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Temp4.9 C
RH12 %
WindENE 14 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

872
FXHW60 PHFO 231949
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
949 AM HST Sun Apr 23 2017

.SYNOPSIS...
Moderate to locally breezy trade winds will continue over the
next couple of days with high pressure northeast of the islands.
A wet trade wind weather pattern is expected to continue today,
with windward showers occasionally spreading leeward, and some
briefly heavy downpours on Kauai. Fewer showers are expected to
ride in on the trade winds from tonight through Tuesday.
Increasing showers are possible from Wednesday into next weekend,
with the potential for unsettled weather toward the end of the
week.

&&

.DISCUSSION...
The high clouds over the islands overnight have thinned, allowing
for a better view of the lower clouds over the islands. Will
likely be making some tweaks to today's forecast over the next
hour or so to better reflect the cloud coverage for today, and
will factor in some tweaks to the PoPs as well.

A 1024 mb high is about 1000 miles northeast of the islands this
morning. The models are in good agreement with the high very slowly
edging to the east throughout the next several days. This will
maintain moderate to locally breezy trade winds across the region
at least into the middle of the week. A northeast to southwest
upper level trough is just west of Kauai this morning, and is
forecast to continue to move to the east through midweek. High
clouds had been streaming over the islands on the east side of
this upper level trough for the last 36 hours or so, but as the
trough edges closer to the islands, the high clouds have begun to
shift to the northeast of the islands. The trough will bring some
colder temperatures over the smaller islands, but the models all
keep 500 mb temperatures around -9/-10 celsius which is normal for
this time of year. The trough is however allowing showers in the
area to become deeper with the higher than normal and weak
inversion. As such, still think a few showers over or near Kauai
could be locally heavy today. Less of a chance elsewhere, but will
be monitoring the situation.

Shower activity is expected to diminish some tonight through
Tuesday with slightly lower moisture riding in on the trade winds.
A front approaching from the northwest will impact the western end
of the surface ridge north of the islands tonight and Monday. This
will help to veer the winds to the east or east southeast. Increasing
trade showers are possible Wednesday night and Thursday, with a
trend toward an unsettled weather pattern toward week's end.

A significant surface trough or front is forecast to be near or
over the islands at the end of the week, supported by an
amplifying trough aloft. A colder upper level airmass could help
to significantly destabilize the island atmosphere. Models are
currently honing in on the Thursday night through Saturday time
frame for the greatest instability, with the ECMWF now more
aggressive in destabilizing the atmosphere. Forecast confidence is
low due to poor run-to-run and model-to-model consistency.

&&

.AVIATION...
A band of low level moisture will affect mainly windward slopes of
Oahu and Kauai today. A surface ridge about 400 nm north of Kauai
will maintain easterly trade winds across the islands, while an
upper level trough centered about 250 nm west of Kauai will keep
slight instability over the western end of the state. This is
creating widespread MVFR ceilings over the terrain and east facing
slopes of Kauai and Oahu. AIRMET Sierra is in effect for these
islands until noon, with improving conditions expected during the
afternoon. Aside from isolated MVFR ceilings across windward
portions of the Big Island and Maui County, expect VFR conditions
to dominate elsewhere.

The easterly trade wind flow is more stable across the eastern end
of the state. The overnight Hilo sounding showed an inversion near
7000 ft, and since east winds at 850 mb (roughly 5,000 ft) may
increase later today and tonight, some low level turbulence will
occur to the west of terrain on the Big Island and Maui County.
However, we do not anticipate needing AIRMET Tango today.

&&

.MARINE...
The Small Craft Advisory (SCA) has been extended through Monday
night for coastal water zones surrounding the Big Island and some
areas around Maui County. See the advisory list below for the
affected areas. The SCA extension was primarily due to strong east
southeast winds interacting with the island terrain and is
adjusted slightly from the typical windy areas due to a slight
veering in the normal trade wind direction. Wind speeds will
maintain through at least Monday night with possible extension for
the same or similar zones into Tuesday.

A number of mainly small swells will affect the state this week
and High Surf Advisories are not expected for at least the first
half of the week. Wind waves, along with a little bit of a trade
wind fetch upstream, will bring small but choppy waters across the
east facing shores through most of the week. Small north and
northwest swells will fade in and out over the next week as well.
Small, long period south swells will also fade in and out this
week.

A stationary upper trough with an axis approximately 250 nm west
of Kauai will trigger scattered thunderstorms in the northwest
offshore waters today and tonight. These thunderstorms are not
expected to reach the western coastal waters zones.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

$$


DISCUSSION...M Ballard
AVIATION...Wroe
MARINE...Foster

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office