Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 231949
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
949 AM HST Thu Feb 23 2017

.SYNOPSIS...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place today with a gradual easing of the winds
through the weekend and into early next week. Clouds and showers
will remain focused over the windward and mauka areas through the
weekend, with rainfall totals expected to remain light through
Saturday. Increasing shower activity is expected starting Sunday
from east to west as additional moisture moves in over the
islands.

&&

.DISCUSSION...
Moderate to locally breezy trade winds will continue today due to
high pressure north of the state. A front to the north of the
islands tomorrow will push the surface ridge closer to the islands
allowing trades to start to weaken, and this trend is expected to
continue into the weekend. Winds are expected to remain out of the
east into the middle of the upcoming week.

Overnight soundings at Lihue and Hilo showed the dry airmass over
the islands with precipitable water (PW) near 0.68 inches at
Lihue and 0.97 inches at Hilo. This matches well with
the satellite derived precipitable water that shows the dry
airmass extends well upstream of the islands. Normal PW values
for this time of year are 1.04 at Lihue and 1.14 at Hilo.

An area of increased moisture is expected to begin to move over
the islands from the south to southwest during the second half of
the weekend. The Big Island will see a return to typical PW values
by Saturday night, and above normal by Sunday. The bulk of this moisture
is currently not expected to reach Kauai, with a return to more
typical values by Monday. The moisture will be dragged northward
in part by the front mentioned above and its corresponding upper
level low lifting to the north.

The ECWMF and GFS are in general agreement with the increase in
moisture, however the ECMWF brings in more than the GFS. Both
models develop a surface low near the Big Island by mid week, but
differ on the placement. This will weaken winds further and turn
them more to the southeast. The GFS lifts the surface low out of
the area quicker, shortening the time period of increased showers.

With the increase in moisture, expect an increase in shower
activity primarily over the Big Island and portions of Maui
County to start the new week. How long and the intensity of the
showers heading into mid week will depend on the surface low
development.

&&

.AVIATION...
A high pressure region north of the state will continue to bring
breezy winds to the Hawaiian Islands for the next 24 hours. Wind
speeds will begin to decrease from Friday afternoon onward as a
low pressure system drops into the region. VFR conditions are
forecast with isolated MVFR in light showers possible.

AIRMET Tango remains in effect for low level turbulence to the
South and West of the terrain likely remaining in place through
Thursday. No additional AIRMETs are anticipated.

&&

.MARINE...
Fresh to locally strong trade winds associated with high pressure
to the north of the state will continue to generate Small Craft
Advisory (SCA) conditions across channels and exposed areas
around Maui County and the Big Island. See MWWHFO (Marine Weather
Message) on our website for affected areas and specific winds and
seas expected. The SCA is in effect through Friday morning with
winds expected to trend down thereafter.

A new long-period west-northwest swell (310 deg) associated with
a recent hurricane-force low that developed over the northwest
Pacific this past weekend continues to fill in across the waters
late this morning. PacIOOS nearshore buoy observations from
Hanalei, Waimea and Pauwela, are running lower than predicted and
that trend will likely hold through the day before this source
begins to ease tonight through Friday. Surf along north and west
facing shores will remain below advisory levels through this time.

Another, slightly larger, west-northwest (310 deg) swell associated
with a large area of gales south of a recent hurricane force low
near the western end of the Aleutian Islands should begin to fill in
through the day Friday, peak Friday night into Saturday, then
gradually ease through the remainder of the weekend. Advisory-level
surf will be likely at and around the peak of this swell along
exposed north and west facing shores. Arrival time of this source
may end up coming later than anticipated, considering recent trends
and current model differences between the ECMWF-WAVE and GFS-
WAVEWATCH III solutions shown (ECMWF is depicting a six to nine hr
later arrival time than the GFS-WAVEWATCH III at a point forecast
near Midway, where the swell is scheduled to arrive within the next
12 hrs). As a result, the forecast reflect a later arrival time
for this swell.

A small northerly swell associated with a compact gale that was
analyzed around 1200 nautical miles north of the islands earlier
will fill in Friday and hold into Saturday before shifting out of
the northeast (020-030 deg) and rising Sunday night into Monday. An
earlier ASCAT pass clipped the western periphery of this system and
showed an area of 35 to 40 kt northerly winds favorable for the
local area. As this north and previously mentioned west-northwest
swells fill on Friday, sea heights in exposed coastal waters
north of the islands may reach the SCA threshold and may need to
issued for those affected waters.

Surf along east facing shores will steadily trend down through the
weekend as the trades gradually weaken. Surf may begin to build once
again Sunday night into Monday as the aforementioned small northerly
swell shifts out of the northeast.

A small long-period (18-20 seconds) south-southwest (220 deg) will
fill in Friday night and hold into Saturday, which may support a
slight increase in surf along south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Southeast Waters.

&&

$$

DISCUSSION...M Ballard
AVIATION...Bedal
MARINE...Foster


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