorms.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
Donaldson


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