Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 250648
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Mon Apr 24 2017

.SYNOPSIS...
A ridge north of the islands will maintain moderate east
southeast winds into midweek. Limited shower activity is expected
through Tuesday. From late Tuesday onward, more windward and
mauka showers are expected, with the potential for a period of
unsettled weather this weekend.

&&

.DISCUSSION...
A 1026 mb high is about 1500 miles 1450 miles north northeast of
Oahu this afternoon, with a ridge remaining about 400 miles to the
north of Kauai. This is maintaining a moderate east to east
southeast flow over the islands.

An upper level low remains over Kauai, and is slowly moving to the
east based on water vapor imagery. High clouds continue to stream
over the Big Island from the southeast in the upper level wind
flow on the east side of the upper low. As the upper low moves to
the east, expect the high clouds to also move to the east
clearing the Big Island.

The afternoon soundings and the satellite derived precipitable
water indicate a dry airmass will remain over the islands through
at least Tuesday morning. By late Tuesday, clouds and showers will
begin to increase over the eastern sides of the islands as low
level moisture rides in on the east to southeast flow. These
passing showers are expected to persist mainly over the windward
and mauka areas of the islands through Thursday.

Heading into Thursday, a cold front will be approaching from the
northwest. Winds will weaken and briefly shift out of the
southeast out ahead of the front on Thursday as moisture tracking
in with the east southeast flow which has the potential to form a
pre-frontal band over the area east of Kauai Thursday night and
Friday. This could bring some heavier rain to those areas. The
front will reach Kauai on Friday with strong north winds filling
in behind it.

There remains uncertainty with the the forecast for the end of the
week, however increased rain is a certainty. Both GFS and ECMWF
track a cold and deep low pressure system towards Hawaii from the
north that would destabilize the atmosphere with a lot of moisture
already in place over the islands. However, the location of the
weather features are bouncing in different places model run to
model run. In general, we anticipate a wetter than normal and
unsettled weather through the weekend over the islands. We will
focus on more details as the event time draws closer.

&&

.AVIATION...
VFR conditions will dominate through Tuesday under a rather dry
and stable trade wind flow. With limited low level moisture,
expect isolated MVFR ceilings to be confined to windward terrain,
as well as leeward slopes of the Big Island during the afternoon
hours.

Border line conditions for turbulence AIRMETs will hold through
Tuesday over some areas. Easterly winds near and just below the
inversion will hover in the 20 to 25 kt range and will produce
some low level turbulence to the west of terrain, especially
around the Big Island and Maui. An upper level trough overhead
will generate diminishing high clouds over the eastern end of the
state this evening and may produce moderate turbulence aloft,
roughly between FL300 and FL400 along and north of the band of
high clouds. We will closely monitor pilot reports.

Otherwise, no AIRMETs are anticipated overnight into Tuesday.

&&

.MARINE...
An ASCAT pass earlier today clipped the eastern half of the island
chain and showed fresh to strong easterly winds. The strongest winds
were shown over the Alenuihaha Channel, near the Hamakua coast and
across the waters south of the Big Island, where a small craft
advisory remains in place through early Wednesday morning. Guidance
remains persistent and depicts the ridge of high pressure north of
the waters weakening Wednesday through Friday as another, late
season, cold front approaches and moves over the waters. Local winds
will respond and become light between Wednesday night and Friday.
Fresh to strong northerly winds will become a possibility over the
northwest waters Friday into the weekend as strong high pressure
builds eastward in the wake of the front.

A small northwest (~310-320 deg) swell has arrived at the nearshore
PacIOOS buoys this evening, which should hold into Tuesday before
dropping off by midweek. Another reinforcement out of the same
direction is forecast to fill in late Tuesday and hold through
midweek before easing into the second half of the week. Strong
northerly winds associated with high pressure building east in the
wake of the previously discussed front moving into the local area
could translate to rising, mainly short period, surf along exposed
northern shores into the weekend.

Surf along eastern facing shores will likely hold steady into
Wednesday before dropping off Thursday and Friday as the local winds
become light.

Small surf along southern shores will hold through midweek, before
rising Thursday through Saturday as a new, long period, south-
southwest (190 deg) swell from the southern Pacific fills in. Surf
will near, if not reach, the high surf criteria (8 ft faces) at some
point from Friday through Saturday. This swell is currently
impacting the American Samoa buoy and will continue to be monitored
through Tuesday night as the swell moves through. If it ends up
peaking slightly higher than predicted, the chances for advisory-
level surf will be higher locally once it fills in.

For more detailed information on the local surf and outlook for
Oahu, see the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu that was issued earlier today.

&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...Jacobson
MARINE...Gibbs



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