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WindNNE 25 mph
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Hawaiian Islands Synoptic Discussion and Guidance

566
FXHW60 PHFO 231347
AFDHFO

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

.SYNOPSIS...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through Friday, with the trades then
easing over the weekend into early next week. Clouds and showers
will favor windward and mauka areas through the weekend, with a
stray shower spilling over into leeward areas from time to time.
Deep tropical moisture is expected to lift northward into the
islands late in the weekend through the first half of next week,
bringing potentially unsettled weather to much of the state.

&&

.DISCUSSION...
Currently at the surface, a 1029 mb high centered around 725
miles north-northwest of Honolulu, is driving moderate to locally
breezy trade winds across the island chain early this morning.
Infrared satellite imagery shows mostly cloudy skies in place in
most windward areas, with partly cloudy conditions in leeward
locales. Meanwhile, radar imagery shows scattered light showers
moving into windward areas, with a few showers spilling over into
leeward areas as well. Main short term concern for the next few
days revolves around rain chances.

Today through Sunday,
High pressure north of the state will keep moderate to breezy
trade winds in place through the remainder of the work week, with
the speeds diminishing slightly each day. The trades are expected
to diminish to light and locally moderate levels over the weekend,
as the area of high pressure weakens and slides eastward around
400 miles north of the island chain. Clouds and showers are
expected to favor windward and mauka areas through the period,
with the occasional shower reaching leeward areas of the smaller
islands. There might be a bit of an increase in the trade showers
late tonight through Saturday, as a dissipated front brings a
slight increase in boundary layer moisture southward over the
islands, and an upper level trough moves overhead. Overall
however, the trade showers as well as rainfall amounts should
remain light through Saturday, as precipitable water values remain
at or below 1 inch.

Both the GFS and ECMWF are now in good agreement showing deep
tropical moisture spreading northward into eastern sections of the
state Saturday night and Sunday, with precipitable water values
climbing to around 1.5 inches. This will likely result in an
increase in shower coverage and intensity particularly across the
Big Island and perhaps as far northward as eastern sections of
Maui. Elsewhere, a fairly typical trade wind shower pattern
should continue.

Sunday night through next Wednesday,
The GFS and ECMWF appear to be coming into better agreement with
respect to the evolution of the large scale synoptic pattern for
early next week. A large ridge of high pressure is forecast to
develop well northeast of the islands with a digging upper level
trough approaching the state from the west. This will assist in
shifting the low level boundary layer flow around to the east-
southeast, transporting deep tropical moisture with precipitable
water values of 1.5+ inches, northward and over the island chain.
There does remain some significant differences in the surface
pressure field between the GFS and ECMWF Monday night through the
middle of next week however. The GFS spins up a surface low which
tracks northward to the east of the islands, while the ECMWF spins
up a weaker low and tracks it slowly northward into the western
islands. Due to the differences between the reliable guidance,
confidence in the forecast remains below normal.

As for sensible weather details, with the increase in deep layer
moisture over the island chain, it appears likely that eastern
sections of the state will be fairly wet Sunday night through
early next week, and possibly right into Wednesday. In fact heavy
rainfall and even thunderstorms are looking to be an increasing
probability for the early and middle portion of next week across
the Big Island. For now will leave thunder out of the forecast
until details become more clear. Elsewhere the forecast is more
uncertain, but given plenty of deep layer moisture and some
forcing for ascent provided by an upper trough moving through the
state, we will need to keep rain chances in the forecast for all
areas.

&&

.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 then begin to decrease from Friday afternoon onward.
VFR conditions are forecast with a few 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 strong trade winds associated with a 1029 mb high
pressure centered north-northwest of the state will continue to
generate rough boating conditions across the exposed waters,
mainly over the channels where the strongest winds will be. High
pressure will weaken Friday through the weekend as a gale-force
low develops to its north and drops south to around 600 nautical
miles north-northwest of the state. This will allow the strong
trades across the channels to trend down into the moderate to
fresh range. A Small Craft Advisory remains up for these channel
waters and has been extended through early Friday morning.

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 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 (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 morning forecast package will
reflect this later arrival time.

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.

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...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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